Q: Dear Jim: We made our house more airtight and efficient, but the air does not seem fresh or healthy indoors. Would installing an efficient fresh-air window ventilator help much or do we need a whole-house model? - Joyce B.

A: Dear Joyce: One of the drawbacks of making a house more energy efficient is that indoor air quality may suffer if you do not also make provisions for fresh air ventilation. An airtight home can result in a "stale air" feeling. On the other hand, an airtight home allows for better control of humidity, dust, allergens, etc.

There are many methods for improving indoor air quality without resorting to just opening a window once in a while during the winter. Actually though, opening a window for a short period of time does not lose a huge amount of heat energy because the heat content of air is relatively low. The key is a very short period and a lot of air movement; a windy day is best.

The efficient window fresh-air ventilation units you mentioned are very effective. I use one in my own office where I am writing this column right now. Larger whole-house ducted models are more effective, but with the furnace blower set for continuous air circulation, my window unit eliminates the staleness throughout my home.

These fresh-air ventilation systems are called heat recovery ventilators (HRV). There is a heat exchanger built into the unit. As the stale heated air is exhausted outdoors, it passes through the heat exchanger first. There it tranters much of its heat energy (up to 80 percent on some models) to the incoming cold fresh air so your energy dollars are not lost.

All the HRV models have effective air filter elements to clean the outdoor air before it enters your home.

Window units look similar to a typical window air conditioner, but they weigh less because there is no heavy compressor used. The best ones use a super-efficient direct current (DC) type motor and use only about 100 watts of electricity (about as much as a light bulb.)

A whole-house HRV model is usually mounted somewhere in the utility room, attic or basement near the furnace. It can be connected to your existing furnace duct system, but it is best to install its own ducting. The stale air inlets are often located in the kitchen and bathrooms and the fresh air outlets can be in a bedroom, living room or hallway.

A variation on an HRV is an ERV (energy recovery ventilator). An ERV also transfers the moisture between the outgoing stale air and the incoming fresh air along with the heat. This can help keep the indoor air from becoming overly dry during the winter and excessively humid during the summer. By maintaining the proper humidity level, you can lower the thermostat (winter) and raise (summer) for a large utility bills savings without sacrificing comfort.

There are several designs for the heat exchangers (crossflow, rotary, heat pipe, etc.) used by the various HRV/ERV manufacturers and they all are effective.

When selecting a HRV/ERV unit, consider the types of controls. Multiple speed settings are a definite plus. This feature allows you to manually adjust the speed for quicker ventilation, but with more noise, or a quieter, slower speed for sleeping or continuous use.

How long and when the unit runs is often determined with a simple timer control. You can also install a more sophisticated control that measures the humidity or other pollutants in the indoor air. When the level you set is reached, the unit comes on automatically. During the summer, a humidity sensor is particularly effective and efficient.

Write for (instantly download - www.dulley.com) Utility Bills Update No. 993 - buyer's guide of 14 whole-house and window HRV/ERV manufacturers listing fresh air capacities, efficiencies, convenience/comfort features, warranties, heat exchangers, and prices. Please include $3.00 and a business-size SASE. James Dulley, Illinois Country Living, 6906 Royalgreen Dr., Cincinnati, OH 45244.

James Dulley is a mechanical engineer who writes on a wide variety of energy and utility topics. His column appears in a large number of daily newspapers.

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