Underfloor heating as primary heating

[Richard]

Hi Christian

Sorry, I intended to convey in my posting that I think a composite system is sensible, but wanted to see why Dr D doesn't.

Richard

[John Stumbles]

I've done a cheap'n'cheerful one in my kitchen. I had to take up the (suspended) floor anyway so I insulated between the joists with rockwool laid on garden netting slung over them, then ran 15mm polybutylene pipe (Hep2O or equivalent) up and down between the joists suspended just below the floor level by tiewrapping it to lengths of screwfix builders band fixed across the top of the joists. Then I laid the new plywood floor and tiled on top of the wood. The pipe is connected directly into the CH circuit via a TRV under the floor set to open when the air temperature in the unheated void below the floor drops to a coldish temperature (I forget what). In practice the heating is a bit patchy with some areas feeling quite warm and some cool (to bare feet or hands) but generally the room is a comfortable temperature in all but the coldest weather.

If I were doing one professionally I'd calculate the heatloss of the room using the standard method (U values of surfaces + ventilation losses) and compare the heat output available from any of the construction methods for UFH for which figures are available to see if it's possible to match the heatloss. If the room has a large heatloss e.g poorly insulated external walls and windows and a small floor area (allowing for the area taken up by units etc) it may not be possible to entirely heat the room with UFH. In this case extra heating from a radiator or kickspace heater may be required in the coldest weater. (Depending on how much of a shortfall there is it may be acceptable to supplement the UFH with an electric fan heater in cold weater, rather than going to the expense of a kickspace heater on the central heating.)

Doing it 'properly' one should also fit a separate pumped thermostatically mixed supply to the UFH so that it runs at a lower temperature than the main CH circuit.

[John Stumbles]

Ovens and hobs (used intensively) yes, but microwaves and washing machines? These should produce a negligible contribution to space heating.

Also because UFH produces radiant heat whereas ovens & hobs heat the air I wonder whether the excess heating would subjectively feel as much as if the central heating were an unregulated radiator or fan heater producing the same comfort levels. I think an argument could be made for UFH as primary heating in a kitchen because of its greater efficiency in areas of more frequent air changes (such as kitchens and bathrooms).

[The Natural Philosopher]

Definitely if the insulation is up to snuff and teh density of pipe is great enough to match the heatloss.

between 50W/sq meter and 200W/sq meter depending on room insulatuon and heatloss.

So between 450W and 2KW...

You MUST insulate the floor though, or it will be bearer 400W/sq meter, with you providing a nice little soil heater for the worms..

[The Natural Philosopher]

It can, but a floor in direct contact with a cold bit of earth is only about 25% efficient..it costs a fortyne and leasds to very heavy pipe density.

Yup.

has some data that I used.

I went for 200mm pipe spacing IIRC.

I think that was around 50W/square meter. Its enough - just - in the very coldest weather. Frankly 100mm spacing might be better in all but modern insulated rooms, and then simply use a thermostat to limit the temp rise.

[The Natural Philosopher]

Yup. Not recommened to run pipe-in-screed much over 45C whereas CH can be up to 70C IIRC.

Simply restricting the flow ain't good enough - you just get high entry and low exit temps.

You need an aux pump, thermo mixing valve and a relay, cos that pump will be running (and need to run) independently of the main CH pump.

[The Natural Philosopher]

Stout lad!

You need AT LEAST 50mm, and the more the better, isnulation in it somewhere. I used blue styrene foam. Celotex type is even better, though not as strong. On a total green field type install its cheaper to use e,.g 75mm blue than 50mm celotex..

Mine was first of all a block and beam concrete floor..yours will probably be a slab, and this MUST finish at least 6" below finished floor. and below damp course level. 8" is better..

THEN you blind that with a dry sand cement mixtuire to level it off and get rid of any sharp edges and lay the insulation - slabs of whatever - and the DPM - big plastic sheet - over it. Take the DPM up to over the existing DPM level up the walls..you cut it off once its all finished - and also run insulation up to finished floor level as well - this allows a bit of expansion of the screed slab and prevents heat leaking into the walls.

What you now have is a bowl shaped insulation layer. 50-75mm thick in the middle, maybe 25mm up the edges. And at least 75mm deep..

Now you have a choice. yoiu CAN buty formers to lay teh plastic pipe into, but I didn't bother. I shad to install metal reinforcing grid anyway, so we simply tie wrapped the pipes to every other rod.

Pipe runs should never exceed 100 meters, and never ever be joined - use a single length of pipe. If they have to pass thresholds slip a bit of flexible conduit over to allow movement.

Pipes are ideally laid in a double spiral - think of it as a loop that is coiled up -look at an old electric cooker element :-)

Once laid fit to your manifold and fill the pipes up and seal, and then use a pressure gauge and pressure pump (I hired on for peanuts for a gew days) to make sure no leaks are there, and *keep the pressure up at around 5-6bar whilst laying screed*. Make sure that pipes are NOT run where there is no need (under kitchen units: These simply will retain the heat under the plinth, unless you punch holes in it) and where you need to bolt or scew anything to the floor.

Then screed up to the finished floor level, cutting any excess DPM and insulation with a stanley knife or serrated kitchen knife.

Add floor of your choice (UHH works REALLY well with stoine and tiles, less well with laminate and vinyl, and is useless with carpet by and large)

The great advantage of UFH we have found is that in winter, its warm and cosy, but in summer also, the slab stays cool..because we have mo insulation between the room and the floor, it acts as a heat bank stablizing temperatures during hot days to the daily mean.

[The Natural Philosopher]

Total bollocks

Its the sole form of heating here apart from open wires we only use on the very coldest days, and they keep 250sq meters totally warm.

Done properly, its better than anything else.

That is mandatory anyway.

>

[The Natural Philosopher]

Actually the best kitchen heater is an Aga.

You can cook on it too.

[The Natural Philosopher]

Not with a ruddy great slab of screed under it. That acts to stabilise the temperatures.

Or 30 sq meter kitchen is totally heated except in the coldest of conditions by an 700W output oil fired aga.

Cooking can kick out up to 2KW. Nothing can cope with losing that apart from ventilation and fans or open windows.

In summer we shut the Aga down, and use the barbecue. Or an electric stove ..The massive floor and its insulation keeps the kitchen cool.

Christian, your logic is wrong. Once the heat output on the room rises above its heat loss, it doesn't matter how its heated. What matters is how is cooled. UFH is ideal for places that are used a LOT. Its not so ideal for - e.g/. a room used occasionally where you want fast heat up and otherwise its left cool.

With the modern trend towards large kitchen/dining areas, UFH is perfect. The kitchen is no longer the place you go into to prepare a meal, it's where you live.

[The Natural Philosopher]

If you are doing it from scratch is is very cheap.

I think that I spent around £300 on the pump/manifold and relay stiff..beyond that teh cost of the pipe was no more than radiators, and piping and the cost of laying it was a day on a computer drawing program to plan the pipes, and about a morning laying it before the screeders got busy. FAR less work than soldering up copper pipework is.

Given the cost of plumbers and the time it takes to lay copper, I think it was actually cheaper.

Wiring it all up took me a day..probably he most challenging part really. Got a Eurobox, DIN rails, stuck some large connector blocks in and a relay, and a 3 zone timer, and spent a merry day connecting it all together. There's a LOT of wire in there..

[The Natural Philosopher]

I don't think the very small advantages are worth the extra cost. If the room is in use and heated the floor will come up to room temp whether its heated or not. Kickspace only useful if you leave it cold, and require rapid heating just when you are using it. I've got fan blown heaters here, and they are great in the bedrooms - take up almost no space, and heat them from icy to cosy in an hour or so..but they are pretty useless if what you want is all day heat. Noisy and sometimes smelly due to dust getting trapped in them.

Dr Drivel can safely be ignored on all issues - I think that the rest of us who may disagree on other things, would agree on that point.

[The Natural Philosopher]

So that the floor is not destroyed...

Yes to all of that.

I run about a three hour lead on hours..strangely enough it only takes about half an hour from switchoff before you notice that its off..the heating is SO constant and even that the slightest change - a door opening etc - makes you realise its changed. In deepest winter I run it

24x7 anyway, because we have so much thermal mass it scarcely makes any difference to the cost.

We find that we don't have any hot or cold spots though - apart from under the sofa. THAT is up to maybe 35C sometimes..the pets LOVE UFH.

[Guy King]

The message from The Natural Philosopher contains these words:

Not to mention being illegal. Though some of the recent rules are rather daft, the one insisting on proper insulation under solid floors makes sense. I've got 4" under the conservatory floor.

[Guy King]

The message from The Natural Philosopher contains these words:

That's important, that bit. Without it you'll end up with one end of the floor getting a lot hotter than the other.

The spiral in then spiral back out again in between the incoming spiral system means that the slab warms up evenly.

[Christian McArdle]

Not quite. Take an example:

The heat loss is 1kW and the UFH is kicking out a 1kW, which is quite reasonable. A nice stable temperature.

Now start cooking at about 1.5kW. You are now shoving in 2.5kW which will rapidly heat the room by 1.5kW excess. Even if the stat kicks in immediately, the UFH will give 1kW for some time.

If you had a fan convector, it would shut down immediately, giving 1.5kW, which gives only 500W too much, which could be easily dissipated by opening an internal door and using the heat in other rooms.

If you had a composite system, you'd probably set the floor temperature to

25C, which is comfortable to the feet, but would not add significantly to the heating of a room already to temperature, perhaps a few hundred watts at most.

Christian.

[Guy King]

The message from "Christian McArdle" contains these words:

That assumes the 1.5kW is constant. In the case of a modern oven it'll only be for a few minutes at the outset, and again every now and again to keep it hot. In the case of boiling something, again you'd be using that much perhap to raise something to the boil but with a lid on you don't need anything like that much to keep it boiling. If the extractor fan's running and removing steam that's much of your energy gone straight away as water vapour carries a lot of heat.

[Christian McArdle]

1.5kW is a nice average for cooking a meal, rather than reheating something in the oven. The peak output could be as high as 10kW for Christmas dinner.

Christian.

[noelogara]

The Natural Philosopher wrote: > >> Christian.

Seems to me that you are dishing out advice to people and you really dont even know what you are talking about. In another thread you were offering your inside wisdom on wood burning stoves and you dont even have one. Then you said you had a large open fire and now you claim to have under floor heating. And you then rubbish other people's advices. I really think you should change that misleading name you put on yourself. If the UFH is so great why do you need a large open fire? If the open fire is so great why do you need the UFH? Your advice is seriously misleading to genuine enquirers so please restrain your advice to matters that you actually know about.

[Christian McArdle]

They do different things. UFH is great at heating a house very comfortably and efficiently.

An open fire is great to look at. Pathetic at providing low maintenance, low cost comfortable day to day heat, but really nice now and again.

Christian.