Trying to understand large thermal mass for underfloor heating

[noobiediyer]

When I had a wet underfloor heating system installed, I was told that a thick thermal concrete mass was required between the UFH pipes and the finished floor. I believe I was told 7cm thick concrete mass I was only able to get 4cm in.

If all else is equal, why is 7cm better than 4cm? Why is 2cm worse than 4cm? I'm really confused.

Speaking generally, a good heating system feels responsive. If you feel cold, and turn up the heat you want to feel that heat right away. Same for when you turn down the heat, you want it to react quicker. But having a thicker thermal mass makes the heating laggy. When you turn up the heat, it takes time to feel the heat because it has to heat up the thermal mass first. When you turn it off, the thermal mass will continue to radiate heat for a long while after.

So I can't quite understand why the thicker thermal mass is better when it just makes changes less reactive. Can anyone help explain this to me please?

 

[Roddy Young]

We built our house 12 years ago, well insulated and airtight with a heat recovery ventilation system.
Our builder had a lot of experience in low energy houses and underfloor heating was taken for granted.
He was very keen, however, to reduce the thermal mass of the poured floors and so used what was, for him, a new technique - self levelling gypsum. It is not very different for thermal lag, though.
The lag effectively means there is no point trying to switch the heating on and off umpteen times through the day.
Our controller is "adaptive" in that it notices the rate at which each room is cooling and decides when the heating should add warmth to the slab(s). Sometimes we are puzzled that the heating comes on when a room seems warm enough but it is clever and, using a datalogger I built, I can see that the system maintains the room temperatures very accurately.
I have set temperatures a few degrees cooler at night but the clever system knows to add heat from about 5am to top up the rooms for breakfast at 7:30!
And that is the whole point of UFH. Even in the depths of winter the difference in temperature between floor level and ceiling is likely only to be 1 or 2 degrees. That is very comfy - no more cold feet and stuffy hot heads.
The floors are never heated above 27 degrees but that's lovely in the bathroom for example. The only downside we could find was that there is no radiator to warm your bum on...... but we're never cold so don't need to!
Thermal lag is something we learned to accept and now are very happy with very even temperatures.
Roddy

 

[Madrab]

I was told that a thick thermal concrete mass was required between the UFH pipes and the finished floor. I believe I was told 7cm thick concrete mass I was only able to get 4cm in.

Nothing to do with thermal mass as such over and above the extra mass it adds to the slab, more to do with loading and support, normal screed imbedded UFH will require a certain finished thickness to ensure there is able support and that the concrete won't crack over time. Modern polymer screed types are more durable and don't need such a large overlay as they are supposedly stronger but they are relatively new and I think the jury's still out on that

When talking about thermal mass' when it comes to 'crete imbedded UFH BTW is the whole slab above the base insulation.

All traditional large concrete slab UFH should be run constantly to maintain the room's air temp at a set medium, so the whole space is kept at it's desired temp. There is always thermal lag when it comes to warming large thermal masses and it just has to be managed in a different way to rads

The only UFH that has a fast heat up are the ones that don't have a thermal mass to warm up, e.g. low level overlay systems, they are one such a system that can be used just as radiators would be.