Heat Recovery Ventilation
From: Roger Diggle (digglemacline.com)
Date: Thu, 3 Nov 94 02:56 CST
> Roger: Please send the info on air to air heat exchangers.
> We are very interested, having first heard of them
> while reading about the Toronto  GreenHome.

> Thanks,
> Stephen Hawthorne
> Blue Heron Farm
> Chatham County, NC

Modern construction practices allow for the construction of very airtight
houses.  The average thirty year old house is leaky enough to provide several
air changes or more an hour if it's very breezy outside.  This is good,
because it removes contaminants from the house, but it's bad because it
causes a direct heat loss when the air you paid to heat leaks out and is
replaced by the 20 degree air that comes in through every crack and crevice. 
It also produces draftiness, which causes discomfort and increases the
temptation to walk over to the thermostat and do the wrong thing.

In a tight house with no ventilation, on the other hand, the humidity will
rise to levels uncomfortable for the occupants and insafe for the structure
and personal belongings.  Double-glazed windows will fog over and wallpaper
will peel.  Other contaminants will also concentrate.  In tight construction,
in most of the country, the amount of ventilation required to remove the
excess moisture will generally reduce any other concentrations to acceptable
levels (though not always).

The nice thing about air-to-air heat exchangers in tight construction is that
they allow *you* to control the amount of air exchanged, rather than
depending on outdoor airspeed to do as it will.

Air-to-air heat exchangers, or, as the industry seems to want to call them
these daze, heat recovery ventilators, get rid of excess moisture and other
indoor air contaminants and provide replacement air that's as fresh and clean
as your outdoors.  Some of the air from your house is blown through a heat
exchanger and then to the outside, carrying nasties with it.  Outside air is
blown through the heat exchanger and into the house, picking up much of the
"waste" heat from the air you are throwing away.  They generally have four
duct connections: two for the air you're getting rid of and two for the fresh
air you're bringing in.  The air to be removed is usually removed from the
bathroom and/or kitchen, and replaced in the main living area, or blown into
the furnace ductwork, if such exists.

Several companies make this equipment.  I'll fish for information about more
of what's around, but here's a run down of what I know at this point:

Altech Energy's design is based on a heat exchanger manufactured by
Mitsubishi, I think (some Japanese company), of a special moisture permeable
paper.  Because of the permeability, no defrosting is required, a plus, but
longer run times are necessary to get rid of a given amount of moisture, a
minus.  They also offer an impermeable heat exchanger core (polyethylene, I
think) that would not be appropriate for Northern winter weather.  Overall, I
think their design is superior to most others.  The heat exchanger core
itself, and the design of the ventilator it is used in, are both very
elegant.  The only mechanical things are the single blower motor that
operates both blower wheels, and the humidity control that turns the unit on
and off.

Altech also offers a recovery ventilator that installs just about like a
bathroom fan, and would move adequate air for a small tight dwelling.  It's
even available with an energy efficient compact fluorescent lamp built in.

Altech Energy
(608) 221-4499
7009 Raywood Rd
Madison, WI


Research Products has dealers all over creation...  many heating / air
conditioning / refrigeration supply houses handle the line.  Their air-to-air
is a very new product, however (about a year, I think) and may suffer in some
places from unfamiliarity.  This company has made very good humidifiers and
high-efficiency air filters for many years.

Research Products' ventilator design (the mechanical layout) is virtually
identical to Altech Energy's, although it's a bit nicer looking and has some
advantages in ease of installation.  Many years of manufacturing experience
pay off.  They can't use the patented Mitsubishi heat exchanger, however, so
they came up with their own exchanger design, also made of moisture permeable
paper.  The design doesn't seem as elegant, but may in fact work just as
well.

Research products also makes a very nice humidity control that automatically
readjusts it's setpoint with changes in outdoor air temperature.  Very nicely
done, and reasonably priced for what it does.  There are other schemes for
controlling the ventilators, such as timers or manually operated switches,
but I think that humidity control is the best method, especially with a
control like this that can reset itself according to outdoor temperature.  If
the ventilator could be sized just right, I suppose that it could run full
time, but that has disadvantages as well.

Research Products
(608) 257-8801
1015 E. Washington Ave.
Madison, WI   53703

The other two basic designs I'm aware of use either moisture impermeable heat
exchangers, usually aluminum, which are generally larger and require defrost
controls, or "heat wheels" which use a rotating heat exchanger.  The heat
wheel exchanger surface, usually a fine, open-ended honeycomb, rotates
through the exhaust air stream collecting heat as the air passes through it. 
It then continues rotating through the intake airstream, giving up the heat
it collected to the incoming fresh air.  They should be self defrosting. 
This is a design that has been used in commercial buildings for some years,
so it isn't as contrapted as it might sound.  Mechanical parts involved are
blower motor(s) and a motor to rotate the heat wheel.  Drain lines would be
required to get rid of the defrost water.  Honeywell (Minneapolis) and Carnes
(Verona, WI), among others, use this design.  

Carnes's unit had something of a reputation for mechanical trouble in the
past, but the problems may be resolved.  It's been around a while.  I'll try
to get some scuttlebutt on it.  Honeywell's unit is a fairly recent entrant,
I think about a year or so ago.  Honeywell typically makes good stuff, though
I have no experience with this unit other than a peek at one at a supply
house.

All the moisture impermeable exchanger models I'm personally familiar with
are large, commercial size, for large buildings, swimming pools and other
specialty applications.  I'm not personally familiar with any of the models
manufactured for residential use, though I'd guess that the largest number of
manufacturers use this design.  These exchanger cores will frost up in cold
weather, and, if not defrosted, will lose exchange efficiency, and eventually
become completely ice-blocked.  The defrost controls typically involve timers
and/or pressure controls that operate dampers to close off or bypass the cold
incoming airstream around the exchanger core, and use the warm discharge air
to melt the ice.  This extra stuff is in addition to the humidity control
that operates the unit in the first place and the blower motor(s) that move
the air.  It's also necessary to provide drain lines to get rid of the water.
 It makes for a more complicated ventilator.  That's why I like the moisture
permeable core designs the best.

One other possible unit to check into is a Therma-Stor unit that combines
heat recovery ventilator with heat pump and water heater.  I've only seen
literature, not the equipment, but I've worked with their combo heat pump /
water heaters a few times, and they seemed to be pretty well thought out. 
Therma-Stor is also, amazingly enough, in Madison...  A division of Dairy
Equipment Company.
Therma-Stor Products Group
(608) 222-5301
2001 S. Stoughton Rd
Madison, WI

Hope this helps...

Roger Diggle

- via BulkRate 2.0

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