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

Dear Joyce: One drawback of making a house more energy-efficient is the 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.

The efficient fresh air ventilation window units you mentioned are effective. I use one in my office in my home. Larger ducted whole-house models are more effective, but with my furnace blower set on continuous air circulation, my window unit eliminates the staleness throughout my home.

These systems are called heat recovery ventilators, or HRVs. There is a heat exchanger built into the unit. As stale heated air is exhausted outdoors, it passes through the heat exchanger. There, it transfers much of its heat energy (up to 80 percent on some models) to the incoming cold fresh air.

During the summer, the outgoing cooled stale air precools the incoming hot outdoor air. All the HRV models have effective air filters to remove particles and allergens before the fresh air enters your home. Window models resemble a small window air conditioner, only they weigh less. You can install one easily yourself. They have multiple speeds and are quiet on the low setting to work well for bedrooms. The best ones use an efficient direct current, or DC, type motor that consumes only about 100 watts.

The whole-house HRV models usually are mounted somewhere in the utility room, attic or basement near the furnace. These can be connected to your existing furnace duct system, but it is best to install its own ducting. The stale air inlets often are 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, which stands for energy recovery ventilator. An ERV also transfers the moisture between the outgoing stale air and the incoming fresh air along with the heat. This keeps the indoor air from becoming overly humid during the summer and they are more popular in hot, humid areas.

When selecting a HRV or ERV unit, consider the controls. These will alter its effectiveness, convenience and your family’s comfort. Multiple speeds are a plus. This allows you to select a high speed for quick ventilation or a quiet, slower speed for sleeping or continuous use. The unit can have a simple timer or a more sophisticated control that measures the humidity or other pollutants in the indoor air to switch it on and off.

Write for (instantly download – www.dulley.com) Update Bulletin No. 993 – buyer’s guide of 14 whole-house and window HRV-ERV manufacturers listing fresh air capacities, efficiencies, comfort-control features, warranties, heat exchangers and prices. Include $3 and a business-size SASE, and send to James Dulley, 6906 Royalgreen Drive, Cincinnati, OH 45244.