Originally Posted by
Nick Ostrowski
I guess I'm getting a little confused here in terms of how much heat this type of unit can generate. It sounds like it's primary function is to introduce fresh outside air to the interior air with heat generation being secondary. Assuming an exterior temperature of 40 degrees and an interior ambient air temperature of 68 degrees, is one of these units when functioning properly enough to serve as the primary heat source for a finished basement (size of unit and size of space considered)?
Air-to-air heat exchangers do not generate heat, they conserve or transfer it. Do not think of heat generation when considering these units.
Residential air-to-air heat exchangers, also known as heat-recovery ventilators, are used to provide fresh air while minimizing heat loss, or in cooling season, heat gain. The heat exchanger in a gas-fired furnace is (duh) an air-to-air heat exchanger. Heat is moved from the warmer air stream to the cooler one without the movement of any of the physical portion of the stream from one side to the other.
Houses require air exchange with the exterior to avoid the buildup of noxious substances, including, perhaps, moisture, in the air inside the house, a general recommendation is for 1/3 of an air exchange per hour. There are two different ways of doing this, which, for simplification and objectivity
, I will label The Good Way and The Bad Way.
I will discuss this in terms of the heat flow during the heating season. In The Bad Way, a house, by design or not, is built with a certain amount of leakage. Driven by the stack effect and wind pressure, cold air flows into the house and warm air flows out. The rate of air exchange increases with decreasing temperature and increasing wind speed. The air exchange rate is not controlable by the occupant for different conditions inside the home (except for opening windows) to adjust for internal contamination conditions, such as cooking or have many guests. Instead, the air exchange rate increase with greater inside-outside temperature differential, meaning that the heat loss increases exponentially with increasing temperature differential, because each unit of air causes more heat loss because of the greater temperature differential and there is more air being lost because of the increase of the forcing of the stack effect. The air transfer movement is determined by where the leaks are. Some parts of the house may have too much ventilation, some too little. This system provides random air exchange with maximal heat loss.
In The Good Way, the house is build as tightly as possible, so that there is almost no air exchange. A whole-house heat recovery ventilation system is installed, with air being brought into the 'clean' rooms, such as bedrooms, and the living room, and exhausted from the 'dirty' rooms, such as bathrooms and the kitchen. The system is designed for no dead spots and the longest possible path between incoming air vents and the exhaust vents. In the heat exchanger itself, outgoing stale air warms the incoming clean air, with about a 75% conversion efficiency. Most systems have two or more fan speeds, for more air exchange when conditions warrent. The system's air exchange rate is effectively independent of outside weather conditions. The system provides the maximum benefits of air exchange with minimal heat loss.
One of the benefits of a heat recovery ventilation system is that is minimizes indoor-outdoor pressure differences. I installed such a system in my house when it was built in 1994. We measured indoor-outdoor pressure differences under four conditions, with the system on and off, and with the exhaust fans (kitchen and the half bath too remote to put into the system) on and off. There was less pressure differential with the system and the exhaust fans on than with neither on. The system, in addition to its other benefits, provides pressure balancing.
The system Nick has shown is a wheel system. Wheel systems generally are used where there is the desire to move water vapor, as well as heat, across the streams. Such systems may have a small amount of cross-contamination. See, for example,
Building Performance Equipment, Inc. - Air to Air Heat Exchangers.
I have a background in thermodynamics, and I cannot imagine building a house without a heat-recovery ventilator. My system is from American ALDES
American Aldes Ventilation Corporation. But I am opinionated, I would not have a scorched-forced-air system either.
If you choose to evaluate such a system, check for airflow at the intake and exhaust vents in the house. If it is very cold, or hot, outside, the incoming air should be closer in temperature to the inside air than to the outside air. Most incoming vents will be at about 20 cfm, if the only ventilation in a bathroom, the exhaust vent must be at least 20 cfm, the kitchen exhaust flow will be much higher, it probably provides most of the exhaust for the house.
- BOB
