Last week I described our state-of-the-art Zehnder heat-recovery ventilator (HRV), explaining its various features and specifications. This week I’ll review what should be a critical step in the installation of any HRV: commissioning, including the critical step of balancing the air flow.

This is absolutely necessary to ensure proper operation and full satisfaction from a Zehnder HRV and most other HRVs.

Why commissioning
is so important

The ducting runs in a ducted HRV system vary in their air-flow resistance. The two fans in an HRV should maintain neutral pressure -- as much outgoing air force as incoming. Otherwise, with negative pressure in the house, radon and other soil gases could be drawn in, or with positive pressure, indoor air could be forced through the building envelope where it could cause moisture problems.

But beyond the two primary fans and pressure-balancing the entire house, the individual registers need to be balanced to ensure that you’re getting proper air flow through each of the supply and return registers. If this balancing step isn’t followed, the HRV might pull a lot more air out of a downstairs bathroom (which is closer to the HRV), for example, than a more distant upstairs bathroom.

Balancing our Zehnder system

Barry Stephens, the Business Development and Technology Director at Zehnder America, came up to Vermont to commission and balance our HRV. I didn’t watch the entire process, but was incredibly impressed at the level of care that he gave to this task.

While the user controls of the Zehnder ComfoAir 350 Luxe are elegantly simple, the behind-the-scenes controls are much more sophisticated -- confirmed by paging through the 40-page installation manual (in English) -- and I was very glad to be leaving the programming to Barry.

He used a hand-held device to measure airflow through the supply and return registers. This is a small hood that fits tightly over the register with an anemometer (wind gauge) allowing the airflow through the register to be measured in cubic feet per minute (cfm).

The flow through the registers (diffusers) can be adjusted in different ways depending on the type of register. Ceiling-mounted supply registers are adjusted simply by rotating the round, screw-mounted cover plate on the unit, which increases or reduces the gap and the airflow.

Wall-mounted supply registers are adjusted by removing the cover plate and installing an insert that restricts airflow. Different-size disks can be added as needed to further restrict flow.

Exhaust ports are adjusted
by moving the center
component in or out.

After a round of adjusting, the airflow tests have to be repeated. Every time the flow through one register is changed it affects the airflow through the others. My sense is that there’s a lot of art involved in these adjustments; after balancing hundreds of systems, Barry and others at Zehnder America have a very good feel of how adjusting some diffusers will affect others.

I think for our system all this took several hours, though I’m sure I slowed Barry down with all my questions.

Condensate drain

We realized before Barry arrived to commission our system that the condensate drain had never been hooked up during the installation. Zehnder HRVs have a sophisticated condensate drain with a specialized trap. Barry was able to carry out this installation quickly, though the fact that the trap hadn’t been installed over the previous several weeks meant that moisture got into the heat exchanger core, and this may have caused the frost protection cycle to work harder that it normally does, increasing electricity consumption.

Experience to date

We commissioned the HRV the same day we set up an energy monitoring system that allows us to track the electrical consumption of key loads in the house, including the HRV. While in normal operation the HRV uses very little energy, the frost-protection cycle does use a lot of energy -- about 800 watts. During the last ten days of January (a very cold spell ), the unit used 65 kilowatt-hours (kWh), while this month (through Feb. 16) the unit has used 53 kWh.

I love the simplicity of operating the HRV. From either bathroom I can either manually change the speed, or click on a clock icon to boost the unit up to the highest setting for either ten minutes (by tapping the button quickly) or 30 minutes by holding it down for three seconds. (Those times can be adjusted by going into the programming.)

As I noted last week, this isn’t the most affordable HRV you can get, but I feel very good about having what I believe to be the best and most energy-efficient model on the market.

Alex Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to alex@buildinggreen.com.