Too much boiler power?

[Rustychain]

I'm getting a Vaillant ecoTec plus boiler, but need to decide on which model. I live in a 3 bedroom semi and I'm in the process of adding a 44 sqm single storey rear extension.

I have heard that a boiler which is overpowered is less efficient when heating a house because it cycles on and off more frequently than a lower powered boiler. Is there any truth to this?

I know the higher power will give better DHW, but we've only one bathroom, so I'm not too worried about that aspect.

Thanks

 

[chrishutt]

With combi boilers you decide on the output purely on the basis of your hot water flow needs. Even the smallest 24 kW output boilers will heat a large house and if you're satisfied with about 9 litres/min of hot water then you might as well go for that, although most of us would say go for 28 or 30 kW, giving about 11 l/min, because they aren't usually much more expensive to buy.

 

[bengasman]

This was true for most boilers designed in the days that a morris minor was a modern car. Like the car, boilers have become more advanced and today oversizing does not mean less efficient.

 

[croydoncorgi]

Any half-decent High Efficiency boiler (combi or not) should modulate automatically down to under 10Kw, if necessary. The better ones even modulate the pump as well as the burner.

 

[mehran]

better ones even modulate the pump as well as the burner.

heat-line do that with there A rated boilers... does that mean they are one of the better ones? i think it is becoming common for A-rated boiler too do it

 

[gas4you]

As ChrisH says, size for your hot water. I never fit less than the ecotec plus 831. If you love baths and only occasional showers then consider the 837.

Your installer will range rate the ch output down to what your property needs when he commissions the boiler, so the worry of oversized and inefficient doesn't come into it with these boilers.

 

[Rustychain]

The Vaillant is fully modulating, so from your responses it doesn't seem that it should be any less efficient.

I'll go for the 837 just to make sure there's enough HW flow (two young kids that need baths).

Thanks for the info

 

[ChrisR]

COMplete waste of time and money to fit a boiler capable of heating more water than your mains can supply - measure yor pressure and flow. Plenty of similar threads...