Balancing Airflow in our Zehnder HRV System
This post is about the adjustments I’ve made to our heat recovery ventilator (HRV) system to balance the incoming and outgoing airflow. While you may find this post helpful, please consult your HRV professional before making any adjustments to your system. I wrote this post mainly to remember what I did.
Our Zehnder ComfoAir 350 HRVwas installed about 6 years ago. Our home, Midori Haus, is a certified Passivhaus so the house is very airtight (0.6ACH50). Having an effective ventilation system is important for comfort and health. My husband and I wanted to install the yellow particle filter, rated at MERV15, near the air intake. After experiencing extended poor outdoor air quality days during the 2018 California wildfire season, we thought this was prudent. Adding an extra air filter would increase the resistance in the incoming airflow. We expected the incoming air flow be reduced, but didn’t know how much, so we measured it.
I did the work myself. Tools used for this work included:
- Ladder – to reach diffusers installed in 9-foot ceiling
- Alnor Loflo balometer (6200D)– which I borrowed from PG&E lending library
The system we have in place is a Zehnder ComfoAir 350 HRV with F7 red filter (MERV 13)for supply, G4 black filter (MERV 7 / 8)for exhaust, and CW-FC 320 activated carbon filterin ComfoWell.
First task was to take a baseline measurement before installing the new filter. I took the balometer to each of the 13 diffusers inside the house and measured the airflow.
Here is the baseline measurement:
Though I suspected there would be a slight negative pressure, I was surprised to see these numbers confirming it. I compared these numbers to the commissioning report from January 2013 and noticed that:
- Return Air (Exhaust) total was much higher (25%) than the original commissioning numbers. I suspect we caused this when we cleaned the dust out of the diffuser, we didn’t place them back to their original setting.
- Fresh Air (Supply) total was slightly lower (2% ~ 6%) than the original commissioning numbers. This can be attributed to the installation of the activated carbon filter near the supply intake. The ComfoWell box was installed in October 2013 and we haven’t balanced the air flow at that time.
Note to self: Next time you clean the diffuser, put them back to the same settings as you found them!
This is what the Return Air diffusers they looked like when I pulled them out, before the adjustment and cleaning:
Adjusted the opening size by pressing the tongue and pushing in the inner fitting. The size of the smaller opening in the photo below is 8 on the gauge. This roughly corresponded to the air volume measured (in CFM) at the medium HRV setting.
Adjust Supply Diffuser
I adjusted the supply diffuser in one room to slightly increase the flow. The smallest ring was in place. I removed the ring and cut it smaller. This increased the flow by 2 CFM.
Install Fine Particle Filter
Next phase of this project required climbing into the attic to Install the yellow particle filter (MERV 15) in the ComfoWell, upstream of the activated carbon filter. While I was in there, I replaced the activated carbon filter. We’ve been calling this “charcoal filter” and can’t get ourselves to call it by the correct name.
Adjust Fan Setting Values
The total fresh air (supply) measurement dropped by about 3 CFM after the installation of the additional yellow particle filter. The total return air (exhaust) measurement was still about 20 CFM greater than the total fresh air (supply) measurement at medium fan setting. Next step was to adjust the fan speed.
I followed the instruction on page 9 of Zehnder Installation Manualto access P3 (ventilation program) menu. I lowered the fan values for exhaust medium setting (P32) and exhaust high setting (P33). I raised the fan value for supply high setting (P37).
Summary of changes made to the P3 menu.
I measured the air flow at each of the 13 diffusers using the balometer and recorded the airflow. It’s pretty well balanced now.
Result: Better Indoor Air Quality
The indoor air quality is better after installing the extra filter. How much better? We haven’t seen numbers this low on the Dylos Air Quality Monitor. We feel better too!