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 ...
1. 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
2. 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?
3. 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!
Thanks for any advice,
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 http://www.tiny.cc/uufgY 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
Many thanks again!
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
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
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
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
1. A thermal mass is heated (floor)
2. Heat is lost at a rate (k value x thickness x temp) through the
rest of the structure
3. 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)
4. 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
5. 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.
Yes and no.
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
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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