ASHP: underfloor heating over suspended timber joist floor?

[Timberframeguy]

I am currently in the process of a designing my extension and am looking at ASHP with the associated grants as we tearing the house apart anyway so may as well use the opportunity to upgrade the CH.

 

My question is would underfloor heating with the pre routed weyroc boards be viable with an ASHP or do you really need a solid concrete floor to achieve the required heating through under floor?

 

Thanks!

[sharpener]

You will not get the thermal mass without the concrete. This means the system will be more responsive, but you will not be as able to bridge across peak electricity periods or charge the slab up only at night as IIRC @JohnMo does.

 

[ProDave]

The important thing is whatever you do, you need a LOT of insulation underneath the UFH.  Easy to do in an extension but a lot harder if you are thinking of retro fitting to the existing part of the house.

[Timberframeguy]

10 hours ago, sharpener said:

You will not get the thermal mass without the concrete. This means the system will be more responsive, but you will not be as able to bridge across peak electricity periods or charge the slab up only at night as IIRC @JohnMo does.

 

This is what i'm concerned about, perhaps UFH without the concrete floor is solely for gas boilers which can run at a higher temp?

 

10 hours ago, ProDave said:

The important thing is whatever you do, you need a LOT of insulation underneath the UFH.  Easy to do in an extension but a lot harder if you are thinking of retro fitting to the existing part of the house.

Yes I am planning to insulate between the joists while the house is a building site.

[sharpener]

10 minutes ago, Timberframeguy said:

This is what i'm concerned about, perhaps UFH without the concrete floor is solely for gas boilers which can run at a higher temp?

 

What is more relevant is that gas costs the same at any time of day! So there is no incentive to turn it off during peak periods which is when a concrete slab can usefully act as a storage heater.

[JohnMo]

On 10/01/2025 at 21:32, Timberframeguy said:

question is would underfloor heating with the pre routed weyroc boards be viable with an ASHP or do you really need a solid concrete floor to achieve the required heating through under floor?

We have thick concrete screed in our house and we did have an overlay system in our summerhouse. The overlay system had 200mm of insulation under it.

 

Overlay system just wouldn't work at house flow temps (from either gas boiler or ASHP) so was capped off. I got up to 38 Deg flow temps and at 5 degs outside summer house was about 16. House was overheating by this point.

 

If your floor isn't designed for UFH, from the start, I am not sure I would add it.

[Beelbeebub]

22 minutes ago, JohnMo said:

We have thick concrete screed in our house and we did have an overlay system in our summerhouse. The overlay system had 200mm of insulation under it.

 

Overlay system just wouldn't work at house flow temps (from either gas boiler or ASHP) so was capped off. I got up to 38 Deg flow temps and at 5 degs outside summer house was about 16. House was overheating by this point.

 

If your floor isn't designed for UFH, from the start, I am not sure I would add it.

Wouldn't the issue more be: the summer house heat losses were too high for the UFH to supply at a flow temperate similar to the house.

 

[JohnMo]

5 hours ago, Beelbeebub said:

Wouldn't the issue more be: the summer house heat losses were too high for the UFH to supply at a flow temperate similar to the house.

 

Did the calculations, pipe spacing was on 125mm centres, (house is on 300mm centres) the panels holding the pipe were aluminium lined also, so in theory had enough output. Output should have been about 50W/m². Testing against an electric panel heater only, the energy usage pretty matched the design heat loss, which is closer to 30W/m².

 

So as mentioned possibly would have been fine at much higher flow temperature but not at lower flow temperature.