relative boiler efficiency

[LindaLancaster]

We are trying to choose a combi boiler to replace a 20 yr old instant hot water boiler and a 25yr old CH boiler that 'lives' in the chimney breast behind a gas fire (Sorry! I don't know what they are called) Both boilers go out on a fairly regular basis and are probably using way more gas than we really need to be using. Also the water boiler is an eyesore, and so is the gas fire in front of the other one.

I've added up the total radiator output (some new, some will be replaced when we get round to it) and it will be 37,000 BTU. The thing is, 2 of these radiators will never be used except to air the rooms occasionally, and 2 more only get used once in a blue moon. 4 of the remaining 5 are always turned down low. In other words, we don't use our heating very much at all, but we do need the facility.

My question is, which would be more efficient - a smaller boiler running 'higher' or a larger boiler running lower? I've looked at the technical specs of Worcester boilers and can't decide whether a '24' or a '28' would be better for our needs.

DHW - we will be running a shower and a bath, plus hot water for the kitchen. I can't see that we would ever need the shower and the bath at the same time.

Hope you can help - you all seem very knowledgeable!

Love Linda

 

[kevplumb]

tis my humble opinion to work them near their limit

 

[DP]

I would keep the back boiler, upgrade system controls so that you can select CH by itself or HW by itself. Get the boiler serviced (not the 15 minute check) Burners, injectors and heat exchanger should be stripped and cleaned. Flue tests carried out and boiler run on maximum and HW cylinder set to 55 degrees. Boiler most probably can be adjusted for your heat load.

Forget the combi boiler. Quite complex, break down often and then you might not get an engineer to repair same. I am talking as a heating engineer who mainly repairs boilers.

fergot to say, a combi will need larger gas line, system will have to be powerflushed (a must) and there is a fair chance there will be leaks once system connected to sealed system boiler.

 

[LindaLancaster]

Hi KevPlumb

would you go for a 24 then? Have to say that's what I'm errring on at the moment

Hi DP

We haven't got a cylinder for water, hot or cold, and the CH boiler only does the CH. Don't understand!! I feel we need to replace this terrible old CH boiler cos it's only about 50% efficient. (and maybe I'm too green for my own good?) If we run the CH boiler on full we cook. Even turning the radiators down as far as they wil go we cannot get it low enough. We currently run the CH boiler at setting 2 out of 5.

Is this of any help?

 

[blahblah]

Linda, with a combi I always recommend going for the best HW flow rate you can get. The output heating side on a decent one can be adjusted to suit (and will modulate anyway if not adjusted) and will also be preset at less than the HW output.