Air Con vs. Heat Exchangers

[Mark Harrison <Mark.Harrison@xxxxxxx>]

OK - Here's part 1, the diagram of how a basic Heat Exchanger based system
works. This is the kind of thing sold by Baxi and the like for a couple of
grand. (See attached JPG).

And the explanation:

A pump, and ducting, takes air out of "hot" rooms - bathrooms,
kitchens
(maybe, sometimes not because of the risk of fire), etc, and runs it
through
a heat exchanger before venting it to the outside.

At the same time, a different pump, and some other ducting, takes
"cold" air
from the outside, runs it through the heat exchanger, and then pushes it
into the "cool" rooms, like bedrooms, sitting rooms.

The heat exchanger acts sort of like a big car radiator. It does _not_ mix
the two air streams, instead it pipes them through two different sets of
pipes, but the pipes get "wrapped around" each other, and are
deliberately
made so that heat passes through them easily. Effectively, the
"hot" air
cools down, by passing heat into the "cold" air.

The point of a system like this is to compy with two different bits of the
Building Regulations:

- The bit that says you have to vent air _out_ of kitchens and bathrooms.
This is the job normally done by extractor fans and kitchen
"hoods" over
hobs.)

- The bit that says you have to have gaps to allow _fresh_ air _into_ other
rooms. This is the job normally done by trickle vents above windows in
newer
houses, and by "leaky construction", quite a lot of which used to
be around
window frames in older houses. I really mean this - anyone who's put sealed
double glazing units into an older house will have seen massive
condensation
problems around them. This is because the previous windows
"leaked" air so
the water-heavy vapour never built up in the same way.

However, this doesn't give you enough to HEAT your house (because the
incoming air can only get as warm as the air you're pumping back out of
your
kitchens and bathrooms, or the air outside, whichever is higher.)

Likewise, this doesn't give you enough to AIR-CONDITION your house (because
the incoming air can only be as cool as the air outside, or the air you're
pumping back out of your kitchens and bathrooms, whichever is lower.)


Some systems also allow you to cut the heat exchanger _out_ of the system,
and for the two air streams to vent directly. You'd do this on warm days,
when you don't want the incoming air to be pre-warmed by the outgoing...

Part 2, the bit where we learn how to add an air-conditioning unit is the
exciting part ;-)

Mark Harrison
Head of Systems, eKingfisher

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Heat Exchanger.jpg