Hello

Some while since I have been here which is an indication of how slow our project is taking.

We have an Edwardian dormer bungalow semi and are soon to be starting on the ground floor by removing the suspended floors and fitting UFH from an ASHP. The floor make up will be sand blinding, DPM, 150mm Jabfloor 70, 50mm PIR then 150mm reinforced concrete slab.

We are thinking on two options for positioning the UFH pipes.
First is to put them onto the PIR using clip rail (which we have) then 70mm chairs to support the reinforcing for the concrete pour. This puts the pipes at the bottom of the 150 slab.
Option 2 is is to put the mesh on the chairs and cable tie the pipe to the mesh.
(Option 1 makes the fitting of the pipes at the correct spacings etc very eay compared with option 2)


I have seen a number of discussions on both methods but they mostly seem to be in houses which are very well insulated. Our situation
is that we are aiming to get the solid walls insulated up to BC renovation standard and very well air sealed (ultimately!). Wall insulation will extend down to the bottom of the floor insulation.
The upstairs has 75mm pir between and 75 under the rafters and well taped and sealed.
So while our insulation will be massively better than it was, it is not "well" insulated by this forum's standards.

My question - will the difference in height of the ufh pipes in the slab have much of an impact in reaction times and could that be a problem? It seems to be a balance between thermal mass which would allow us to run the ASHP at lower temps but for longer for efficiency versus the reaction time if the temperature drops. But does the pipe at the bottom v the pipe in the middle of the slab make much difference?

We do have a wood burning stove for a top up on winter evenings.

Grateful for your thoughts!

"Why the mix of EPS and PIR insulation?"

To improve U value a bit and gives a better surface if we use the clip rail which has a double sided tape on the base.

I am fairly sold on using the slab as a thermal store so (at least a fair part of) the heating can be done off peak at night.
I guess I am interested to know thoughts on how having the pipes at the bottom or in the middle of the slab woul affect the heating response.
Putting the pipes under the reinforcing obviously reduces the need to walk on it but maybe that is not a huge problem if it is well supported...

Having dug out the floor of the barn by about half a metre (depth constrained by foundations on the uphill side), we did 150mm of EPS in 2 x 75mm thick layers straight onto a 150mm layer of compacted crushed recyled concrete (most of which came out of the barn), then DPM, then 100mm concrete slab, then we laid reinforcing mesh on top and cable tied the UFH pipes to it at 100mm centres (the mesh was not for reinforcing so much as to tie the UFH pipes to), then sand & cement screed into which we laid the brick floor directly. so the thermal mass part of our floor is about 200mm thick.

I made the U-value about 0.188

GSHP and it’s toasty in there. We also ran pipes in Aluminium spreader plates under the upstairs oak floorboards.
I don’t have any detailed data or anything but it seems to stay at a constant temperature.

The larger the thermal mass of the floor, the less reactive it will be. Ideally you'd run it constantly with a variable flow temperature driven by the outdoor temperature (which would keep the floor temp very close to the target room temperature.

Ours has been running like the for 8 years now and it works really well, resulting in very stable internal temps.

However, if your insulation levels aren't good that might result in slightly higher heating costs vs a reactive system (low thermal mass, higher flow temperatures) because it DOESNT really work if you want a different house temperature at different times of the day. (it's too slow to react to have cool in day time unoccupied/warmer in evening)

Yes, definitely has Weather Compensation.
See this video
https://www.youtube.com/watch?v=-983aLzVZ_I

Note that in the comments the author of the video says
"Mine was at 50c at -3c, I now have it at 35c at -5c and it's saved me a fortune and giving me an internal temp of 22c all day and night. I have all my stats set high to 28c, knowing that with such a low WC curve, my house will never reach 28c. I now use the WC to control my internal house temp, not the thermostats. With the stats set to 28c they are always calling for heat, so the hp is always running which means less chance of the hp short cycling. To many regular short cycling is not good for the life of the pump or your wallet as it uses a lot of kw each time it restarts. So try to keep the hp running for as long as you can, but tweak the WC to as low as you can but where it keeps the house warm. If your home is well insulated you will probably find after several days of running the pump 24/7 you have to lower the WC again due to the house holding the heat"

I don't have any internal stats at all - system runs solely on the weather compensation. It does a bit of tweaking through the first winter or two but I've not touched it since. The aim is to get the flow temperatures as low as possible (and you can't set that up if internal stats are turning the system off)

Bit late to the party, but...

- I always fix UFH pipes to the mesh, on supports, then pour concrete. It might take a couple hours longer to tie the pipes than punch-clip to insulation, for an entire GF of a house, but it gives a good result.
- I use a193 mesh, which is a bit thicker gauge than the usual a142, and so is easier to walk on for placing pipes.
- I also prefer the metal mesh supports, that look lick a wire-frame Toblerone. They're more stable for walking over than the plastic stools.
- Air pressure test the UFH pipes, and leave on for a day or so, including when conc is poured.
- builders only every like to do what they've always done. "Do what you always did, get what you always got."

Thanks GreenPaddy and Revor.

Useful tips on the heavier chairs and wire. I have seen some folk suggest padding between the wire and pipe at the cable tie point but plenty seem to say don't bother so interested in views on that. I saw a video where they were using small pieces of pipe lagging for this.
I should be able to pressure test with water easy enough.

We are having large radiators upstairs but hoping they won't have to do much work. We have managed to insulate upstairs pretty well and being dormer bungalow it's a relatively small volume to heat.

Lost count of the number of times builder No1 has said "never done it like that before" and "why do you want that much insulation, you can get away with...', and I am not talking about a lot either.

I'm told it will be worth it in the end!