Combi Boiler with UFH

[wcavanagh]

So many of these what boiler questions, this is not quite that....

I want an oil combi for various reasons, now the issue is sizing. To get a somewhat decent 22L / min DHW I need a large 28Kw to 33Kw boiler. Now with a whole house of UFH I dont need anything near that. So I am struggling to find if say a Grant 28Kw will modulate down. How do I find this information?

Cheers
Will

 

[John D v2.0]

Yes it's impossible to find a house that need a fixed amount of heat on every heating day so every central heating system can modulate.
Can modulate the burner power, the burner on/off, and presumably you'll have a thermostat on the ufh zone with boiler interlock.

 

[wcavanagh]

Understand that, problem is will it modulate down enough to closer to 10Kw. I don't seem to be able to find this info.

 

[Dan Robinson]

They won't modulate. Just switch off. This if the inherent problem with oil combis. Have you space for a buffee vessel?

Hedging said that, my brother tube his Grant oil boiler with Evohome, and not had any Ill affects yet.

 

[John D v2.0]

Didn't spot you said oil, but my point was purely that even a thermostat is a form of PWM iee pulse width modulation, so you needn't match your heating load to the burner output.

 

[wcavanagh]

So maybe a heat store between the boiler and UFH?

 

[Dan Robinson]

So maybe a heat store between the boiler and UFH?

A buffer tank, like I suggested.

 

[wcavanagh]

What exactly is a buffer tank, and what will that do?

 

[Dan Robinson]

It's a tank that acts as a buffer between the system and the boiler. Your installer should work out how big it needs to be based on the desired burn time and output of the boiler.

 

[Dan Robinson]

I would suggest a minimum burn time of 6 minutes, but @muggles is far better versed in the ways of oil then I am. Listen to him over any of the wannabe armchair googleers on this site.

 

[wcavanagh]

How is this different to a Heat Store? Sorry so many questions, haha

 

[wcavanagh]

In would suggest a minimum burn time of 6 minutes, but @muggles is far better versed in the ways of oil then I am. Listen to him over any of the wannabe armchair googleers on this site.

Many Thanks, im getting so much conflicting advice from installers

 

[Dan Robinson]

A great store can be anything you want it to be including a buffer. Not a plate heat exchanger to the side and it can do your hot water too. I have one in the workshop that is 300 litres, with a 100kw plate heat exchanger on the site that can in theory give 30+ lites a minute. I had it at home for a while to do some experiments. Took it out in favour of a small unvented once if finished " playing".







It's looking for a new home

 

[wcavanagh]

Ahh I see. The principle is all the same try and get a volume of water upto temp with as little cycles as possible, as each cycle wastes a bit of oil.

I wonder what your wee 300L would cost to get shipped up here

 

[John D v2.0]

With oil?

Of course, thermostat is just a switch after all