UFH with suspended floor

[mike]

How practical / economical is fitting wet UFH under an existing suspended wooden floor?

A friend has just bought a renovated 1900s terrace and is thinking of doing so. He also has a builder about to remove some of the downstairs wall to open up the reception rooms/hallway.

He reckons the upstairs and loft conversion feel warm but downstairs stays quite cool and thinks UFH might improve this.

Does UFH not work best when there's a large thermal mass (i.e. concrete slab)?

In these circumstances, would it improve comfort without necessarily being an improvement economically?

[Michael Chare]

Personally I would prefer the heat to be delivered directly into the room via a radiator rather than through a wooden floor. Wood being quite a good insulator which is why it gets used for saucepan handles.

There is quite a wide choice of radiator type things. You are not limited to the conventional metal panels.

[The Natural Philosopher]

pretty good really.

no. Thtas a fallacy. What you need is nothing more than a high conductivity path upwards and a low one downwards. Yes concrete conducts better than wood but concrete is thicker and with a carpet on its still pretty poor!

Te secret here with suspended wood floor is massive insulation downwards under the heating coils.

So its

- lift the floor completely

- intstll at least 6" of celotex between joists set down an inch or two and seal with mastic foam or WHY.

- either use premade foam pipe guides or some way to get hot water pipe into the gap.

- peressure test pipes.

- relaly floor.

- pressure test again in case you nailed through it

- hook up to new CH circuit and thermostat

- enjoy.

Done properly especially with lo thermal inertia wood, its actually very efficient. IN addition a warm floored room is more evenly heated and so it feels warmer.

[amcmaho]

No personal experience but I read that some contractors were getting unsatisfactory results unless they used heat transfer plates between the pipes and floor boards. The 'staple-up' method wasn't as good.

There's no other reason why it shouldn't work. Insulation under pipes is required as mentioned above.

[The Natural Philosopher]

What you need to understand is that the heat will always get up there eventually. If you don't use plates simply use more pipe. In the end that simply makes the air under hotter, and you get the same heat transfer. The limit on that is when the air temp approaches UFH pipe limits. which are only 55C IN SCREED which can crack.

You can calculate hear transfer into the room using standard U values for wood.

say the desired room is 20C and you have 3/4" of hardwood. And the air underneath is at say 45C.

I have from building regs a k value of 0.14 for wood, so 20mm of wood will have a U value of 0.14/0.02 or 7.. so for a temperature difference of 25 degrees that's 175W/sq meter.

Notably one inch of wood is MORE conductive than 4" of screed..

A typical well insulated room only needs 50W/sq meter. So the wood here is not the limiting factor. Probably the pipe surface area is.

So add more pipe. I'd personally go for 4" spacing which should give you around 100W/sq meter.

you can always turn heating down, but unless you run it dangerously hot, you cant turn an under piped system UP.

[harryagain]

The floor is probably the problem. There will be an air flow beneath it and likely draughts/air leaks though it.

As a minimum it needs insulating and draught proofing before you start. Probably best to fit normal radiators rather than UFH.

Alternative is to take up timber floor and replace with concrete (insulation below) & then UFH.

UFH works best with tiled floors etc (ie no insulation on top.) But plenty underneath.

[dennis]

Heating works best when you have proportional control and a low thermal mass with no insulation between it and the occupants. However its difficult to do and most of them compromise by having more thermal mass to slow down changes. Many boilers makers suggest fitting a buffer (heat store) tank to hold the heat for the UFH so the boiler doesn't cycle too much.

If the UFH is under the floor boards you may want to fit heat spreaders to get the heat through the wood without having to increase the flow temperature. Put plenty of insulation under the heating elements.

You may be better off installing it on top of the floor and covering it in a new floor if you have the room height to do so. You still want plenty of insulation under it.