New boiler - what level of output needed?

strawman_special

I have just received quotes from four different gas safe registered companies to replace a back boiler system with a new condensing combi boiler. I live alone and my house is a two bedroom semi detached with seven radiators. I have a bath maybe once a week and use an electric shower at all other times.

My question is what power output should I expect to need to fit? They have quoted me for 24kW, 28kW and 33kW. What are the main factors in deciding this? Is it better to go bigger rather than smaller? How does it affect the running cost, is a 28 always more expensive to run that a 24 or is it cheaper because it does not have to work as hard? I did ask for them to allow for me fitting a new shower in the future which runs off the combi boiler.

Any help would be gratefully received. I have had no luck in finding any resources that answer this question.

Cardew

The maximum output of a combi boiler is used to heat DHW(Domestic Hot Water) rather than being required to heat the house.

If you intend to run a shower from the combi, you will get a better flow of hot water with the bigger boiler; albeit it will use more gas to obtain that flow.

Boilers 'modulate' their output(i.e. they turn down the wick!) to meet the demand, so there should be very little difference in the efficiency of those boiler sizes if the system is properly set up.

gas4you

Personally I would recommend the 28kw to let you run your bath quicker in winter.

There sounds no need for you to need a 33kw. 24 Kw would be ok if you don't mind having to wait for the bath to take longer to fill in winter.

All will cope easily with your heating requirement.

Pincher

I wouldn't worry too much about it, a combi is two contradictory devices forced together to save space. What you actually want is a 12kW central heating boiler and a 30kW water heater. A modern combi turns the flame up or down to suit the activity. Some of them are cleverer at this than others. So a 30kW that can modulate down to 12kW is better than a 24kW that only modulates down to 18kW.

If you don't want to think about it too much, grab a 24kW Potterton Promax combi; not because it's technically advanced, but because it's a decent brand with good parts availability and good engineers that make house calls.

joop1977

A 24 kw boiler would be fine for you,get your shower changed for a mains fed thermostatic