I've been reading the various posts over the years on wet UFH, which I am having installed in a new extension in a few weeks time. I have a few questions ...
I am unsure about the flooring to lay on top. Part of the new build is an extension of the existing lounge. I want the screed above the pipes to be flush with the suspended floorboards of the old part of the house so that I can fit my new super-large lounge with the same flooring throughout, for continuity. I am thinking of some Kahrs 10mm Linnea 1-strip engineered flooring (which apparently they have just started to make), because presumably the conductivity of the heat will be better through this than the 15mm version, but it will be more hardwearing than the 7mm version. My worry is this: it will be sitting on two very different surfaces and so will respond differently in terms of expansion, warping, wear etc because the old half is floorboards + rads, the new half is concrete + UFH. Is it unwise to do this? Should I consider the orientation of the new flooring as well? I want to lay along the length of the room, which will also be perpendicular to the old floorboards. This means that each run of the flooring will go across the join between old and new. [Does this make sense?!]
In the kitchen, presumably you don't lay the pipes under the part of the kitchen where the units are going to sit. Does this mean you should decrease the spacing of the pipes so that the total length is equivalent to having laid across the entire floor?
Given the depth of the concrete slab that the builder has already poured in, I reckon I have room for about 100mm of celotex/kingspan boards leaving 65mm for the pipes & screed and 10mm for the flooring. Do these sound reasonable thicknesses?
I am keen to look at flooring before buying (I'm in the Cambridge area) so if you think I am mad going for the Kahrs, let me know something else that mainstream shops might have in stock!
10mm will be pretty nice. Make sure you have insulation under the screed. Don't worry about the temp differences..its the humidity that will move the kahrs - not the heat. In summer I had about 6mm expansion over a 6 meter wide floor that is about 0.1%..allow for 1% nd you are away, and you may get away with 0.2% or 0.5%..
Essentially yes. I would lay them right TO the unit edges..I didn't and there is a perceptibly cool strip running round the edge..total pipe length is more or less the way you control total heat input..
Generous. I ended up with 50mm polystyrene and 75-100mm screed. In your case I would go or 745mm polystyrene - thats enough..and 90mm of screed and so on.
I guess you COULD use celotex if you don't mind the price.
I ended up with kahrs, also in cambridge area..well west suffolk anyway. I bought it online tho.
If you want to visit and have a look and feel of how it turned out, put suitably disguised e-mail address in a post here..
Hmm, I'm not sure that 50mm polystyrene will meet the building regs, and 75mm will only just do it. For our P/A ratio, the guy who did the plans calculated 80mm now that the U-value for floors has dropped to
0.22. I can't find similar details on the Cambridge City Council website, but Bath have some intereresting guidelines on meeting the new Part L requirements at
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in the first PDF link on the left.
So, perhaps I should go for 80 mm polystyene and 85 +/- typical error screed. If you go for much thicker screed, don't you end up with the floor taking ages to preheat?
I would be very interested in looking at your floor and getting any further tips on .. for example .. location of the manifold, ways to prevent my condensing boiler cycling on and off ... and associated pitfalls that I am bound to make! My email address should be in my profile.
Many thanks again! Lotty.
kahrs, also in cambridge area..well west suffolk anyway.
You may well be right. My floor just met 2000 regulations, and with hindsight I would have put in more insulation than that. P/A ratios do not work when you have a raised concrete floor with a 30 mph wind at -5C under them..
In an ideal world, go for as much as you can afford.
You do anyway. Mine does about 1 degree C per hour. Its massive and not that well laced with pipes. That plus the massive brick chimneys means that it is very hard to get more than a 4-5 degree C variation between day and night..which is a total boon in very hot weather..we open the windows at night and leave the curtains drawn back, then by day shut the windows and the curtains..we never went much over 26C inside last summer when the outside air temperature was up to 30C plus..
My manifold is in a wall with cupboard doors on it. I used the Polyplumb kit that you can get from Ridgeons. If you use the recommended setup with temp reducing valve and a separate pump, it all works fine..short cycling haha. The boiler comes on at about 3 pm, and spends the next two hours getting the floor warm..19C is the off temperature, but if we then draw the curtains and switch the lights on, the room continues to rise to about 20C in the evening, and the boiler never gets called on to heat again!
I only calculated the pipe spacing for 100W/sq meter, that being enough to keep the room warm..I hadn't considered heating up times. In any case if te floor gets warm enough to generate more heat output than that, its likely to stay warm well beyond the point at which the heating gets switched off. And overheat the room. The answer is to think of UFH as a
24x7 system that modulates between warm by evening, and cooler by morning. And get plenty of thermostats controls ob the zones.
Main gotcha to remember is to get the pipes laid in continuous lengths on a carefully worked out plan, and to pressurise them before screeding, and hold the pressure during screeding. Pumps can be hired for this.
I had reinforcing mesh specified for te screed to prevent cracking on the he large slabs, so we simply used tie wraps to lace the pipes to the rebar, and then pumped the lot up, checked it overnight, and then screeded.
The screeding was a pain..they couldn't do it in one day, we should have used premix, and the labourers who mixed it, mixed it all slightly different.
T result was that the final surface was uneven..it had slumped..and it cracked btween the two days and parts were even slightly loose.
I ended up pouring PVA into the cracks to bind it all together, and using levelling compound to get it almost level. It wasn't because that stuff isn't self levelling.
The most amusing thing was to see the self levelling drying out with te UFH on..you could see the dry araes marking out te exact pipe runs..
SWMBO is happy to have visitors..e-mail on its way.
Wow... not sure I should get into this one but generally I am happy to see people take an interest in UFH. Its weird the way people have there own opinions on this form of space heating even if they do not have it, have not experienced it, have heard bad things about it, will dismiss it out of hand, etc...
Lets try to keep it simple
A thermal mass is heated (floor)
Heat is lost at a rate (k value x thickness x temp) through the rest of the structure
If the rate of heat input is more than the rate of heat loss the temperature will increase (as it gets warmer this rate increases)
The aim is to heat the mass (floor) to a surface temperature of only a few deg C above the desired air temperature (say 20 +/- 2 deg C)
Depending on the amount (thickness) of thermal mass heated this will retain the heat inputed and release it as a thermal store which can only be controlled at the rate the heat can be released. i.e. time control and to a lesser extent thermostats are....not ideal.
There is something to be gained by not running 24x7 in a room that is only used in the evening..or in the daytime in a more active way.
So a timer has some use..BUT the thermal time constants are very large, and in my case the room only fluctuates by about 5C over a whole day at the very most. And since I have a lot of UNHEATED mass, I tend to get temperature overshoot on bringing the room up to temperature.
This actually works pretty well. Morning temps are around 16-18C..and if there is sunlight the room tends to come up to 18-19C in winter. On cold days I put the UFH on at midday, on less cold days at 3pm - thats timed.
The room then slowly rises to around 19C at around 7pm, when the stat kicks out, and overshoots to 20C or sometimes 21C before starting to cool back, and is well on its way when we go to bed ..its back down to just over 19C then.
The UFH comes on for an hour in the morning, but that tends to heat the corridoors where the pipe runs are, more than the living areas..which is fine. We come down onto warm feet there, and the kitchen has an aga which IS 24x7, so thats that.
If its christmas with a house full of people loafing around being idle, then I simply put it on 24x7 and leave it..it uses very little more oil
What I HVE noticed though, and demonstrated to myself..is that NOT running the UFH and lighting a bloody great fire, I had cold feet unless the stat indicated 22C or more.
In short if you do have a massive screeded floor, you are quite likely to be running the room hotter than it needs be, to get comfortable feet and not feel cold.
Unless you have massive fitted carpets etc.
So really you end up with two distinct approaches to heating.
One is the low mass and insulation inside of it approach..the rooms heat up fast, and are very suitable for fitted carpets, radiators and so on.And timers for those who leave the house empty by day make for good economy. However such houses become very hot in summer. They may actually need air conditioning..
The other is the high mass inside the insulation scenario. Such houses tend to average out temperature wise. This means in winter they are at a higher average temperature than they need to be, but conversely in summer they can stay very cool in the afternoon heat. Timers are not a great deal of use, but help a little. Stats are useful, but 'intelligent' stats are actually better. UFH works well in these houses and fitted carpets possibly do not. In fact this is fairly typical of a southern european house - massive walls and rooves and deep overhanging eaves to keep out high sun, (but let in lower winter sun) and provide shade, and tiled concrete floors..and 24x7 heating in winter.
Which type of heating you prefer and which is most economical depends a lot on personal taste. My main point being that these are different STYLES of house, which each have advantages and disadvantages, and both need to be operated intelligently to et the best ourt of them. I have been very comfortable in both these types of houses..but what you do NOT want to do in my opinion, is have a high mass floor that is NOT either underfloor heated OR has a damn thick carpet on top of it or you WILL get cold feet. Unless you run the heating 24x7 anyway,which rather negates any advantages a radiator system has.
Personally I think I prefer the high mass UFH heated house..and no fitted carpets. Easier to keep clean and much more comfortable in summer. Downside is its a bit of a luxury if you leave it empty during the day..but the general feeling of it is that its both warm and cool together..I don't feel cold at far lower temperatures than I used to aim for in a house with cold floors and/or floor level draughts.
By the way, be also aware that sticking furniture with low hanging valances on a UFH floor reduces the heat output quite a bit. Except for kittens, who crawl under it and toast their tummies in 30C temperatures..
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