Wet underfloor heating advice for extension and existing area

[sheffdave]

I would appreciate some advice about the best way to install UFH for our extension and existing area to create a single zone.


The areas marked on the picture are as follows…

Yellow – existing timber floor with approx. 1.2m void

Green – existing solid concrete floor

Blue – new extension


I assume the extension and existing concrete floor will be simple enough, though I’m assuming the concrete will have to be dug out to a certain depth.

With regards to the yellow section, I think I have 2 options, backfill the void or batten and insulate running the pipes along the existing joists?

Bearing in mind that the yellow section will have a kitchen and large island on it which won’t require the pipes to be run under.


Thanks in advance for any advice!

 

[Madrab]

Unfortunately, you cannot run a suspended timber floor UFH the same way you would run a concrete base with screed. They need to be be heated in completely different ways. A large concrete thermal mass in screed needs to be kepy at a constant temp to maximise output and efficiency. A timber/suspended floor would be treated more like radiators whereby the heat would be directed to that area as and when it's needed to heat the space as it reacts much faster if using spreader plates.

You also have the issue whereby the existing concrete may not be insulated properly with a suitable depth of insulation below the slab, nor in the extension?

Traditional UFH may not be an option then and an alternative approach, like overlay, may need to be looked at.

 

[sheffdave]

Thanks Rob,

I think the idea for the existing concreate floor was to excavate to 360mm: lay 60mm screed on 100mm polyurethane insulation on 100mm concrete on 100mm hardcore.

 

[Madrab]

And the extension or is that not built yet? Is there 360mm of concrete in the existing, has that been checked?

If all the concrete was insulated then yes a traditional screed approach to UFH could be used but again the timber suspended floor would have to be on its own zone as it would need to be controlled differently. Huge amount of work and upheaval though and there are very effective alternatives.

 

[sheffdave]

Thanks again Rob,

The extension isn't built yet.

Nope 360mm is an assumption so could be incorrect!

If we were to do the overlay systems would we do this throughout the yellow and green section then just standard pipework in the extension? I assume we would have a floor height difference when entering the open planned area? (I've previously looked at Nu Heat LoPro and they quote 15mm height build up).

Sorry for all the questions this is a bit of a learning curve for me!

 

[Madrab]

If we were to do the overlay systems would we do this throughout the yellow and green section then just standard pipework in the extension

Nope, overlay throughout, then it could all be on a single zone, anything else would need separate zones due to the difference in heat up - one way or another. You may have to factor in a little styrene insulation below.

What you would look to do would be to level the floor throughout, using ply or cementitious board on the suspended and then either re-finished and/or self level on the concrete. Overlay then sits on top of that throughout. If it's using an EPS400 overlay (+20mm), then any finished floor surface can be used directly on top (carpet would require at least a 6mm cementitious overlay before low tog underlay and carpet)

 

[sheffdave]

Nope, overlay throughout, then it could all be on a single zone, anything else would need separate zones due to the difference in heat up - one way or another. You may have to factor in a little styrene insulation below.

What you would look to do would be to level the floor throughout, using ply or cementitious board on the suspended and then either re-finished and/or self level on the concrete. Overlay then sits on top of that throughout. If it's using an EPS400 overlay (+20mm), then any finished floor surface can be used directly on top (carpet would require at least a 6mm cementitious overlay before low tog underlay and carpet)

thanks Rob, do you think it would it be possible just to have the new extension with UFH and the existing (green and yellow) with a couple of rads? Our final floor finish would be laminate or vinyl so nothing too cold under foot!

 

[Madrab]

Of course you could, no issue with that at all. The UFH zone would use it's own stat so could be controlled as a separate zone. I guess the question would be if you could have UFH throughout then why wouldn't you. So much better and more efficient at heating the space.

 

[sheffdave]

I guess what puts me off are people's opinions on an overlay system not being very good.

 

[Madrab]

I guess what puts me off are people's opinions on an overlay system not being very good.

Really .... who? I've installed a fair number of those systems and I also have it installed in my own place. I live in an area that is perfectly suited to retrofit overlay systems as they all have concrete founds that are insulated above rather than below the slab and it can be done without much if any loss in ceiling height.

I also have run a case study with NoMorePly where I've used 6mm board over an overlay system and then topped that with low TOG underlay and carpet and it all works very well. In fact it's actually saved me in gas bills with better overall heating (lower flow temps) in the downstairs of the house.

 

[Higgy28]

I guess what puts me off are people's opinions on an overlay system not being very good.

We had exactly the same situation as you - part suspended, part old solid and new solid extension. We went with a low profile system (~20mm build up) throughout. As mentioned, it works differently to "conventional UFH", more like the fast response and low thermal inertia of a radiator, but it works well.

We had considered the backfill the suspended (~50cm) and excavate the old solid floor, but were advised against it by architect and builder on cost grounds.

 

[sheffdave]

Really .... who? I've installed a fair number of those systems and I also have it installed in my own place. I live in an area that is perfectly suited to retrofit overlay systems as they all have concrete founds that are insulated above rather than below the slab and it can be done without much if any loss in ceiling height.

I also have run a case study with NoMorePly where I've used 6mm board over an overlay system and then topped that with low TOG underlay and carpet and it all works very well. In fact it's actually saved me in gas bills with better overall heating (lower flow temps) in the downstairs of the house.

Thanks Madrab, good to heard a positive, it was a couple of builders who I spoke to that didn't think it would work that well. Which system would you recommend? NuHeat LoProMax was recommended to us.

Thanks!

 

[sheffdave]

We had exactly the same situation as you - part suspended, part old solid and new solid extension. We went with a low profile system (~20mm build up) throughout. As mentioned, it works differently to "conventional UFH", more like the fast response and low thermal inertia of a radiator, but it works well.

We had considered the backfill the suspended (~50cm) and excavate the old solid floor, but were advised against it by architect and builder on cost grounds.

Thanks Higgy, good to hear this worked for you, what system did you go for?

 

[Madrab]

Thanks Madrab, good to heard a positive, it was a couple of builders who I spoke to that didn't think it would work that well. Which system would you recommend? NuHeat LoProMax was recommended to us.

Builders are good at building and they should really stick to that, central heating specialist (qualified plumbers) are good at central heating.

NuHeat lowpro max seems to be a great system, the only issue with it is it doesn't add any insulation and if the existing slab is not insulated then there is still the problem of downwards heat soak. They suggest that this may be limited to between 6-10% loss, which they explain as - "Due to the thermal characteristics of a solid sub-floor, downward heat loss is limited to between 6% and 10% of the total heat output which represents a small percentage of the annual heating costs for the property"

10% heat loss is not small beer as far as I'm concerned, certainly not at the moment and they say that this is recouped by having lower stat temps and the system running cooler but that saving should be in the clients pocket, not used to offset the heat loss in the system. It is also a traditional egg tray UFH system that use a fill overlay, the only difference is that instead of a traditional 50>60mm screed, it uses a ~20mm polymer compound. TBH it is new and I'm never sure about an overfill that is structural like that @only 20mm thick, with a plastic tray beneath it. Not sure how long this has really be in play and if there are any long term studies. Not suggesting it isn't suitable, just suggesting that the jury may still be out.

The Low Pro Lite is the same EPS panelling overlay system as I am suggesting, albeit it it only uses a 10mm pipe and it seems that there is a premium cost for use a 'branded' system rather than generic. ~£2.5K for a 70m2 system is pretty pricey and that's using a small bore system with reduced output per m2 than say against a 16mm system that can have much more flexibility.

What this comes down to is, I'd never recommend just jumping at a 'branded', one fit, solution as typically they can cost a lot more.

 

[sheffdave]

Builders are good at building and they should really stick to that, central heating specialist (qualified plumbers) are good at central heating.

NuHeat lowpro max seems to be a great system, the only issue with it is it doesn't add any insulation and if the existing slab is not insulated then there is still the problem of downwards heat soak. They suggest that this may be limited to between 6-10% loss, which they explain as - "Due to the thermal characteristics of a solid sub-floor, downward heat loss is limited to between 6% and 10% of the total heat output which represents a small percentage of the annual heating costs for the property"

10% heat loss is not small beer as far as I'm concerned, certainly not at the moment and they say that this is recouped by having lower stat temps and the system running cooler but that saving should be in the clients pocket, not used to offset the heat loss in the system. It is also a traditional egg tray UFH system that use a fill overlay, the only difference is that instead of a traditional 50>60mm screed, it uses a ~20mm polymer compound. TBH it is new and I'm never sure about an overfill that is structural like that @only 20mm thick, with a plastic tray beneath it. Not sure how long this has really be in play and if there are any long term studies. Not suggesting it isn't suitable, just suggesting that the jury may still be out.

The Low Pro Lite is the same EPS panelling overlay system as I am suggesting, albeit it it only uses a 10mm pipe and it seems that there is a premium cost for use a 'branded' system rather than generic. ~£2.5K for a 70m2 system is pretty pricey and that's using a small bore system with reduced output per m2 than say against a 16mm system that can have much more flexibility.

What this comes down to is, I'd never recommend just jumping at a 'branded', one fit, solution as typically they can cost a lot more.

Great reply - thanks for spending the time to do this!

As the concrete floor area is approximately only 15% of the total floor area maybe I shouldn't worry about this too much?

Is there a system you recommend which would be more cost effective?

thanks!