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In my topic Cause & effect (funny story) I mentioned how I worked around a problem with heating a single UFH zone with 30kw heat only boiler. That all worked out but I also face a similar problem when choosing a heating system for a newly extended outbuilding.

This extension is on a double garage & workshop separate from the house. It already has a gas supply line (I had it put in when the house was built just for this eventuality) so I'm inclined to use that although society seems to be heading in a different direction these days. Already installed is a single UFH zone on the ground floor. This room is 24m2 although some area is taken up with a staircase. Upstairs is a room-in-roof with a floor area of 30m2. First-fix pipework for radiator(s) has also been laid up there.

Energy-wise the areas to be heated are reasonably well-insulated (150mm PIR in roof, 100mm in floor, 75mm in cavity walls) and the windows are all double glazed. I estimate that we might need as little as 5kw for our intended uses. A 2kw eclectic convector has been maintaining an acceptable level although so far this year outside temperatures here have been at least a few degrees above freezing at worst. So I would like some advice here on choosing a suitable heat source that would make the most economic sense..? And if gas is the answer, do boilers modulate down to lower levels these days?
 
A 12 kW boiler will modulate down to 2 kW.
Hi oz-plumber, a 6:1 modulation ratio would be a great improvement over my 3:1 glow-worm (the only modulating boiler I've known). However, I know that some 'smaller' boilers are just range rated in firmware so don't necessarily maintain that ratio across their range of products. Can you point me to the 12 kw boiler you're thinking of?
 
Most boilers specified with a 12kW output will reduce down to between 2 & 3 kW.
Many boilers can be range rated from a 24kW or 30 kW boiler can be range rated down to a 12 kW boiler.

I would stick with a boiler specified to 12kW and if necessary range rate it from there.

I quickly looked at the Baxi boiler specifications for you purpose - not for any reasons bar the fact I had the specs on hand
 
I will mention electric boilers.
You'd be mad to get one, especially if you have an existing gas supply. Why you would even consider it is beyond me.
Absolutely. However... there is talk of the UK green energy surcharges set to switch from electric to gas bills. It seems like we need a crystal ball to choose a boiler that will be the most economical in, say, five years time. ASHP would probably hit that mark but the up-front cost is unrealistic.

Gas is therefore probably still the best option but there is one thing going for electric... the boilers are half the price of gas and anyone handy can legally install and commission them themselves. On this basis there's possibly £1K spare to put towards the leccy bill. Maybe.
 
All sorts of things may or may not happen in future. I'd feel more comfortable basing decisions on the status quo.
A leccy boiler has quite a bit of regulation associated and at least will more than likely require it's own circuit from the consumer unit.
 

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