RE: Air Con vs. Heat Exchangers

["Kenneth Watt" <kennethwatt@xxxxxxx>]

Correct, and that's where the clever bit comes in!

If you install an evaporator capable enough on the incoming duct from the
outside air source you can then chill the air going into the rooms that you
are cooling. The problem being that systems cabable of this are usually
rather chunky and need to be rather powerful in order to cool the ambient
air quickly enough and keep it cool till it reaches the destination room so
a difference is given between the ambient and the incoming air feed.

Usually these types of system are employed in large buildings, noticed the
vents that blast hot or cold air, that's basically how they work, simple
and
effective. BUT, there had to be a but, this used to use all sorts of
gubbins
on the roofs etc to make it work properly and get the massive cooling power
required and it was in some of these cooling plants that the salmonela
virus
managed to breed in hospitals and the likes. So there are pros and cons of
using this type of system.

A salesman will *NOT* tell you that though when you are waving a big cheque
under his nose!

K.

-----Original Message-----
From: Mark Harrison [mailto:Mark.Harrison@xxxxxxx]
Sent: 18 May 2001 10:52
To: 'ukha_d@xxxxxxx'
Subject: [ukha_d] Air Con vs. Heat Exchangers


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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