Boiler sizing from cubic area, or actual usage

[Kaymo]

Getting a replacement boiler, current one around 30 years old. The new boiler has been sized to the cubic area of the house, and I understand that this is the correct way to do it, and appreciate the one out of three engineers who bothered to do this, and to make sensible recommendations about moving the boiler etc. Worcester CDi, 30kW heating output has been recommended. If I count up the radiators in the house 17, on average around 1000x700 double panel, then again that 30kW is probably about right.

However, I am thinking about this slightly differently. For the lifetime of the boiler there are only going to be 3 of us in the house, most of the radiators are in rooms that are empty so they are either off or low. We do have visitors using the spare rooms but they only ever come in summer. Most of our time is spent in an L shaped extension with 2 radiators in it, and there is usually one, to at most three other radiators on high, with maybe another few on for background heat. My gas bill last year was only about £600 which shows we don't use a lot of energy, and we are about to make a number of insulation/glazing improvements. Even if I am being generous I can't see a scenario where we would be using more than 20kW and so I was thinking of a 24kW heart output boiler as I don't want to lose efficiency by over sizing it for its normal use.

Is there anything wrong with my thinking here?

 

[Dan Robinson]

Is it a combi? In which case he is sizing for the hot water rather than the heating, which is always going to be over powered in a combi situation.

Either way, as a professional one would size heating to the property, not the current occupants. If the occupants want to do otherwise then that is up to them.

 

[Kaymo]

The boiler is a combi, but it is 30kW for heating, and various options for hot water from 30-42kW.

I've no complaints about the engineers methods, I am thinking about what is right for our usage. My understanding is that if a boiler is over or under sized it won't be efficient.

 

[Dan Robinson]

That's true, but, if you're only using two or three rads, it won't matter what sized combi you go for really - it will be massively over sized regardless and therefore relatively inefficient. The extra kW or two of turndown, and the saving of £100 or so of boiler cost might not be worth it.

Besides, you'll probably find a 24kW boiler is enough any way fro the heating. BUT, I have not seen the house. However, it sounds like it is only a room or two bigger than mine, and that has a 15kW heat loss when calculated. When guesstimated using the various short hand methods favoured by some here, it comes out a good couple of kW's higher.

 

[Kaymo]

Once the new heating system is installed, and TRV's added, I'd say that when the heating first comes on there could be up to about 10 or so radiators coming on, but most of these will be set to low on the TRV, and so will switch off soon after, thereafter coming on and off as the TRV makes. I'd be thinking that I would have to size the boiler for the number of radiators that come on at startup, in normal operation, and that would probably be about, 10, maybe a couple more if its very cold and the boilers set to frost or 1 come on. I was thinking that a 24kW boiler is likely to be able to cope with 10-12 radiators, and that a 24kW will be more efficient for 10 rads or less, than 30kW boiler?

However, looking at the boiler specs I might have been misunderstanding how over sizing impacts boiler efficiency. If I am reading below correctly from the data sheet it seems that efficiency increases as the output lowers? If thats true then running at a third of the capacity might be more efficient than running at full load.

Useful efficiency
At rated heat output and high temperature regime η4 % 87,7
At 30 % of rated heat output and low temperature regime η1 % 99,4

 

[John D v2.0]

Neither, you should size your boiler and rads to the heat loss of the rooms at the desired temperature difference plus warm up factor.
The size of the house is not relevant as a poorly insulated old smallsmall bungalow will be far worse than a new town house with the same footprint but 3 floors and high ceilings.
The normal sizing would be for a room temperature of 18-22c depending on the purpose of the room, but you can specify your own target temperature and the engineer can work that in.
As Dan says the output will be based on hw requirements for a Combi, also the amount of efficiently lost by over sizing won't be much. Just over size the radiators and turn the ch water temp down and you'd save more.

 

[Kaymo]

Thanks John, I'm aware of all of this, but as I rarely use the attic room, one of the spare rooms, another room downstairs etc, another way of doing this is, as you say, to specify those rooms as being ~5deg, i.e. off. I'm a professional engineer, so I understand heat transfer etc. I have been in the house just over a year so I know the heat I use, roughly in watts, based on the number of radiators that are normally on.

What you guys are talking about is a theoretical need, based on room size, heat loss etc, and I completely understand why you do this, and are right to, there is no other way to do that with a customer. I already know that there is no way that use anywhere near 30kW of heat, ever, what I am trying to think through, is sizing the boiler for best efficiency, specific to how I use it. Hope that makes sense.

 

[Dan Robinson]

You obviously think you know better and won't listen to us so just crack on.

 

[John D v2.0]

I'm not a pro and I think your problem is your don't know what your question is.
Best efficiency will become by oversizing your rads as much as possible and having the water temp as low as possible.
You'll find adding insulation and draughts proofing will improve efficiently a lot.

 

[cross thread]

some combis can be range rated to suit heating output so perhaps thats the answer

 

[bernardgreen]

some combis can be range rated to suit heating output so perhaps thats the answer

Might be if the range is wide enough.

It would be worth looking at the efficiency of the boiler when operating at the ends of the range. ( if the manufacturers will release that data to you )

 

[Wildcat1961]

At 30 % of rated heat output and low temperature regime η1 % 99,4

Gross?

 

[D_Hailsham]

However, looking at the boiler specs I might have been misunderstanding how over sizing impacts boiler efficiency. If I am reading below correctly from the data sheet it seems that efficiency increases as the output lowers? If thats true then running at a third of the capacity might be more efficient than running at full load.

You are still reading the information incorrectly. It's the temperature regime which affects the efficiency, not the percentage output.

Useful efficiency
At rated heat output and high temperature regime η4 % 87,7
At 30 % of rated heat output and low temperature regime η1 % 99,4

The efficiencies quoted are gross, not net. The net equivalents are: 97.34% and 110.32%

 

[bernardgreen]

The net equivalents are: 97.34% and 110.32%

110.32% suggests more heat out than is avialable in the gas consumed. Misleading to the general public

 

[Kaymo]

Hi D_Hailsham,
At rated heat output and high temperature regime η4 % 87,7
The spec I took this from gave the rated heat output as 24kW, i.e. the maximum heat output of that particular boiler. So, I took this % to be the efficiency when running at 24kW and a high temp.
At 30 % of rated heat output and low temperature regime η1 % 99,4
I therefore took this, to be 30% of 24kW, so when running a heat output of 7.2kW, and a lower temp (presumably at the temp where the boiler is condensing)

Is this wrong?

And, as Bernard says greater than 100% efficiency is not possible, so what does net mean in this context?

Thanks.