After a series of issues with a Baxi combi I am ready to throw in the towel.

This combi has a water leak, that despite a new (plastic) valve block
still leaks. It also has a E78 failure that can either be a pressure
sensor failure of a gas valve issue. Neither seem relevant when the
fault occurs.

By way of example, a new valve body is >£300.

Anyway, I was thinking of a very simple heat only boiler, external pump,
diverter and a flat plate heat exchanger to imitate a combi, without all
the other issues and the cost where everything is off the shelf.

So a very simple boiler, but the remaining causes for concern is:

1) Target water temperature, do systems allow for different DHW and
central heating water temperatures?
2) If not are there standards for temperature compensation I can fudge?
2) Ramping up of temperature. The current boiler has an ignition fan
speed of 50% and goes up or down from here. I am aware some boilers like
to ramp the water temperature up gently to reduce room thermostat
cycling. Is there a way of overriding this?

Finally, any heat only boilers that could be recommended?

On 18/07/2023 23:43, Fredxx wrote:
> After a series of issues with a Baxi combi I am ready to throw in the
> towel.
>
> This combi has a water leak, that despite a new (plastic) valve block
> still leaks. It also has a E78 failure that can either be a pressure
> sensor failure of a gas valve issue. Neither seem relevant when the
> fault occurs.
>
> By way of example, a new valve body is >£300.
>
> Anyway, I was thinking of a very simple heat only boiler, external pump,
> diverter and a flat plate heat exchanger to imitate a combi, without all
> the other issues and the cost where everything is off the shelf.
>
> So a very simple boiler, but the remaining causes for concern is:
>
> 1) Target water temperature, do systems allow for different DHW and
> central heating water temperatures?
> 2) If not are there standards for temperature compensation I can fudge?
> 2) Ramping up of temperature. The current boiler has an ignition fan
> speed of 50% and goes up or down from here. I am aware some boilers like
> to ramp the water temperature up gently to reduce room thermostat
> cycling. Is there a way of overriding this?
>
> Finally, any heat only boilers that could be recommended?

Have a look at Intergas boilers.... only 4 parts in total (apparently)

On 18/07/2023 23:43, Fredxx wrote:
> After a series of issues with a Baxi combi I am ready to throw in the
> towel.
>
> This combi has a water leak, that despite a new (plastic) valve block
> still leaks. It also has a E78 failure that can either be a pressure
> sensor failure of a gas valve issue. Neither seem relevant when the
> fault occurs.
>
> By way of example, a new valve body is >£300.
>
> Anyway, I was thinking of a very simple heat only boiler, external pump,
> diverter and a flat plate heat exchanger to imitate a combi, without all
> the other issues and the cost where everything is off the shelf.
>
> So a very simple boiler, but the remaining causes for concern is:
>
> 1) Target water temperature, do systems allow for different DHW and
> central heating water temperatures?
> 2) If not are there standards for temperature compensation I can fudge?
> 2) Ramping up of temperature. The current boiler has an ignition fan
> speed of 50% and goes up or down from here. I am aware some boilers like
> to ramp the water temperature up gently to reduce room thermostat
> cycling. Is there a way of overriding this?
>
> Finally, any heat only boilers that could be recommended?

Simple boilers need hot and cold wter tanks. Do you have both tanks or
plans to install them?

--
Michael Chare

On 18/07/2023 23:43, Fredxx wrote:

> After a series of issues with a Baxi combi I am ready to throw in the
> towel.
>
> This combi has a water leak, that despite a new (plastic) valve block
> still leaks. It also has a E78 failure that can either be a pressure
> sensor failure of a gas valve issue. Neither seem relevant when the
> fault occurs.

Sometimes a supplier of second hand / recon parts can make sense.

>
> By way of example, a new valve body is >£300.
>
> Anyway, I was thinking of a very simple heat only boiler, external pump,
> diverter and a flat plate heat exchanger to imitate a combi, without all
> the other issues and the cost where everything is off the shelf.

Vented or unvented?

> So a very simple boiler, but the remaining causes for concern is:
>
> 1) Target water temperature, do systems allow for different DHW and
> central heating water temperatures?

Yes some do (many don't) - they have separate "call for heat" inputs for
each.

> 2) If not are there standards for temperature compensation I can fudge?

Again some support and some don't. You will likely find the ones that do
weather compensation are likely to also support split temperature operation.

> 2) Ramping up of temperature. The current boiler has an ignition fan
> speed of 50% and goes up or down from here. I am aware some boilers like
> to ramp the water temperature up gently to reduce room thermostat
> cycling. Is there a way of overriding this?

Pass - you would have to study the details of the boiler in question.
Again it will tend to be those with more sophisticated controls that
allow tweaks of that type.

> Finally, any heat only boilers that could be recommended?

Not really got experience with enough heat only boilers to comment. The
only one I have used was a Vaillant 400 series in an open vented
configuration. That does support split temperature operation. (and
weather compensation with the appropriate Vaillant controls)

Personally I would go with a system boiler rather than a combi. But if
going that far it seems a shame to go to extra effort to simulate the
biggest failing of combis (poor DHW performance) if you have the option
not to.

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On 18/07/2023 23:43, Fredxx wrote:
> After a series of issues with a Baxi combi I am ready to throw in the
> towel.
<snip>
> Anyway, I was thinking of a very simple heat only boiler, external pump,
> diverter and a flat plate heat exchanger to imitate a combi, without all
> the other issues and the cost where everything is off the shelf.
>
> So a very simple boiler, but the remaining causes for concern is:
>
> 1) Target water temperature, do systems allow for different DHW and
> central heating water temperatures?
> 2) If not are there standards for temperature compensation I can fudge?
> 2) Ramping up of temperature. The current boiler has an ignition fan
> speed of 50% and goes up or down from here. I am aware some boilers like
> to ramp the water temperature up gently to reduce room thermostat
> cycling. Is there a way of overriding this?
>
> Finally, any heat only boilers that could be recommended?

Heat only boiler and a storage tank is one way (the one I chose last
year) but you need to make space for tank etc which is why a conbi. was
typically fitted in the first instance, and a bit of jiggery-pokery to
reconfigure stuff.

My next bestm or even a better solution is the Intergas DHW/Heat
Combi-boiler.

There are no valves or plate heat exchangers to clog up, just 2 copper
pipes snaking through a big slab of aluminium which is the Main (only)
heat exchanger, the fan, burner, and a flow sensor as main components
(off the top of my head)
The only thing in contact with the aluminium is the dry heat from the
burner, and condensate.

Two things to consider are that the central heating temp. ideally
shouldn't be set higher than 60 degrees because the DHW if drawn off
while heating is on will pull heat from the heat exchanger pre-heated to
C/H temps.
That said, unless your radiators are particularly under-sized and your
house particularly un-insulated and draughty then CH running at 60
degrees C should be plenty hot enough and reduces the risk of overshoot.
Obviously 55 degrees or lower would be a better C/H temp. (bring in the
weather comp.)

What else? I believe Intergas boiler has weather compensation in-built
but you just have to buy the external sensor.

In a hard water area because you're heating the incoming water directly
without any form of inhibitor it is strongly advised to fit a
"combimate" phosphate dosing system to prevent limescale forming in the
DHW line within the boiler. This is only because the cold water is
heated directly so I understand although I can't really see the
difference as the big slab of Aluminium would distribute heat very
evenly to the incoming water.
https://www.combimate.co.uk/
When I bought my house 20 years ago, the very antiquated combi-boiler
fitted had a phosphate dosing contraption. I'd never seen one before but
it was still called a combimate back then and looked the same as they do
today so it's been a thing for a very long time is seems.

When I replace the pre-condensing Valiant boiler at work I'll be fitting
an intergas combi (with weather comp.) + combimate. No hesitation
because every other combi-boiler out there inherently has a diverter
valve P.O.S. and heat exchangers that are prone to blocking up on C/H
and DHW sludge and scale.

Cheers - Pete

On Tue, 18 Jul 2023 23:43:26 +0100, Fredxx wrote:

> After a series of issues with a Baxi combi I am ready to throw in the
> towel.
>
> This combi has a water leak, that despite a new (plastic) valve block
> still leaks. It also has a E78 failure that can either be a pressure
> sensor failure of a gas valve issue. Neither seem relevant when the
> fault occurs.
>
> By way of example, a new valve body is >£300.
>
> Anyway, I was thinking of a very simple heat only boiler, external pump,
> diverter and a flat plate heat exchanger to imitate a combi, without all
> the other issues and the cost where everything is off the shelf.
>
> So a very simple boiler, but the remaining causes for concern is:
>
> 1) Target water temperature, do systems allow for different DHW and
> central heating water temperatures?
> 2) If not are there standards for temperature compensation I can fudge?
> 2) Ramping up of temperature. The current boiler has an ignition fan
> speed of 50% and goes up or down from here. I am aware some boilers like
> to ramp the water temperature up gently to reduce room thermostat
> cycling. Is there a way of overriding this?
>
> Finally, any heat only boilers that could be recommended?

Perhaps bear in mind that you chose a less reliable brand of Combi.
Our daughter had a Baxi installed back in the day and it failed after a
few years.

Our Worcester Bosch has been running for more than 10 years and is still
(as of today) going strong.

Having said that, I regret not installing a system boiler, but only
because of low water flow rates which we didn't have at the last house
with a Combi.

Cheers

Dave R

--
AMD FX-6300 in GA-990X-Gaming SLI-CF running Windows 7 Pro x64

On 19/07/2023 10:13, Michael Chare wrote:
> On 18/07/2023 23:43, Fredxx wrote:
>> After a series of issues with a Baxi combi I am ready to throw in the
>> towel.
>>
>> This combi has a water leak, that despite a new (plastic) valve block
>> still leaks. It also has a E78 failure that can either be a pressure
>> sensor failure of a gas valve issue. Neither seem relevant when the
>> fault occurs.
>>
>> By way of example, a new valve body is >£300.
>>
>> Anyway, I was thinking of a very simple heat only boiler, external
>> pump, diverter and a flat plate heat exchanger to imitate a combi,
>> without all the other issues and the cost where everything is off the
>> shelf.
>>
>> So a very simple boiler, but the remaining causes for concern is:
>>
>> 1) Target water temperature, do systems allow for different DHW and
>> central heating water temperatures?
>> 2) If not are there standards for temperature compensation I can fudge?
>> 2) Ramping up of temperature. The current boiler has an ignition fan
>> speed of 50% and goes up or down from here. I am aware some boilers
>> like to ramp the water temperature up gently to reduce room thermostat
>> cycling. Is there a way of overriding this?
>>
>> Finally, any heat only boilers that could be recommended?
>
> Simple boilers need hot and cold wter tanks.  Do you have both tanks or
> plans to install them?

The primary system can normally be pressurised or fed with a header
tank. I presume the former to lose a header tank and in my case there's
nowhere to put it.

There is no need for cold water storage as the flat-plate heat-exchanger
would be fed with mains.

On 19/07/2023 08:37, SH wrote:
> On 18/07/2023 23:43, Fredxx wrote:
>> After a series of issues with a Baxi combi I am ready to throw in the
>> towel.
>>
>> This combi has a water leak, that despite a new (plastic) valve block
>> still leaks. It also has a E78 failure that can either be a pressure
>> sensor failure of a gas valve issue. Neither seem relevant when the
>> fault occurs.
>>
>> By way of example, a new valve body is >£300.
>>
>> Anyway, I was thinking of a very simple heat only boiler, external
>> pump, diverter and a flat plate heat exchanger to imitate a combi,
>> without all the other issues and the cost where everything is off the
>> shelf.
>>
>> So a very simple boiler, but the remaining causes for concern is:
>>
>> 1) Target water temperature, do systems allow for different DHW and
>> central heating water temperatures?
>> 2) If not are there standards for temperature compensation I can fudge?
>> 2) Ramping up of temperature. The current boiler has an ignition fan
>> speed of 50% and goes up or down from here. I am aware some boilers
>> like to ramp the water temperature up gently to reduce room thermostat
>> cycling. Is there a way of overriding this?
>>
>> Finally, any heat only boilers that could be recommended?
>
>
> Have a look at Intergas boilers.... only 4 parts in total (apparently)

I will have a look, thanks

On 19/07/2023 11:50, John Rumm wrote:
> On 18/07/2023 23:43, Fredxx wrote:
>
>> After a series of issues with a Baxi combi I am ready to throw in the
>> towel.
>>
>> This combi has a water leak, that despite a new (plastic) valve block
>> still leaks. It also has a E78 failure that can either be a pressure
>> sensor failure of a gas valve issue. Neither seem relevant when the
>> fault occurs.
>
> Sometimes a supplier of second hand / recon parts can make sense.

Except in this case I assume the replaced part has the same issue!

>>
>> By way of example, a new valve body is >£300.
>>
>> Anyway, I was thinking of a very simple heat only boiler, external
>> pump, diverter and a flat plate heat exchanger to imitate a combi,
>> without all the other issues and the cost where everything is off the
>> shelf.
>
> Vented or unvented?

Unvented, I was thinking of using a flat-plate heat exchanger but
outside of the boiler.

>> So a very simple boiler, but the remaining causes for concern is:
>>
>> 1) Target water temperature, do systems allow for different DHW and
>> central heating water temperatures?
>
> Yes some do (many don't) - they have separate "call for heat" inputs for
> each.

Ah, I wasn't sure, any names and models I can look up? Is there some
phrase I can lookup for this feature?

>> 2) If not are there standards for temperature compensation I can fudge?
>
> Again some support and some don't. You will likely find the ones that do
> weather compensation are likely to also support split temperature
> operation.

That sounds good.

>> 2) Ramping up of temperature. The current boiler has an ignition fan
>> speed of 50% and goes up or down from here. I am aware some boilers
>> like to ramp the water temperature up gently to reduce room thermostat
>> cycling. Is there a way of overriding this?
>
> Pass - you would have to study the details of the boiler in question.
> Again it will tend to be those with more sophisticated controls that
> allow tweaks of that type.
>
>> Finally, any heat only boilers that could be recommended?
>
> Not really got experience with enough heat only boilers to comment. The
> only one I have used was a Vaillant 400 series in an open vented
> configuration. That does support split temperature operation. (and
> weather compensation with the appropriate Vaillant controls)

That's good to known.

> Personally I would go with a system boiler rather than a combi. But if
> going that far it seems a shame to go to extra effort to simulate the
> biggest failing of combis (poor DHW performance) if you have the option
> not to.

I can't afford the space for a HW cylinder, hence this approach, and my
dislike for combi boilers! It's only me and if there's enough heat for a
power shower, then that is sufficient.

BTW, what is the difference between a 'system' boiler and a 'regular'
heat only one?

On 19/07/2023 13:57, www.GymRatZ.co.uk wrote:
> On 18/07/2023 23:43, Fredxx wrote:
> > After a series of issues with a Baxi combi I am ready to throw in the
> > towel.
> <snip>
> > Anyway, I was thinking of a very simple heat only boiler, external pump,
> > diverter and a flat plate heat exchanger to imitate a combi, without all
> > the other issues and the cost where everything is off the shelf.
> >
> > So a very simple boiler, but the remaining causes for concern is:
> >
> > 1) Target water temperature, do systems allow for different DHW and
> > central heating water temperatures?
> > 2) If not are there standards for temperature compensation I can fudge?
> > 2) Ramping up of temperature. The current boiler has an ignition fan
> > speed of 50% and goes up or down from here. I am aware some boilers like
> > to ramp the water temperature up gently to reduce room thermostat
> > cycling. Is there a way of overriding this?
> >
> > Finally, any heat only boilers that could be recommended?
>
> Heat only boiler and a storage tank is one way (the one I chose last
> year) but you need to make space for tank etc which is why a conbi. was
> typically fitted in the first instance, and a bit of jiggery-pokery to
> reconfigure stuff.

Unfortunately I don't have room for tanks. I was thinking of an unvented
system and a flat-plate heat-exchanger for the DHW.

> My next bestm or even a better solution is the Intergas DHW/Heat
> Combi-boiler.

I keep in mind I might still have to accept a combi solution,. despite
my dislike for them!

> There are no valves or plate heat exchangers to clog up, just 2 copper
> pipes snaking through a big slab of aluminium which is the Main (only)
> heat exchanger, the fan, burner, and a flow sensor as main components
> (off the top of my head)
> The only thing in contact with the aluminium is the dry heat from the
> burner, and condensate.
>
> Two things to consider are that the central heating temp. ideally
> shouldn't be set higher than 60 degrees because the DHW if drawn off
> while heating is on will pull heat from the heat exchanger pre-heated to
> C/H temps.
> That said, unless your radiators are particularly under-sized and your
> house particularly un-insulated and draughty then CH running at 60
> degrees C should be plenty hot enough and reduces the risk of overshoot.
> Obviously 55 degrees or lower would be a better C/H temp. (bring in the
> weather comp.)
>
> What else? I believe Intergas boiler has weather compensation in-built
> but you just have to buy the external sensor.
>
> In a hard water area because you're heating the incoming water directly
> without any form of inhibitor it is strongly advised to fit a
> "combimate" phosphate dosing system to prevent limescale forming in the
> DHW line within the boiler.  This is only because the cold water is
> heated directly so I understand although I can't really see the
> difference as the big slab of Aluminium would distribute heat very
> evenly to the incoming water.
> https://www.combimate.co.uk/
> When I bought my house 20 years ago, the very antiquated combi-boiler
> fitted had a phosphate dosing contraption. I'd never seen one before but
> it was still called a combimate back then and looked the same as they do
> today so it's been a thing for a very long time is seems.
>
> When I replace the pre-condensing Valiant boiler at work I'll be fitting
> an intergas combi (with weather comp.) + combimate. No hesitation
> because every other combi-boiler out there inherently has a diverter
> valve P.O.S. and heat exchangers that are prone to blocking up on C/H
> and DHW sludge and scale.

Thanks for the insight. Looks simple. The water here is pretty soft too.

On 19/07/2023 14:16, David wrote:
> On Tue, 18 Jul 2023 23:43:26 +0100, Fredxx wrote:
>
>> After a series of issues with a Baxi combi I am ready to throw in the
>> towel.
>>
>> This combi has a water leak, that despite a new (plastic) valve block
>> still leaks. It also has a E78 failure that can either be a pressure
>> sensor failure of a gas valve issue. Neither seem relevant when the
>> fault occurs.
>>
>> By way of example, a new valve body is >£300.
>>
>> Anyway, I was thinking of a very simple heat only boiler, external pump,
>> diverter and a flat plate heat exchanger to imitate a combi, without all
>> the other issues and the cost where everything is off the shelf.
>>
>> So a very simple boiler, but the remaining causes for concern is:
>>
>> 1) Target water temperature, do systems allow for different DHW and
>> central heating water temperatures?
>> 2) If not are there standards for temperature compensation I can fudge?
>> 2) Ramping up of temperature. The current boiler has an ignition fan
>> speed of 50% and goes up or down from here. I am aware some boilers like
>> to ramp the water temperature up gently to reduce room thermostat
>> cycling. Is there a way of overriding this?
>>
>> Finally, any heat only boilers that could be recommended?
>
>
> Perhaps bear in mind that you chose a less reliable brand of Combi.
> Our daughter had a Baxi installed back in the day and it failed after a
> few years.

It wasn't my choice of install and was here before me. It's a Baxi with
an all plastic water/hydraulic body. I accept there are more reliable
makes and you intimate.

> Our Worcester Bosch has been running for more than 10 years and is still
> (as of today) going strong.
>
> Having said that, I regret not installing a system boiler, but only
> because of low water flow rates which we didn't have at the last house
> with a Combi.

Pump?

On 19/07/2023 14:35, Fredxx wrote:

>> When I replace the pre-condensing Valiant boiler at work I'll be
>> fitting an intergas combi (with weather comp.) + combimate. No
>> hesitation because every other combi-boiler out there inherently has a
>> diverter valve P.O.S. and heat exchangers that are prone to blocking
>> up on C/H and DHW sludge and scale.
>
> Thanks for the insight. Looks simple. The water here is pretty soft too.

Splendid.

:)

I don't think the Intergas unit should be classed as a "combi boiler" as
such as it's brilliantly simple, but I suppose same scenario, if the
fan, burner or flow switch failed you're left without C/H or DHW.

I fitted an ion-exchange water softner at home when I replaced the
original combi. boiler with a new (and expensive) German M.A.N. boiler
so no concerns about scale (hard water).
Managed to keep that boiler going for about 16 years. 3 diverter valves
and a couple of pumps was all I had to fit. No "servicing" through its
life. In the end the back plate which formed the water passage for
condensing rotted out.

Far happier with the current solution of a heat store, heat only
Intergas boiler and wood burner boiler even if it meant losing 2 kitchen
cupboards and a sink, although the un-used sink and cupboards made space
for a washing machine so a far better solution in the end.

:)

On 19/07/2023 14:31, Fredxx wrote:

> BTW, what is the difference between a 'system' boiler and a 'regular'
> heat only one?
>

Internal pump on a system boiler, oh, also perhaps an internal expansion
vessel.

Heat only is literally just that. heat and a heat exchanger, everything
else has to be added on externaly.

On 19/07/2023 14:25, Fredxx wrote:

>> Simple boilers need hot and cold wter tanks.  Do you have both tanks
>> or plans to install them?
>
> The primary system can normally be pressurised or fed with a header
> tank. I presume the former to lose a header tank and in my case there's
> nowhere to put it.
>
> There is no need for cold water storage as the flat-plate heat-exchanger
> would be fed with mains.

But you're still incorporating a diverter valve and that is by far the
greatest point of failure in my experience, so all you're achieving is
slightly better access to the same components that you'd have in a
regular combi. surely?

www.GymRatZ.co.uk <noreply@where.cron> wrote:
> On 19/07/2023 14:25, Fredxx wrote:
>
> >> Simple boilers need hot and cold wter tanks.  Do you have both tanks
> >> or plans to install them?
> >
> > The primary system can normally be pressurised or fed with a header
> > tank. I presume the former to lose a header tank and in my case there's
> > nowhere to put it.
> >
> > There is no need for cold water storage as the flat-plate heat-exchanger
> > would be fed with mains.
>
> But you're still incorporating a diverter valve and that is by far the
> greatest point of failure in my experience, so all you're achieving is
> slightly better access to the same components that you'd have in a
> regular combi. surely?

I know nothing about combis, but I'm curious whether there's any more
control system in them than:

- DHW flow switch closes, switch diverter to DHW, fire boiler at high power
- DHW flow switch opens, switch diverter to CH, boiler reverts to previous
state

I could imagine something fancier, eg varying the gas flow to control the
temperature ramp. For example I'd be worried that a DIY combi running flat
out is at risk of overheating if there's any drop in the water pressure.
You want the target temp to be 60C, but not 90 and definitely not 110 if
there's a sudden drop in water pressure like somebody flushing a toilet. On
a regular combi I'd guess there's something that adjusts the gas flow so the
output temp is right, and to make sure the temp never overshoots (don't want
to go above 60 even for a short time, and even a short burst of 110 is Very
Bad).

Theo

On 19/07/2023 14:31, Fredxx wrote:
> On 19/07/2023 11:50, John Rumm wrote:
>> On 18/07/2023 23:43, Fredxx wrote:
>>
>>> After a series of issues with a Baxi combi I am ready to throw in the
>>> towel.
>>>
>>> This combi has a water leak, that despite a new (plastic) valve block
>>> still leaks. It also has a E78 failure that can either be a pressure
>>> sensor failure of a gas valve issue. Neither seem relevant when the
>>> fault occurs.
>>
>> Sometimes a supplier of second hand / recon parts can make sense.
>
> Except in this case I assume the replaced part has the same issue!
>
>>>
>>> By way of example, a new valve body is >£300.
>>>
>>> Anyway, I was thinking of a very simple heat only boiler, external
>>> pump, diverter and a flat plate heat exchanger to imitate a combi,
>>> without all the other issues and the cost where everything is off the
>>> shelf.
>>
>> Vented or unvented?
>
> Unvented, I was thinking of using a flat-plate heat exchanger but
> outside of the boiler.
>
>>> So a very simple boiler, but the remaining causes for concern is:
>>>
>>> 1) Target water temperature, do systems allow for different DHW and
>>> central heating water temperatures?
>>
>> Yes some do (many don't) - they have separate "call for heat" inputs
>> for each.
>
> Ah, I wasn't sure, any names and models I can look up? Is there some
> phrase I can lookup for this feature?

The phrase "split temperature operation" seems to be one common
expression. Vaillant & Bosch do it - plus others - I don' have a mental
exhaustive list.

>>> 2) If not are there standards for temperature compensation I can fudge?
>>
>> Again some support and some don't. You will likely find the ones that
>> do weather compensation are likely to also support split temperature
>> operation.
>
> That sounds good.
>
>>> 2) Ramping up of temperature. The current boiler has an ignition fan
>>> speed of 50% and goes up or down from here. I am aware some boilers
>>> like to ramp the water temperature up gently to reduce room
>>> thermostat cycling. Is there a way of overriding this?
>>
>> Pass - you would have to study the details of the boiler in question.
>> Again it will tend to be those with more sophisticated controls that
>> allow tweaks of that type.
>>
>>> Finally, any heat only boilers that could be recommended?
>>
>> Not really got experience with enough heat only boilers to comment.
>> The only one I have used was a Vaillant 400 series in an open vented
>> configuration. That does support split temperature operation. (and
>> weather compensation with the appropriate Vaillant controls)
>
> That's good to known.
>
>> Personally I would go with a system boiler rather than a combi. But if
>> going that far it seems a shame to go to extra effort to simulate the
>> biggest failing of combis (poor DHW performance) if you have the
>> option not to.
>
> I can't afford the space for a HW cylinder, hence this approach, and my
> dislike for combi boilers! It's only me and if there's enough heat for a
> power shower, then that is sufficient.
>
> BTW, what is the difference between a 'system' boiler and a 'regular'
> heat only one?

System boilers typically contain the key bits of a system in one box. So
for an unvented system it will include the expansion vessel plus the
over temp / pressure relief valve. They also usually include the pump.
Zone valves, room and cylinder stats are still external.

In most respects you can think of a system boiler as being like a combi
but without the diversion valve, plate heat exchanger, and the DHW flow
sensing.

For example, here is a Vaillant 800 series combi:

https://wiki.diyfaq.org.uk/index.php/File:VaillantEcotec824HE.png

Here is the equivalent 600 series system boiler:

https://wiki.diyfaq.org.uk/index.php/File:EcoTec%2B_624_AirIntakeOff.jpg

As you can see, not much difference!

--
Cheers,

John.

/=================================================================\
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| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On 19/07/2023 15:05, www.GymRatZ.co.uk wrote:
> On 19/07/2023 14:25, Fredxx wrote:
>
>>> Simple boilers need hot and cold wter tanks.  Do you have both tanks
>>> or plans to install them?
>>
>> The primary system can normally be pressurised or fed with a header
>> tank. I presume the former to lose a header tank and in my case
>> there's nowhere to put it.
>>
>> There is no need for cold water storage as the flat-plate
>> heat-exchanger would be fed with mains.
>
> But you're still incorporating a diverter valve and that is by far the
> greatest point of failure in my experience, so all you're achieving is
> slightly better access to the same components that you'd have in a
> regular combi. surely?

Yes, but readily available at the usual places like Screwfix and at a
fraction of the cost.

On 19/07/2023 16:02, Theo wrote:
> www.GymRatZ.co.uk <noreply@where.cron> wrote:
>> On 19/07/2023 14:25, Fredxx wrote:
>>
>>>> Simple boilers need hot and cold wter tanks.  Do you have both tanks
>>>> or plans to install them?
>>>
>>> The primary system can normally be pressurised or fed with a header
>>> tank. I presume the former to lose a header tank and in my case there's
>>> nowhere to put it.
>>>
>>> There is no need for cold water storage as the flat-plate heat-exchanger
>>> would be fed with mains.
>>
>> But you're still incorporating a diverter valve and that is by far the
>> greatest point of failure in my experience, so all you're achieving is
>> slightly better access to the same components that you'd have in a
>> regular combi. surely?
>
> I know nothing about combis, but I'm curious whether there's any more
> control system in them than:
>
> - DHW flow switch closes, switch diverter to DHW, fire boiler at high power
> - DHW flow switch opens, switch diverter to CH, boiler reverts to previous
> state
>
> I could imagine something fancier, eg varying the gas flow to control the
> temperature ramp. For example I'd be worried that a DIY combi running flat
> out is at risk of overheating if there's any drop in the water pressure.
> You want the target temp to be 60C, but not 90 and definitely not 110 if
> there's a sudden drop in water pressure like somebody flushing a toilet. On
> a regular combi I'd guess there's something that adjusts the gas flow so the
> output temp is right, and to make sure the temp never overshoots (don't want
> to go above 60 even for a short time, and even a short burst of 110 is Very
> Bad).

Most modern combis will allow you to set an upper fixed point on the DHW
temperature. However most don't have any form of flow regulation. They
usually do have output power modulation though.

That means that they can produce DHW at the requested set point for
moderate flow rates. If you lower the flow rate, then the boiler can
modulate down to prevent it exceeding the set point. If you go to low,
then it will have to cycle the burner once the maximum turn down limit
is reached on the modulation.

If you demand more flow rate and the boiler is already at full power,
then the temperature will fall (laws of physics and all that!). For
boilers that have DHW flow regulation, they can limit the flow to
prevent a large temp fall.

If implementing a DIY combi with external DHW, then much the same
modulation should still occur. At low DHW flow rates the Plate Heat
Exchanger will be unable to draw much energy from the primary flow. The
boiler should respond by modulating down as its return temperature
increases. If it increases further once at minimum output power it will
cycle off. However here the behaviour may diverge if the boiler has an
anti cycling timer to prevent short cycling. That will block the
generation of any more DHW completely for several minutes.

--
Cheers,

John.

/=================================================================\
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| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On 19 Jul 2023 at 14:16:38 BST, David wrote:

> On Tue, 18 Jul 2023 23:43:26 +0100, Fredxx wrote:
>
>> After a series of issues with a Baxi combi I am ready to throw in the
>> towel.
>>
>> This combi has a water leak, that despite a new (plastic) valve block
>> still leaks. It also has a E78 failure that can either be a pressure
>> sensor failure of a gas valve issue. Neither seem relevant when the
>> fault occurs.
>>
>> By way of example, a new valve body is >£300.
>>
>> Anyway, I was thinking of a very simple heat only boiler, external pump,
>> diverter and a flat plate heat exchanger to imitate a combi, without all
>> the other issues and the cost where everything is off the shelf.
>>
>> So a very simple boiler, but the remaining causes for concern is:
>>
>> 1) Target water temperature, do systems allow for different DHW and
>> central heating water temperatures?
>> 2) If not are there standards for temperature compensation I can fudge?
>> 2) Ramping up of temperature. The current boiler has an ignition fan
>> speed of 50% and goes up or down from here. I am aware some boilers like
>> to ramp the water temperature up gently to reduce room thermostat
>> cycling. Is there a way of overriding this?
>>
>> Finally, any heat only boilers that could be recommended?
>
>
> Perhaps bear in mind that you chose a less reliable brand of Combi.

It does appear so. My last Ideal boiler was still going strongish after 12
years. Admittedly it had needed about £300 worth of parts at the 10 year mark.
The Vaillant combi in my current house is at least as old. It too has needed
parts recently - but well under £100 and easy to fit. I'll probably replace it
with another combi come the time, although I will be looking at air source
heat pumps.

That said, I am looking to install an electric shower over the bath as a
backup.

--
Cheers, Rob, Sheffield UK

On 20/07/2023 00:23, John Rumm wrote:
> On 19/07/2023 14:31, Fredxx wrote:
>> On 19/07/2023 11:50, John Rumm wrote:
>>> On 18/07/2023 23:43, Fredxx wrote:
>>>
>>>> After a series of issues with a Baxi combi I am ready to throw in
>>>> the towel.
>>>>
>>>> This combi has a water leak, that despite a new (plastic) valve
>>>> block still leaks. It also has a E78 failure that can either be a
>>>> pressure sensor failure of a gas valve issue. Neither seem relevant
>>>> when the fault occurs.
>>>
>>> Sometimes a supplier of second hand / recon parts can make sense.
>>
>> Except in this case I assume the replaced part has the same issue!
>>
>>>>
>>>> By way of example, a new valve body is >£300.
>>>>
>>>> Anyway, I was thinking of a very simple heat only boiler, external
>>>> pump, diverter and a flat plate heat exchanger to imitate a combi,
>>>> without all the other issues and the cost where everything is off
>>>> the shelf.
>>>
>>> Vented or unvented?
>>
>> Unvented, I was thinking of using a flat-plate heat exchanger but
>> outside of the boiler.
>>
>>>> So a very simple boiler, but the remaining causes for concern is:
>>>>
>>>> 1) Target water temperature, do systems allow for different DHW and
>>>> central heating water temperatures?
>>>
>>> Yes some do (many don't) - they have separate "call for heat" inputs
>>> for each.
>>
>> Ah, I wasn't sure, any names and models I can look up? Is there some
>> phrase I can lookup for this feature?
>
>
> The phrase "split temperature operation" seems to be one common
> expression. Vaillant & Bosch do it - plus others - I don' have a mental
> exhaustive list.

That was a good start. Other terms I have no come across ate "Hot Water
Priority" and "PDHW" (Priority Domestic Hot Water).

>>>> 2) If not are there standards for temperature compensation I can fudge?
>>>
>>> Again some support and some don't. You will likely find the ones that
>>> do weather compensation are likely to also support split temperature
>>> operation.
>>
>> That sounds good.
>>
>>>> 2) Ramping up of temperature. The current boiler has an ignition fan
>>>> speed of 50% and goes up or down from here. I am aware some boilers
>>>> like to ramp the water temperature up gently to reduce room
>>>> thermostat cycling. Is there a way of overriding this?
>>>
>>> Pass - you would have to study the details of the boiler in question.
>>> Again it will tend to be those with more sophisticated controls that
>>> allow tweaks of that type.
>>>
>>>> Finally, any heat only boilers that could be recommended?
>>>
>>> Not really got experience with enough heat only boilers to comment.
>>> The only one I have used was a Vaillant 400 series in an open vented
>>> configuration. That does support split temperature operation. (and
>>> weather compensation with the appropriate Vaillant controls)
>>
>> That's good to known.
>>
>>> Personally I would go with a system boiler rather than a combi. But
>>> if going that far it seems a shame to go to extra effort to simulate
>>> the biggest failing of combis (poor DHW performance) if you have the
>>> option not to.
>>
>> I can't afford the space for a HW cylinder, hence this approach, and
>> my dislike for combi boilers! It's only me and if there's enough heat
>> for a power shower, then that is sufficient.
>>
>> BTW, what is the difference between a 'system' boiler and a 'regular'
>> heat only one?
>
> System boilers typically contain the key bits of a system in one box. So
> for an unvented system it will include the expansion vessel plus the
> over temp / pressure relief valve. They also usually include the pump.
> Zone valves, room and cylinder stats are still external.
>
> In most respects you can think of a system boiler as being like a combi
> but without the diversion valve, plate heat exchanger, and the DHW flow
> sensing.
>
> For  example, here is a Vaillant 800 series combi:
>
> https://wiki.diyfaq.org.uk/index.php/File:VaillantEcotec824HE.png
>
> Here is the equivalent 600 series system boiler:
>
> https://wiki.diyfaq.org.uk/index.php/File:EcoTec%2B_624_AirIntakeOff.jpg
>
> As you can see, not much difference!

Hmm, I see your point. But why put expensive valve gubbins in the boiler
when it could be outside leading to a smaller and less complex boiler. I
saw a Youtube clip where the guy was installing an Intergas boiler, and
it seemed the valving was external but the pump still internal,
presumably to ensure run-on.

On 20/07/2023 11:02, Fredxx wrote:
> On 20/07/2023 00:23, John Rumm wrote:

>>> BTW, what is the difference between a 'system' boiler and a 'regular'
>>> heat only one?
>>
>> System boilers typically contain the key bits of a system in one box.
>> So for an unvented system it will include the expansion vessel plus
>> the over temp / pressure relief valve. They also usually include the
>> pump. Zone valves, room and cylinder stats are still external.
>>
>> In most respects you can think of a system boiler as being like a
>> combi but without the diversion valve, plate heat exchanger, and the
>> DHW flow sensing.
>>
>> For  example, here is a Vaillant 800 series combi:
>>
>> https://wiki.diyfaq.org.uk/index.php/File:VaillantEcotec824HE.png
>>
>> Here is the equivalent 600 series system boiler:
>>
>> https://wiki.diyfaq.org.uk/index.php/File:EcoTec%2B_624_AirIntakeOff.jpg
>>
>> As you can see, not much difference!
>
> Hmm, I see your point. But why put expensive valve gubbins in the boiler
> when it could be outside leading to a smaller and less complex boiler.

The only extra valve in the combi is the DHW diverter... In many combi
installations with only a single heating zone (by far the most I would
expect) there are no external valves - it is basically W plan with the
zone valve in the boiler. So it makes the whole system very quick and
easy to install, and quite neat and self contained for the end user.
Hence the popularity.

For a system boiler they will be external, but it will be far more
common to have multiple heating zones, and that would be difficult (or
at least not cost effective) to provide for those in the boiler itself
anyway.

> I
> saw a Youtube clip where the guy was installing an Intergas boiler, and
> it seemed the valving was external but the pump still internal,
> presumably to ensure run-on.

Many heating only boilers with external pumps support run on. The power
to the pump is provided by the boiler and not the zoning wiring loom.

Where the bits are is in some ways less important than how serviceable
they are. A well laid out combi (see the Vaillant piccie) is relatively
easy to work on - the major components can be got at and swapped out
without needing to dismantle to much else to get there. Alas some boiler
designs seem designed to make simple jobs as much of a ball ache as
possible! (For example, it used to be common to stick the expansion
vessel right at the back behind everything else!). Same goes for
external valves etc. In a cramped space, wedged between a HW cylinder
and a wall at floor level is far less nice to work on than one
surrounded by free space.

--
Cheers,

John.

/=================================================================\
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|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On Thursday, 20 July 2023 at 18:39:45 UTC+1, John Rumm wrote:

> Many heating only boilers with external pumps support run on. The power
> to the pump is provided by the boiler and not the zoning wiring loom.

If needed it's doable to use a bathroom fan overrun timer to do this.

On 21/07/2023 00:34, Animal wrote:
> On Thursday, 20 July 2023 at 18:39:45 UTC+1, John Rumm wrote:
>
>> Many heating only boilers with external pumps support run on. The power
>> to the pump is provided by the boiler and not the zoning wiring loom.
>
> If needed it's doable to use a bathroom fan overrun timer to do this.

Indeed, examples here:

https://wiki.diyfaq.org.uk/index.php/Central_Heating_Controls_and_Zoning#Adding_Pump_Overrun

--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

On 20/07/2023 07:23, RJH wrote:
> On 19 Jul 2023 at 14:16:38 BST, David wrote:
>
>> On Tue, 18 Jul 2023 23:43:26 +0100, Fredxx wrote:
>>
>>> After a series of issues with a Baxi combi I am ready to throw in the
>>> towel.
>>>
>>> This combi has a water leak, that despite a new (plastic) valve block
>>> still leaks. It also has a E78 failure that can either be a pressure
>>> sensor failure of a gas valve issue. Neither seem relevant when the
>>> fault occurs.
>>>
>>> By way of example, a new valve body is >£300.
>>>
>>> Anyway, I was thinking of a very simple heat only boiler, external pump,
>>> diverter and a flat plate heat exchanger to imitate a combi, without all
>>> the other issues and the cost where everything is off the shelf.
>>>
>>> So a very simple boiler, but the remaining causes for concern is:
>>>
>>> 1) Target water temperature, do systems allow for different DHW and
>>> central heating water temperatures?
>>> 2) If not are there standards for temperature compensation I can fudge?
>>> 2) Ramping up of temperature. The current boiler has an ignition fan
>>> speed of 50% and goes up or down from here. I am aware some boilers like
>>> to ramp the water temperature up gently to reduce room thermostat
>>> cycling. Is there a way of overriding this?
>>>
>>> Finally, any heat only boilers that could be recommended?
>>
>>
>> Perhaps bear in mind that you chose a less reliable brand of Combi.
>
> It does appear so. My last Ideal boiler was still going strongish after 12
> years. Admittedly it had needed about £300 worth of parts at the 10 year mark.
> The Vaillant combi in my current house is at least as old. It too has needed
> parts recently - but well under £100 and easy to fit. I'll probably replace it
> with another combi come the time, although I will be looking at air source
> heat pumps.
>
> That said, I am looking to install an electric shower over the bath as a
> backup.
>

that reminded me of Triggers original brush that he had for ever, except
it had had new brush heads and quite a few handles.

On 18/07/2023 23:43, Fredxx wrote:
> After a series of issues with a Baxi combi I am ready to throw in the
> towel.
>
> This combi has a water leak, that despite a new (plastic) valve block
> still leaks. It also has a E78 failure that can either be a pressure
> sensor failure of a gas valve issue. Neither seem relevant when the
> fault occurs.
>
> By way of example, a new valve body is >£300.
>
> Anyway, I was thinking of a very simple heat only boiler, external pump,
> diverter and a flat plate heat exchanger to imitate a combi, without all
> the other issues and the cost where everything is off the shelf.
>
> So a very simple boiler, but the remaining causes for concern is:
>
> 1) Target water temperature, do systems allow for different DHW and
> central heating water temperatures?
> 2) If not are there standards for temperature compensation I can fudge?
> 2) Ramping up of temperature. The current boiler has an ignition fan
> speed of 50% and goes up or down from here. I am aware some boilers like
> to ramp the water temperature up gently to reduce room thermostat
> cycling. Is there a way of overriding this?
>
> Finally, any heat only boilers that could be recommended?

I thought I would say that I purchased an Intergas Rapid boiler,
essentially on the advice of many here. It certainly wasn't the cheapest
boiler on the market but it does some good features and I admire it's
simplicity. So few moving parts and the only water connections are
direct to the heat exchanger.

Many thanks for all those who contributed to this thread, very much
appreciated.