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!

 

[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.