Running a combi boiler with just two radiators

[kennyd]

Hi, I've a large house, heated with a 42kw combi boiler.

My 89 year old mother lives with us and uses two down stairs rooms. She is always cold (we live in Scotland) so even during the summer months I have to heat her two rooms.

I use two electric oil filled heaters at the moment but wondered if it would be cheaper/OK to switch on the central heating with just her two radiators opened and all the other radiators turned off.

One of the her radiators has straight through valves and the other a TRV.

I've tried a search but cant find this info, so would appreciate any advice.

 

[newcastle phill]

Personally id get 2 fan assisted convectors.
That boiler will be inefficient with 2 rads

 

[kennyd]

Thanks for the reply.

The cost of my electric heaters is high. I have two 3kw heaters set on medium, running 15 hours a day and they only just get the rooms to 21 degrees because of the 12' high ceilings and single glazing of our old house.

I make that about £45 a week to run these heaters. I suppose the only way to find out if heating with the boiler would be cheaper is if I try it.

My concern is whether it could harm the boiler with only having two radiators on.

 

[gmartine]

Assuming your actual rads have a combined output of say 4 - 6 kw, your boiler would have to have a modulation of around 10:1 to run efficiently. If it doesn't it'll constantly cycle and therefore greatly increase wear and tear.

What boiler do you have?

 

[newcastle phill]

Well you dont say what boiler it is but I doubt it will modulate down to much lower than perhaps 9kw or if an older boiler maybe 14kw
2 heaters on medium will probably be on the 2kw setting.

What rating in Kw are the radiators ?

 

[kennyd]

I had a Baxi Duo-Tec 40 fitted iast December. Not the best I know, but fitted for £1500 all in with a 7 year guarantee seemed a good deal.

The Radiators are both 1400 x 600 (2.5kw each)

(Just looked up the Modulation rate 1 : 7)

 

[lame plumber]

ran a saunier dauval combi for 2 years on 2 rads, its still going strong 13 years on, touch wood

 

[newcastle phill]

Well it can modulate down to about 7kw so there you go.
Youve answered your own question.

But you still only have 5kw of heat emitting from rads and the same high ceilings.
If youve done the maths - there you go, job done

 

[kennyd]

So it's OK if the lowest modulation output the boiler is higher than the combined output of the radiators?

I'm not too bad with pipe work but not very knowledgeable about boilers.

 

[solutions]

So it's OK if the lowest modulation output the boiler is higher than the combined output of the radiators?

I'm not too bad with pipe work but not very knowledgeable about boilers.

Idealy the lowest modulation wants to be below the kw output of the rads and pipework losses so the boiler can run without short cycling. Otherwise no sooner has the boiler fired up and it will be shutting down again as the return temperature will be too high and then the cycle repeats.

 

[gmartine]

Open up one more rad in the hallway?

 

[newcastle phill]

And whe they cycle they are very inneficient

 

[kennyd]

Thanks everyone for your help and advice.

It always lifts me that folk will freely give there time and knowledge to assist others.

I'll open another radiator so the boiler is working within it's range and give it a go.
If i keep an eye on my gas consumption I'll soon find out if it is more economical than the electric heaters.

 

[Worcester]

Ideal use for a buffer tank simples

 

[Worcester]

Ideal use for a buffer tank simples.

Just get the boiler to heat up a 300litre cylinder / buffer tank, get the bolier to manage that just like a DHW cylinder (tank stat, two port valve) and then use the 300 litres of hot water as the 'boiler' for the rads, just call for heat from the buffer as you like, it's only two rads.

Boiler will work efficiently and won't short cycle. - this is the reasons buffers became so popular before when boilers didn't modulate.

 

[scorps]

Ideal use for a buffer tank simples.

Just get the boiler to heat up a 300litre cylinder / buffer tank, get the bolier to manage that just like a DHW cylinder (tank stat, two port valve) and then use the 300 litres of hot water as the 'boiler' for the rads, just call for heat from the buffer as you like, it's only two rads.

Boiler will work efficiently and won't short cycle. - this is the reasons buffers became so popular before when boilers didn't modulate.

He'll never recover the cost of that installation though,so I would'nt think that will be an option

 

[Sparkgap]

Any TRVs on rads which might shut down if it gets warm? Auto bypass valve needed to keep flow through boiler. Also, being a combi, have you got it set for hot water preheat? In that case it'll short cycle anyway keeping itself warm for the hot water so I don't see a problem.