Boiler Burner Cycling question/modulation

[AnthonyL]

Baxi 105 HE Combi. Recently installed a Drayton Wiser thermostat
which is set to come on at 0630 to 0830 and call for a temperature of
19.5C in the lounge.

It started off at the right time with a room temperature of just below
17C and nearly reached 18C by 0830.

What I don't understand is the the burner would cycle on and off. Why
isn't it on all the time till the temperature is reached?

The boiler output temperature is set for 50C. It is low as I
understand from here that boiler efficiency increases with lower
return temperatures.

It may be too low to achieve the desired room temperature but then I
would expect the burners to be on all the time.


--
AnthonyL

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

[Spike]

Sounds like the boiler is cycling due it’s own thermostat being set too
low. You could be wasting gas due to this.

You need to set the flow temp to at least 60degC and preferably slightly
higher than that to improve radiator output and kill off Legionnaires - and
your lounge will also heat up faster.

--
Spike

[Andy Burns]

Spike wrote:

> AnthonyL wrote:
>
>> It started off at the right time with a room temperature of just below
>> 17C and nearly reached 18C by 0830.

1 °C in 2 hours is quite lethargic.

>> What I don't understand is the the burner would cycle on and off.
>

> Sounds like the boiler is cycling due it’s own thermostat being set too
> low.

Turning the flow temp up would be my first change.

[alan_m]

Your boiler is probably oversized for 50C.

The flow temperature reaches 50C and at this time of year if the boiler
continued to burn the temperature would have to rise above 50C - but you
have told it not to. The return flow is probably also getting close to 50C

This time of year your whole house may only have a heat loss of 2kW to
but if you have an oversized boiler, say, 25kWh for heating it may only
be able to modulate its burner down to 6kWh.

If you have a radiator that was sized for 1kWh at a 70C flow and you
have now reduced this to 50C then the maximum output of the radiator
falls to possibly below 0.6kwh.

If the system has been designed for a 70C flow temperature then it is a
balancing act between radiator sizes and flow temperatures to get the
the flow temperature down.

I'm experimenting at the moment having established that my original
system was designed for a 70C flow but now with windows changed from
single to double glazing and much more insulation that my radiators are
around 40% oversized for a 70C flow.

Currently I have a 60C flow with a design aim of 40C return but the
return is likely only to be 40C when the weather gets colder. If I have
the heating on for say an hour in the morning my boiler will also cycle.
To prevent rapid cycling it has a lockout period after the burner has
turned off before it can fire again.

As posted a few weeks ago - look at the size of the TWO radiators in
this room with a heat loss of 1.7KWh with a flow at 50C
https://youtu.be/ZEJ0VUINV0s?t=538


see the link below which may explain better
https://www.youtube.com/watch?v=wFOpHNz7Fdc




--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

[mechanic]

On Wed, 16 Nov 2022 10:29:12 GMT, AnthonyL wrote:

> Baxi 105 HE Combi. Recently installed a Drayton Wiser thermostat
> which is set to come on at 0630 to 0830 and call for a temperature of
> 19.5C in the lounge.

Baxi 105 combi is a 30kw burner machine so should be able to raise
the domestic temp up quickly. It seems to be short-cycling i.e.
turning off too soon. Are there room thermostats turning off the
rads too soon?

> It started off at the right time with a room temperature of just below
> 17C and nearly reached 18C by 0830.
>
> What I don't understand is the the burner would cycle on and off. Why
> isn't it on all the time till the temperature is reached?

The boiler is turning off when the rad circuit gets too high - check
whether the rads are heating up as expected.

[Brian Gaff]

Sounds like some form of servo hunting, or hysteresis in the control loop to
me.
Brian

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[John Rumm]

On 16/11/2022 10:29, AnthonyL wrote:

> Baxi 105 HE Combi. Recently installed a Drayton Wiser thermostat
> which is set to come on at 0630 to 0830 and call for a temperature of
> 19.5C in the lounge.
>
> It started off at the right time with a room temperature of just below
> 17C and nearly reached 18C by 0830.
>
> What I don't understand is the the burner would cycle on and off. Why
> isn't it on all the time till the temperature is reached?

It sounds as if there is insufficient load at that flow temperature.

The load will be a function of the total radiator area, and the
difference between the flow temp and the average house room temp.

If the boiler is operating at minimum power, and the flow temp exceeds
the flow temp you have set, then it will be forced to cycle since that
is the only way it can reduce its output to less than its minimum output
when firing.

This probably indicates that your radiators are undersized for that
combination of flow temp, and room temp, and/or the boiler is oversized.

If you increase the flow temp limit a bit, then the rads will be hotter
and also able to dissipate heat faster.

> The boiler output temperature is set for 50C. It is low as I
> understand from here that boiler efficiency increases with lower
> return temperatures.

It does - but also lower flow temperatures will result in lower rates of
heat movement (i.e. power) into the house. So if the total rad load is
on 5.5kW at 50 deg into a house at 17 deg, and the boiler output at
maximum "turn down" rate (i.e. modulated as low as it can go [1]) is
11kW, then it will need to cycle at a 50% duty rate to achieve an
average power output of 5.5kW.

> It may be too low to achieve the desired room temperature but then I
> would expect the burners to be on all the time.

Only *if* it can dump all the heat it is producing.

[1] The Baxi 105HE is very powerful boiler, with a heat output to the CH
of between ~11 to 31 kW. That *minimum* output power is very high (many
houses would be ok with a maximum output of that much!)

How many rads / rooms is it heating?

If it is less than 30, then I would suggest doing some heat loss calcs:

http://wiki.diyfaq.org.uk/index.php/Heat_loss

(and/or you could measure all the rads and go and lookup the typical
output power based on the size and delta T - ideally the total output of
all of them added together needs to be near the upper end of the
boiler's max power)

Minor rant: heating installers used to do proper heat loss calcs to work
out the required boiler size - since going too large meant lots of
cycling and inefficiency, less comfort for the owners since you got
larger temperature fluctuations, and not to mention the excess wear and
tear on the system resulting in lower reliability and shorter life
expectancy of the kit.

Then boilers got all clever and gained the ability to modulate their
output - at which point it seems that many installers decided - there
was no need to bother sizing it "correctly" any more, just leave it to
the modulation to match the demand. Alas that only works if the typical
heat demand falls into the output range that the boiler can actually
manage.



--
Cheers,

John.

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\=================================================================/

[AnthonyL]

On Wed, 16 Nov 2022 11:41:10 +0000, alan_m <ju...@admac.myzen.co.uk>
wrote:

Thanks.

The boiler is 30kW. I doubt that it has been engineered. The
property is a bungalow which originally had a system boiler in the
loft (1968), but the building has been extended on its side and to the
rear and the combi boiler installed 2006 in the rear extension.

The building is fully PVC double glazed, and PVC doors. The middle of
the loft is boarded over the original insulation to the ceiling
joists. I've just added 200mm to the remaining insulation under the
eves.

In the past 3 hours with an outside temp of 9-10C the lounge
temperature has dropped 1degC from the 18 it had reached at 0830. A
bit of sun now in the south facing large lounge window so it'll soon
warm up leaving me cold in the rear study :(

The video does not address combi boilers and the size needed to supply
instant hot water in particular for the shower which is in the same
area as the boiler.

All the radiators have TRVs and those that can be turned down have
been turned down. The hall radiator and the original bathroom
radiators have lockshield valves. As far as I know all the radiators
are on the same circuit and I've turned off the hall radiator. This
no doubt has an influence on the return temperature but there are 11
radiators in total.

The Baxi 105 HE manual states:

<quote>
1. Central Heating: The central heating hot water
flow temperature can be adjusted between 30° C
(ą 5° C) minimum and 85° C (ą 5° C) maximum.
2. Turn the control knob clockwise to increase the
temperature (Fig. 9).
3. In normal winter usage we recommend that the
central heating temperature be set at maximum.
</quote>

I've not tried turning the temperature to a maximum and it seems to go
into the face of condensing efficiency.

I need to test the output and return pipe temperatures. Will just
holding a digital kitchen probe thermometer closely against the pipes
be a sufficient guide?

[AnthonyL]

Thanks John.

Many of the questions you've asked are in my reply to alan_m which
crossed over with the timing of yours.

Your input into that would be welcomed.

[John Rumm]

It seems unlikely that the place will get close to the kind of heating
load that would need the full output of the boiler.

Combis are often selected for the power they can deliver to DHW rather
than heating. The Baxi seems unusual in that it can output the same
power to the CH as it can the DHW (Many combis will provide less heating
power to the CH. For example the 35kW Combi installed at my previous
house would delivery a max of 24kW to the CH).

> In the past 3 hours with an outside temp of 9-10C the lounge
> temperature has dropped 1degC from the 18 it had reached at 0830. A
> bit of sun now in the south facing large lounge window so it'll soon
> warm up leaving me cold in the rear study :(
>
> The video does not address combi boilers and the size needed to supply
> instant hot water in particular for the shower which is in the same
> area as the boiler.

A 24kW Combi will supply between 10 and 14 lpm (depending on cold water
temperature), which is usually more than adequate for most showers
unless it has multiple heads and body jets etc). So 30kW will be over
specced most showers (but would manage two moderate ones at once, and be
"ok" for bath filling)

> All the radiators have TRVs and those that can be turned down have
> been turned down. The hall radiator and the original bathroom

Which lowers the maximum power output the house can absorb lower.

> radiators have lockshield valves. As far as I know all the radiators
> are on the same circuit and I've turned off the hall radiator. This
> no doubt has an influence on the return temperature but there are 11
> radiators in total.

FWIW I am running 19 rads on a 24kW system boiler that can modulate down
to just under 6kW - that is more than adequate in a not particularly
well insulated solid wall property...

>
> The Baxi 105 HE manual states:
>
> <quote>
> 1. Central Heating: The central heating hot water
> flow temperature can be adjusted between 30° C
> (ą 5° C) minimum and 85° C (ą 5° C) maximum.
> 2. Turn the control knob clockwise to increase the
> temperature (Fig. 9).
> 3. In normal winter usage we recommend that the
> central heating temperature be set at maximum.
> </quote>
>
> I've not tried turning the temperature to a maximum and it seems to go
> into the face of condensing efficiency.

It does... it is just a "rule of thumb" that ensures the place can get
hot enough in the depths of winter (and is somewhat assuming that the
boiler is sized appropriately for the property). It probably cuts down
on the tech support calls they get from people who can't get the house
hot enough.

> I need to test the output and return pipe temperatures. Will just
> holding a digital kitchen probe thermometer closely against the pipes
> be a sufficient guide?

Stick a bit of masking tape on each pipe, and point a IR thermometer at
the tape (or find a bit of painted pipe) - don't take a reading from
bare metal since they emissivity of the copper will not be what the
thermometer is assuming.

Ideally you will need to check each rad for balance - with the TRV heads
full open to start with. A traditional setup would be balanced for a 11
degree delta. Condensing systems would be balanced for a bigger drop -
typically 21 degrees.

I expect you will find that the boiler is significantly oversized all
the time - even with -3 degree outside temps. So you may need to run
higher flow temps and sacrifice some condensing efficiency.

Installing larger rads could help, or even consider using a thermal
store to buffer the heating load (i.e. the CH output connected to a
large direct cylinder of water, which is then used to heat the CH via a
heat exchanger (plate or coil) and a pump). That way the boiler could be
set to fire in bursts of high output to replenish the store, and not
need to try and match the significantly lower rate of transfer into the CH.

[alan_m]

On 16/11/2022 13:13, AnthonyL wrote:

While the linked video doesn't necessarily cover combi water
temperatures it fully explains what you are seeing with your central
heating.

The minimum you boiler can output is around 8KWh. Today your house
possibly only needs much less than 4kW and this is if you heat the whole
house to, say, 19/20C. As John has stated in his post the boiler can
only achieve this by
i) Modulating down to the minimum output
ii) Having a on/off cycle of 50/50%

Further, if you have a radiator of nominally 1kWh designed for 70c flow
and 55C return and you then use 50C flow and 30 return it now is a
0.4kWh radiator. You could have reduced the output of all the
radiators in your house by 60% by using such a low boiler flow
temperature. Also your TVRs may have closed down some radiators as well
as the one you have turned off yourself. What this could mean is that
your radiators cannot deliver the heat loss of the building at a 50C
flow. You may now have a building heat loss of 4KWh, the active
radiators only able to achieve a 2KWh output and a boiler that cannot
output less than 8KW. The boiler will cycle and be off 75% of the time.


You may/will not be able to achieve return temperatures of 40 or less
with an oversize boiler and even if you change the boiler for one with a
lower minimum output (which usually means lower maximum output) return
temperatures of 40C may/will only be achievable with your radiators
perhaps being 3x the current size. One reason the use of "gigantic"
radiators (under floor heating) is recommended for air source heat pump
systems where the flow is around 50C.

You are probably better off setting the flow temperature first at 70C
and see how the CH performs, then reduce it to 65C and then 60C,
checking the performance each time. You may/will find as the temperature
outside falls you may be getting return temperatures of 55C or less.
Maybe for comfort you may have to set the flow up a bit during the
coldest few weeks of winter.

Consider how much efficiency you are chasing and have a realistic gaol -
don't chase the unobtainable unless you are perhaps going to make
expensive changes to your system.

[AnthonyL]

On Wed, 16 Nov 2022 19:31:33 +0000, alan_m <ju...@admac.myzen.co.uk>
wrote:

I tried it on full on when it came on at 1630 (house at 17C).
Obviously radiators very hot as the output temp was 80C. Checked the
flue and quite a big flume so wound back to 2/3rds which gave the 70C
you suggest above and hardly any sign of plume.

>Consider how much efficiency you are chasing and have a realistic gaol -
>don't chase the unobtainable unless you are perhaps going to make
>expensive changes to your system.
>

I'm chasing reduced costs. I don't propose any significant changes to
the system, at least not this winter. I want to see how having a
thermostat, improving the insulation and turning off the radiator in
the large hall/corridor space works out.

Of course at the moment month on month (last year v this year)
comparisons are a bit meaningless due the mild weather so far. But at
least I have an idea of what I'm trying to do and why.

[AnthonyL]

In the 6yrs we've been here, East Midlands but relatively high up, the
boiler has always been able to keep the house warm and I don't think
I've turn the dial to much more than 1/2 way. It's a bit more than
that now but the room thermostat should stop overheating.

>Installing larger rads could help, or even consider using a thermal
>store to buffer the heating load (i.e. the CH output connected to a
>large direct cylinder of water, which is then used to heat the CH via a
>heat exchanger (plate or coil) and a pump). That way the boiler could be
>set to fire in bursts of high output to replenish the store, and not
>need to try and match the significantly lower rate of transfer into the CH.
>
>

Thanks. I accept the boiler is oversized. Given that it is ~16yrs
old I've asked three service engineers in the past few years who have
given quotes/estimates without looking at the property and its layout.
Eg:
1) Boiler: Worcester 29cdi Condensing Boiler
2) Baxi 800 / Baxi 600 (estimates so no mention of capacity)


An ideal replacement for us would be able to:

1) Provide instant hot water for the shower next to the boiler

2) Provide hot water for a cylinder above the kitchen to avoid the
20m+ run that currently exists. The debate is to whether it is a cost
efficient solution rather than a convenience solution. Mainly
required in the winter so bleeding off some of the CH supply would be
a possibility. None of the boiler service engineers have been
enthusiastic about looking at this option.

3) Keep the house warm as needed.

The bath runs from the combi and takes some balancing of the
temperature to get right but at two or three baths/year significant
expense is not warranted. An electic shower in the main property
bathroom (the combi shower is in the utility room alongside the
boiler) is not currently functional. If I do get it working it will
probably remain electric. We still have a cold water tank in the loft
(needs replacing as it is rusting) and my gut reaction is to retain
that rather than go mains pressure throughout, though it only supplies
the toilet, cold water taps in the bathroom and an adjacent bedroom
washbasin.

The property does lend itself for a heat pump in as much as the boiler
is already on a back wall. But the work for the rest of the house
doesn't. Any piping from the boiler goes through a flat roof
extension and then awkwardly into the loft. Fully fitted carpeted
floors need to be shifted for pipework which I suspect is mostly too
small. Tile flooring lifted. Ugh!!

[John Rumm]

On 16/11/2022 20:33, AnthonyL wrote:

> Of course at the moment month on month (last year v this year)
> comparisons are a bit meaningless due the mild weather so far.

If you track down a table of "degree days" then that will allow you to
factor the variations in weather out of the sums.

[alan_m]

On 16/11/2022 21:07, AnthonyL wrote:

> The property does lend itself for a heat pump in as much as the boiler
> is already on a back wall. But the work for the rest of the house
> doesn't. Any piping from the boiler goes through a flat roof
> extension and then awkwardly into the loft. Fully fitted carpeted
> floors need to be shifted for pipework which I suspect is mostly too
> small. Tile flooring lifted. Ugh!!
>
>

Surface mount pipes on walls can be done quite neatly. Friends of mine
had a complete system fitted in a property with solid floors so pipes
run at skirting board level and in the corners of the room. Another
option is pipes running inside (hollow) skirting boards.

https://www.traderadiators.com/media/amasty/blog/uploads/2020/10/pipes-skirting.jpg

Smaller and neater if the original skirting is removed first allowing
pipes to be fitted lower and allowing the covers to be smaller.

What you really need if replacing is properly designed system. Also bear
in mind that the heat loss is usually calculated for the expected
coldest days of the year and for most of the late autumn/winter/early
spring seasons your boiler only needs to work at a quarter to half of
its maximum capacity.

[alan_m]

On 16/11/2022 13:13, AnthonyL wrote:

>

> I need to test the output and return pipe temperatures. Will just
> holding a digital kitchen probe thermometer closely against the pipes
> be a sufficient guide?

Buy a 4 off
https://www.ebay.co.uk/itm/255637631779
you may find them cheaper on Ebay/Amazon (from a non-China seller).

Also order some replacement LR44 batteries (2 off per module) as the
ones they come with may have been in there some time.
Although these modules possibly may only have an accuracy to 2C select
the two that give that best match and show the (near) same temperature.
With a bit of luck all four will show the same temperature.

Tape one probe to the flow and another to the return pipes somewhere
near the boiler. As the radiators are heating from cold you will first
see a large difference in temperature but check when all the radiators
that are turned on are hot and have been in that state for 15 minutes -
and the boiler is still firing.

You can also check the temperature difference between the flow and
return pipes on the radiators in the same way BUT checking individual
radiators doesn't necessarily give you the same results as the flow and
return to the boiler.

[AnthonyL]

On Wed, 16 Nov 2022 21:39:44 +0000, John Rumm
<see.my.s...@nowhere.null> wrote:

>On 16/11/2022 20:33, AnthonyL wrote:
>
>> Of course at the moment month on month (last year v this year)
>> comparisons are a bit meaningless due the mild weather so far.
>
>If you track down a table of "degree days" then that will allow you to
>factor the variations in weather out of the sums.
>

I'd often idly wondered whether such information was available to Joe
Public - thanks very much, my search has stumbled upon
https://www.degreedays.net/

I'm only after indicative figures and in any event have only just
started using a thermostat which is based on inside lounge
temperature.

Additionally and like most I guess the target internal temperature
varies depending on the time of day, so currently:
Deg C
0630-0830 - 19
0830-1620 - 15
1630-2230 - 19
2230-0830 - 15

I suppose I could do an internal degree day calc but broadly it is 33%
at +4 (above base) but no idea what last year was.

Gas for October 2021 - 1292 kWh
Gas for October 2022 - 840 kWh

Degree day difference between Oct 21 and Oct 22 -12%

1292 * .88 = 1136 = like for like for Oct 22
Actual 840 is (1136 - 840)/1136 = 26% reduction (due to thermostat)

I understand this is very much a first order approximation and it will
be interesting to see how the figures look as the winter draws in and
the effect (hopefully) of improving the insulation kicks in.

Certainly we are being much meaner with the amount of time the boiler
is on if only because we are more focused on the lounge temperature.
Before it was a matter of looking at the thermometer in the large
empty hall and seeing it it was around but not over 20C. That large
radiator is now off.

So without the degree day figure I could have easily been misled that
the savings were bigger due to my alterations, though this post only
exists as I was aware this October was warmer (and now I can quantify
that) compared to a year ago.

The equipment and insulation plus some tools have cost around £300.
Last year's gas cost (to end of Oct'22) ~£1100. It looks feasible to
have a payback within 2 years if not sooner. Or I've got some figures
wrong :)