Heating a Slab & Levelling Compound

[Spinny]

So my last lot of floor levelling compound was put down 27 days ago and has been left to go off and dry naturally in that time. All be it chunks of the floor have had floor mats and kitchen parts all over them for a couple of weeks of that. Kitchen now sitting on legs awaiting parts.

 

Floor makeup - 120mm concrete slab with heating pipes around midpoint, approx 25mm to 5mm of levelling compound on top in 2 coats, top coat between 15mm and 3mm.

 

Time to run some heat through the slab, but it is June.

Unheated the surface of the levelling compound is around 24 today anyway - using my heat gun.

(Presumably temp is lower deeper within the slab ?)

I have turned down the flow temp to Min - probably about 25C ? ? - and just turned on the underfloor heating.

 

Advice please on how long to run it and how to increment the water temperature over the days to come ?

(LVT to be laid in 26 days time)

Edited Wednesday at 17:51 by Spinny

[JohnMo]

If your a well insulated house, your flow temp isn't going to much higher than about 28 at -lots outside, so zero point going beyond that. We never bothered doing any drying cycling as we didn't have a heat source at the time.

 

Your heat pump is pretty unlikely to fire up, if the floor is already warm. It will or should have a drying programme, so run that and limit max temperature otherwise you'll just melt.

[Spinny]

Ah. No heat pump, gas boiler.

There is a mixer valve on the manifold. I am not actually sure how accurate those things are but in the winter we had it set at 45C, albeit we still had no insulation around the bifolds. Boiler output is at 60C to rads. Floor temperature got up to 26C then.

 

I guess Min on the valve is perhaps equivalent to off ? i.e. cold water feed temperature ?

Tweaked it up a fraction, heat gun has pipes at 22C.

 

The concrete slab itself is well dried out, been heated through two winters.

Edited Wednesday at 18:38 by Spinny

[Nickfromwales]

Just time the heating to do an hour on then 2 hrs off, then repeat. 

[Spinny]

Hmm. I have been doing something similar to what you suggest @Nickfromwales

I suspect that as the surface is around 22C, then it might well be that heating the slab to 22C (though it may not be getting to that) has no effect whatsoever on the levelling compound on top. Essentially the heating below is no different from the heating the levelling compound is receving from the environment above it.

I shall gradually increase the flow temperature over the coming days.

[Nickfromwales]

4 minutes ago, Spinny said:

Hmm. I have been doing something similar to what you suggest @Nickfromwales

I suspect that as the surface is around 22C, then it might well be that heating the slab to 22C (though it may not be getting to that) has no effect whatsoever on the levelling compound on top. Essentially the heating below is no different from the heating the levelling compound is receving from the environment above it.

I shall gradually increase the flow temperature over the coming days.

Don’t overheat it. Slow and steady wins the race.

[Super_Paulie]

with your mixer at 25 and the boiler at 60, does it not short cycle straight away or do the rads come on at the same time? id just leave it at 45 and then whack it on for an hour every few hours.

[Spinny]

Have tweaked up the mixer a little, but if using my heat gun on the pipes at the manifold is anything to go by I seem to have an input flow at around 25C and a return flow at around 24C. The actual surface temperature of the floor is around 23C (though did open the door for a bit earlier). So I guess not much appears to be happening because that is all close to the same temperature - very gentle heating if anything at all.

 

Meanwhile there is no visible change at the surface. I have some marks/surface colour differences where I had mats down on top of the floor during the kitchen install. Lighter colour at the joins where presumably some moisture could escape between the mats. See photos...

 

Should I actually be expecting the whole floor to turn to the same colour ? It has had pronounced colour variations by pour since it was levelled.

[Nickfromwales]

It may be possible that you’re over-thinking the 💩 out of this…..

 

It’s likely to have ‘done’ whatever it needs to do by now anyways.

 

I’ve not poured one yet where it’s a perfect finish, as it varies with thickness etc.

 

Go to the pub instead 🤝👍

[saveasteading]

12 hours ago, Nickfromwales said:

done’ whatever it needs to do 

Exactly so.

The hardening is mostly chemistry, not drying.

Changes in colour are normal too, depending on water content, being laid straight or worked extensively, thickness, and any objects or joints below.

[Spinny]

Yes OK thanks.

However what bugs me most is that you can clearly see the pattern in the surface colour in the first two photos from the floor mats and their joins. You would think this would be due to greater drying out where the mat joins were. Therefore removing the mats would see these areas gradually dry to the same colour.

 

The surface of the floor has been at 24-25C for periods because that has been the ambient temperature, and the doors have been opened to create air flow over the surface. In fact when sunlight somes through the rooflights or bifolds the sunlit floor surface temp gets up to 30/31C.

 

What the temperature is lower down in the slab I have no way of knowing.

What I don't want is to turn the u/floor heating on at 45C when winter comes, producing a floor surface temp of 26C, but driving moisture out of the floor leveller causing cracking and ruining an LVT floor. That could be £10k and more down the drain and a ruined room.

[Spinny]

PS Found yet another hidden bodge today which doesn't help my paranoia.

Top board on the omnie u/floor heating fitted by our original builder - was sticking up vs adjacent boards, so took it up - turns out the board was wrongly cut so the pipes didn't match with the routing in the top boards. Whoever screwed it down must have known - but why give a shit when you are being paid to work on someone else's house eh. Don't have a router or a spare board, so chiselled out a new groove to put it right.