Resizing a boiler after insulation works

H.E. Pennypacker

Our house had a Grant Vortex boiler fitted around 9 years ago when we had a BER of D3. It’s a 50/90 model and was set to the factory default 23kw output. Following recent insulation works we’ve a BER of B2 and are heat pump ready in terms of heat loss. When we got the boiler serviced we asked the serviceman about reducing the boiler output as the heat demand is much lower. He reduced it to the lowest output (15kw) on the basis that it’d be easier to work back up in terms of sizing if it was too low but wasn’t clear as to how we’d tell. It works fine, we’ve plenty of heat although it takes about twice as long, say 30 mins) to heat the 11 double radiators initially. It’s a 135sq meter bungalow so there’s a long pipe run.

My question is: how do you pick a point where the boiler is at its most efficient and uses the least amount of oil. Would it be more efficient at the next step up (20kw output)? I’ve tried timing firing cycles but it’s very hard to tell as the boiler doesn’t come on very much at all once the house is warm and runs a lot until it it’s fully heated up.

kirving

If it's outputting enough heat to keep the house warm, and to get to that level in a reasonable timeframe, I think the lowest setting is fine in terms of efficiency.

There isn't a crazy amount you can really do to improve much further, as the flowing water will be enough to transfer as much heat as possible away from the boiler as opposed to it being dumped outside.

At the end of the day, it's kW boiler output = kW heat loss.

Idbatterim

Is it an oil boiler ? As gas would be self modulating afaik!

H.E. Pennypacker

Idbatterim wrote: »

Is it an oil boiler ? As gas would be self modulating afaik!

Its kerosene. We live out in the sticks so there's no mains gas, unfortunately.