a breath of fresh air

There are times when I'm very detail oriented, and, well, there are other times. I know a good deal about the mechanical systems of homes, and am learning more now. The main thing I have learned in the last 6 weeks is that I know significantly less than I thought, and less than I want.

We're using a heat recovering ventilator (HRV), in our case by Fantech. Unlike other mechanical systems, we've spec'd and installed it ourselves. (Or just about, as we've just a couple of more ducts to run before insulation plugs up our wall cavities on Wednesday.) The idea is that homes that are well built don't have enough natural air changes to dissuade indoor air pollution and built-up moisture caused by offgassing surfaces, cooking, showering, and living our dang modern life.

An HRV takes air from the bathrooms, laundry room, and kitchen and sends it outside through a central ventilator that at the same time is bringing fresh air in and supplying it to living and sleeping areas. The cold, dry, fresh outside air is pre-warmed by the stale but warm indoor air with the use of air-to-air heat exchange. They claim about 65% efficiency, which I guess means that about about 2/3rds of the latent btu's in the outgoing air are recovered in the pre-warming.

Maybe. This air can feel cold to me. One mistake I made years ago was to bring it into a closet. Wasn't long before I got a call wondering why there was mildew on the clothes. Turned out that the cooler air in the volume of the smaller space couldn't hold moisture suspended, so the dew point was being reached, causing condensation which in turned caused fungal growth.

Still, you have to have fresh air, bathroom fans are wasteful, in most climates keeping the windows open enough isn't practical, and 65% isn't all that bad, really. I figure almost all homes built today are too tight to assume enough natural ventilation, so as long as you have to buy the equipment anyway, it may as well REALLY have a job to do! For years we have tested for envelope leakage using blower doors to put the home under a vacuum and found that our homes hover around one tenth of an air change (.1 ACH) per hour in natural pressure. This is extraordinarily low, and can account for some tremendous energy savings. ASHRAE standards suggest that this be boosted to a controlled ACH between .25 and .35.

In a home with air ducts for heating and cooling, we have tapped directly into those for distributing the fresh air. The Vermont Street Project is foregoing air conditionong and is heating with a combination of radiant tubes in the basement slab and European-style wall hung radiators in the main two floors. With no ducts already there, we've run our own 4 and 5" rigid pipe, winding around the plumbing (oh why didn't we beat them to that joist space?), always counting our static pressure (SP) losses, and adding a Panasonic low sone booster fan in the main bath, which is also the farthest spot with the highest SP, or resistance to the airflow.

The unit runs all the time, at a low rate of 60 cubic feet per minute (CFM). When you step into the bath room, you hit a remote timer which kicks the unit up to 150 CFM for either 15, 30 or 60 minutes. Neither 60 or even 150 CFM is much when spread throughout an average house volume, yet the job will get done, noise is all but eliminated, and fresh air is inexpensively introduced.

Like all high performance operations, this house will require tuning. We're sure to experience some unbalanced airflow, and I'm undecided whether to run the unit in good weather, when windows really will be open. We'll watch this system and make our call as we live the home.