WB combi boiler not heating rads properly

[petebill]

MY 3 bed semi has always been chilly.
7 yrs ago I decided to replace the heating system.
I was advised to get a much bigger boiler and was recommended a WB CD42i (42KW) combi.
All 7 rads in the house were replaced. Overall cost was £5k

Firstly I should point out that I have a one-pipe CH system

Problem is that when the heating comes on, the rads start to warm up, then the boiler switches off for a period of time (even though the stat is still calling the flame) and the rads start to cool. Then the boiler comes on again and the rads start to warm up, but then the boiler shuts off again, so the rads never get properly hot and with outside temps around 5C it will take approx 16 hours to raise the temperature from 16c to 22c. So it's nice and warm just as I go to bed!
My gas bill is frightening!

Any advice much appreciated
Thanks
Pete

 

[muggles]

So you spent all that money and didn't bother to have it changed to a two-pipe system? More fool you...this is most likely the cause of your problem. Large boiler, very few rads and a one pipe system are all combining to give you slow heat up times and high gas bills. The boiler is cycling because it can't get rid of the heat it is generating fast enough, so the boiler gets up to temperature and shuts off before the rads are even warm

Re-pipe required

 

[petebill]

I have been quoted prices of between £10k and £12k to install a 2-pipe system and make good.

So it's hardly a case of not 'bothering' - I would LOVE a 2-pipe system, but I don't have £10k+ to do it.

 

[muggles]

That sounds excessive for a 3-bed semi if you're just looking to re-pipe the system and not change radiators etc. Should be looking at £3k max I'd have thought. You're not going to see any improvement in either heat output or gas bills without doing it anyway

 

[Agile]

The average het loss of a three bed semi is about 11 kW but you have chosen a 42 kW boiler.

That is going to seriously cycle as you have discovered.

There may be minor adjustments that can be done to improve the situation like reducing the boiler power output to the minimum.

I cannot imagine the thoughts of any installer who advised that size boiler unless you specifically told him your priority is to have a good hot water flow rate.

If you had plenty of space there could be other options involving a thermal store to act as a heat buffer to keep the boiler firing for longer.

Tony

 

[petebill]

Thanks to both of you.
Cost is so high as the oak floor would have to be replaced rather than re-laid over the whole ground floor and the same with the travertine in the bathroom.

I know nowt about heating, I'm a LAN engineer.
It was the 'experts' who advised on a huge boiler.
True that I get a fantastic shower, but the heating is a real pain.
My new boiler man said he usually sees such boilers fitted in schools etc, so it's obviously got way too much power.

2 questions - sorry if they sound daft
The boiler has a max/min setting for temperature of HW and CH. Would it help if I turned the CH to min? or would the differential remain the same?

There's a balancing valve fitted between the supply and return on the pipework close to the boiler which is currently closed. Anything I can do with that to help?

Thanks again
Pete