Cohousing-L Mailing List: Re: Heat-exchanging venntilators

[Roger Diggle (diggle]

On 11/4/94, Dan Robinson wrote:

> To Roger Diggle:
> 
> I've always assumed that carbon dioxide is a major pollutant in a 
> "tight house", but you don't mention it. Not so? 

It certainly is a problem in large buildings, where %CO2 is the first thing
measured to test for adequate ventilation.  However the uses of the spaces
are different, which leads to controlling the ventilation differently.  In
commercial space, where people don't generally cook or take baths much,
moisture is not typically a problem.  But the occupants still insist on
exhaling, and in a crowded office, this makes for many exhales/sq. ft./hr. 
So the codes usually require # cubic ft. per minute per occupant (or per
floor space with assumption of a maximum occupancy rate).  In a house, fewer
occupants and exhales per square foot, but large cooking and bathing moisture
loads, mean that if you control for moisture in the air you are probably
controlling nicely for CO2, if only by accident.  CO2 isn't usually a problem
in houses, but, yes, it is important.

> Is aluminum (a very good heat conductor) or other metal a problem 
> because it corrodes too fast?

Aluminum is a good choice in metal exchangers...  It will last nearly forever
in this kind of service.  It just has the frosting problem in COLD weather. 
Ferrous metals would be a bad choice due to corrosion.

> I like the idea of the "heat wheel", but preferably with flat 
> surfaces instead of honeycombs. Then brushes could isolate the two 
> air streams, and also keep the surfaces clean.

The honeycomb cells are parallel to the airflow, and are very small, so that
brush seals would in fact work.  The entering and leaving faces of the wheel
are flat.  I can't remember how the airstreams are leak-sealed, so I'll try
to look in the next few daze and report.

> To be efficient, it must must have a lot of heat- 
> exchanging surface, which would be hard to keep clean. Do present 
> versions do anything about this?

Several of the units incorporate filters, but I can't speak for them all. 
The heat exchange cells on come exchangers are also very small, and can
actually filter the big particles pretty nicely all by themselves.
 
> Another important factor, which I assume commercial versions 
> incorporate, partly since it would be more complicated not to, is 
> the counter-current effect. Two bodies of still air, or air currents 
> moving in the same direction, of different temperatures, with a heat
> conducting surface between them, will both approach a medium 
> temperature. 0 degree air and 70 degree air will both approach 35 
> degrees. Two air currents moving in opposite directions will each 
> approach the original temperature of the other. The 70 degree air 
> will approach 0 degrees, and vica versa. Just how much heat is 
> exchanged depends on how long they share the heat-conducting 
> surface. 

Counterflow heat exchange is the best from the standpoint of most complete
heat exchange, but it has some practical difficulties.  It is very hard to
make a compact, inexpensive, counterflow heat exchanger to handle low
pressure gasses.  As a result, many of the designs have the air streams
moving through the exchanger at right angles to each other.  This allows
making duct connections to the airstreams more easily, and the exchanger can
be so much more compact that it is feasible to increase surface area to get
better exchange.

> Now that I've said my piece, and hopefully impressed you all, I must 
> also say that it seems like this subject would be more appropriate 
> under something like "building environmental design".

Appropriate, but I don't know about MORE appropriate...  One of the
interesting/frightening/fun things about cohousing is that it encompasses so
much of life...  many of the things discussed here would be appropriate in
other forums, but we'd all have to weed through hundreds of messages from a
large number of forums to get the specific information we pass around here. 
I think it keeps us all thinking, realizing just how formidable and wonderful
is the mental calculus of making good living decisions.

- via BulkRate 2.0

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