If you have lots of radiators in your house...

[minesadecaf]

What size boiler do you have? We currently have 12 radiators but an extension will add two more. Just wondering if during the building work it would be prudent to replace the boiler with a bigger wattage.

will you be using a hot-water cylinder? Or a combi boiler?

A combi has to be sized to heat the bath or shower water at 12 or more litres per minute, so will often be 30kW or more; far more than is needed for heating.

Depending on age and insulation of the house, it will probably have a heatind demand around 12kW.

I did once have a house with a 30kW heating demand, it was built in 1905, five bedrooms, two bath, solid walls, detached. I no longer live with such profligacy.

Observe your boiler. In cold weather, does it constantly fire at full power? Or does it fire enough to get the radiators hot, then throttle down to a small flame and turn itself on and off intermittently?

1931 2000sq ft detached soon to have a 7x4m extension added.

Currently got a 3yo 18kw boiler installed by previous owner which has given various problems and costs since we've been here.

So you have a hot-water cylinder.

What colour is it?

Sounds like 18kw might be tight if it's really badly insulated or has a large wall area for the floor space. But the extension might actually lower the heat demand.
We had an original 1920s solid wall house, normal L shape, and we had the back and side filled in and the floors all insulated. Because so many walls had been enclosed by modern cavities, and there was little extra area, the actual heat demand dropped a bit.
We have a 24kw boiler for about 120sqm and it doesn't even fire on minimum rate (7kw) continuously once it gets up to temperature as it starts to overshoot the radiator temperature after a while.

We have 13 radiators. Two are small for downstairs loo et. The remainder are pretty good size and 6 are doubles as well . In addition we have a large 70m extension with underfloor heating. Note that the demand from the extension for heat is low as the insulation is exceptional . Our boiler is apparently 30kw and easily copes.

@MarieG10

I suspect that you have a combi boiler. Is that right? They are sized big to supply the hot water. It is most unlikely that you need anywhere near that power to heat the house.

Homes with a hot-water cylinder need much less power.

Like Johnd2, I have an 8-24kW boiler (and I have an HW cylinder).

Although , if everything is cold, it runs hard for a few minutes when first turned on, it mostly modulates itself to the lowest power.

My old boiler was 15kW, which exceeded the heat demands of the house even on the coldest winter nights. But it is well-insulated. If you look at your gas meter-readings for winter, you can work out how much heat it;s providing. One cubic metre on the gasmeter is about 11.2 kWh. Have you ever used more than that?

@PigletJohn. No we have a normal water tank. It is a big house and yes copes easily but wound want any smaller.. BG were trying to persuade us to change to a newer one but said 30kw was right for us

Not that I would have them do a new boiler...outrageous quote and I looking to stop the Homeserve. Only kept on with the heating cover as they always come out quickly and have the parts but recent call out...they were shocking . Scheduled for 36 hours and then rang and said too busy with emergencies....vulnerable people apparently.

Looks like the cheaper opposition,will get a chance

We have a similar foot print and had a worcester boiler tank and unit and it is toasty. We have underfloor heating plus rads in kitchen (extended) and both bathrooms
Rads match room size so long ones in all reception rooms and the big rooms medium in the upstairs hall and box room.
I hate being cold so if you can afford it is upgrade the boiler

I think there are some online calculators or an installer should be able to work out the kW required. It think it can vary a bit depending on whereabouts you live (a sheltered, sunny spot in London vs an exposed, shaded site in a cool part of the country), and the output of your radiators etc. To me 18kW sounds a little small as the range I’ve seen has been 12-50kW and your house is quite big so I’d expect a larger kW output.

I have been trying to find a free, public-access heat loss/boiler sizing calculator without success today.

But, most boilers are oversized. This doesn't affect the purchase price very much, but it reduces efficiency. I like to drive my supercharged V8 limo to the shops because I know I have power in reserve and the engine is not stressed, right?

If your current boiler heats the house adequately, you know it has sufficient power. If it doesn't, start bt seeing if the radiators are too smal (housebuilders use over-optimistic calculations and fit small radiators because they're cheaper). If the radiator is fully hot all over, and the room is cold, then the radiator is too small and a bigger boiler will not help.

Bigger (wider) radiators can run warm not hot, and keep the room more comfortable at a more even temperature. They do not use more energy, because energy load is driven by heat loss.

For a real-world test of heat loss in your house by experiment, wait for a cold, frosty night. Go and read the gas meter. Turn up the heating and put on radiators so that all rooms are comfortably warm. Go and read the meter again after at least an hour.

One cubic metre of gas on the gas meter is about 11.2kWh.

So if you use exactly two cubic metres in one hour, you are using 22.4kW of heat. I bet you're not. But if you are, and you house is comfortably warm, a 24kW boiler is enough.

If you want a combi, you will need a bigger one. Not due to heat loss, but due to the demands of running a hot bath or shower.

I used to use the Baxi Whole House Calculator, but it seems no longer to exist.

The other links I have found push you towards boiler sales pages, without saying how many kW you need.

This one www.glow-worm.co.uk/homeowner/advice-knowledge/boiler-size-calculator/ correctly told me I need a 12kW boiler, but I had to push it into a Regular Heat-Only boiler to make it tell me I needed a 12R.

I did find this calculator for a single room, it seems to be about right. It would be a bit tedious to calculate for every room and add them together.

Though I like to have radiators 50% greater so they can warm rather than very hot. You don't need 50% more boiler, as that would make the rooms too hot.

@flouncyfanny thanks, I think I used that one but with a simple app.

Luckily I have a special Babbage Engine that can perform calculations with great accuracy and speed...

just need Ada Lovelace.

I have a 34 combi boiler, 5 beds, large open plan extensio

Vitodens, excellent boiler. Have one myself.

try

Every time we have done an extension the builders have wittered on about needing a bigger boiler. We haven't needed one yet.

We started with a 4 bed house.
Added a study with 1 radiator.
Built out over garage + 2 more radiators.
Loft conversion + 3 more radiators.

So that is 6 added with no issues with boiler.

However we don't tend to have all rooms in use so only very rarely (e.g. Christmas with visitors staying) have all radiators active.

I'd not bother changing the boiler unless and until it becomes an issue.

yes, I think a phone call about 5pm may work, and text your address later in case they forget.

We are now approaching the time of year when people start to think about boilers.