Under Floor Heating System for Suspended Floors
Polyplumb floor central heating on suspended floors can be installed using a variety
of methods. Pipe can be laid along the joists using double heat spreader plates or
across joists on battens using single heat spreader plates. Pipe only systems can
be used where heat outputs are less critical.
| Key Design and Installation Information |
| Maximum heat output |
|
Approx. 70w/m2 (see output table) |
| Recommended design flow temp. |
|
60°c (see output table) |
| Maximum circuit length |
|
80m (15mm pipe) 100m (18mm pipe) |
| Maximum coverage per circuit |
|
25m2 at 300mm centres (15mm pipe) |
| Single heat spreader plates |
|
30m2 at 300mm centres (18mm pipe) |
| Maximum coverage per circuit double heat spreader plates (15mm) |
|
17m2 at 225mm centres |
| Material Requirements (approx) |
| |
Single heat spreader plates 15mm and 18mm) |
Double heat spreader plates for 15mm pipe only) |
| Pipe |
3·1m/m2 |
4·5m/m2 |
| Heat spreader plates |
3 plates/m2 |
2 plates/m2 |
Installation and application using double
heat spreader plates
 (spans of 450mm to 350mm)
Where joist centres are 450mm to 350mm double
heat spreader plates can be used to span the joists
and support the pipe at 250mm centres (225mm
average spacing).
These plates run along the joists and are positioned
to allow the return bends to cross joists through
pre-prepared notches.
Installation and application using single
heat spreader plates
(spans greater than 450mm or less than 350mm)
Where it is not possible or desirable to drill or notch the
floor joists, and the floor height can be raised, spreader
plates can be used in the following way:
1) To prevent downward heat transmission, insulate
between the voids in the joists with appropriate
mineral wool or foam insulation.
2) Lay 75mm x 25mm battens across the existing joists
at 300mm centres. Trim the ends of the batten back
to the last joist at alternative ends.
3) Lay spreader plates between the battens and pin
in position. Lay pipe into the spreader plates.
 Pipe only systems
The use of heat spreader plates is recommended,
as they assist the response time and heat spread.
However, in some circumstances, simply laying pipe in
an air void between foil faced reflective insulation and
the underside of the floorboard will be adequate.
The space between the top of the insulation and
the underside of the floor deck should be kept to a
minimum (20mm) and care should be taken to ensure
that the insulation is sealed at the edges and ends.
The pipe only application would be suitable in areas
where the heating response is less important, eg
bedrooms, and is not recommended in living spaces
or over unheated spaces, eg rooms above garages.
Enhancing pipe only systems
The effectiveness of pipe only systems can be
enhanced by laying a weak (1:6) dry mix screed
approximately 25mm thick between the joists.
In order to apply this extra support may be required
below the insulation. Although pipe only systems
are designed at 60°C flow temperature, the flow
temperature may be reduced when applying this detail.
Before this system is applied it is suggested you
consult a structural engineer to confirm suitability.
I beam joists
Where I beam joists are used plumbing and heating
systems are installed from below through holes in the
joist. Floor heating can be installed from below and
insulation laid after the pipe is clipped in position.
Application of Timber floors over Suspended floor systems
The application of timber flooring is possible on
suspended floor heating systems only where the timber
floor is laid directly over the under floor system.
There should never be two layers of wood, i.e.
chipboard overlaid with timber, as the resistance of the
two layers would be too great for the heat passage.
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