"BlueJohn" wrote in message
...
We've just had an extension built and we specified wet underfloor
heating for it (by Osma/Wavin). The plumber's installed it, the
manifold gets warm, as do the flow and return pipes, but they're not
nearly as hot as the rads, and I can barely detect any heat emerging
from the floor at all even if I turn the wall thermostat up to 25C.
I've never seen the ambient temp in the room go higher than 17C even
with the boiler temp full blast at about 80C.

On top of the heating boards is 17mm plywood, some thin underlay
(recommended by the manufacturers for UFH) and then B&Q click-to-fit
wooden boards (also said on the pack that they would be OK for UFH).

We've got a beefy combi boiler (15 lit/m) and the rest of the heating
in the house is fine. I balanced the rads a couple of weeks ago to see
if that might help, but it didn't.

The plumber says it's working, and that UFH is just "very subtle." I
agree there is some heat going into the floor, and I realise the floor
won't be as hot as a radiator or anything, but I was expecting at
least *discernible* heat from the floor when walking on it in bare
feet. Is that not the case?

If anyone has any tips, I'd be grateful. I know next to nothing about
plumbing though.


Well for sure something's wrong as the room isn't up to the design
temperature which will likely be 21C with an outside air temp of -1C, ie. a
22C difference. So either the design is wrong, or the installation is
wrong, or the setup or the usage.

Presumably your system designer did heat loss sums for the extension. You
don't tell us much about it but let's assume it's a fairly normal room built
to today's insulation standards, not stupidly long and thin or uninsulated
etc. UFCH product from the major players claims about 100W/m2 which for
most normal rooms works well. UFCH is getting a bit like lego now, buy the
standard bits, plug them together and it simply works, so it's hard to see
how the designer could have gone very wrong.

Your builder had more latitude to mess up, often they are not experienced in
exotic things like UFCH. The spacing between the pipes is important as too
wide a spacing limits heat output. Screed depth on the solid floor is also
critical as too thick a screed makes the system very slow to respond,
although it should still get there in the end. That said, provided he used
the standard bits and bothered to read the instructions it should simply
work.

Setup is something which is often ignored as it takes time and is a bit
fiddly, something that builders are often not good at. There will be a
thermostat on the manifold which controls the water flow temperature, it
does this by mixing colder return water from the UFCH pipes with hotter
water from the boiler. Obviously this has a major effect on the temperature
of the floors. On my Polypipe system the thermostat is a red knob which
clicks 1 notch per degree, on the OSMA I think (but I'm not sure) it looks
like a horizontally mounted radiator valve, a big white thing with numbers
and a pointer on it. Try turning yours to a hotter setting, the flow pipe
should get hot. It should get to a temperature you don't really want to
hold on to for very long (say 65C-ish).
Each individual heating circuit (one per room usually) will have a way of
restricting its flow in order to balance them. Its quite possible they've
been turned down too much but I don't know how its done on the OSMA. From
memory that system lacks any flow indicators so you may end up having to
measure the temperature of the flow and return of each circuit with a
thermometer (or hand), they should be roughly 11C different. Mind you for
your purposes simply opening them up would at least prove it is capable of
heating the rooms in the short term.
The UFCH is pumped separately form the main heating so I suppose the pump
could be faulty or indeed it could be adjustable and set too low.
There will also be a couple of isolating valves which could restrict the
flow if they weren't open properly.
Wow, I didn't realise there was so much potential to mess up.

Usage of UFCH is a little different than radiator circuits in that it needs
a little while to heat up. People have suggested it needs all day, not on a
properly installed system it shouldn't. Mine gets the screed noticeably
warm in about 30 minutes and fully up to temperature in about 60. The
wooden floors are quicker.

Hope those ramblings help.