Blog Post

Commissioning Our Home's Heat-Recovery Ventilator

To function properly, any ducted HRV has to be balanced after installation

Barry Stephens measuring the airflow through a ceiling register of our HRV.Photo Credit: Alex Wilson


After choosing and installing our state-of-the-art heat-recovery ventilator (HRV), we completed 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.

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.


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The hand-held anemometer in a hood used to measure airflow.Photo Credit: Alex Wilson


Balancing our Zehnder system

Barry Stephens, the business development and technology director at Zehnder America, came up to Vermont to commission and balance our Zehnder HRV. I didn’t watch the entire process, but was very impressed at the level of care that he gave to this task.

Barry 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.

Measuring airflow through a wall register.Photo Credit: Alex Wilson


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 and controls

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 system to work harder that it normally does, increasing electricity consumption.

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, though I’ll need to dig into those instructions when I want to change something.

We have two of these wireless controllers—one in each of our bathrooms.Photo Credit: Alex Wilson


Experience to date

We commissioned the HRV the same day we set up an eMonitor 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 intermittent 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 February 16th) the unit has used 53 kWh.

I'm surprised at how high this consumption is and hope that some of it has to do with moisture having gotten into the heat exchanger core before the condensate drain was properly installed. I'll be very interested to see the annual consumption.

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.

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By the way, Eli Gould (the designer-builder of our home) and I will be leading a half-day workshop at the NESEA Building Energy Conference in Boston on Tuesday, March 4, 2014. In this workshop, “What Would the Founder of Environmental Building News Do? Adventures on the Cutting Edge of Green Building,” we’ll be reviewing product and technology choices, describing lessons learned, presenting data on performance, and discussing, in a highly interactive format, some outcomes from this project that can be applied much more affordably in deep-energy retrofits. This should be informative and a lot of fun. I’ll also be presenting in the main conference, March 5-6, on “Metrics of Resilience.”  Registration information can be found here.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.

Published February 18, 2014

(2014, February 18). Commissioning Our Home's Heat-Recovery Ventilator. Retrieved from

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April 8, 2015 - 1:56 pm

Hi Alex,
I've been following your experience with the Zehnder unit with interest. By the way, my understanding is that Barry Stephens is brother to Northwest Energy Efficiency Alliance's Charlie Stephens.

Ten years ago we installed a UltimateAir DX200 ERV in our new home in Richmond, VT. It has worked fairly well, though the three motors all needed replacement at around year 5 and there have been issues with the exchange medium ending up where it shouldn't. Also, because our house is so tight the humidity recovery during the winter ended up being more of a problem than a solution.

The DX-200 is now about at the end of its service life and I'm planning to replace it this summer with an HRV, rather than run the risk of going through a winter like last one with the danger of a system failure. I like the sound of the Zehnder, but am wondering whether it could work in a retrofit application. When we installed the DX200 it did not come with dedicated ducts or vents. Everything outside the ERV is standard metal HVAC. Any thoughts?