Boiler size - is bigger better?

Hi,
I'm in the process of deciding between some quotes for a replacement heat only boiler. Some people have quoted for a much larger boiler whilst other have quoted for something closer to my current boiler - which is 23.4 kW.

Is it better to have spare capacity because the boiler will modulate lower; or is it better to have a boiler close to the required output so as to work it harder? Like a lot of the stuff they've been telling me, I just don't know who to believe any more.

Any plans to extend ? Larger capacity would accommodate additional rads .

close to capacity means will struggle a bit in extremes off cold
but a say 20% over spec will meet all need s but perhaps a very slighty reduce efficiency costing perhaps £40 a year but extra capacity when you need it

A gas boiler today will modulate, so it has a range of output, for example 8 to 28 kW, in the main most houses end up with that size so it's big enough to heat water for a shower, over the 8 kW minimum it can end up switching off/on when drawing domestic hot water, so in real terms 6 kW would be better, the minimum output is more important to the maximum output.

With stored domestic hot water the boiler can have a smaller range, oil boilers often have stored DHW and 18 to 25 kW you can get modulating oil but not to the extent of gas.

So if your option is 6 kW to 25 kW or 6 kW to 35 kW then you may as well go for the big one, however if the option is 6 kW to 25 kW or 12 kW to 35 kW then it would be better to go for the smaller boiler, as large one will not modulate enough so you would need to turn taps on full every time to stop water alternating between hot and cold.

Theory is heart of winter boiler never turns off, it just turns up and down to match demand, this means the thermostat must also turn up/down with some thing like OpenTherm. As to if this actually works not so sure, the Worcester Bosch boiler will not work with OpenTherm last time I looked, they have their own Wave system, so it does not link to TRV heads so only one room is A1. I am running on oil now, and using a Nest thermostat which is linked to the TRV heads so three down stairs rooms will likely be spot on, but it is using mark/space control not modulation. Not done a winter with oil yet, so not sure how it will work.

There is a trade off, every time a boiler switches off it cools through flue to outside, so less it turns off/on the less heat is lost outside, but also the larger the off time the greater the hysteresis of the temperature. So really boiler should be only just big enough, if you go over size then likely there will be a bigger hysteresis.

So boiler in first house 3 bedroom semi was 24 kW this was with no double glazing or cavity wall insulation, and you felt the difference between just switched off, and just about to switch on again, next house had 8 to 28 kW modulating combi boiler detached 3 bedroom it was too warm, the insulation was really good, so boiler was modulated most of the time, and sun through bay windows made house far too warm. This house 18 to 25 kW oil, not a clue if big enough, but a 5 bedroom house, living room, dinning room, two kitchens, and a second small living room on three levels. However built 1985 so if not big enough one would have expected previous owners to have done something, it was toasty warm in Feb this year when we viewed it, and the open fire clearly had not been lit for some time.

You need to use a whole house boiler sizing calculator to determine the correct size boiler for your house. An oversized boiler will suffer from short-cycling and increased wear & tear. Installing a boiler with OpenTherm controls will help to keep the boiler and system at its most efficient

DursleyDavid said:

Hi,
I'm in the process of deciding between some quotes for a replacement heat only boiler. Some people have quoted for a much larger boiler whilst other have quoted for something closer to my current boiler - which is 23.4 kW.

Is it better to have spare capacity because the boiler will modulate lower; or is it better to have a boiler close to the required output so as to work it harder? Like a lot of the stuff they've been telling me, I just don't know who to believe any more.

Click to expand...

Speak to the manufacturer.
I spoke with Viessmann the other day in relation to fitting a system boiler on a job I priced up. He said best not to go too big and size a boiler that has to work hard. On the other hand don't undersize too much.

Don't forget that the boiler will be working at max output for only a few days of the year; it's the rest of the year which is just as important.

Say your heat loss is10kW boiler (assuming 0C outside and 20C inside). You have a choice of two boilers: 10kW, which can modulate down to 3kW, or a 20kW, which can modulate down to 8kW. The 10kW will be only run in on/off mode if the outside temperature is above 14C; but the 20kW boiler will run in on/off mode if the temperature is above 4C

.

It's a bit of a balance.

For starters some assumptions need to be made. ie I'm assuming its a combi boiler you have. 24kw is generaly speaking the smallest common size for a combi. Assuming the house isn't huge with poor efficiency then it would likley be enough for the heating load. Combis are sized to accomodate hot water usage.

You don't want to oversize by much to try keep efficiency as best as you can. But you could find your current boiler is smalle than what suits you. Best way to check is to find what your water flow rate is, theres no point putting a 35kw boiler onto a system that only has 9 litres a minute mains flow.

You could find that the cold flow is pretty high, and your current boiler is set to slow the water enough to heat it enough, and therefor you could have a better hot water flow rate with a bigger boiler if the cold mains is available.

If you find the current boiler provides enough hot water and heating for your requirements, then I'd suggest replacing with a similar size, go to a 28kw and that would shave off any cooler hot water you might expirience in colder temps in the winter. Or if your cold mains is noticably better than the hot flow, a 30kw or higher could be justified.

ScottishGasMan said:

For starters some assumptions need to be made. ie I'm assuming its a combi boiler you have.

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But if your assumption was wrong?
It's a heat only. It's quite a large house, with 20 rads and a big cylinder. But many of the rads are tiddly little things which heat en-suites and the like. So the fitter who estimated 1.5 kW per radiator + 4 for the tank came up with 35 kW, which is way over the top. I calculated that I have about 19 kW of radiators, and that is ample, even for my large 5 bed house. It has very well insulated walls - thermalite blocks and cavity wall insulation.
Anyways, I've settled for a 26 kW Viessmann open vent. I hope I don't have to replace it when the winter comes.

DursleyDavid said:

But if your assumption was wrong?
It's a heat only. It's quite a large house, with 20 rads and a big cylinder. But many of the rads are tiddly little things which heat en-suites and the like. So the fitter who estimated 1.5 kW per radiator + 4 for the tank came up with 35 kW, which is way over the top. I calculated that I have about 19 kW of radiators, and that is ample, even for my large 5 bed house. It has very well insulated walls - thermalite blocks and cavity wall insulation.
Anyways, I've settled for a 26 kW Viessmann open vent. I hope I don't have to replace it when the winter comes.

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Does the existing 23.4kW cope OK? If so no reason for a big increase and 26 kW sounds like a good choive.
With 20 rads your house looks bigger than average. Mine is 3/4 bed semi, 10 rads, heat-only boiler, rated 15.3kW, acctual output about 14kW (gas pressure at burner slightly below design for historical reasons) and it works fine.

DursleyDavid said:

But if your assumption was wrong?
It's a heat only.

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Which is what you said in the first sentence of you opening post!

I calculated that I have about 19 kW of radiators, and that is ample, even for my large 5 bed house. It has very well insulated walls - thermalite blocks and cavity wall insulation.

Click to expand...

If the rads were in situ before the insulation was improved, the house may not even require 19kW to heat it. But that doesn't matter; it just means the rads will be able to run at a lower temperature, which will increase the time when the boiler is actually condensing.

I had a similar situatiion at my last house (moved two weeks ago). There was a 15kW boiler and 13kW of rads. After I had insulated the loft and walls the heating requirement was only 9kW. Apart from when the cylinder was being heated, and possibly in the early morning when the house was being heated from cold (17C), the rads rarely got above 60C. The whole house stayed at a comfortable 21C all day long.