Do you need to range-rate a modulating boiler?

[Poogle]

Hi folks, just a regular member of the public here, learning all he can before having a new combi-boiler and radiators installed in my old victorian flat. For years I've just had some fan heaters! But now it's time to upgrade.

I'm fairly techie, but I can't see why your would range-rate a boiler, if it modulates anyway. I've read and watched as much as I can; but the answer eludes me. I'm most likely misunderstand something, so would be so grateful for your help.

Thanks!

 

[Brambles]

Modulating boilers do not need to be range rated. Some manufacturers (Vaillant and others) allow “Engineer settings” to limit the maximum output for heating (only), rather than heating and hot water - I think on combis.

Having said that get a reputable installer to confirm that a Combi will deliver the hot water flows to showers et al that you require

 

[oz-plumber]

Heat only boilers are generally sized to the heatload requirements.
Combi boilers are sized to the hot water requirements.

You may need a 30 kW combi boiler for your hot water requirements, but only 12kW for your heating requirements.
So you would adjust the boiler to perform at the required level for heating and adjust the boiler to perform at the required level for hot water.

So to answer your question, yes, a combi boiler needs range rating.
Non condensing boilers via manometer on burner pressures.
Condensing boilers via a Flue Gas Analyser

 

[Dr Doolittle]

British heating is just so far behind the world its embarrasing. Just go with a boiler that has the ability to switch between room temperature and system flow temperature heating methods. This allows the use of low temperature water flow which will reduce your gas consumption by at least 40% while giving your home a lovely steady constant and even heat but has the added benefit of being able to heat really quickly when needed. These "smart" boilers have built-in weather compensation capability so you can even select the "you sort out the heating for me" button. If you want the best system, then split off the heating and DHW with 2 boilers. This means heating is always on (not with a combi) and hot water is unlimited at whatever temperature you set. If you have the space, this is the way forward. Tankless DHW heater and combi would still be less than £1,500

 

[doitmyself]

I can't see why your would range-rate a boiler, if it modulates anyway.

It all depends on the internal control logic of the boiler.

Let's say the boiler has a much higher output (say 30 kW) compared to the system requirement (e.g. 15 kW). If the boiler''s logic makes the boiler go to max output every time it re-lights, the flow temperature will rise very quickly and the boiler will be turned off by the internal stat and start cycling on and off. So it takes a very long time for the water to get up to the required temperature. And don't forget that, for most of the year, the requirement will be much less than 15 kW. If, however, the boiler restarts at minimum output, or even at the same output as when it was turned off by the internal stat, the cycling will not occur so often and the water will reach the required temperature much faster. Range-rating the boiler which goes to max on restart will have a noticeable effect.

Modulation was not designed to provide the engineer with a way of avoiding setting the boiler output to meet the system requirements..

 

[John.g]

Accepting the above but shouldn't a 30 kw boiler with a system requirement of 15 kw modulate down before cutting out as the cut out is normally SP+5C, I've watched a relations 20kw Vokera firing up on HW duty only when the coil demand is just above the min boiler output of ~ 5.5 kw and its quite impressive as it only overshoots the SP by 2C or so and then runs away at the SP, even when the cylinder is almost up to temperature where the coil demand is lower than the boiler min output it will still modulate down to minimum and after a few minutes will creep up to its cut out of SP+5C.

 

[Dr Doolittle]

Look like the obvious has to be stated then....did you calculate the peak demand required @ (-2)? If not then its always a guess
[automerge]1606850911[/automerge]

Accepting the above but shouldn't a 30 kw boiler with a system requirement of 15 kw modulate down before cutting out as the cut out is normally SP+5C, I've watched a relations 20kw Vokera firing up on HW duty only when the coil demand is just above the min boiler output of ~ 5.5 kw and its quite impressive as it only overshoots the SP by 2C or so and then runs away at the SP, even when the cylinder is almost up to temperature where the coil demand is lower than the boiler min output it will still modulate down to minimum and after a few minutes will creep up to its cut out of SP+5C.

set point 18 degrees. Overshoot by 5 degrees is nearly 30% differential...that's way off. Thats why creating the heating flow SP is the only way forward.

 

[doitmyself]

Accepting the above but shouldn't a 30 kw boiler with a system requirement of 15 kw modulate down before cutting out as the cut out is normally SP+5C,

Yes it should do that; but the problem arises when the flow temperature rises faster than the boiler can modulate down. When the anticycle time is taken into account, the aveage water temperature is low, so the house does not rise very vast, with the inevitable result that the occupants complain about the house taking ages to heat up and never gets up to temperature.

Vaillant 400 series were notorious for this, particulrly the versions with an "Auto" setting in the range-rating parameter.

 

[John.g]

Yes, the modulating down time from fire up seems to be a bigger problem for some makes, wonder is it anything to do with the fan speed etc on start up which is ~ 70%? of max output so even with de rating the start up settings are probably unchanged to obtain stable/safe light up. Also it looks like some manufacturers (all?) seem to employ the same size burner for a range of power outputs, I think I have seen power outputs ranging from say 38 kw to ~ 20 kw with the same minimum output which implies that the same size burner is employed and is actually range rated before it goes to the customer, the fact really is that gas boilers do not like cycling because of the high output start up coupled with the very small volume of water in the Hx, so no buffer like a oil fired boiler with a Hx of 20/25 litres.

 

[Poogle]

Many thanks for your kind replies, and although I confess I don't grasp some of the more detailed tech lingo, it sure gives me confidence that people here know their stuff!

I'm going for a Vokera Vision Plus 30C (combi-boiler), CH 25kW, DHW 31.6kw, Flow Rate 12.9L/min. My flat is 2 very large bedrooms, 1 very large living room, and 1 shower room. The estimated heating requirement at delta 50 is approximately 16kW (3.4m high ceilings!). So I easily could go for the 25C, which is CH 20kW, DHW 25kW, Flow Rate 10.2L/min. But I thought I would go for the better flow rate; my water pressure is approximately 20L/min.

According to the tech spec of the Vision Plus:

Vision Plus C Data Sheet

This does make sense to me. If there was no modulation, then I could see why you would want to limit the max heat output of the boiler. But with modulation, I can't see why you would do this. Indeed, in some circumstances you would want to boiler to go to max.

eg: You come back from holiday in mid-Jan, and it's minus 5C outside. The heating has been off for 3 weeks, and everything is stone cold. So you want to heat the CH water as quickly as possible, and the radiators are shedding large amounts of heat, as the rooms are so cold. The boiler can work at it's max, until the temp starts to climb, and then the modulation can throttle the boiler back, to ensure a gentle arrival at the set-point. If the boiler had been range-rated at say 70%, then even with or without modulation, the property would take much longer to heat-up.

In practice though, I guess one should at lest set the heating to at least a level to protect against frost; but even then, the max output will help. And yes, I'm getting a fancy Wiser control kit, so I can turn on the heating at home when I take off from the Bahamas!

As it's now December 1st, I wish you all a very Merry Christmas, and a Happy New Year.

 

[John.g]

The vision 20S restricts the output to 75% for 15 minutes on each start up so a automatic measure of output, desirable to you or not.
You want to check this out on your combi model because you certainly require full output for your shower.

"With the selector switch in the heating & hot water position and any additional controls (time clock, programmer, room thermostat, etc.) calling for heat, the appliance will operate in the heating mode. The pump and fan will be activated via the flow temperature sensor. When the fan is sensed to be operating correctly (tacho signal), the ignition sequence commences. Ignition is sensed by the electronic circuit to ensure flame stability at the burner. Once successful ignition has been achieved, the electronic circuitry increases the gas rate to 75% for a period of 15 minutes "

 

[Brambles]

It is worth fitting the external temperature (weather) compensation sensor to the Vokera - it is a simple plug in connection. That then adjusts the boiler water temperature ( heating only not hot water) according to the outside temperature

 

[John.g]

The vision 20S restricts the output to 75% for 15 minutes on each start up so a automatic measure of output, desirable to you or not.
You want to check this out on your combi model because you certainly require full output for your shower.

"With the selector switch in the heating & hot water position and any additional controls (time clock, programmer, room thermostat, etc.) calling for heat, the appliance will operate in the heating mode. The pump and fan will be activated via the flow temperature sensor. When the fan is sensed to be operating correctly (tacho signal), the ignition sequence commences. Ignition is sensed by the electronic circuit to ensure flame stability at the burner. Once successful ignition has been achieved, the electronic circuitry increases the gas rate to 75% for a period of 15 minutes "

Just checked out your model, as I thought it doesn't limit the output to 75% on HW but does on CH (on each start up) for 15 minutes so you will have to put up with that on a cold morning.

 

[Poogle]

Just checked out your model, as I thought it doesn't limit the output to 75% on HW but does on CH (on each start up) for 15 minutes so you will have to put up with that on a cold morning.

Many thanks! So interesting. Could you be so kind to explain (like you're talking to a 5 year old ) why the boiler does that? What is it aiming to achieve?

 

[John.g]

That would be quite a good question from a 5 year old, the short answer is that I don't know, I was a bit surprised when I read that re my relation's Vokera 20S. Technically, I can maybe see a reason for it being limited for a minute or two because it fires up at approx that output, but not for 15 minutes, I would suggest that your installer speak to Vokera as its possible that they will instruct him on how to override this except that there is something specific to these boilers.

 

[Poogle]

That would be quite a good question from a 5 year old, the short answer is that I don't know, I was a bit surprised when I read that re my relation's Vokera 20S. Technically, I can maybe see a reason for it being limited for a minute or two because it fires up at approx that output, but not for 15 minutes, I would suggest that your installer speak to Vokera as its possible that they will instruct him on how to override this except that there is something specific to these boilers.

Thank you for that advice, I will have my installer speak to them. Here are some other snippets from the Vokera Compact installation & operational manual. I assume it's the same for the Vision Plus range. It's a lot to piece together:

 

[Brambles]

John,

With the Vokera Vision , I think the 75% max for the first 15mins is linked to the type of primary heat exchanger that they use. It is (or appears to be) machined and welded from two blocks of an aluminium alloy. The control system maintains quite a tight limit on the differential temp between flow and return ( 15 degrees I think) - once exceeded it trips requiring a manual reset. My guess is that is all part of the control to limit that differential - on HW it is not an issue because of the high flow rate and quick return from the secondary.

The condensate also produces a white milky water with white grains - which if not flushed annually - eventually blocks the discharge line ahead of the trap. I am told this initially stems from the fluxes used in the manufacturing process - but is a sign of other issues if it continues after the first couple of years.

Having said that - other than that one nightmare, all the others I have dealt with have been fine - but then when you have stripped and rebuilt one a few times, you become a pocket expert.

 

[John.g]

Interesting because I also discovered that whoever installed the relations 20S installed the magnetic filter adjacent to the HW cylinder and its installed as a full fledged bypass ie its connected across the flow and return (presumably by mistake) so no problem with excessive delta T there!!, strangely enough even though the system is regularly run with only 2 rads (zoning, in a small house) with a combined output of ~ 2.5 kw causing the boiler to cycle regularly, it never causes any problems, just a 3 minute anti rapid cycling time and as stated previously only exceeds the SP by 2C for a few minutes before going on to cut out at SP+5C maybe 5 minutes or so later. Whatever about boiler efficiency due to the higher return temperature there may in fact not be a huge difference in the delta T (vs with filter installed properly) due to the much higher circulating flow but the overall temperature rise in very rapid which one might think wouldn't be great for the Hx.

 

[Poogle]

John,

With the Vokera Vision , I think the 75% max for the first 15mins is linked to the type of primary heat exchanger that they use. It is (or appears to be) machined and welded from two blocks of an aluminium alloy. The control system maintains quite a tight limit on the differential temp between flow and return ( 15 degrees I think) - once exceeded it trips requiring a manual reset. My guess is that is all part of the control to limit that differential - on HW it is not an issue because of the high flow rate and quick return from the secondary.

The condensate also produces a white milky water with white grains - which if not flushed annually - eventually blocks the discharge line ahead of the trap. I am told this initially stems from the fluxes used in the manufacturing process - but is a sign of other issues if it continues after the first couple of years.

Having said that - other than that one nightmare, all the others I have dealt with have been fine - but then when you have stripped and rebuilt one a few times, you become a pocket expert.

Many thanks for this info.

I have purchased a Wiser smart thermostat and WiFi TRV's. I think it's the case, that if the Wiser Hub is just connected as a standard on/off connection to the boiler, then all the boiler modulation and control specifics you have outlined above will still be retained?

The Vision Plus has OpenTherm, as does Wiser; but when I spoke with Vokera, they said that most 3rd party smart systems with OpenTherm are not compatible with the Vokera OpenTherm. Well so much for the "open" communication protocol! But again, would OpenTherm instructions override the boiler modulation and other Internal controls?

 

[Brambles]

My experience with Opentherm with Vokera is not very positive. I have tried on three separate systems to integrate the former Honeywell Evohome system with Vokera products. The results have been unpredictable and in one system led to lockouts on overheat.

I am sure that these issues are resolvable - but it is not plug and play - which is what I need to sell systems that are seamless in their compatibility.

Others probably have had better experiences

 

[Poogle]

My experience with Opentherm with Vokera is not very positive. I have tried on three separate systems to integrate the former Honeywell Evohome system with Vokera products. The results have been unpredictable and in one system led to lockouts on overheat.

I am sure that these issues are resolvable - but it is not plug and play - which is what I need to sell systems that are seamless in their compatibility.

Others probably have had better experiences

Thank you for this info. I think I will just leave it un-connected. With the modulation and other control aspects of the boiler, together with the Wiser system, I think I will be 95% of the way there.

 

[doitmyself]

eg: You come back from holiday in mid-Jan, and it's minus 5C outside. The heating has been off for 3 weeks, and everything is stone cold. So you want to heat the CH water as quickly as possible, and the radiators are shedding large amounts of heat, as the rooms are so cold.

You are forgetting that a heating system is sized for exactly those conditions (though usually -3C outside). It doesn't assume that the internal temperature has only dropped (e.g) five degrees. If the heat loss calculation says you need a 15kW boiler, you need a 15kW boiler;.fitting a 30kW will not achieve anything. But if you have a combi which produces 30kW for heating, then range rating down to near 15kW would be a sensible thing to do.

 

[John.g]

We shouldn't forget that if the system is down overnight then the boiler must reheat say 75/100 litres of water from say 20C to say 70C which requires ~ 4.4 kwh minimum, as this is being heated up, the rads will start emitting heat as well but it does take extra time to achieve full operating temperature, my ~ 15 kw heating load with 85 litre of water heated by a 20kw oil fired boiler takes 25 to 35 minutes in the morning before reaching 70C and cut out.

 

[Poogle]

You are forgetting that a heating system is sized for exactly those conditions (though usually -3C outside). It doesn't assume that the internal temperature has only dropped (e.g) five degrees. If the heat loss calculation says you need a 15kW boiler, you need a 15kW boiler;.fitting a 30kW will not achieve anything. But if you have a combi which produces 30kW for heating, then range rating down to near 15kW would be a sensible thing to do.

This is such an interesting conversation, and much appreciated.

From my (limited) understanding, my property's heat loss, is calculated at an external temp of -3C and in internal temp of 20C. Let's say for example the total is 13kW. My boiler will need to run, on average, at 13kW to maintain this constant internal temp of 20C, assuming all other factors, including the external temp of -3C remain constant.

But if we now consider the conditions I outlined above namely: -5C outside and -5 inside — due to leaving the house empty and unheated during a 2 week cold-spell. With this scenario, when I return from holiday, I now wish to heat the house as quickly as possible to reach 20C, and so the boiler can ramp-up to 30kW, and then modulate down to 13kW when 20C is reached?

In practice, the boiler would only increase its output so the flow temp maxes to say 80C; and the radiators would be able to shed far more heat, as the delta is now D78 and not D50 (or even lower). (Av rad temp of say 75C, and an internal temp of say -3, gives a delta of D78).

Any thoughts? Is this a correct analysis?

Hence why, it seems, that when you have a modulating boiler, you don't need to range-rate it, and indeed if you did so, it would not make the boiler as useful and as quick to rapidly heat the house when you needed to.

Of course, I understand that in practice, you would be better to perhaps leave the heating on at a low-level whilst you are away for a number of reasons.

Thanks!

 

[John.g]

If you de rate the boiler to match the heating load, say 13kw above and you come home to a freezing house, and because you want a reasonable quick heat up in some rooms initially, then simply just heat the ground floor for a hour or two, you will then get a very rapid heat up to operating temperature and then the boiler will modulate to maintain this heat load of ~ 50%, then switch in your second zone and this will heat up, at a slower rate admittedly but you won't suffer any great discomfort IMO. In practice, I don't think anyone really sets the boiler output to exactly match the heating load. If it was my house and I had a 30kw boiler with a 13kw heating load I would de rate to maybe 18 to 20kw.
Also the de aerating isn't written in stone and is settable and I think in some cases the user can select his heating output from a menu?.

 

[Chuck]

But if we now consider the conditions I outlined above namely: -5C outside and -5 inside — due to leaving the house empty and unheated during a 2 week cold-spell. With this scenario, when I return from holiday, I now wish to heat the house as quickly as possible to reach 20C, and so the boiler can ramp-up to 30kW, and then modulate down to 13kW when 20C is reached?

Assuming the system stills works at all and hasn't been destroyed by freezing, it would depend on the design of the radiator system, i.e. size of radiators and how they are piped. A system designed to emit 15kW max (at say 60°C flow) is going to struggle to emit more than about 23kW max (at say 80°C flow)

The 'perceived temperature' of a house depends roughly 50:50 on the air temperature and the temperature of the surfaces. You can get the air temperature up fairly quickly (hours) but warming the fabric of the building can take several days.

I'd recommend that you leave the house with the heating set at 10°C and the stopcocks off.

 

[doitmyself]

From my (limited) understanding, my property's heat loss, is calculated at an external temp of -3C and in internal temp of 20C. Let's say for example the total is 13kW. My boiler will need to run, on average, at 13kW to maintain this constant internal temp of 20C, assuming all other factors, including the external temp of -3C remain constant.

The boiler will not need to run at 13kW once the house is up to temperature.

Let's say the house has got up to temperature so the room stat has turned the boiler off. The room temperature starts dropping and when the temperature has dropped, say 2C, the room stat turns the boiler on. The boiler now needs to raise the temperature by 2C, not by 23C (from -3 to 20), so only 1.13kW is required (2/23 x 13). Unfortunately most boilers only modulate down to about 5KW.

 

[Poogle]

Thank you for all your kind advice and input — gives me lots to think about!

 

[John.g]

The boiler will not need to run at 13kW once the house is up to temperature.

Let's say the house has got up to temperature so the room stat has turned the boiler off. The room temperature starts dropping and when the temperature has dropped, say 2C, the room stat turns the boiler on. The boiler now needs to raise the temperature by 2C, not by 23C (from -3 to 20), so only 1.13kW is required (2/23 x 13). Unfortunately most boilers only modulate down to about 5KW.

But 13 kw is still required to maintain the house at 20C (from -3)? , last night (ambient -1C) with downstairs zone only on and with roomstat set to 22C, my 20kw boiler (oil) was cycling and burner firing for 35% of total cycle time which means my heating demand was 7 kw, this was with the roomstat switching between 22C/19C, if I had had a modulating gas boiler then I would expect it to operate continuously at 7 kw output with a boiler temperature (my SP) of 70C and would obviously stop firing with roomstat switch off, then resume firing when roomstat again calls for heat and would fire at 100% output until boiler SP again reaches reaches 70C so IMO yes it will only take a few extra kw(H) to restore the balance but at no time will its demand fall below 7kw? as long as the ambient conditions don't change, the same should apply to any heating demand.

 

[Dr Doolittle]

why not just switch to a Japanese combi and be done with any guessing. Everything is included in the price and so much more. Prices are around £1000 inc VAT and delivery (and flue and fitting kit and IoT controller (that even talks to you lol)

 

[Brambles]

Which ones (Japanese) are you thinking about? I assume not the fuel cell ones.

 

[doitmyself]

But 13 kw is still required to maintain the house at 20C (from -3)?

No. All that is required is sufficient heat to make up for the drop in temperature due to the hysteresis of the room stat.

If you need 13kw to raise the temperature 23K (-3C to 20C) that is 13/23 = 0.565kW/K. So for a room stat with a 3K hysteresis that means 3 x 0.565 = 1.7kW (approx). But you say you need 7kW. How did you measure the 35%?

 

[John.g]

Are you saying that it only takes 1.13 kw to maintain the house at a average room temperature of 19C ((20+18)/2) with a outside temp of -3C, once its up to temperature.

I'm afraid I don't get that, think of it another way, we are told that a house needs 13kw to maintain the house at 20C., IMO if you rise the roomstat slightly above 20C then the boiler still requires to run at a constant 13kw to maintain 20C, if you reduce the roomstat to 20c then the boiler and circ pump will cut out but the rads are still emitting a lot of heat, 13 kw at the time of cut out but the emittance will then fall with reducing rad temperature leading to reduced room temp and the boiler will cut in again at 18C and will again eventually get the room temp up to 20C and so on but the average energy required is still 15kwh or slightly less or the required power/hour is 15kw and not 1.13kw.

Re my measurements, if the roomstat isnt satisfied I just measure the burner run time (its quite noisy) and the off time, I do this for just a few cycles and divide the total run time by the total cycle times and that gives me a % which I X by the boiler output of 20kw. If I want to measure the average demand with the roomstat cutting in/out, I do it over a extended period of say 1 hour with a plug in energy monitor which measures the fan/burner power for that 1 hour run period and from that I can calculate the kw(h) required which is a bit less but only a 1 or 2 kw(h) or so over that period.

 

[doitmyself]

we are told that a house needs 13kw to maintain the house at 20C.

That's not how I understand it. 13kW is the boiler output required to raise the temperature from, say -3C to 20C. It's the temperature difference which is important, not the target. So if the outside drops below -3C, say to -6C, the room will only get up to 17C, not 20C.

IMO if you rise the roomstat slightly above 20C then the boiler still requires to run at a constant 13kw to maintain 20C, if you reduce the roomstat to 20c then the boiler and circ pump will cut out but the rads are still emitting a lot of heat, 13 kw at the time of cut out but the emittance will then fall with reducing rad temperature leading to reduced room temp and the boiler will cut in again at 18C and will again eventually get the room temp up to 20C and so on but the average energy required is still 15kwh or slightly less or the required power/hour is 15kw and not 1.13kw.

Sorry, but I just don't follow your logic.

 

[John.g]

That's correct, as the boiler output was based on the heat loss based from -3C to 20C, if you think the temp might drop to -6C you size accordingly plus as I suggested a margin of ~ 5 kw or so as you must heat up the system contents as well each time from cold and also it must be borne in mind that after the rads are up to full temperature the house temperature is not so the rads will emit even more, hence the need for that margin. IMO.
So again, based on the specified 13 kw requirement from -3C to 20C, do you still reckon that it only takes a little > 1 kw to maintain this house temp, I wish I had a house like that.

My logic tells me that once my house, once the system contents are up to temperature, that it will require 13kw to maintain it as it is loosing 13kw to the outside.

[automerge]1607110765[/automerge]

You are forgetting that a heating system is sized for exactly those conditions (though usually -3C outside). It doesn't assume that the internal temperature has only dropped (e.g) five degrees. If the heat loss calculation says you need a 15kW boiler, you need a 15kW boiler;.fitting a 30kW will not achieve anything. But if you have a combi which produces 30kW for heating, then range rating down to near 15kW would be a sensible thing to do.

Here is the specification based on the heat loss..

 

[Ben-gee]

Some fundamental misunderstanding here. 13kw is the amount of energy required to maintain the internal temperature at 20 when the outside temperature is -3.
It is the heat loss of the building.
I’m afraid your “only needs 1.13kW “ is incorrect - assuming your numbers were right it would be 1.13 + 13 approx; not just the 1.13
Have another think...

 

[Poogle]

Some fundamental misunderstanding here. 13kw is the amount of energy required to maintain the internal temperature at 20 when the outside temperature is -3.
It is the heat loss of the building.
I’m afraid your “only needs 1.13kW “ is incorrect - assuming your numbers were right it would be 1.13 + 13 approx; not just the 1.13
Have another think...

Many thanks for that, and even I, a humble member of the public agree.

And surely it must also be correct, that if this delta 50 heat loss of the building is 13kW (-3 temp outside, 20 inside), then if you need to heat the whole house from -3 to 20, then have a boiler that can run at 30kW is going to help. And it is going to be able to run at much more than 13kW, because the delta of the rads is now 73, and even 83 if you up the flow temp to the max of say 80C. ie: the rads are going to be able to shed far more than 13kW of energy, until the inside temp reached 20C.

Hence, if you have a self-modulating boiler, then you not only don't want to range-rate it; but doing so will actually slow down the speed at which you can heat-up a really cold house.

 

[John.g]

Some fundamental misunderstanding here. 13kw is the amount of energy required to maintain the internal temperature at 20 when the outside temperature is -3.
It is the heat loss of the building.
I’m afraid your “only needs 1.13kW “ is incorrect - assuming your numbers were right it would be 1.13 + 13 approx; not just the 1.13
Have another think...

Or maybe, at 18C the heat loss is "only" 11.87kw and 13 kw will restore the balance again at 20C.

 

[doitmyself]

Some fundamental misunderstanding here. 13kw is the amount of energy required to maintain the internal temperature at 20 when the outside temperature is -3.
It is the heat loss of the building.
I’m afraid your “only needs 1.13kW “ is incorrect - assuming your numbers were right it would be 1.13 + 13 approx; not just the 1.13
Have another think...

If 13kW was requred to maintain 20C continuously inside, the boiler would have to run continuously. But it doesn't. The boiler is turned off when 20C is reached, and turned back on when the temperature has dropped, say 2C. All that is now needed is sufficient heat to raise the temperature by 2C; which isn't 13 kW.

If you wanted to heat the house to 18C, instead of 20C, (raising by 21K instead of 23K) you would only need 13 x (21/23) kW = 11.87 kW. If you now wished to raise the temperature to 20C, do you have to supply another 13kW on top of the 11.87, i.e 24.87 kW?. No. you have to supply an extra 1.13 kW.

As an analogy, think of a car. To get from 0 mph up to 70 mph, in a reasonable time, requires much more energy then maintaining a constant 70 mph. And if the speed drops by 2 mph you only have to supply a little bit extra via the accelerator to get back up to speed again.

 

[John.g]

"If you wanted to heat the house to 18C, instead of 20C, (raising by 21K instead of 23K) you would only need 13 x (21/23) kW = 11.87 kW. If you now wished to raise the temperature to 20C, do you have to supply another 13kW on top of the 11.87, i.e 24.87 kW?. No. you have to supply an extra 1.13 kW."

Reflect on this:

You range rate your boiler to 11.87kw and wind the room stat up to 30C, what happens?, the boiler will run continuously until the house temperature reaches 18C but because the house is loosing exactly the same, then the boiler will still run continuously to maintain this 18C but the temperature cannot rise any further.
You now want a house temperature of 20C so you wind up the boiler to 13kw, what happens?, the boiler now has another 1.13 kw to get the house temp to 20C, from 18C (which requires 11.87kw continuous) it can't rise the temperature any higher after reaching 20C but will continue to run continuously at 13kw as the house is loosing exactly the same.
Having the room stat set to 20C will result in a average house temperature of 19C with a resultant average boiler demand of 12.15kw but in cycling mode.