Hybrid Systems: Ghost-Busting Heat
Transfer In Return Air Plenums Under Radiant Floors
© 2014, Robert Bean, R.E.T.,
P.L.(Eng.). All rights reserved. Edited and
originally published in
Common sense, a bit of heat transfer
knowledge and strict adherence to specifications go a
long way to improving the air/hydronic dynamic.
One of the
radiant based HVAC systems using
(de) humidification and decontamination is the disconnect
often created between those responsible for the air
system and those responsible for the hydronic system.
Both of them are installing heat exchangers of one form
or another. These heat exchangers are intimately
connected by the on-site assembly of their respective
One hazard that has crept up occasionally
over the years has to do with the poor practice of
flashing joist bays for return air plenums. When air is
circulated through an un-ducted and un-insulated return
air plenum under a heated or cooled floor, the laws of
heat transfer will definitely tell you that you are
about to have an expensive problem to solve.
Here is what can happen: the call comes
in from the client complaining about discomfort, so you
head on over to the job site. Lo and behold your client
has a case. You start your diagnostics and discover the
supply air being delivered to the space is 15°F (8.3°C)
higher than design setpoint of 72°F (22°C). You discover
that a control valve you expected to be stuck open is in
fact in the closed position.
Now you ask yourself where is the air
picking up heat if the valve is closed. One possible
source is valve leakage. I know it is a shock but valves
leak, some more than others. Even with a valve in the
closed position and its actuator torque of sufficient
strength to close off against pump head – yes you can
still have leakage.
How much? Well, it all depends, but in
this case the leakage was too small to be of any
significance so that was not the prime problem. You
think to yourself it has to be coming from the radiant
floor, but how could that be when
your specs called for
insulated ducts? Your brain is now doing mental
gymnastics until the light comes on.
"I bet the sheet metal guy didn't look at
the spec for insulated ductwork and just flashed the
bottom of the joists," cause that is what a
seasoned ghostbuster would think. You have the general
contractor take down a small piece of drywall and your
suspicions are confirmed (see Figure 1).