Underfloor Central Heating

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I am in the process of a big renovation project and we are having underfloor heating installed. We have suspended timber floors and they have been prepared to take the underfloor heating by having Celotec insulation fitted between the joists.
We have had several plumbers come along to look at the job of laying the underfloor heating (uponor) on the celotec- some say we need screed on top of this and some say not. Does anyone out there have any views on this?
 
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Hi, I work for a leading underfloor heating company and there are arguments for and against both sides of the story...

In terms of heat output then if you have pipework just clipped to the insulation with an airgap then what you'll effectively do is heat that air gap... no where near as much heat will be transferred through the floor finish. However, if you seal the ends of the joists so there is no air change then this will drastically increase performance. Many of the larger UFH companies are now installing like this, such as warmafloor and uponor. This is mainly to cut cost in an increasingly competitive market. The UHMA (underfloor heating manufacturers association) have not yet released heat output expectations on this method, but with a F/R of 55/45°c, a floor surface limit of 29°c and a target air temperature of 21°c, you're looking at approximately 50w/m².

On the otherhand, if you use screed then you can drastically increase performance. the UHMA have tested and prove a solid floor system should easily give you up to 100w/m² based on the above criteria. HOWEVER - there is the extra cost, and the question wether or not your joists/ceilings will take it. If you can use a screed then i would strongly recommend doing so.

The other system that a lot of manufacturers do is a plated system, where the pipe sits in an aluminium tray between the joists, on top of compacted soft "loft" insulation. This should give you approximately 70w/m², though i suspect if your pipe is already down then this is not really an option!

What spacing is your pipe at? I'm guessing 200mm centres between 400mm joists?

If you have any questions then give me a shout :) good luck!
 
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Who installed the Celotex and was this on their own initiative or following a plan supplied to them ?
 
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Two options Aluminium Heat Emission Plates or Screed Infill.

Both options require insulation.

The heat emission plates are pre-channelled ready to hold the pipe, fitted with no air gaps underneath. Lay a thin layer of mineral wool, filling the gap, between the plates and polystyrene insulation. This also acts as a cushion as the plates are pinned/stapled to the joists. The floor can then be laid with its underside in contact 100% with the plates

Screed infill. The polystyrene insulation is supported, with the top of the insulation 25mm down from the top of the joists. The UFH pipes are run, and stapled, in the meander pattern, to the supported insulation. This 25mm gap is filled with a biscuit mix of 8 to 1, level to the top of the joists. Edging strip must be used where the gap reaches the walls. This helps stop cold bridging and takes up expansion and contraction.

Both installations are based on the joists at 400mm c/c
 
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Use above figures from lumon. Then get your heat loss calc from your architect. If heat loss is equal to or less than ufh output then it's ok. If heat loss is more than heat output then ufh becomes background heating and you should also have additional heating, ie rads
 
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@lumon

How can you quote outputs without giving pipe-spacing interval ?

...if you use screed then you can drastically increase performance. the UHMA have tested and prove a solid floor system should easily give you up to 100w/m² based on the above criteria.

If they used the spacing you suppose ( 200 mm) , then they would be looking at less than 50W/m2.

I am not a professional but when i laid my own system, then I found figs of 100W/m2 with 10 cm spacing and this would not be available to the OP because of the insulating effect of the wooden-floor

If they used the spacing you suppose ( 200 mm) , then they would be looking at less than 50W/m2
 
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mointainwalker:

The figures that i have quoted are just ballpark estimates. In realitiy there are many more factors that will effect output other than just pipe spacings. for example - flow & return temperature, floor covering type / thickness, loop lenths, pipe diameter (frictional loss), amount of air gap, Delta T, floor suraface temperature, etc, etc... The chances are that the system, in its current form, could probably emit anywhere between 25 and 65w/m² depending on a multitude of factors.

The figures that i have quoted for the other systems are relevant to this to give "bossbabe" an educated idea of potential outputs.
 
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Who installed the Celotex and was this on their own initiative or following a plan supplied to them ?

This is a project that has gone wrong! The original builder had the Celotex put down- I think on his own initiative as he'd used the same approach elsewhere and said it worked really well? We have 3 floors- one has had the pipework laid - the others have the Celotex in the spaces ready for the pipework.
 
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Hi, I work for a leading underfloor heating company and there are arguments for and against both sides of the story...

In terms of heat output then if you have pipework just clipped to the insulation with an airgap then what you'll effectively do is heat that air gap... no where near as much heat will be transferred through the floor finish. However, if you seal the ends of the joists so there is no air change then this will drastically increase performance. Many of the larger UFH companies are now installing like this, such as warmafloor and uponor. This is mainly to cut cost in an increasingly competitive market. The UHMA (underfloor heating manufacturers association) have not yet released heat output expectations on this method, but with a F/R of 55/45°c, a floor surface limit of 29°c and a target air temperature of 21°c, you're looking at approximately 50w/m².

On the otherhand, if you use screed then you can drastically increase performance. the UHMA have tested and prove a solid floor system should easily give you up to 100w/m² based on the above criteria. HOWEVER - there is the extra cost, and the question wether or not your joists/ceilings will take it. If you can use a screed then i would strongly recommend doing so.

The other system that a lot of manufacturers do is a plated system, where the pipe sits in an aluminium tray between the joists, on top of compacted soft "loft" insulation. This should give you approximately 70w/m², though i suspect if your pipe is already down then this is not really an option!

What spacing is your pipe at? I'm guessing 200mm centres between 400mm joists?

If you have any questions then give me a shout :) good luck!


Thanks for all this- I think the spacing you say is right. The Celotec does have a reflective surface that the pipework goes on to- what would you recommend re screed- the joists have had small battens put under them- (running parallel with them) to support the celotec
 
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Two options Aluminium Heat Emission Plates or Screed Infill.

Both options require insulation.

The heat emission plates are pre-channelled ready to hold the pipe, fitted with no air gaps underneath. Lay a thin layer of mineral wool, filling the gap, between the plates and polystyrene insulation. This also acts as a cushion as the plates are pinned/stapled to the joists. The floor can then be laid with its underside in contact 100% with the plates

Screed infill. The polystyrene insulation is supported, with the top of the insulation 25mm down from the top of the joists. The UFH pipes are run, and stapled, in the meander pattern, to the supported insulation. This 25mm gap is filled with a biscuit mix of 8 to 1, level to the top of the joists. Edging strip must be used where the gap reaches the walls. This helps stop cold bridging and takes up expansion and contraction.

Both installations are based on the joists at 400mm c/c

Thanks- what is a biscuit mix?
 
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In terms of heat output then if you have pipework just clipped to the insulation with an airgap then what you'll effectively do is heat that air gap... no where near as much heat will be transferred through the floor finish. However, if you seal the ends of the joists so there is no air change then this will drastically increase performance. Many of the larger UFH companies are now installing like this, such as warmafloor and uponor. This is mainly to cut cost in an increasingly competitive market. The UHMA (underfloor heating manufacturers association) have not yet released heat output expectations on this method, but with a F/R of 55/45°c, a floor surface limit of 29°c and a target air temperature of 21°c, you're looking at approximately 50w/m².

!

If that method is used then as a result of the lower heat transmission through the air gap, presumably a full boiler output flow temperature can be used. That could make a further saving as no second pump and temperature control valve would be needed.

The usual 40-50 C maximum is primarily to prevent the floor getting too hot.

What do you think about that?

Tony
 
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A friend of mine has recently done up a house with insulation between the joists with a gap of about 30mm above which contains the heating pipe which is then filled with concrete to help transfer the heat.

Its not quite up and running yet because he fell into his empty basement swimming pool and shattered a vertebrae and had to have metal bars screwed to two above and two below. Finished very neatly though.

Tony
 
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In terms of heat output then if you have pipework just clipped to the insulation with an airgap then what you'll effectively do is heat that air gap... no where near as much heat will be transferred through the floor finish. However, if you seal the ends of the joists so there is no air change then this will drastically increase performance. Many of the larger UFH companies are now installing like this, such as warmafloor and uponor. This is mainly to cut cost in an increasingly competitive market. The UHMA (underfloor heating manufacturers association) have not yet released heat output expectations on this method, but with a F/R of 55/45°c, a floor surface limit of 29°c and a target air temperature of 21°c, you're looking at approximately 50w/m².

!

Don't know if this helps but this renovation has been designed to make the house very thermally efficient- as well as a new roof with the necessary insulation and all new double glazed windows we are having the external cladding and insulation which we are told will give us a u value of around 2.5w/m2
If that method is used then as a result of the lower heat transmission through the air gap, presumably a full boiler output flow temperature can be used. That could make a further saving as no second pump and temperature control valve would be needed.

The usual 40-50 C maximum is primarily to prevent the floor getting too hot.

What do you think about that?

Tony
 
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A friend of mine has recently done up a house with insulation between the joists with a gap of about 30mm above which contains the heating pipe which is then filled with concrete to help transfer the heat.

Its not quite up and running yet because he fell into his empty basement swimming pool and shattered a vertebrae and had to have metal bars screwed to two above and two below. Finished very neatly though.

Tony

Poor chap - I hope he gets better soon so he can get his heating working and use his pool.
 
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I had plans to underfloor heat the upstairs of my property during an extension but decided against it due to the cost and inefficiency of the floorboard job. I spent extra money on insulation and with only two small radiators with trvs it's toasty. I did as much of the downstairs as possible using the screed method and it's great
 

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