Castellated panels will likely 'breath'

[MortarThePoint]

I hate working this sort of thing out as sometimes ignorance is bliss.

 

For a gas, pressure (P) is proportional to temperature (T) in Kelvin for a closed volume. Therefore if there is a dT there is a resulting dP. With the UFH off for some time the floor will be at the same temperature as the room. With it on, the temperature will rise above the room temperature by dT (e.g. rising to 36C giving dT=15C=15K). That's going to create a pressure difference of dP = (15/300) * 1Atm = 1/20 Atm = 5kPa in any air trapped below the floor. The weight of 50mm thick screed is around 2.4kg/m3 * 0.05m * 10N/kg = 1.2kPa. Presuming a continuous air gap under the screed that would lift the screed if it didn't leak out, so it will leak out. That's tiny you'll think, but most castellated panels increase the volume of air involved greatly. The volume of the castellation and ridge per unit cell is probably around 25ml (a shot glass) per 70mm x 70mm unit cell, so 5.1l per m2. So, a 20m2 floor will breath 5.1l per cycle.

 

So what? I don't know, but I thought it was interesting. I had planned to glue down my castellated panels, but this makes me think of the air going in and out pushing out any warmed up fumes from the glue. Probably no biggy as the temperatures aren't very high.

 

Has anyone thought about this before? @Nickfromwales you normally like talking sense against this sort of nonsense.

Edited July 21, 2021 by MortarThePoint

[PeterW]

They don’t and can’t move and all you end up with is an increase in air pressure in the cell using a straight line PVT calculation. 

[MortarThePoint]

3 minutes ago, PeterW said:

They don’t and can’t move and all you end up with is an increase in air pressure in the cell using a straight line PVT calculation. 

 

But unless there is a sealant, it's going to leak out. If not, it would raise the floor by approximately (1/20) * 25ml / 7cm * 7cm = 1/40 mm which is admittedly tiny, but probably enough to open up an air leak to release the pressure.

Edited July 21, 2021 by MortarThePoint

[PeterW]

Ok but you have to also look at air leakage as you haven’t got a perfectly sealed load cell, so you will have voids and also other leakage routes and won’t create the lift you are anticipating. You’re also only using self load, so haven’t included any additional static load or additional downforce such as fixings. 
 

And you can’t measure 0.025mm in a building. 

[MortarThePoint]

14 minutes ago, PeterW said:

Ok but you have to also look at air leakage as you haven’t got a perfectly sealed load cell, so you will have voids and also other leakage routes and won’t create the lift you are anticipating. You’re also only using self load, so haven’t included any additional static load or additional downforce such as fixings. 

 

I agree and the leakage is what will allow it to 'breath' and push air in and out of the living area.

 

15 minutes ago, PeterW said:

And you can’t measure 0.025mm in a building. 

 

Yes, it's nothing. I'd prefer that to the air going in and out.

[Russell griffiths]

So if you had clipped your pipes into rails you wouldn’t be wasting your time thinking of this. ??

Are you sure on your weight calculation for the screed, mine weighed something like 144kg per m. 

[markc]

1 minute ago, Russell griffiths said:

So if you had clipped your pipes into rails you wouldn’t be wasting your time thinking of this. ??

Are you sure on your weight calculation for the screed, mine weighed something like 144kg per m. 

Decent sand/cement screed is 1800kgs a cube, 75mm thick would be 135kgs/m

[Russell griffiths]

1 minute ago, markc said:

Decent sand/cement screed is 1800kgs a cube, 75mm thick would be 135kgs/m

Spot on, mine was 80mm. 

[MortarThePoint]

Those are around 1.4 kPa of screed weight.

[Nickfromwales]

Ok, lol, so my 2 penneth is to Foxtrot the castellated panels right off and do as the angry cup ninja above suggests and use clip rails. You want that void full of screed and not part full of worthless air anyhoo, as the screed is your heat emitter.

Oh, and all this nonsense about air lifting and whatnot.........time for more tonic / less Gin   

 

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