What goes more than 100mm deep into the slab.

[MikeSharp01]

I am just laying out our UFH, on the drawings, ready to put them into the slab and I am trying to work out where they cannot go. I appreciate that services coming up through the slab must be avoided but my worry was what fixings might need to go more than 100mm, our slab is 150mm thick with the 16mm UFH at the bottom, into the slab and therefore must be avoided. I was making a list:

 

1. Fixings for the sole plate (outer walls).

2. Fixings for sole plate of internal stud partitions.

3. Fixings for the internal steel columns.

4. Fixings for the toilet pans if I cannot sort wall hung (or some later occupier might want to fit floor standing).

5. Fixings for the base of the stairs.

6. Fixings for the kitchen island unit.

7. ???

8. ???

....

 

Other questions which feel like I know the answers as to where the pipes must go, under the downstairs wet room shower tray former and into the downstairs WC.

 

[Russell griffiths]

From my point everything you have mentioned will not have heating pipes below them 

when you get the layout mark on it all the things to avoid and the ufh guy will design around them

hope that makes sense. 

It sounded better in my head than it looks on the screen. 

[Dee J]

Don't think there'd be any fixings which would need to be more than 75mm. The bigger problem is accidentally over-deep drilling. An sds drill pushing an 8mm bit with gravity assistance can easily run way too deep. Depth stops or short bits may be useful.

Edited June 7, 2018 by Dee J
Typos

[willbish]

Spacer + mesh + pipe <50mm?

 

[Onoff]

12 hours ago, MikeSharp01 said:

Other questions which feel like I know the answers as to where the pipes must go, under the downstairs wet room shower tray former and into the downstairs WC.

 

 

I presume you'll insulate the pipes under the trap to mitigate any drying out effect on the shower trap?

[MikeSharp01]

10 minutes ago, Onoff said:

I presume you'll insulate the pipes under the trap to mitigate any drying out effect on the shower trap?

Very good point, The pipes actually go around the trap itself but I agree it needs to be kept well away from the trap or well insulated.

[Ferdinand]

if your staircase is a separate module, floor fixings for that?

[Dudda]

Consider what rooms do and don't need UFH. I won't go putting heating into a pantry for example. Having that 2-3 degrees cooler for storing food is useful. Similarly you don't need UFH in a plant room as it will be hot enough with all the manifolds and kit in it. The plant room is one area where you could end up bolting random items to the floor.

[Nickfromwales]

100mm fixings through a 44mm soleplate will keep you safe and fixed sufficiently well. For added pazzaz you can use a D4 PU glue to bond the timber to the screed. 

TBH, after the glue goes off you could take the fixings back out. 

WC pans go down with CT1. 

 

Oh, and ffs please stop getting bogged down with what potential future occupants may do with a drill bit. . 

 

[epsilonGreedy]

1 hour ago, Nickfromwales said:

WC pans go down with CT1.

 

 

Is this the stuff?

 

http://www.ct1ltd.com/product-applications/info-on-ct1-sealant/

 

Ceramic to concrete screed?

[PeterW]

4 minutes ago, epsilonGreedy said:

 

Is this the stuff?

 

http://www.ct1ltd.com/product-applications/info-on-ct1-sealant/

 

Ceramic to concrete screed?

 

Yep

 

And it’s pretty much anything to anything ...! You normally tile under the pan anyway so it’s ceramic to porcelain/ceramic/stone etc

[Nickfromwales]

Your WAY over my head. 

I go into my merchant and look at the shelf. There's clear, black, white or grey. 

I grab the one that I need that day. . 

[Onoff]

Do you hoover the joint before CT1'ing if rough surfaces? 

 

Two smooth say tiled faces I imagine it's just Multi Solve.

 

I mean say you're sticking a pan down to stone. I hoovered the concrete before I placed the Aqua Panel edge down on it (Sika EBT in this case). Just figure the more loose bits you can get up...

[mvincentd]

On 07/06/2018 at 17:56, MikeSharp01 said:

our slab is 150mm thick with the 16mm UFH at the bottom

I thought it was usual to place ufh near the top....whats the rationale for it being at bottom, out of interest?

 

Mine is 40mm below surface in a (min)200mm slab.  Next process was drilling 50 x 100mm holes for temporary wall construction supports.....very stressful praying we missed all the ufh, which it appears we did!

 

All my internal walls are stud and will be glued down.....but I won't use a topical sealer on the slab where i'm gonna glue.

[MikeSharp01]

43 minutes ago, mvincentd said:

I thought it was usual to place ufh near the top....whats the rationale for it being at bottom, out of interest?

The slab is heavily insulated below the UFH, to which it will be pinned, and so I guess getting it lower down helps make the slab more controllable in terms of heat flow - upwards and outwards so hotspots round the pipes are eliminated when heating and when cooling, which ours will be, we take heat out from the bottom of the slab. There may also, depending on the engineering of the slab, be a structural issue requiring it to be at a given height such as on or close to the neutral axis - not an issue for us. 

[Jeremy Harris]

The position of all our internal walls, kitchen units, kitchen island, ground floor WC, utility room units etc were all accurately spray paint marked on the insulation under our slab before the UFH pipe was laid, so we could be certain that we had no pipes underneath anywhere where we might later need to drill any holes.  Our pipes are also towards the top of the slab; the reinforcing fabric is in the centre of the 100mm thick slab and the 16mm pipes are cable tied to the top of this, to get them closer to the upper surface.

 

Concrete has a reasonably good thermal conductivity, but even so I'd want the pipes nearer the top surface than the bottom.  There will always be a thermal gradient inside the slab, and even though there is insulation underneath it I'd still prefer the thermal path to the upper surface to be as short as possible.

[MikeSharp01]

41 minutes ago, JSHarris said:

Our pipes are also towards the top of the slab; the reinforcing fabric is in the centre of the 100mm thick slab and the 16mm pipes are cable tied to the top of this, to get them closer to the upper surface.

Interesting, I will take a look and build a thermal model of this to see what might be optimal. We have three levels in the slab, the base and then two layers of mesh, at which the pipes could be pinned and I felt the bottom was the best but it may be that there is an optimal level.

 

As a starting point I found THIS document which sort of shows my initial thinking for the model but as you move the pipes up in the screed the thermal dynamic changes and modifies a lot of features EG response times of the slab to both heating from the pipes but also the dynamic ability of the pipes to transfer solar heating. When operating in cooling mode things may change again and it may be that the best solution, largely impractical for cost reasons, would be to have two layers of pipes which could provide more optimal matching for the conditions met in the annual heating / cooling cycles.

[MikeSharp01]

Thinking again - the two layer concept has other possibilities. In those areas where Solar heating occurs the pipes might be better at the top of the slab, where they could react more quickly to the heating and in those areas where it does not occur they could be down in the slab so as to bury the warmth from the solar heated areas into the depths of the slab. We have to accept, it is what it is, that a concrete slab is not a perfect conductor so the radiation pattern is everything. More thinking required....

Edited June 9, 2018 by MikeSharp01
typo

[Jeremy Harris]

My experience (hard won, and at odds with my natural instincts) is that it is very easy to over-think stuff that has next to sod all real-world impact.  The sad thing is that I'm very definitely an applied, rather than pure, scientist, so you'd think I'd be more inclined to just accept an empirically derived solution!  The bottom line is that there are loads of hard to quantify variables that could be included in a model, but that have a negligible effect in practice.

 

In practice I've found that our UFH is pretty slow to respond, with the upper surface of the pipes around 30 to 40mm below the top of the 100mm thick slab.  I think that placing the pipes lower would have two negative effects; it would make the lower surface of the slab warmer then the upper surface, so increasing downward losses through the insulation (by increasing the Δt) and it would slow the rate of change of temperature of the upper floor surface, so slowing the time taken for either heating or cooling to transfer heat in or out of the room.

[MikeSharp01]

7 minutes ago, JSHarris said:

My experience (hard won, and at odds with my natural instincts) is that it is very easy to over-think stuff that has next to sod all real-world impact. 

Yep. 

9 minutes ago, JSHarris said:

The sad thing is that I'm very definitely an applied, rather than pure, scientist, so you'd think I'd be more inclined to just accept an empirically derived solution! 

Applied engineering in my case.

8 minutes ago, JSHarris said:

In practice I've found that our UFH is pretty slow to respond, with the upper surface of the pipes around 30 to 40mm below the top of the 100mm thick slab. 

Ours is 150mm thick so much more of a sink below if we had the pipes just 30mm down - so counter factually you might think it better to have them even higher to speed up the response. Have you take an thermal image of the surface when it is running does it show the pipe runs clearly?

[Jeremy Harris]

I have taken several thermal images of the floor, both in the areas where the flooring is travertine and the areas where it's bamboo (both bonded directly to the slab).  None seem to show any indication as to where the pipes are, and our pipes are on 200mm centres.

 

The other thing that's noticeable is the time lag between the in-floor temperature sensor, which is located under a stud wall, about 200mm horizontally from the nearest UFH pipe, and the heating or cooling coming on.  This lag is around 2 to 3 hours, with the measured temperature only getting within about 0.1 deg C of the surface temperature after around 6 to 8 hours.  Given that the heat loss from the surface of the slab above the sensor will be a fair bit lower (because it's under a stud wall that's filled with rockwool), I was surprised at just how slowly this sensor responded to change when the heating/cooling turned on.  This delay was one of the reasons I really struggled to get the floor temperature control system to work, and why I ended up reverting to a simple room thermostat.

[MikeSharp01]

12 hours ago, JSHarris said:

This delay was one of the reasons I really struggled to get the floor temperature control system to work, and why I ended up reverting to a simple room thermostat.

I guess this delay is systemic in that it would be the same wherever it was placed provided the distance to the pipe was the same although the wall will have some effect - perhaps it insulates the slab at that point slowing the response time. It is essentially, in effect, the lag of the whole slab.  Is it near a return run / an outward run or at the midway point in the run of pipe as I guess this must have an effect in the ramp up / down phases but probably drops out during the steady state phase once the steady state is reached. One thing is clear and that is that your experience tells us that using the slab temperature as a proxy for the room temperature is in effective because of the lag.

[Jeremy Harris]

I chose to place the sensor where I thought it would be most likely to accurately sense the slab core temperature, at the same depth as the UFH pipes.  Putting it under the stud wall was an attempt to not have local slab surface heat loss effect the measurement too much.  Whether this was right or wrong I don't know, but I do know that the slab surface temperature tends to be higher than the temperature at the slab sensor for an hour or two, until the heat travels sideways to the sensor (the sensor is well away from the edge of the slab).

 

The slab surface temperature tends to be very even, except in the doorway between the utility room and the kitchen, where there are 6 UFH pipe runs out to the rest of the slab, so the spacing is a lot closer there than elsewhere.  The slab rate of change of temperature seems a lot faster than the house, but very small changes in slab temperature have a large impact on the house temperature.  The latter effect is down to the heat power output from the slab relative to the temperature differential between it and the room.  Given that the house rarely needs more than about 1 kW of heat, and most of the time in the heating season it barely needs more than a couple of hundred Watts, it's easy to see how this sensitivity to slab temperature arises.  Here are the heat output numbers for our 75m² heated floor area for a small range of room and slab surface temperatures: