Realistic UFH Outputs - Overlay Systems

[Barry1965]

Hello all,

 

There are several overlay panel systems that we are looking at (Wunda, Ambiente, Nu-Heat etc) that give expected heat outputs per m2 for their systems.

 

For example, Ambiente claim that their AmbiDeck 20 Pro gives out 70W/m2 at 35C MWT with a 150mm pipe spacing and a 0 tog floor tile finish in a 20C heated room.

 

How realistic are these claims and are there any important assumptions behind these figures that we should be aware of.

 

Our system is to be installed on concrete slab (no insulation below) and running 24 hours a day at flow temperatures between 30-40C.

 

Many thanks,

 

Baz

[JohnMo]

Worth a watch

 

 

What ever you do do a screed on top of the pipes. I tried an overlay system it was rubbish compared to a screed system.

 

Floor output chart h use a straight line between points - to get MWT, pipe spacing and correct output.

 

[Barry1965]

Thanks John,

 

What was the overlay system that you used? Was it rubbish in terms of outputs or were there other issues too?

 

We are restricted to a build up of 40mm max so are unable to use a screed system unfortunately. Are there similar output charts to the one you included for overlay systems? Presumably this output also depends on the insulation below the installed system?

 

Thanks,

 

Baz

[JohnMo]

52 minutes ago, Barry1965 said:

What was the overlay system that you used? Was it rubbish in terms of outputs or were there other issues too

It was an EPs with thick aluminium and OSB over the top (with an additional 250mm else below). Output wasn't good enough to keep room warm with out super high flow temps. Stopped using it was so bad.

 

Output shouldn't change with insulation by much but energy input will to get same room temperature will.

 

I would add 

52 minutes ago, Barry1965 said:

build up of 40mm max

Add insulation and radiators - is it a boiler or heat pump?

 

Or the one in the video could be screeded and within 40mm. 

Edited Wednesday at 10:42 by JohnMo

[Barry1965]

Thanks John,

 

We're using a heat pump so want to keep flow temperatures as low as possible.

 

Do you think the OSB had an impact on the poor performance of your system?

 

Thanks,

 

Baz

[SteamyTea]

OSB has a k-value around 0.13 W.m^-1.K^-1 at a density of 650 kg.m^-3.

 

[ProDave]

1 hour ago, Barry1965 said:

We are restricted to a build up of 40mm max so are unable to use a screed system unfortunately.

 

What restricts you to 40mm?  What is the bottom of that 40mm?

 

25mm battens UFH pipes between pug mix and 20mm engineered wooden floor is what we have, only 5mm "too thick" for you.

 

What insulation and floor build up below your 40mm starting point?

[JohnMo]

4 minutes ago, Barry1965 said:

Do you think the OSB had an impact on the poor performance of your system

Yes, wood is an insulation compared to screed. Screed also gets full contact with pipes so maximum heat transfer.

[SteamyTea]

Cement screed has a thermal conductivity of about 1.1 W.m^-1.K^-1.

Liquid screed is about double, so 2.2.

 

Probably best never to put timber above UFH.

[MikeSharp01]

12 minutes ago, SteamyTea said:

 

Probably best never to put timber above UFH.

Don't tell me this, we have 20mm Ash / ply engineered floor, 1 Tog, over ours, pipes are about 80mm down in the slab at 150mm centers. The floor people want us to keep the slab temperature below 27deg C.  Doing a quick line across JohnMo's chart we should be on about 25W / m2 and we have 100 m2 of UFH so 2.5kW and we only need about 2kW so we should scrape in on worst days. 

[SteamyTea]

5 minutes ago, MikeSharp01 said:

1 Tog

Horrible unit.

A TOG is thermal resistance, so the R-Value.

1 TOG is equal to R-Value 0.1 m².K.W^-1.

Or in Roman Catholic U-Value 10 W.m^-2.K^-1.

 

Why don't we all just use the easy to understand SI units, for everything. Not as if there are many if them, or difficult to visualise.

[Barry1965]

Thanks for your replies.

 

We don't have any insulation below the proposed overlay system. The floor is currently concrete slab and we don't really want to start digging down, and existing doors and fittings limit us to about 40mm build up.

 

If we were to put down an overlay system on the concrete slab then pour a thin layer of liquid screed over the top, what sort of outputs could we expect if we topped it with 10mm tiles.

 

(assuming 20mm XPS boards, 16mm pipe at 120 spacings)

 

Thanks again for your help,

 

Baz

[ProDave]

31 minutes ago, Barry1965 said:

We don't have any insulation below the proposed overlay system. The floor is currently concrete slab and we don't really want to start digging down, and existing doors and fittings limit us to about 40mm build up.

 

If we were to put down an overlay system on the concrete slab then pour a thin layer of liquid screed over the top, what sort of outputs could we expect if we topped it with 10mm tiles.

Don't fit UFH in this situation.  Your overlay boards will not have anything like enough insulation to sit directly on an uninsulated floor.  If you do proceed expect a lot of your heat to go down and very poor performance.

[SteamyTea]

7 minutes ago, Barry1965 said:

20mm XPS boards

Has a thermal conductivity of about 0.035 W.m^-1.K^-1.

So a U-Value of 1.75 W.m^-2.K^-1.

So a lot of power will go into the ground.

If I was at home, with a blank spreadsheet open, I would take a stab at how much goes into the room, and how much into the ground.

But let us say you have 20mm of liquid screed above the pipes (same thickness as XPS below them).

The U-Value would be 110 W.m^-2.K^-1.

 

Now the temperature differences have to be taken into account.

Initially let use say that the mean flow temperature though the pipes is 30°C, ground temperature is 8°C and room temperature is 20°C.

Power to ground will be 1.75 [W.m^-2] x 1 [m²] x (30 - 😎 [K] = 38.5 W.m^-2.

Power to room will be.

110 [W.m-2] x 1 [m²] x (30 - 20 [K] = 1100 W.m^-1.

 

Now for some reason those numbers do not look right to me, so maybe I have made a mistake somewhere (very likely) or I have picked up some duff k-Values.

A 3 to 1 difference just does not look right.

But if it is right, then a third of your cash is warming worms.

[JohnMo]

1 hour ago, Barry1965 said:

we were to put down an overlay system on the concrete slab then pour a thin layer of liquid screed over the top, what sort of outputs could we expect if we topped it with 10mm tiles.

 

(assuming 20mm XPS boards, 16mm pipe at 120 spacings)

Use what is linked in the video. 20mm insulation is a waste of time in the grand scheme of things. Adding the insulation in this case just decouples the existing slab from the UFH. You are better having it coupled, then run low and slow per video. You will suffer downwards heat loss, but that is true with any of these systems.

 

Not sure I would trust a super thin layer of screed on top of the 20mm insulation. Cracks etc.

Edited Wednesday at 12:57 by JohnMo

[JohnMo]

1 hour ago, MikeSharp01 said:

Don't tell me this, we have 20mm Ash / ply engineered floor, 1 Tog, over ours, pipes are about 80mm down in the slab at 150mm centers. The floor people want us to keep the slab temperature below 27deg C.  Doing a quick line across JohnMo's chart we should be on about 25W / m2 and we have 100 m2 of UFH so 2.5kW and we only need about 2kW so we should scrape in on worst days. 

We also have 20mm (oak) flooring, but pipes at 300mm centres. There is zero issues I generally flow at 35 degs, floor temp has never been above about 24. The temperature through the pipes is dispersed via distance, as heat radiated outwards. Floor temp and flow temp are different things

[MikeSharp01]

5 minutes ago, JohnMo said:

We also have 20mm (oak) flooring, but pipes at 300mm centres. There is zero issues I generally flow at 35 degs, floor temp has never been above about 24. The temperature through the pipes is dispersed via distance, as heat radiated outwards. Floor temp and flow temp are different things

Thanks for the reassurance John. Our wood floor supplier, who has done a great job of getting us into a position to go for a wooden floor, tiles being our previous choice, wants us to install a temperature probe directly below the wood to allow us to ensure we don't go over the maximum. They are very keen not to dry out the top surface, 6mm of Ash, not sure the ply below it is anything of an issue.  Anyway if it is flow temp then we can get to 60 W/m2 on your diagram, which is stacks.

[Nickfromwales]

15 minutes ago, MikeSharp01 said:

Thanks for the reassurance John. Our wood floor supplier, who has done a great job of getting us into a position to go for a wooden floor, tiles being our previous choice, wants us to install a temperature probe directly below the wood to allow us to ensure we don't go over the maximum. They are very keen not to dry out the top surface, 6mm of Ash, not sure the ply below it is anything of an issue.  Anyway if it is flow temp then we can get to 60 W/m2 on your diagram, which is stacks.

Running your particular floor / slab ‘ling & low’ means you’ll be fine at prob 25°C max UFH flow temp. These types of homes don’t really know if it’s winter outside, with the only real negative impact being from the MVHR sucking in ice cold air.

 

You’ll be fine, chalk n cheese to what’s being discussed here. 
 

@Barry1965 have you tried lifting 1m2 of floor area in the corner of a random room, to see if this is a 50mm topping layer over a rough cast sub slab? That’s what I found in a 60’/70’s bungalow, giving opportunity to put 40mm XPS boards down in tile adhesive and then the overlay could go onto those. 
 

If not then I’m with @JohnMo in that you should just nail clip it to the slab and run 24/7 with a much lower flow temp.

 

 

This is one where there was 180mm or 200mm hollow core concrete deck, then I installed the UFH, and the around 65mm of screed went over the entire area. Then engineered wooden flooring over that (bonded down NOT floating!). 
 

You’d have to accept that if you REALLY want UFH then you’ll just have to live with around 25p in the £1 going to Mother Earth.

 

Theres a company who can router out the existing slab and lay the pipe straight into that, which is what I’d do to offset a lot of expensive carpentry and fixtures alterations, perhaps, but you 1000% want to go for tiles not wood or carpet, and have one floor type for the entire area so you get same results per m2 which will make controlling it a bucketload easier.

 

I’d have the UDH tbh, as the heating system is prob high temp rads and costing a fair bit to run anyways, so switching to a good quality heat pump will help (but I’d prob advise you stayed on gas unless you have polar PV and a big battery (to chip in to the overall annual electrical running costs).

 

The heart wants what the heart wants, and sometimes the head will come second place. 

[Nickfromwales]

This kind of thing, random internet grab, no affiliation / recommendation. 
 

https://www.heatflowinstallations.co.uk/floor-milling.html

[Barry1965]

Thanks for your replies @ProDave@SteamyTea@JohnMo,

 

@Nickfromwales We were looking at the overlay systems because we want to avoid any digging up if possible, and as we have ASHP we would be looking at running it continuously with flow temps of max 35C. 

 

Having done a bit more reading around yesterday, I went back to our original heat loss report and calculated the following:

 

For the areas we are looking to install UFH, floor heat loss is currently ~500W or about 6% of total house heat loss, so assuming a doubling in floor heat loss in these areas due to hotter UFH heated floors (and ignoring any insulating effect of overlay panels) would give an overall increase in heat loss of ~6%.

 

Our heat loss report gave a figure of ~5000KWH for annual heat pump energy use for space heating (in reality this has been much lower), which at ~20p/KWh would mean an additional cost of ~£60 per year if we used UFH in these areas... which seems very low. Or have I missed something or made any daft assumptions?

 

In conclusion, as we are only proposing to install UFH to ~28% of floors and because downward heat loss is only a minor part of total heat loss in these areas, then the additional heating cost shouldn't be too extreme.

 

Thanks again for your help,

 

Baz

[Nickfromwales]

Seems a lot of ball ache for such a small area (don’t shout, just saying  ). 

[Nickfromwales]

On 05/03/2025 at 13:33, Nickfromwales said:

This kind of thing, random internet grab, no affiliation / recommendation. 
 

https://www.heatflowinstallations.co.uk/floor-milling.html

👆?

[Barry1965]

30 minutes ago, Nickfromwales said:

Seems a lot of ball ache for such a small area (don’t shout, just saying  ). 

 I completely agree! But it's this or rethink the kitchen layout to fit in some massive radiators...

[Nickfromwales]

9 minutes ago, Barry1965 said:

 I completely agree! But it's this or rethink the kitchen layout to fit in some massive radiators...

Feck. If it’s a kitchen then you’ll have far less heated floor area so far less output. 
 

Have you looked at hydronic plinth heaters? These bolster smaller radiators when you need a bit more oomph. 
 

Also, consider electric under tile heating for a bit of comfort, but NOT for use 24/7. 
 

I assume you say HUGE rads because it’s a heat pump? Is the heat pump already installed?

[Barry1965]

Yes, heat pump is already installed and house is now mostly well insulated too.

 

Although it's the kitchen, because we are going for free standing units we were planning on installing UFH to all areas. We hadn't thought of plinth heaters before but they do look a good option for an occasional boost, thanks.