On 2 Dec 2023 at 10:59:15 GMT, AnthonyL wrote:

>> You can get temperature fluctuation problems with some combis at low
>> rates of DHW demand since the boiler may not be able to keep within the
>> specified max DHW flow temp - even when modulated down (assuming it is a
>> modulating boiler) to minimum output. At that point the only option that
>> have is to cycle off.
>>
>
> As I understand it the boiler will not modulate low enough. A more
> modern one will.

IME/IIUC, it all depends on what you try to do. 'Low enough' will be too high
for 2 or 3 decent sized radiators. If you're only heating one smallish room
I'd have thought an electric heater would be best. A modern gas boiler will
simply not modulate down to a low load.
--
Cheers, Rob, Sheffield UK

On Sat, 2 Dec 2023 12:29:10 -0000 (UTC), RJH <patchmoney@gmx.com>
wrote:

>On 2 Dec 2023 at 10:59:15 GMT, AnthonyL wrote:
>
>>> You can get temperature fluctuation problems with some combis at low
>>> rates of DHW demand since the boiler may not be able to keep within the
>>> specified max DHW flow temp - even when modulated down (assuming it is a
>>> modulating boiler) to minimum output. At that point the only option that
>>> have is to cycle off.
>>>
>>
>> As I understand it the boiler will not modulate low enough. A more
>> modern one will.
>
>IME/IIUC, it all depends on what you try to do. 'Low enough' will be too high
>for 2 or 3 decent sized radiators. If you're only heating one smallish room
>I'd have thought an electric heater would be best. A modern gas boiler will
>simply not modulate down to a low load.

9 radiatiors, 7 rooms.

--
AnthonyL

Why ever wait to finish a job before starting the next?

On 02/12/2023 10:59, AnthonyL wrote:
> On Sat, 2 Dec 2023 04:08:03 +0000, John Rumm
> <see.my.signature@nowhere.null> wrote:
>
>> On 01/12/2023 19:19, AnthonyL wrote:
>>> On Tue, 28 Nov 2023 21:28:13 +0000, John Rumm
>>> <see.my.signature@nowhere.null> wrote:
>>>
>>>> On 28/11/2023 21:09, AnthonyL wrote:
>>>>> On Mon, 27 Nov 2023 16:51:56 +0000, John Rumm
>>>>> <see.my.signature@nowhere.null> wrote:
>>>>>

>>>>>> The fault light on the boiler does not sound like something that could
>>>>>> be as a result of anything related to the stat. I suspect you have
>>>>>> unrelated fault that has shown up at the same time by coincidence.
>>>>>>
>>>>>> Could it be as simple as the system pressure is low? What does it say on
>>>>>> the pressure gauge?
>>>>>>
>>>>>
>>>>> I don't like "coincidences". The one day x happens y also happens
>>>>> despite a year of operation (ok not much happend over summer).
>>>>
>>>> Can you posit a way in which doing something with the call for heat on
>>>> the boiler could result in a pressure / pump warning?
>>>>
>
> I'm trying to posit a way in which it doesn't happen :)

Well that is quite easy - your boiler has a fault - although marginal as
it is not stopping it completely. It seems that any prolonged period
running, or any period of inactivity is causing it to not work as it
should. The stat may be the thing that set when and for how long it
works, but that is not going to be the "cause" of the fault - just a
trigger. Likely you would see the same result with the stat replaced by
a loop of wire.

>> or possibly the radiator output vs. the rate of heat loss from the room...
>>
>
> If I get a new system in I'll have the radiator sizes and balancing
> re-checked. I think my study radiator, against and outside wall in a
> flat roof extension, is undersized. But the boiler, in trying to get
> to an unachievable temperature, ended up with the fault light so there
> was a weakness then.

I don't think the boiler "cares" whether it s being asked to do
something unachievable - it just runs in response to a call for heat,
and operates within the parameters that have been set like flow temp,
and heating load. It should run reliably 24/7 if requested to do so.

>>> FWIW I gave the boiler interior (not the burner) a bit of clean,
>>> cobwebs, spider skins, and brushed some of the crud off of the DHW
>>> sensor. I also dropped the pressure (at cold) from 0.8 to 0.6, didn't
>>> want to go any lower.
>>
>> IME boiler pressure warnings are not complaining about higher than
>> expected pressure, but lower pressure. A sealed system boiler will
>> include an over pressure relief valve to vent water outside should the
>> pressure ever get too high - so it is not something most boilers would
>> report as a fault code. A low pressure reading however will get
>> reported. Generally you would set the pressure at the boiler to between
>> 0.7 and 1 bar when cold. Depending on the total system water volume and
>> the size of the expansion vessel - you would see it rise as it warms -
>> but to usually no more than say 2.5 bar. The pressure relief valve will
>> normally be set to vent at 3 to 3.5bar
>>
>
> See response below. I don't think the pressure is, or has been, out of
> bounds, either cold or operating (currently at about 2bar when
> running).

I was more curious why you felt the need to lower the system pressure?
The manual shows the minimum system pressure as 0.5 bar. The pressure
relief is set at 3 bar.

One of the lights on the front has the description "Fault on Pump or Low
System Pressure" - so definitely not an over pressure indication.

>>> Whilst the boiler worked fine for a day the room was not reaching
>>> target temperature so I turned the control up from the 4 'o' clock
>>> position to 6 'o' clock. Here is where strange things happend. At
>>> the 4 'o' clock position the neons (in 10deg increments starting at
>>> 30deg) get to 60deg. At the 6 'o' clock position they only went up to
>>> 40deg. Needless to say the lounge did not warm up so returned to the
>>> 4 'o' clock position.
>>
>> What temperature are the neon indicators reporting - flow or return? If
>> reporting the return temp then it is entirely possible that you get
>> temperatures reported that don't necessarily match what you have set as
>> a target flow temperature, since the heating load can vary significantly
>>from one time to another.
>>
>
> Flow as best I understand it, see response below.

The manual has this to say on anti-cyling etc:

Central Heating Mode (Fig. 2)
1. With a demand for heating, the pump circulates water
through the primary circuit. At a pre-determined flow
rate the central heating flow switch operates, initiating
the ignition sequence.
2. The main burner ignites at low rate, then the gas valve
controls the gas rate to maintain the heating
temperature measured by the temperature sensor.
3. When the flow temperature exceeds the setting
temperature, a 3 minute delay occurs before the burner
relights automatically (anti-cycling). The pump continues
to run during this period.
4. When the demand is satisfied the burner is
extinguished and the pump continues to run for a
period of 3 minutes (Pump Overrun).

>
>>>
>>> System ran last night all right until again 8am with the fault light
>>> flashing. Reset and back up running until not needed at 1030.
>>>
>>> An hour later I decided to have a shower and that was a nightmare.
>>> Ran, then hot, then off. This happened a couple of times, fault light
>>> flashing then after a few moments back on again until after about the
>>> 3rd time it was all fine and I enjoyed a nice hot shower. I don't
>>> usually encounter this but I think my wife has complained of it and
>>> I'm wondering whether the difference is that she tends to have her
>>> showers when there is no call for heat whereas I have my showers early
>>> evening. Thermal shock? Steady state?
>>
>> Combis will normally give priority to DHW - so even if the heating was
>> on, drawing DHW will in effect turn off the heating for the duration.
>>
>
>> You can get temperature fluctuation problems with some combis at low
>> rates of DHW demand since the boiler may not be able to keep within the
>> specified max DHW flow temp - even when modulated down (assuming it is a
>> modulating boiler) to minimum output. At that point the only option that
>> have is to cycle off.
>>
>
> As I understand it the boiler will not modulate low enough. A more
> modern one will.

The minimum on that boiler appears to be 11kW which is fairly high.

Also if in a hard water area, scaling in the DHW plate heat exchanger
can cause issues with power transfer to the water.

>
>>> So this afternoon, I decided to have another go at reaching the target
>>> temperature and turned the dial to 4.5 'o' clock. It took about
>>> 3.5hrs to get from 17.5deg to 19.5deg and then the boiler started
>>> cycling as opposed to be continuously on as the call for heat had
>>> dropped.
>>>
>>> I observed a couple of cycles:
>>>
>>> 1815 – timed a cycle with target temperature reached (19.5° with
>>> outside at 0°)
>>> 1. Approx 5’30” on, neons at 60°
>>> 2. ~ 3 mins no burner then pump off
>>> 3. Fault light flashes, 3 neons, then another 1” before cycle
>>> restarts
>>
>> What make / model of boiler is it?
>>
>>> I'm sure I've read that it is better for the burner to cycle than be
>>> constantly on.
>>
>> With a modern condensing boiler I would say the reverse is true. The
>> ideal situation is to have it run with nice long burns with its output
>> modulated such that it closely matches the heating load. That places
>> least wear and tear on the boiler (less thermal cycling, less valve
>> operation, less stress from ignition, fan proving etc), and gives a nice
>> stable room temperature with no overshoots. It also maximises
>> condensing efficiency, since lower return temperatures will allow
>> greater latent heat recovery.
>>
>
> I don't think this boiler will modulate enough to be running
> continuously. I went through this last year (thread somewhere in this
> NG) and it was ascertained that at the lowest modulation (6kW?) it
> would still need to cycle.

In article <kt0gkpFnv86U3@mid.individual.net>, alan_m
<junk@admac.myzen.co.uk> scribeth thus
>On 02/12/2023 04:08, John Rumm wrote:
>
>> On old fixed output boilers, the temperature "control" was basically
>> just a thermostat, that would set the temperature at which the burner
>> would stop firing. So if you turned it down it would (crudely) lower the
>> average flow temperature by cycling the burner. Generally it is better
>> to have them set fairly high since you are not going to gain any
>> condensing efficiency at lower temperatures, and if it did start
>> condensing then it would cause accelerated corrosion in the boiler anyway.
>>
>> On more modern modulating boilers the flow temp will set a desired
>> output flow temp. The boiler will then run at high output until that
>> flow temp is achieved, and then start to reduce output power to maintain
>> it at that level. As the house warns and the return temp increases, then
>> they will modulate down further. If there comes a point where the
>> heating load is so low that the flow temp would exceed what you have set
>> even with the boiler at its lowest output power, then it would behave
>> like the older fixed output boiler and cycle on the internal stat.
>>
>
>If the boiler is oversized, which seems to be common when a like for
>like boiler replacement has been undertaken without re-evaluating the
>heat loss of the house, the boiler often cannot modulate down far enough
>and will always short cycle. Modern boilers do tend to have lock out
>periods to try and minimise the shortness of the cycle. After the flow
>temperature is exceeded on the lowest boiler modulation the burner will
>turn off. The boiler then locks out for x number of minutes before it
>will fire up again. The pump continues to run during this lockout and
>the flow temperature may drop quite a way below the set temperature
>before the boiler fires up again. The boiler will always fire up at the
>highest burner rate before modulating down.
>
>If the OP has set the flow temperature too low on an oversize boiler
>then he is possibly running into this lockout period. It may be better
>for heating comfort and boiler opperation to set a higher flow
>temperature rather than slavishly following the generic advice to have a
><60C flow on a boiler that doesn't run efficiently with a lower flow
>temperature
>
>
>When replacing an old boiler don't assume that the new boiler needs to
>have the same heat capacity as the old boiler. The house may have been
>improved with double glazing or better insulation in the intervening
>years. Perhaps 10 or 20 years ago the heating "engineer" may have
>performed a house heat loss calculation and just added x2 factor for the
>boiler specification. With modern condensing boilers its better to match
>the boiler more closely to the house heat loss or select a boiler with a
>much better modulation range.

Might have to look at replacing a Boiler as a heat pump won't hack it;!
and to that end anyone know of a good heat loss boiler size calculator
as i thing this boiler was too big originally!..

Cheers..
--
Tony Sayer

Man is least himself when he talks in his own person.

Give him a keyboard, and he will reveal himself.