What size heat-only boiler?

[gerbils]

@gerbils

I believe both the Plus 418 and the Pure 418 have the same aluminium heat exchanger these days. I think the only heat only boilers currently with a stainless steel heat exchanger are Viessmann and Alpha. The stainless steel Alpha heat exchanger seems identical to the one Vaillant used to use (and still use in some of their other boilers). It's used by other manufacturers as well. Intergas gets a good reputation. Their heat exchanger is totally different to anybody else's.

Thank you. Is there any benefit in 25KW vs 18KW - they are the same cost but I was thinking 18KW will be more efficient.

 

[wannocks]

The new Ecotec plus (system) has a stainless steel heat exchanger, as did the previous one.
The open vent version has an aluminium heat exchanger.

 

[wannocks]

Our water flow is 12L/Min & 2 Bar - not enough for a vented system I have been told and borderline for a combi.

The water flow rate is irrelevant to the size of the coil.
You can have an unvented (mains pressure) cylinder, or a vented (tank fed) cylinder with a large coil hat will heat quickly.
Agreed, if your pressure/ flow rate is poor then a combi or mains fed cylinder would not be great.

 

[wannocks]

Thank you. Is there any benefit in 25KW vs 18KW - they are the same cost but I was thinking 18KW will be more efficient.

There is no benefit in significantly oversizing a boiler. It will increase the minimum output of the boiler and you can end up with a boiler that cannot modulate low enough in mild weather.

 

[MNW67]

Thank you. Is there any benefit in 25KW vs 18KW - they are the same cost but I was thinking 18KW will be more efficient.

From everything you've said, I think 18KW would be plenty. Too much power might possibly cause problems with cycling in some situations. If your house is slow to heat, it might be the radiators are not big enough. You would need to investigate to find the limiting factor.

Also, I was only commenting that Plus and Pure have the same heat exchanger. The rest of the Plus may be superior. I have no knowledge of that.

It looks like you can get Viessman for about the same price. Also Intergas a bit cheaper. Just some thoughts. Beware of the January price rises inc. the heat pump levy. Some may not include price rise yet.

Viessmann Heat Only Boilers

Viessmann Heat Only Boilers

viessmanndirect.co.uk