UFH Design Review, Passive Slab

[Nickfromwales]

7 hours ago, Duncan62 said:

I have avoided under shower and kitchen cupboards (some of them) simply because I have 3 x 100m loops, which works nicely for buying and fitting.

I shall not do serpentine as it looks (much) more difficult for this mortal to install

OK.

I think you should have more loops / zones. If I was designing this as' your M&E guy', I'd have a loop per space. The bathroom should be at 100mm o/c and be as fully populated as is practicable. Space A-01 should be fed by taking the loops under the wall. Space A-02 is fine. A-07 should also be at 100mm o/c as it is a rising space to FF.

 

Consider the effects of the days solar gain, and how it will affect each space. IMHO, the current design does not. You have some small, confined spaces, which will 'out-perform' the other, larger open areas.

[John Carroll]

1 hour ago, Duncan62 said:

 

Thank you Terry: I plan to use the bottom side wall in the plant room which is 900mm wide. Enough for a 3x Out and 3x Return manifold + pump?

 

Yours doesn't look too big?

 

 

The Willis should be installed  with the heating element underneath otherwise air can build up causing problems.

[Mike]

2 hours ago, Duncan62 said:

I plan to use the bottom side wall in the plant room which is 900mm wide. Enough for a 3x Out and 3x Return manifold + pump?

Mine just fits in 900mm, excluding the filling loop (though that could be repositioned), but doesn't have a plate heat exchanger like @TerryE

[Duncan62]

What UFH pipe do people recommend for installation direct to mesh, 75mm below the surface with 200mm total depth of slab?

PEX AL PEX or PERT AL PERT?

16mm as standard?

and why?

Edited April 24 by Duncan62

[JohnMo]

8 minutes ago, Duncan62 said:

What UFH pipe do people recommend for installation direct to mesh, 75mm below the surface with 200mm total depth of slab?

PEX AL PEX or PERT AL PERT?

16mm as standard?

and why?

 

I like Pert-al-Pert as it has no memory so is easy to work with. Modern UFH I would only use 16mm - easily available, carries plenty of energy, low pressure drop. Anything else size wise is just a bad choice.

[Duncan62]

1 hour ago, JohnMo said:

 

I like Pert-al-Pert as it has no memory so is easy to work with. Modern UFH I would only use 16mm - easily available, carries plenty of energy, low pressure drop. Anything else size wise is just a bad choice.

 

great, thanks, like this?: https://underfloortradesupplies.co.uk/product/komfort-16mm-x-100m-pert-al-pipe-5-layer-barrier-white-multi-layer-ufh-pipe/

[JohnMo]

10 minutes ago, Duncan62 said:

 

great, thanks, like this?: https://underfloortradesupplies.co.uk/product/komfort-16mm-x-100m-pert-al-pipe-5-layer-barrier-white-multi-layer-ufh-pipe/

Something like that

[TerryE]

On 05/04/2024 at 00:10, John Carroll said:

The Willis should be installed  with the heating element underneath otherwise air can build up causing problems.

 

I have often pondered about this one.  It was first pointed out by JSH referencing the manufacturer installation instructions, and IIRC he got a bit heated about this point.  Why?  The primary use of the Willis is to act as an external heater on a copper vented (unpressurised) H/W cylinder.  Here we have open-loop heating constantly replenished mains-fed water.  This carries an element of dissolved air which will tend to come out of solution during heating and build up in inverts -- hence the recommendation to fit the Willis inverted.

 

TBH, even if I was redoing my CH system, I would still use a Willis and I would still fit it heating element up, because this issue doesn't apply for this use: the water is a small closed volume running in a pumped circuit.  In our case I filled our UFH loops 7 years ago, topped up with inhibitor / unfreeze, and then pressured the closed system to 1 bar.  Occasionally over the next month or so, I bled the system at the manifold to let out any air coming out of solution.  In the process the tiny amount of water lost when bleeding was taken up by the expansion tank dropping the pressure to 0.8 bar or so.  I then topped up the system back to 1 bar, and that's it: 7 years later, the system is still at 1 bar.  There is no more air to come out of solution.

 

So air build up is just not an issue here, and the Willis works happily in any orientation, and I am happier with the wiring and thermostat setting on top  where I can see them and have easy access.  So yes, for me heating element up works best.

 

Edited April 26 by TerryE

[Nick Laslett]

On 24/04/2024 at 13:12, Duncan62 said:

What UFH pipe do people recommend for installation direct to mesh, 75mm below the surface with 200mm total depth of slab?

PEX AL PEX or PERT AL PERT?

16mm as standard?

and why?

Some earlier discussion here. 
 

 

According to Wundatrade they recommend their Pert-Al-Pert for over floor systems. They recommended their PexB-Al-HDPE for screed. When I was researching this topic, I found very little to confirm a specific pipe for going into a concrete insulated slab foundation. 
 

https://www.wundatrade.co.uk/shop/home/water-underfloor-heating-solutions/joist-floor-heating-pipe/16mm-hdpe-al-pex-pipe/

 

[John Carroll]

On 26/04/2024 at 11:23, TerryE said:

 

I have often pondered about this one.  It was first pointed out by JSH referencing the manufacturer installation instructions, and IIRC he got a bit heated about this point.  Why?  The primary use of the Willis is to act as an external heater on a copper vented (unpressurised) H/W cylinder.  Here we have open-loop heating constantly replenished mains-fed water.  This carries an element of dissolved air which will tend to come out of solution during heating and build up in inverts -- hence the recommendation to fit the Willis inverted.

 

TBH, even if I was redoing my CH system, I would still use a Willis and I would still fit it heating element up, because this issue doesn't apply for this use: the water is a small closed volume running in a pumped circuit.  In our case I filled our UFH loops 7 years ago, topped up with inhibitor / unfreeze, and then pressured the closed system to 1 bar.  Occasionally over the next month or so, I bled the system at the manifold to let out any air coming out of solution.  In the process the tiny amount of water lost when bleeding was taken up by the expansion tank dropping the pressure to 0.8 bar or so.  I then topped up the system back to 1 bar, and that's it: 7 years later, the system is still at 1 bar.  There is no more air to come out of solution.

 

So air build up is just not an issue here, and the Willis works happily in any orientation, and I am happier with the wiring and thermostat setting on top  where I can see them and have easy access.  So yes, for me heating element up works best.

 

Not a problem, as you say, with a pumped Willis, but certainly (IMO) will tend to cause problems with the normal installation as the outlet temperature will run much higher due to the very slow circulation rate and release more air?,  I have read of the heating elements failing due to air build up if installed with the immersion on the top, 

[Duncan62]

Questions: Installation time.

 

Lay/connect the pipe loops to rebar.

Install manifold to 3x loops.

 

Then connect garden hose to pressurise the system?

 

Will pressurising the pipe work (16mm pert-al) mean I can walk on the pipes without damaging them against the rebar?

 

If not, how on earth do I protect the pipes when the concrete contractors trample all over it?!

[garrymartin]

5 hours ago, Duncan62 said:

If not, how on earth do I protect the pipes when the concrete contractors trample all over it?!

The pipes are pretty bulletproof and shouldn't be damaged during the pour under normal circumstances.

 

There were some questions in other threads about abrasion against the mesh as people are walking on the pipes. I saw the below use of pipe lagging in a Youtube video and initially thought it was a great idea, but then wondered whether it would cause any issues with such a low compressive strength of the pipe insulation in the slab itself...

 

[saveasteading]

Repeated friction could wear the plastic coating off. The screeders  shouldn't need to walk directly on it and the concrete or screed should flow ahead of them.

If in doubt then put a plank down to spread the load. Once concrete is in, even the wet stuff will spread the load and form some cushion between pipe and rod.

 

The use of the foam in the picture effectively forms a void in the concrete, which isn't ideal for a few reasons.

 

[Dave Jones]

thats old gas boiler UFH design.

 

You want 100mm spacing and every inch of the slab piped. No missing out baths, under cabinets etc.

 

No zones either.

[JohnMo]

1 hour ago, Dave Jones said:

thats old gas boiler UFH design.

 

You want 100mm spacing and every inch of the slab piped. No missing out baths, under cabinets etc.

 

No zones either.

No zones yes

 

Everything else is nonsense. I am on 300mm centres, have missed everywhere, where there is something on the floor - cabinet, beds etc. max flow temp has been 32 at -9

[TerryE]

The max flow temp depends on average heat flow into the house.  For example for our Passive-class house in Dec-Feb, we need about 2 kW heat input into the environment peak to sustain temps.  IIRC our radiant floor area is about 72m² × ~ 7 W/Km² or ½kW/K so we need the slab surface to be on average ~ 4 °C warmer than the internal temp.  A more typical  modern build might need 3× this delta.

 

So if I was using a 3kW ASHP then it would be running at a mark:space ratio of 2:1, and as @JohnMo says you could probably get away with 300mm centres.  IIRC ours are at 150mm, but we use a Willis with block heating at the cheapest Agile half hour slots.  Even so our manifold out into the slab rarely gets about 30°.

[Nick Laslett]

20 hours ago, Duncan62 said:

Questions: Installation time.

 

Will pressurising the pipe work (16mm pert-al) mean I can walk on the pipes without damaging them against the rebar?

 

If not, how on earth do I protect the pipes when the concrete contractors trample all over it?!

This was my main concern. There is a post somewhere from @Nickfromwales in one of the insulated slab threads where he talks about how robust these pipes can be. I tried to find a UFH pipe where they specifically said it could be tied to rebar in a concrete slab, but was not very successful. Even the question “is pert more robust than pex?”, or the other way round is very difficult to find an answer to. I believe the HDPE outer layer of the Wundatrade pipe is why they claim it is okay to go in screed. Another outfit, Optimum Underfloor Heating which uses PEX(b) pipe, in their installation instructions has an install in concrete slab. Page 4 of their installation guide. 
 

https://www.optimumunderfloor.co.uk/s/Optimum-Installation-Guide.pdf

 

This is not to say that PERT-Al-PERT is not good for this requirement. I’m sure it is excellent and will do a good job. It would just be reassuring to have a manufacture confirm this specific use case, without having to scour the internet with a fine tooth comb. It might just be as a homeowner, I don’t need to see this info and the installers have a handle on which is the best pipe.
 

Unless I have missed the particular thread in question, I don’t believe on BuildHub anyone has shed any greater light on this specific use case, but I could be mistaken. Very happy to be corrected as this question comes up a lot, and things like ease of installation and pipe memory, are secondary in this particular use case, where robustness is the biggest concern. 

*Edits

Here is a good discussion from the early days of BuildHub about the robustness of various UFH pipe. 
 

Here is the post I was thinking of from Nick. 

 

Quote

If they use proper Pex-Al-Pex pipe then you'll only seriously damage it if you take a hammer to it. My screeders regularly run barrow-fulls of screed over the pipes and then one of them will up-end the barrow on partially visible pipes but still with no ill effect. The stones in the concrete are of absolutely zero concern I assure you ( and I've been here more than once over the last 23 years ).

 

Edited May 25 by Nick Laslett

[garrymartin]

13 minutes ago, Nick Laslett said:

I tried to find a UFH pipe where they specifically said it could be tied to rebar in a concrete slab, but was not very successful.

 

 

I'm pretty sure most would be fine tied to rebar in a concrete slab, but if you want an absolute guarantee that that's OK from a manufacturer, here's at least one.

 

WarmUp Forte Grid System - https://www.warmup.co.uk/underfloor-heating/water/forte-grid-system

 

 

 

https://www.warmup.co.uk/wp-content/uploads/Warmup-TS-Forte-2016-09-29.pdf

https://www.warmup.co.uk/wp-content/uploads/Warmup-TS-AL-PE-RT-v1.0-2016-09-29.pdf

 

[Nick Laslett]

5 minutes ago, garrymartin said:

 

I'm pretty sure most would be fine tied to rebar in a concrete slab, but if you want an absolute guarantee that that's OK from a manufacturer, here's at least one.

 

WarmUp Forte Grid System - https://www.warmup.co.uk/underfloor-heating/water/forte-grid-system

 

 

 

https://www.warmup.co.uk/wp-content/uploads/Warmup-TS-Forte-2016-09-29.pdf

https://www.warmup.co.uk/wp-content/uploads/Warmup-TS-AL-PE-RT-v1.0-2016-09-29.pdf

 

That is a great find. Bit annoyed all my searching in 2019 didn’t unearth this. I did look at Warmup, but must have not looked hard enough. 

[Dave Jones]

16 hours ago, JohnMo said:

No zones yes

 

Everything else is nonsense. I am on 300mm centres, have missed everywhere, where there is something on the floor - cabinet, beds etc. max flow temp has been 32 at -9

That’s fine but you are wasting efficiency for no gain.

[TerryE]

6 hours ago, Dave Jones said:

wasting efficiency

 

How? What measure of efficiency? You've lost me here.

 

@Nick Laslett MBC have been doing this with their slabs for a decade.  

Edited May 26 by TerryE

[Dave Jones]

8 minutes ago, TerryE said:

 

How? What measure of efficiency? You've lost me here.

 

@Nick Laslett MBC have been doing this with their slabs for a decade.  

think of it as trying to only heat the top left corner of a radiator. The rest leeches form it and therefore requires more energy killing efficiency.

 

Same with a slab. not rocket science.

[JohnMo]

7 hours ago, Dave Jones said:

That’s fine but you are wasting efficiency for no gain.

No not true I already flow at my lowest achievable flow temp to sub zero if I run on WC mode.

 

11 minutes ago, Dave Jones said:

think of it as trying to only heat the top left corner of a radiator. The rest leeches form it and therefore requires more energy killing efficiency.

 

Same with a slab. not rocket science.

Obviously is rocket science, because your analogy is poor. It more akin to a thick towel always over the radiator and only heating those areas not covered by all the towel. The towel will get warm but you are not wasting energy heating it unnecessarily.

 

What point does heating the underside of my kitchen units serve - none.

[Iceverge]

3 hours ago, JohnMo said:

What point does heating the underside of my kitchen units serve - none.

 

Added thermal storage?

[JohnMo]

6 minutes ago, Iceverge said:

 

Added thermal storage?

Not sure about that. I'm either ticking away in WC mode or batch charging. WC mode you really don't need more thermal storage. Have about 60Te.  Batch charging will last just fine between E7 periods, so no added value.

 

Think we have a total of 670m of pipe and 7 loops in around 195m of 100mm concrete with pipe clipped to 200mm of insulation. Works fine and dandy. You can stuff pipes at 100mm centres, but even at that I wouldn't be able to flow any lower temps, because the floor is at 22 to 24 and the heat pump runs at 5dT and requires a dT of between 6 and 8 to restart after a heat cycle.  So if I flow at 25 to 26, the heat pump runs once than cannot decrease return temperature enough to get a restart permission. So min practical flow temp is 28-29.

Edited May 26 by JohnMo