Slab Surface Temperature

[worldwidewebs]

Looking at engineered wood flooring options and the spec sheets give a maximum allowable surface temperature of the slab. What would be considered normal or high? It's an MBC slab if that makes a difference 

[Jeremy Harris]

The maximum our slab (also MBC) surface temperature reaches, with the UFH running, is about 23 deg C, there's some data here from some recent measurements:

 

[jack]

I actually measure the surface temp at one point of our slab (under our kitchen island) using a one-wire sensor held against the slab and covered with a weighted-down offcut of PIR.

 

From memory, the highest temp we've seen is 24C.  That was fairly early on, before I did anything at all about temperature control (not that I've actually done much more since!)  It's amazing what a subjective difference even one degree of slab temperature makes to the perceived temperature of the house.  24C left the house feeling uncomfortably warm.  23 is too warm, 22 is borderline, 20-21 is pretty comfortable, 19 starts feeling a little cool but okay, below that seems too cold.

[Jeremy Harris]

It'll be interesting to see how long it takes for the UFH component suppliers and designers to get their heads around UFH systems that operate at just a degree or two above room temperature.  There seems to be a fair bit of concern about things like floor covering temperature tolerance, based on conventional UFH systems that run at higher temperatures, but even so I find it hard to understand why there's so much concern, as the sun shining on a bit of floor will often make it a great deal warmer than any UFH system.  There's a bit of the kitchen floor in our old house (that has a solid concrete slab with no insulation under it) that gets too hot to walk on in bare feet in summer, just from the sun shining on it.

[worldwidewebs]

Thanks for the replies chaps. Out of interest, do you take readings in areas where you may have a concentration of UFH pipework (such as doorway into the plant room for example)? I wondered whether it was possible to see significantly higher readings there.

[Jeremy Harris]

It'll be a tiny bit warmer, but no warmer than standing on a bit of floor in bare feet would make it.  There's really no problem, in terms of potential floor covering damage, as the temperature will always be a great deal cooler than with relatively high temperature UFH. 

[SteamyTea]

Bit here about putting in temperature sensors:

 

[TerryE]

Andrew, how hot your slab gets depends of your UFH system.  Conventional UFH systems in normal houses might heat the floor up to 30°C or more.  However the rule of thumb is that your floor will chuff out 7-10W/Km².  In an MBC house you will slowly start to cook if you let the slab get much more than a few degrees warmer than room temperature.  So your issue isn't worrying about whether your floor is going to be too warm for the engineered flooring,  it's that you'll need to throttle your heat input into the slab right back.  That's the subject of a couple of Jeremy's blog posts and also my  topic that he linked to.

[worldwidewebs]

Cheers, Terry. I've never had UFH (nor lived in an efficient house!) so it's a bit of a leap into the unknown for me. The bigger worry is spending £20k on a floor and finding it fails due to the heat

[TerryE]

You have to get the control system right. Do a design review here if you are uncertain.