boiler sizing

we’re having a large extension done which is expanding our house from 130->250m2 – so it’ll end up really quite large. The plumber has started (subbed from the main builders) and is telling us that our existing 15kw (system) boiler (which looking at the ranges for sale, appears to be really quite small) is “stretched at the moment” and no way will heat the house when its done. On the face of it this seems reasonable, we are almost doubling the size of the house.

Looking at a couple of high level guides like British Gas, they’re sizing in a 30kw boiler, which (roughly) lines up with what the plumber has said. https://www.britishgas.co.uk/boiler-marketplace/type-of-home

however if you go somewhere like heat geek https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/ or – heating hub – https://www.theheatinghub.co.uk/what-size-boiler-do-I-need both are coming out at 10kw.

A key difference might be taking into account the build of the house – it is a well insulated new build from 2014 and the extension is also insulated to the latest standards. We’re detatched, in the south east (which none of the guides ask), but even that doesn’t seem to explain a tripling in boiler size recommendation??

I’ve also sanity checked this by looking at peak gas-use-day. In the last year the worst day used 100kwh of gas, and on a summers day we use about 25kwh – so lets assume 75kwh is the worst case for heating the current house. Am I right in thinking you could run 100kw through a 4kw boiler (assuming some gas is used for the hob)? obviously you have peaks and troughs in real life use, but this seems a long way from the 30kw being suggested..

all signs point to keeping a small boiler, but my wife will string me up if its cold in the house so double-checking 🙂

Do you need to upgrade the boiler immediatley?

I’d be tempted to suck it and see if you are happy with your existing boiler. It might very well turn out it’s not up to the job…and if you destroy it in the process, well, you needed a newer higher capacity one anyway, so nothing lost?

That would be my logic anyway..good luck!

Rule of thumb for new build is 36kw per m2, this is based on lots of actual heat loss calculations that we undertake. That would lead you to a 9kw boiler.

it will also be pointless installing a 30kw boiler unless the pipework is increased to probably 35mm.

that wouldn’t be a bad idea as you would be looking at heat pump size pipework so whilst you are at it increase the rad sizes too, it will increase the efficiency of your gas boiler even if you don’t go for a heat pump. You do know you could probably get £7500 of grant money for a heat pump as well as zero rating VAT for anything to do with the heating.

30 kw is 125w per m2, that is insane

How big is your hot water cylinder? If it’s 200 litres a 10kW boiler will take over an hour to recharge it from 10degC to 60degC.

Agree that 30kW is overkill.

Get a better cylinder if it takes that long, my 10kw heat pump takes less than an hour for 170 litres, probably more like 40 mins.

thanks for all your help.

Do you need to upgrade the boiler immediatley?

I’d be tempted to suck it and see if you are happy with your existing boiler. It might very well turn out it’s not up to the job…and if you destroy it in the process, well, you needed a newer higher capacity one anyway, so nothing lost?

this is my plan even if everyone says I need to upgrade, as I haven’t got the cash to do it right now anyway. The plumber is saying that because the system is drained down as part of the work, it’s cheaper to do now, which I suspect is technically true, however the cost to drain/refill a system as a %age of the total boiler install cost is going to be pretty small

How big is your hot water cylinder? If it’s 200 litres a 10kW boiler will take over an hour to recharge it from 10degC to 60degC.

tank is 210l, we have the water set to constant and never run out. Our water use wont increase as part of the work. Doing the maths that’d be 40 minutes working the boiler flat out to refill it from empty (10->50C https://immersionheaters.uk/heating-calculators/calculate-the-time-required-to-heat-a-volume-of-water/ ) – obviously there’s risk that with more boiler output going to the rest of the house there’s less available to heat the tank, so we run out a little more quickly, but we tend to wash at different times of the day (kids evening, adults morning), and even when we have guests it seems unlikely to be a real issue.

Worst case scenario, we’re also fitting a stw-spec log burner, so can just huddle round that 🙂

As Bear says, your existing boiler will be fine, the ‘plumber’ hasnt got a clue.
I presume he hasnt done a heat loss survey, which he should have done before doing any work. Its in the building regs, plumbers dont bother, heating engineers do!
If that is his advice so far, I’d be going elsewhere, as he wont be bothering to get the correct size radiators.

Your cylinder should be storing water at 60degC or at least get above that on a regular basis for legionella control. It’ll take a bit longer to fully recover but that will be a very rare event if you’re not running out of water at the moment.

The plumber is saying that because the system is drained down as part of the work, it’s cheaper to do now

BS.

If the system is drained for whatever reason, you just fill it back up from the mains loop with a bottle or two of inhibitor, job jobbed.

I run 10kw in a 125m2 house built in 1980 with 275mm roof insulation, 85mm graphite poly bead cavity insulation and argon filled windows from 2013. Its fine unless it gets down to -7, when it begins to struggle coming back from a 16 degrees overnight setback temperature. I suppose I could overcome that by leaving it to run longer with a lower flow temp perhaps.

I really can’t see you running something twice the area on 10kw unless you are running to passive house standards and have MHVR. That said, 30kw does seem OTT.

It’s all about the heat loss. What is your heat loss? I imagine on a modern build it will be quite low.

Am I right in thinking that surface area matters not volume? A pair of semis has approx 65% more surface area than one of them would detached..

According to heat geek, a renovated house needs approx 50% more heating than one built to the latest standards, so very simple maths would suggest..

120sqft 80s house needs 10kw
240sqft 80s house needs 17kw
240sqft 2010s house needs 11kw

Add a bit for a system that never struggles, subtract a fair bit as it never gets that cold down here. We look at -2 maximum, so a 20C gap from outside to inside temp, at -7 there’s a 25C gap so we need 20% less heating at peak (maybe?)

Yes, surface area is where the heat is lost to the atmosphere so a mid terrace would lose less heat than an end of terrace (assuming size and construction are equal). I wouldn’t look too much into the heat geek estimate of 50% as it has so much to do with the construction of the building and we have a wide variety of building stock in this country.

For your extension, as long as you’re not adding a significant amount of glazing there shouldn’t be an issue. If you have an existing 15kW boiler you’ll even have about another 5kW available for when it’s really cold.

Heat loss from n buildings (and heat transfer in general) is quite complicated when you start looking at it.

I’m not sure your figures are quite right, a semi house only has one wall attached to the next door, so roughly 25% less heat loss through the outer walls, so long as next door is heated.
Each house is individual, thats why a correct heat loss survey is done, and is required to both size the boiler, and the radiators/UFH. It is both the surface areas, and the volumes that are required for the heat losses, through the fabric of the building, and via ventilation/opening outside doors etc. On modern houses, the ventilation losses are usually bigger than the losses through the fabric of the building, hence why MVHR is going to be used a lot more in the UK, in mainland Europe, we are 20 years behind them with ventilation and heat recovery.
The heat loss is worked out so that the house/rooms get to the required temperature at the expected minimum outdoor temperature.
Typical ranges are -3 to 21 degrees. It is modelled on where you live,and what internal temperature you require. A small amount of extra could be added on if there are once every 5 years extra low temperatures etc, this is discussed when your start the heat loss survey.
The ‘ball park’ figures on the heat geek site are a good starter, but shouldnt really be used to size a heat source, as, above, all houses are different. Get the heat source too small, and it wont heat up the house to the required temperature, get it too large, and you will be paying more for the gas/electric, as it will be running inefficiently. You also have individual rooms to account for, if you get the right size heat emitter for that room, it will make the boiler more efficient, thus saving heating costs.
Just a note about design temperatures, if it is designed to heat your house to 20 degrees at -3 outside, it will only get to 17 when it is -6 outside. The heat source is designed to raise the water temperature by a set amount of degrees, -3 to 20 is 23 degrees, that is all it’ll do if it is designed correctly. But, in the real world, heat loss is not very accurate, there is usally a bit of leeway, and extra capacity is built in.

But, in the real world, heat loss is not very accurate, there is usually a bit of leeway, and extra capacity is built in.

For “usually” read always, no heating engineer, building services engineer and least of all a plumber is going to risk sailing too close to the wind resulting in their customers not being able to get their property up to temperature… so everyone adds their “just in case” figures and virtually all systems end up oversized to a degree, because there is no risk to the designer of spec’ing an oversized system – hence why OPs plumber comes up with 30kW…..

We have a shonky old microbore system with a top big boiler on it, in heat leaking 1970’s semi. Boiler size was the advice of a plumber, I should have ignored. Our modest insulation upgrades means it now kicks out way too much heat, cycling on and off yet also managing to ‘over heat’ spaces.
Get a proper calculation done.

Had a similar issue here…had a 25kW boiler and was told it would be too small. It would have been more than fine but gave out late in the extension building anyway.

That said I did end up getting a 27kW boiler for the upgraded house as I was slightly paranoid as we were getting a lot of underfloor heating(*). I figured modulating it could go much lower anyway, although ours only goes down to 8 or so which can be too much too, so mistakes have been made, and mainly the cost difference isn’t that much when you add everything up. I think you can get ‘infinitely’ modulating boilers now if it comes to that so you could cover all bases.

But TLDR I reckon your 15kW will be fine. Changing it up after the fact is no biggie either.

(*) due to timescales heat pump was not an option at that point.