Suspended Tube Radiant Heating System
The ability of
Pex-AL-Pex to handle relatively high water temperatures makes it
possible to install a suspended tube system. The tubing is placed within
the air cavity between the floor joists. The tubing gives off direct
radiant energy to the surfaces within the joist cavity. The outside of
the tubing also gives off heat to the surrounding air, establishing a
gentle convective circulation within the joist cavities. The warm air
flows across the underside of the subfloor transferring more heat to it.
 Suspended tube systems have
some unique benefits. They don’t require heat dispersion plates and thus
reduce installation cost. They operate at high water temperatures under
design load conditions and thus can often be piped directly to a boiler
without needing a mixing valve. When the tubing is suspended below the
subfloor, it is not subject to puncture from the nail points associated
with installation of hardwood flooring.
Pex-AL-Pex piping is ideal
for suspended tube systems. Its aluminum core provides the structure
that prevents the tubing from sagging between supports when operated
with high water temperatures.
As with all floor heating
systems, it’s imperative to install underside insulation.
This must be a reflective
insulation system meaning that there is a shining reflective metal
surface facing the pipe. There has to be an air gap between the pipe and
the reflective layer minimum 2” or more.
Foil faced batting
insulation or the aforementioned “bubble” insulation can be used. The
“bubble” insulation is different from the one used with concrete. The
aluminum layer is exposed on one side minimum and is always facing the
piping. The insulating layer can be one or two layers of plastic
“bubble” depending on the amount of insulation required.
When the space below the
heated floor is also heated, use a minimum of R-11 underside insulation.
If the space below the floor is partially heated, install a minimum of
R-19 insulation. If the space below the heated floor is an unheated
crawl space, install a minimum of R-30 underside insulation. Although
these suggested underside R-values are conservative, the installer
should verify they meet or exceed local energy code requirements.
Threading the pipe into the
joist space is identical to the method explained under the section
discussing joist space heating with heat transfer plates.
The fastening of the pipe
is different in this case. There are three main ways to secure the pipe;
stapling to the underside of the subfloor; using a pipe hanger to
suspend the pipe in the joist cavity; or use a nail clip to nail the
pipe directly to the side of the joist.
Stapling to the floor is
very simple, however the pipe is close to the surface and can be
punctured easily from above. The other two overcome this problem, but
an extra item pipe hanger or nail clip is used. In high heat load
installations, the direct stapling to the underside can result in high
and low temperature “lines” on the floor.
The fastest and easiest to
install is the nail clip method. They all have their advantages and
disadvantages.
 It’s
possible to staple
Pex-AL-Pex directly against the underside of the subflooring without
using heat dispersion plates. As discussed above this approach is only
suggested for low heating load situations such as rooms that have
minimal if any exterior exposure. Without either a slab or aluminum heat
dispersion plates, the floor’s ability to spread the heat laterally away
form the tubing is more limited. Still, when the design heat load of the
space doesn’t exceed 15 Btu/hr/sqft, this installation method can
deliver adequate heat output at reasonable water temperatures.
As a
summary, it is probably fair to say that piping can be fitted into any
floor surface and there are numerous variations to fit the project
circumstances. It should also be clear that there are important
differences between these methods and some are better suited than the
other for effective heat transfer.
The following image
illustrates the heat transfer process during joist space installation
using heat transfer plates or direct staple up. The image speaks for
itself and gives very good reasons to consider using the
heat transfer plates wherever it is possible.
More Information
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