Wet underfloor heating and stone

[Sonyaclegg]

We are having underfloor heating in the kitchen, hall and utility room and I'm yet to decide on a floor covering. I would like a yorkstone paving slab floor but I would like to know if this is a good option for under floor heating. The plumber says it will be good conductor of the heat. The builder does not agree. I guess much will depend on the thickness of the stone and I'm looking at a 30 mm flagstone. Does anyone have any experience of underfloor heating with a stone covering?

 

[Louis Henry]

It will turn it into a sort of storage radiator, slow to heat up and slow to cool down. Steady heat like this is good for old houses but not if you're in a hurry for the warmth!

 

[Sonyaclegg]

Thanks Louis. I've researched it and it seems stone is a very good conductor of heat but not for quick heat, as you advise.

 

[DJH]

It's important that the stones are laid on a continuous bed of mortar with no air gaps or spaces between them and the substrate as if there is it will greatly affect heat transfer to them..

 

[Sonyaclegg]

Because air is a good insulator, Doug?

 

[DJH]

Not too sure about that but underfloor heating is low temperature water and you need everything above them to be touching so that heat transfers efficiently up through them. Well, that's how I remember it from many years ago before I became a real plumber.

Doug

PS Any news about your pipes!!!

 

[Sonyaclegg]

Kevin came and gave us his thoughts. I fed those back to the CO who was not keen on the spouts being opened up from the underside. I have to wait for Kevin's quote and plans and then seek the views of English Heritage. Meanwhile, we are to put temporary plastic sleeves over the spouts to get the water down the wall. They will look awful but it can't be helped. I don't anticipate EH and the local council will make a decision quickly so those plastic sleeves will be up a while. Kevin was a great chap and we really enjoyed meeting with him. We were left with a lasting impression that if he does the work, we will be in totally safe hands. Thank you for the recommendation.

 

[DJH]

That's good. I can't recommend him highly enough.

Doug

 

[MdB]

We have 75mm of cement screed and 10mm of slate on top of our underfloor heat pipes. Not much different between that and stone.

As the others have said you want the following:

1) As much insulation under the pipes as possible. And then some more. We have 100mm of Celotex.

2) As little insulation as possible above the pipes. Our cement screed has quite a lot of air in it which is bad and we are going to try for something a bit more solid for our extension. The amount of heat travelling into the room depends on the surface temperature of the floor. This in turn depends on the temperature of the water in the pipes and the thermal conductivity (insulation) of the floor above the pipes. With a lot of insulation, the pipe temperature has to be very hot to transfer heat into the room and this often makes the heating system inefficient.

3) If you run your system off Economy 7 or Economy 10 electric (like us) then you want a big thermal mass as your floor (i.e. make it thick) so that the heat stored during the night is released during the day. We have this and you basically don't notice any temperature change in the room day or night. Conversely it is useless at heating the room quickly so if we go away for the weekend for example, we normally don't switch the heating off.

4) If your house is poorly insulated or you want the room to heat up quickly when the heating comes on, you want a small thermal mass in the floor (i.e. make it thin). This will heat the room quickly but may also cause the floor temperature to get uncomfortably hot. I've heard of this happening but I don't know how bad an issue it is.

5) With good insulation under the floor and a big variably heated lump floating on top, you have expansion / contraction issues to worry about. To solve this, fit an flexible insulative strip around the edge of the floor slab to allow a bit of movement. Our underfloor pipe supplier (Uponor) sold this.

 

[Sonyaclegg]

Thank you MdB. Glad to know it works well. We have two rads in the room but thought we should put underfloor heating in whilst we can. Better to have it and not need it than regret not putting it in after the kitchen has gone in

 

[DJH]

Best not to put the pipes under the kitchen units as it's not good for the groceries. An option is to put in a separate loop under them and leave those pipes disconnected until you decide you want to make the kitchen either bigger or into a living room. The cost of the pipe is minimal.

 

[Sonyaclegg]

I believe so, DJH, but I'm very lucky to have a huge walk in pantry to put all the groceries in. Infact, I don't really need any cabinets in the kitchen!

 

[JoceAndChris]

The size of your house, Sonya, reckon you might need a few kitchen areas dotted here and there ! :lol:

I've no experience of UFH but it sounds lovely and toasty and perfect for those houses missing their original floors. Wouldn't dare risk lifting the stone here, which is a good 5 inches thick, but there's one tiny patch in the hallway where the flue comes up from the basement kitchen and that patch is bliss.

Hope it all goes ahead successfully.

 

[LadyArowana]

I believe so, DJH, but I'm very lucky to have a huge walk in pantry to put all the groceries in. Infact, I don't really need any cabinets in the kitchen!

I have had to get out of the habit of keeping anything vaguely food like outside the fridge. With the tropical climate we are prone to bugs and beasties ( even in an airconditioned and fairly sealed apartment ) and they get really excited given the whiff of something to munch on. Only yesterday His Lordship found he was hosting an early Christmas 'ant party' due to slight spillage of a soft drink. Of course it's possible to deploy chemical weapons against them, but that's a little unkind and neither of us fancy breathing the stuff. Left to their own devices they usually move along, at the moment rather than using coasters we have small dishes of water with another plate inside to put the glass on, the ants get to the edge of the moat, hang around for a while and then go home. So our pantry is full of dishes, the fridge is where all the food lives and much of the rest of the kitchen storage seems to have become and expanded 'man drawer'. Will Mr Sonya be allowed a 'man drawer' or are you going to allocate him an area of the pantry?

 

[AMc]

We have wet underfloor heating. I echo most of what's been said above.

You need a thin dense mortar bed with no gaps to help heat transfer. We have one room where the mortar bed is very thick to accommodate differences in the thickness of the floor bricks and pamments. The effect of the 1" thicker mortar vs. dense ceramic is very noticeable underfoot. If I were doing the rooms again I would go for the thinnest bed I could use, probably a tile adhesive rather than simple lime mortar.

The problem with big thermal mass in the floor is that it cools & warms very slowly. As the sun sets and the air temp drops the room gets colder, the thermostat calls for heat and the floor warms up but there is a lag while it catches up during which time the room gets cooler. I would (within the constraints of a stone floor) make the floor as thermally light weight as you can to aid response time.

In one room we have a thermostat probe in the floor as there was no suitable wall for an air thermostat. This is actually more laggy than the others and was a mistake.

Don't be tempted to skimp on the pipe, it's very cheap. If you have too much in there they you can reduce the flow temp or pump speed, but if you don't have enough then you can't raise the flow temp much above the design temp without risking problems with things on the floor being affected and the floor being unpleasant to walk on. You can't easily fit more pipe if it turns out you don't have enough

Heat loss calculations from ready reckoners generally assume modern building standards of air tightness and insulation so will underestimate your heat loss unless you adjust for solid walls and single glazing. It is noticeable how much wind outside affects air temperature inside and again slow response is a pain if the wind changes as the sun sets.

Pay attention to edge insulation of the slab, you don't want your slab to bridge heat into the walls esp. if you have solid walls as they will radiate your heat into the night as effectively as the room.

In addition to good insulation It's worth mentioning that underfloor heating works better in larger rooms than smaller ones...
A 1m x 1m room with 3 external walls has 1m2 of heat emitter and 3m of edge and wall to lose heat.
A 3m x 3m room with 3 external walls has 9m2 of heat emitter and 9m of edge.
A 4m x 4m room with 3 external walls has 16m2 of heat emitter and 12m of edge.

Avoid putting down rugs and carpets to feel cosy as they insulate the floor from the room! In one room which we struggled to heat (small, wood floor, 2 external solid, uninsulated walls) we removed the rug and gained >1C in less than an hour.

Finally get the most sophisticated controls you can and try to see them in action. Ours is very programmable and features a learning weather compensation feature that measures the difference between the time to call for heat and reaching the set point, it then adjusts the heating times for the next 24hrs based on what happens today. This is pretty good but I really wish it had an external temperature probe to deal with cold snaps and maintain constant heat when the temp outside is below 0C by itself (as opposed to be manually overridden by me). While I can programme it it is very fiddly (11 zones, with 7 days and settings by the 20minute period) so I can press a lot of buttons to make the heating change by a single hour in a couple of zones.

I still love having warm feet & no ugly radiators stealing wall space.

 

[DJH]

Those are very good points AMc and worth watching out for. When we first bought our house we intended it to be more of a holiday home and we decided against installing a radiator driven heating system mainly because it was too much of a risk if the boiler went to lockout for some reason and the pipes froze in an unheated house while we were in our nice warm house in the UK. We installed night storage heaters on the assumption that if the external electricity supply failed at all the storage heaters would just turn back on when the the power was restored. If ro some reason they didn't then there would be no pipes in the house to freeze with no resultant water damage as there would have been with radiators.

However,as we worked on the house ( 48 ferry trips in just under two years!!) it became clear that we would just have to live here so we left the storage heaters in place. Since then I've installed a bath and shower room in what was a downstairs bedroom and added a living room on as an extension and I've put some pipework in for radiators (which we might have later or not but we have the facility now to do so) and pipework for underfloor heating. Non of this is working yet, (the boiler has been in it's packaging for 6 years) as we have two stoves which do the work of heating the place wonderfully.

Just one more bit of advise and that is if you intend to cover an under heated floor with wood then you should glue the wood to the floor with a good quality adhesive.
expensive.

I'm allowed a man drawer, here's a picture of it. When Mrs DJH retired she decided to help with my office work and brought her own tool kit which went into the top drawer of my desk. Lipstick,emery board, nail stuff, more nail stuff, a little bag. a bigger bag, Tic Tacs, hand cream, nail files (different sizes) a mirror with two sides, a comb, a funny looking comb like a garden rake, sweeteners, some more nail stuff.............

 

[Jellyfire]

Hmmm, Im a little disappointed in your man drawer DJH, in my opinion they should always have slightly too much in them and be above a cupboard/other drawer so that when you do finally manage to prise them open, the string/wire/extinct battery or whatever you are looking for can finally be located in the lower drawer or cupboard.
Yours looks far too sensible.

 

[MdB]

The problem with big thermal mass in the floor is that it cools & warms very slowly. As the sun sets and the air temp drops the room gets colder, the thermostat calls for heat and the floor warms up but there is a lag while it catches up during which time the room gets cooler. I would (within the constraints of a stone floor) make the floor as thermally light weight as you can to aid response time.

We don't have this problem as we have good insulation everywhere and although it does take ages to cool down / heat up, we use this to our advantage by heating mainly overnight.

Don't be tempted to skimp on the pipe, it's very cheap. If you have too much in there they you can reduce the flow temp or pump speed, but if you don't have enough then you can't raise the flow temp much above the design temp without risking problems with things on the floor being affected and the floor being unpleasant to walk on. You can't easily fit more pipe if it turns out you don't have enough

Very much this. We put in the recommended spacing for heat pumps - 150mm and I would now go for 100mm I think. On the coldest days, we can't get enough heat out of the floor.

Finally get the most sophisticated controls you can and try to see them in action.

We have a simple controller and a single temperature sensor - though it is easy to fit more if needed. This has worked fine for us - again due to the high thermal mass and the good insulation.

I still love having warm feet & no ugly radiators stealing wall space.

Us too. Underfloor is lovely.

 

[DJH]

Hmmm, Im a little disappointed in your man drawer DJH, in my opinion they should always have slightly too much in them and be above a cupboard/other drawer so that when you do finally manage to prise them open, the string/wire/extinct battery or whatever you are looking for can finally be located in the lower drawer or cupboard.
Yours looks far too sensible.

Maybe I should 'fess up' here. It's more like Mrs DJH's man drawer than mine. She keeps some of my stuff in it that she thinks I can use straightway rather than tell her it's in the workshop the particular tool that I need to do her job for her..

Doug

 

[LadyArowana]

Maybe I should 'fess up' here. It's more like Mrs DJH's man drawer than mine. She keeps some of my stuff in it that she thinks I can use straightway rather than tell her it's in the workshop the particular tool that I need to do her job for her..

Doug

She is lucky to be blessed with a husband that is capable of fixing things, I have to keep His Lordship well away from the toolbox. Many years ago we finally decided that either I fix things or we pay someone else. This was as a result of what has become known as "the unfortunate fax incident". He tried to change the plug, but cut cut off the pre formed end that goes into the machine, requiring a whole new lead. He complained to me once that his PA wouldn't let him have the pointy scissors at the office, and I told him she was clearly a very very wise woman :lol: