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Re: [Phys-l] Toying with Domestic Waste Heat Recovery

Back in the 50s I heard about a Canadian school that used the body heat from
the students to heat the classrooms. When students were in the cafeteria,
the huge amount of heat was pumped back to the classrooms. I don't know
whether they were using heat pumps or more passive systems. Unfortunately
there are no basements in the Gulf coast and 90% of the year you need
cooling and dehumidification. Well maybe only 85% of the year.

We usually do not need waste heat recovery, but waste heat and moisture
disposal.

John M. Clement
Houston, TX (on the Gulf coast)


In the period since the idea of domestic energy conservation was
discussed here, I have put some thought into the topic.
I was surprised at a scratch calculation concerning the cost of
mass flow of heated and cooled air into living quarters.

It appears that a rather small temperature differential ( <2degC)
easily covers the electrical cost of motivating that flow with a fan.

The TMP36 is a precision temperature sensor that costs under $2.
A microcontroller can simply convert its 10 mV/degC output.
Solid state relays driven by 5 volt logic, each capable of
controlling up to 25 amps of line power, cost around $7.
(Though this is very much more than the power required to motivate
the desired flows)

The programmed controller is a $40 expense.
This approach seems to provide a very straight forward differential
temperature logic to drive a few strategic air pathways.
One envisions sensing attic, room, basement
and outside air temperatures to activate fans which can
provide outside air, attic air or air routed through a basement
(or sub-soil) duct into the living area, and to control an attic
exhaust fan (for secondary control of ceiling heat flux.)

The attic air would provide the solar gain, and the basement air
would offer a modest thermal sink in its shielded position.
Outside air can also serve to provide a further opportunistic way
of delaying the turn on of a main heating and cooling supply
along with warm attic air and cool basement air.
I am of course concerned that radon infiltration would make it
unwise to use direct basement air, rather than ductwork.

From my current efforts, it seems that the largest single
impediment is the labor needed to install the ducting, the fan,
and the wall or ceiling apertures to allow duct access to
the living quarters.

As a trial, I fitted an exhaust fan for a bathroom/shower
ceiling which was ducted to the eves - a facility my house
formerly lacked.
The materials are not expensive. But, this was a full day's labor
for the ductwork,fan, grille, switch and wiring.

That would weigh heavily against having this sort of
arrangement commercially installed, I imagine.

Otherwise, the economics look attractive, so far....
The technology is now very, very accessible.

Brian W