Hi Mark,
I suspect my thermal transfer is rubbish compared with the underfloor
systems buried in a concrete slab.. But it does not need to be able to
supply a lot of heat in a short period, as the heating is on 24/7. Just
trickling in enough heat to replace that lost through the building
fabric.
You will need to tread carefully when it comes to carpets and floor
coverings in general. Probably best of all would be to just polish the
floorboards if they are in good condition. I have used modern Caberfloor
composite boards as the original floorboards had rampant woodworm and
were in a generally bad state. New floorboards would have decimated our
small budget, but looked a lot better. Ended up using carpet tiles with
a low insulation factor, which are stuck with a tackyifying adhesive so
they can be easily removed if required.
So Basically the floor sandwich looks like this. Between each set of
joists, there is a sheet of 100mm thick PIR foam with a foil vapour
barrier facing each side. Itās easily cut to a push fit with an old
woodsaw, but needs to be done individually as joist spacing in an old
building tends to vary, and we are trying to get a push fit. To get an
airtight seal, itās further attached with buildersā foam to close any
gaps and get a good seal. To hold up the foil-backed sheets temporarily
until the builders foam sets there are a few pieces of lath nailed to
the underside of the joists every 3 feet or whatever is convenient.
The areas of foam are then joined across the top of the joists using
aluminium tape so that the floor is completely sealed from the
crawlspace below. Note that you MUST ensure good airflow in the
crawlspace with outside air via the airbricks to prevent potential
moisture and rot problems in the joists as they can no longer dry out
via the house.
On top of the foil on the joists I screwed 20mm roofing laths, 1 per
joist with gaps to route the underfloor pipes between each section. The
pipes sit on top of the PIR sheets held in place with plastic pegs at
the ends where the pipes turn the corner. Aluminium spreader plates are
clipped on and could be screwed to the joists but they can also just sit
there.
The caberfloor sheets are then screwed to the laths on the joists which
gives us a 20mm for the pipes and spreaders. Screws were preferred to
nails as its then possible to take up the caberfloor at a later date or
tightened at a later date if any squeeks arise.
Using flooboards instead of composite panels would probably be better,
as hot air from the 20mm gap would then seep up between the boards,
improving the heat transfer. Since the gap underneath is sealed, there
would be no reason to seal between the floorboards to improve building
air tightness.
Low-insulation carpet tiles finish the job.
While I did all this myself and I am a retired computer programmer, I
suspect professional underfloor heating engineers would be rather
reluctant to do it this way as it is quite time-consuming, not hard to
do but time-consuming.
Beware heating engineers who normally do gas boiler-powered underfloor,
as its a different concept from a different world. Any hint of a mixing
valve then run as you want a low temperature system, not a hot one
blended down. Heat pumps run best with weather compensation, and the
underfloor will spend most of its time at about 30c or lower anyway. A
mixing valve just gets in the way and means you have to run your
heatpump higher just to give the mixing valve something to mix down
with. Even a valve with a thermostat controlling the mix will only end
up duplicating the weather compensation, and if they are not both
singing from the same hymn sheet the result will be chaos.
To answer your couple of questions below about effects from
thermal/humidity cycling. The answer is none that I can tell. Since itās
all running 24/7, there is little or no cycling of anything. And if
there are seasonal cycles, they will be measured in weeks. Temperature
cycles do happen in spring and autumn when the heat demand is so low the
heatpump cannot modulate down low enough, and it has to switch on and
off. Max is about 4-degree variation in system flow temp over several
hours. Not noticeable given the thermal mass of the building.
Regards
Nigel