Boiler size for underfloor heating vs radiators

[andyscotland]

Another question on my mother in law's build. She wants underfloor heating in the extension (30m2). The architect has raised a concern that the existing boiler might not be big enough for UFH, and so she may have to have radiators or upgrade the boiler

 

I am confused by this. I can't understand why the heat delivery method would affect the power demand in any significant way. As far as I can see either the boiler is big enough to cope with the extra heating load of the extension, or it isn't?

 

I know there is a higher loss to the ground, but the slab is going to be very well insulated (overall floor u-value of 0.13) and running this through @Jeremy Harris floor heat loss spreadsheet I get a loss of about 4.5% which seems fairly negligible.

 

Surely the heat demand is governed by the size and thermal performance of the space - it will require the same input whether it's from UFH or radiators? Indeed if anything the boiler power rating could be lower for UFH since it is essentially on more continuously rather than in peaks and troughs like a conventional system?

 

When I asked the architect he just said it was to do with UFH operating at a lower temperature, but I still don't follow why that would be (I know it can have an impact on the boiler in terms of cycle time / running time etc).

 

Am I missing something?

[ProDave]

Yes you will lose some heat to the ground, but if that is enough to force a larger boiler, I would say it does not have enough capacity anyway to deal with a cold snap.

 

I suspect your architect is used to retro fitted UFH with no insulation?  I would take what he says with a very large pinch or salt.

[andyscotland]

6 minutes ago, ProDave said:

I suspect your architect is used to retro fitted UFH with no insulation?  I would take what he says with a very large pinch or salt.

 

Thanks @ProDave that was my hunch, or just that he wasn't totally up to speed with it. Good to have a second opinion!

[SteamyTea]

There may be a issue with the different temperatures used between radiators and UFH, though I think this can be over come with 'mystical' plumbing devices.

I think @ProDave have the right answer.

[andyscotland]

25 minutes ago, SteamyTea said:

There may be a issue with the different temperatures used between radiators and UFH, though I think this can be over come with 'mystical' plumbing devices.

I think @ProDave have the right answer.

 

I think that's just a question of a valve that lets enough hot water from the main/radiator loop to top up the water in the UFH loop and keep it at the lower target temp. I do know that means the boiler will potentially have to do more running "little and often" than it would with just radiators alone.

[Temp]

+1

 

Its quite possible to have a mixed system. Most UFH manufacturers have a manifold with valve/mixer on the front end that is designed to be connected to a rad based system, typically its connected to the nearest rad to the extension. 

 

Personally I wouldn't bolt on UFH like that. I would run separate pipes from the boiler to the UFH so the UFH can be on its own zone/stat. That way it can call for heat independently to the rads.  eg UFH can call for heat when the rads are off.

 

 

 

[ProDave]

9 minutes ago, Temp said:

+1

 

Its quite possible to have a mixed system. Most UFH manufacturers have a manifold with valve/mixer on the front end that is designed to be connected to a rad based system, typically its connected to the nearest rad to the extension. 

 

Personally I wouldn't bolt on UFH like that. I would run separate pipes from the boiler to the UFH so the UFH can be on its own zone/stat. That way it can call for heat independently to the rads.  eg UFH can call for heat when the rads are off.

 

I would say that is a must, so the UFH can also be on a different time controller. You might want the UFH to come on earlier (because it takes longer to warm up) and then go off earlier in the evening for the same reason.

[andyscotland]

Thanks all - yes absolutely I think the plan is to plumb it back to a separate zone for control/timing reasons.

 

The only sticking point really is whether it's going to make a difference to whether a boiler upgrade is required. As the architect has given the steer that there's quite a cost difference for UFH + a boiler vs just an extra radiator off the existing boiler, but my thinking is the boiler sizing for the house is essentially a separate question.

 

So my advice to MiL is essentially going to be to crack on on the basis she'll have the UFH she wants. Then when the heating engineer eventually gets involved, if they're confident she needs a new boiler to manage the extension she can get one, if they think it'll potentially be OK she can give it a go without and get the boiler upgraded later if it turns out to be a bit under-powered.

 

[SteamyTea]

17 minutes ago, andyscotland said:

Then when the heating engineer eventually gets involved

They can always fit radiators, hard to fit UFH after the build.

[andyscotland]

30 minutes ago, SteamyTea said:

They can always fit radiators, hard to fit UFH after the build.

Exactly my thinking

[Mr Punter]

The new 30m2 extension will probably have a peak load of less than 1.5kW so unless the boiler is currently struggling it should be fine.