New boiler time - KW rating

Yaayuh! · 13 Oct 2020 at 09:42

I've had a quote for a new boiler (and rads), I won't go into too much detail but I was surprised that the boiler recommended was 25KW (Vaillant 25kw Eco Tec Plus).

Our house is a (cold) 3 bed detached and the boiler would supply 10 rads and 1 bathroom. I was expecting a higher KW rating, I know things have got far more efficient and all but does this sound about right?

bremen1874 · 13 Oct 2020 at 10:22

My understanding was that the size doesn’t really matter for the heating side as they’ll all handle it.

The rating is more related to on demand hot water.

When I had mine done (similar sounding house) they installed a 30kW.

Xyclops · 13 Oct 2020 at 10:28

We had a 35Kw installed in our house. It is a combination boiler. Just about powerful enough to run 2 showers simultaneously.

NathG79 · 13 Oct 2020 at 10:32

A higher KW rating will give you a better flow rate on the hot water side.

personally, I would be going for a 28-32KW. (Depending on your budget)

the boiler won’t have to work so hard either, plus you have scope for extra/bigger rads.

seems a little undersized if you ask me. Maybe they need the work and have come in low to try and get you to think it’s a good price.

23 years in the trade.

Yaayuh! · 13 Oct 2020 at 10:59

NathG79 said:
A higher KW rating will give you a better flow rate on the hot water side.

personally, I would be going for a 28-32KW. (Depending on your budget)

the boiler won’t have to work so hard either, plus you have scope for extra/bigger rads.

seems a little undersized if you ask me. Maybe they need the work and have come in low to try and get you to think it’s a good price.

23 years in the trade.

Cheers

I might ask them to quote for the 32KW version.

jpaul · 13 Oct 2020 at 11:13

the ideal logic+ 35KW I have will only do a single shower, NO simultaneous kitchen use.

on my new combo boiler wishlist -
I'd like preheat to be controllable on a timer, to avoid consuming gas, in keeping it warm, when you are not in the house, or, the middle of the night.

Xyclops · 13 Oct 2020 at 11:46

jpaul said:
the ideal logic+ 35KW I have will only do a single shower, NO simultaneous kitchen use.

on my new combo boiler wishlist -
I'd like preheat to be controllable on a timer, to avoid consuming gas, in keeping it warm, when you are not in the house, or, the middle of the night.

Ours is ATAG 35Kw. If someone uses another tap it may momentarily go colder but then it catches up and becomes warm again but flow rate lowers. We do have special shower heads fitted though that reduce water use by releasing the water as droplets rather than a stream.

NathG79 · 13 Oct 2020 at 11:47

Yaayuh! said:
Cheers

I might ask them to quote for the 32KW version.

Good boiler manufacturer choice though.

I would definitely go for a Vailant..

panthro · 13 Oct 2020 at 12:23

I have a 4 bed detached house with a gravity fed water supply. The house is 1970's and not particularly warm. We fitted an 18kW Ideal system boiler and it's more than sufficient to heat 12 radiators and the hot water tank simultaneously.

Haggisman · 13 Oct 2020 at 12:43

We only have 6 radiators and a shower and were recommended a 28kw, but ended up going for a 30kw instead (Worcester-Bosch Greenstar 30i). Keeps the house nice and toasty and the shower is lovely, probably wouldn't want anything weaker though. If I remember correctly, the price difference was negligible (<£100) when compared to the overall cost

Simon42 · 13 Oct 2020 at 17:07

A key question is whether this is a combi system or not as you need a very significant pump/flow rate benefit from higher KW boilers for just heating and stored hot water.

For example on our non combi system we've got 18 radiators (many quite large and K2), three bathrooms and have a 30KW Vaillant ecoTeEC Plus with a good sized HW cylinder. The boiler never works that hard in any circumstances and we can use all showers at the same time (for several showers each) and they're all mains pressure with a new large cold supply pipe to the house so the flow is excellent.

After installation I calculated how much heat our HW cylinder and rads can extract from given the pump flow rate (and we're on 28mm primary circuit/large bore pump) and on the current pump speed settings and its displayed flow rate the system can only used 29KW maximum and that's with boiler temp at 75 degrees.

For combi use a higher output may be needed but not for a regular system so that's a key first point as over sizing just adds cost and reduces efficiency if not needed.

bayo000 · 13 Oct 2020 at 19:17

We've just been quoted £2800 for Worcester 8000 Life 35kw combi to replace the existing one. This includes replacing the flue as it's not compliant, new magnetic filter and replacing lead pipes (about 5 metres). Does that sound reasonable?

Xyclops · 13 Oct 2020 at 19:55

Prices probably depend on where you are located, as hourly rates change per region.

Ours was £4000, but that included relocating the boiler from downstairs to the airing cupboard upstairs, changing system from conventional boiler and tank to a combi, removing tanks from loft, removing tank from airing cupboard, adding a flue to the roof (which required a roofer to do). They also upgraded water pipes between where they enter the property and the new boiler to use wider pipes. We live in South England just 30miles outside of London if that makes a difference.