maximum heat output per m2 from underfloor heating

caz26 · 15 Feb 2010

Would I be correct in saying it's unlikely to be able to get more than 100w /m2 from an underfloor heaing system? Planning to have tiled or stone floor but don't see any system that will provide more than this. Am I correct?

Thanks
Caz

Agile · 15 Feb 2010

This is little understood !

UFH will give up to 100w/sq m but the customers floor coverings usually reduce that to a maximum of about 50w/sq m.

Its an expensive mistake to make not getting it right.

Manufacturers like to be optimistic to sell their products. So are some installers just to get the job!

Tony

caz26 · 15 Feb 2010

Tony,

Thanks for your prompt response. You did reply to my previous post and I did get in touch with my structural engineer who said if done carefully should be ok-but he was talking only in terms of load on the floor and not sufficient heat output to heat the rooms.

Your last comment is however my exact fear. I've a recommended plumber bringing someone that does underfloor heating round. I'd shown him an estimate from Jupiter systems and he said "the heat output wouldn't be sufficient from this" But I reckoned it was based on 100w/m2 so didn't figure this could be bettered anyway. He replied by saying they put the pipes closer together but this is starting to sound like pants to me. If as you say output would likely be nearer the 50w/m2 in a draughty hall I'm thinking this might still be baltic and therefore an expensive waste of time.

Certainly don't mind paying to have it done but as you point out, it would be an expensive mistake if not sufficient.

Thanks again
Caz

mointainwalker · 15 Feb 2010

I disagree.

Stone/tile have very little insulation value - typically only 2-5 % the value of exp polystyrene. Screed has about 10% the insulation value of exp poly.

If you have a build up of 60 mm(say ) of poly, then the pipes then 60 mm of screed then tiles, the insulation below the pipes is R3, above is about R 0.15 i.e. 20 times less.

In such a situation you will get nearly all of your 100 W input at the surface of the floor.

I don't know why Agile brings in the irrelevant and confusing remark about customers floor coverings reducing this when you said initially that you are planning to use stone/tile.

P.S. Just about anything you put in a draughty hallway will be useless. Moving cold air currents will always win over heating.

caz26 · 15 Feb 2010

can I perhaps ask a different question then? Am I right in saying that I'd not get more than 100w/m2 no matter what the system?

Also, we have high ceilings, but I read that this is irrelevant (or at least not so important) when calculating heat loss from the room for an underfloor system. If so, how can i calculate heat loss without taking height into consideration??

Thank you all
Caz

big-all · 15 Feb 2010

underfloor heating is only good for background heating unless you plan on leaving it on most off the time
a cold room will take several times to warm up over the the same room with just a radiator

so iff you plan on running your heating for 2 hours in the morning and 3 in the evening for example underfloor will need a radiator to suppliment it

njk · 15 Feb 2010

You won't have any problems getting 100 w/sqm on a concrete floor and tiled/stone floor. You can even get higher than that, but it is not recommended since your floor will exceed comfort levels.
If your house is a new build it should not need more than 75 w/sqm anyway.

It is funny how sceptical people are about the ufh and its output. They are installed in majority of houses in Germany, Sweden and Iceland, (and many more), and they work very well. They main reason people in the u.k. are afraid of it is because often ufh is installed in the u.k. by diyers and plumbers not understanding the difference between ufh and radiators.

Most call outs I am getting these days is because of badly insulated and installed ufh. Sometimes you can even feel the cold winds coming in under the skirting boards.

Get someone competent and make sure the house is well insulated.
Done properly, your ufh will have no problems to keep you and your family warm.

mointainwalker · 15 Feb 2010

Don't bother about the temp gradient. It is much more even than it would be with rads and I would bet that in the history of CH, there has never been a plumber who did a heat loss calc using a temp gradient for rads !!

I would echo the advice to get someone competent, but for me that means someone competent to design it themselves and not just go and buy a high-priced kit and rely on the suppliers tech dept and not be able to answer any of your queries themselves.

My system is laid out for 100 W/M2 with 16 mm pipes at 100 mm centres.

gassafeengineer · 15 Feb 2010

yet to see a happy customer with underfloor heating

njk · 15 Feb 2010

we have been installing underfloor heating for over 20 years. Yet to see an unhappy customer.

Agile · 16 Feb 2010

There you go! Several conflicting answers!

The reality is that it can work well in a situation where heating is wanted all day long as in libraries, old peoples homes and for retired people. It has to be installed in an already well insulated building where its limited heat output is adequate. Poorly insulated properties or if the floor is not going to be insulated properly are unlikely to be satisfactory. Its rarely adequate in small rooms as found in the typical three bed semi.

Another problem is that people in the UK have become accustomed to having carpets on the floor which also reduces output.

But many of the people who are advocating it are those profiting from its use or installation. Similar situation arises with solar.

Be wary of Mountainwalker ( he spells it wrongly ) as he casually mentions that his system is fitted at 100 mm centres whereas most UK systems are based on 200 mm centres. 100mm spacing will increase output but doubles the cost of the tube.

There is someone posting on the forum tonight who has an UFH system installed without the proper control system.

Its actually very easy to install but not so many understand the design or specification aspects.

Tony

mointainwalker · 16 Feb 2010

@ Agile

I take offence at this comment about being wary of me because " I casually mention 100 mm centres : That is a truly stupid remark.

I don't ".. casually ..." mention it at all. I deliberately do so to give complete information because to say you "get 100W/m2 " is just as silly as to say a radiator will give you 1,000 W.

Some will, some won't , it depends on their size and equally when people airily talk of 50W/m2 for UFH , they should always give the pipe-spacing to complete the spec.

You don't seem to like UFH, but that's no reason for your put-down asides e.g. "...its limited heat output ..."

I have to say another silly comment. Everything has a limited heat output or are you able to supply a source of heat with unlimited output ? If so, please specify.

If the heat loss calc shows you need 2 kW and you have 20 or so m2 fine, if not, rethink needed.

Agile · 16 Feb 2010

I have nothing against UFH at all PROVIDING its properly designed, installed and assessed as being suitable for the application. This is generally where the building will be in use for much of the time and is well insulated..

Just like solar and heat pumps, there are too many suppliers and installers claiming many benefits and heat outputs because they want to get a profit. I have no vested interest.

If a high output ( usually more than 50w/m² ) is needed then special precautions like the closer pipe spacing you have are required.

For most people I would still not advise UFH as the main heat source in the bedroom because most people like it warm when undressing for bed and to cool shortly afterwards when they are in bed. Its not nice for me to have to wake two hours later to adjust the duvet when the room has finally cooled down.

Tony

caz26 · 16 Feb 2010

Now now Men, let's not be falling out! I read this all with interest and appreciate both sides of the argument.

We are thinking of this in two rooms, 1) the hall, which is draughty, but principally because we currently have flagstones under the carpet (I believe largely messed up ones at that due to channeling for pipe work etc) with large cellars underneath. Therefore currently we have what I would describe as the opposite of underfloor heating with a chilled floor. So my thoughts were that if we remove the flags and insulate above the cellar laying UFH would be a great improvement with stone or tiles to finish. Also considering a wood burner stove in room upstairs that we may be able to duct to the hallway to augment the heat there (just a thought, the hallway is large and runs through the middle of the house)
2) the kitchen. Although as Agile points out, UFH seems appropriate for a low level of constant slow to alter temperature so how would this work when I'm slaving away in the kitchen with the gas hobs and ovens on (I do like to cook!) i.e. increasing the temperature of the room quickly-would it get too hot in there??
Regards,
Caz
PS the house is ancient with single glazing (listed) so I'm also wondering in principle does this rule out UFH

mointainwalker · 16 Feb 2010

The smart idea that is becoming more and more recognised, is that before looking at heating you look at insulation.

You are in the very fortunate position of having cellars, so you could insulate the joists from below with very little effort and cost and then re-evaluate the situation.

If you then decided you still needed heat of some sort ,the insulation would not be wasted and would not need to be touched whatever you did ( excluding UFH between the joists with spreader plates )

Ducting stove- heat from up to down. Have major reservations about feasibility and results of this.

Kitchen. I personally have the kitchen heated to about 13 C because....

a) There is the added heat you speak of and ..
b) You are moving around and as long as it is not "cold" , my experience is that I do not notice the odd couple of degrees +/-

It is generally acknowleged that UFH has a significant time-lag vis-a-vis rads. This is correct, however I was surprised to find that with my system it is only 30 mins from boiler start-up to warmed tiles ( The other surprise was that the tiles never feel warm to the hand because they are at less than body -temp)

The above is just a comment as I am not advocating frequent intervention to UFH temps.

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maximum heat output per m2 from underfloor heating

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