Ventilating With a Super-Efficient ERV

Well, it’s official.  It may have taken 3.8 years to make it happen, but my Zehnder ComfoAir Q600 energy recovery ventilator (ERV) is running now!  I started it up on 29 December, so it’s been helping to clean my indoor air for a month now.  I wrote about the installation blitz we did back in April, so I won’t go into those details here.  But let me give you a little introduction to life with an ERV.

The basics

Here’s a short video I made to show how this ventilation system can pull cold outdoor air in and deliver it to the house at close to room temperature.

As you saw in the video, there are two air streams.  They pass through the capillary core, a heat and moisture exchanger.  The cold outdoor air being brought in picks up heat from the warm indoor air being exhausted to outdoors.  Most people just talk about supply and exhaust with these systems, but Zehnder actually uses four names, as you see below.  It took me a while to get used to them, but now they make perfect sense.

The graphic below shows what the four temperatures were one chilly morning shortly after I started running the system.

And for those of you living outside the US, here are the Celsius temperatures.

The temperatures above came from the Zehnder ComfoControl app on my phone.  Because an ERV exchanges both heat and moisture, the app also shows the relative humidity in each of the four places.  I haven’t dived into calculating efficiency yet, but you can see just from the temperatures above that the efficiency of heat exchange is impressive.

Getting ready for startup

After the April installation blitz, the system was about 75 percent complete.  Here’s what I still had left to do before I could start it up:

• Install the basement extract and supply register boxes, ducts (ComfoTube), and valves (vents)
• Install the two big ducts to the outdoors for the outdoor and exhaust air with the exterior terminations
• Get 240 V electrical connection

I started working on the first item in October.  You can see the register box (called a boot in heating and cooling duct systems) and ComfoTube (the white ducts) for one of the basement bathroom extract vents below.  I kept the red cap on it until startup to keep the inside clean during construction.

I installed the big duct for outdoor air in November.  As you can see below, it’s quite a long run, but I didn’t have any better options.  The first two photos in the next section below show the soffit we built around the duct.

Finally, on 28 December I got the last vent and ducts installed.  That’s me finishing it up in the encapsulated crawl space in the photo below.

The next day I plugged it into the 240 V outlet the electricians had installed for me in the fall.  Then I turned it on and watched it go through the startup process.  One of the first things it did was ramp up the motors to see how close it could get to the maximum rated air flow, which is 600 cubic meters per hour.  It hit that number and has been running ever since.

After startup

Although I was able to start up the system, not everything was complete yet.  Here’s the list of things that were left to do at that point:

• Trim the duct extensions on some of the basement register boxes
• Install boost switches in the bathrooms and kitchen
• Install the ComfoSense C67 controller
• Commission the system

I’ve trimmed the three ducts that extended below the ceiling plane.  Unlike the ones we installed in the attic, we didn’t have the space to install these so they’d be flush with the ceiling without having to trim them.  So I put a new metal-cutting blade in my handy-dandy oscillating multitool and made them flush.  You can see before and after shots of one of them below.  (That soffit along the edge of the walls is where the outdoor air duct hides.)

We’ve got five extract vents in the house:  one in each of the four bathrooms and one in the main kitchen.  The nice thing about Zehnder ERVs is that they can run continuously at one rate and then boost to a higher rate when you need more ventilation.  You can initiate a boost through the app, on the unit itself, or by pushing a boost button installed convenient to places where the extract vents are.

I’ve installed three of the five boost switches.  So far, I’ve got them in the two basement bathrooms and in the primary bathroom on the main floor (photo above).  Currently, the boost button ramps up the air flow for 30 minutes.  I’ll get the other two installed when I can.

I’ve been talking with Gary Baggett about commissioning the system but haven’t made that happen yet.  Likewise, the ComfoSense C67 controller is still in the box.

My initial take on the Zehnder Q600

The unit does make a little bit of noise, but it’s not bad at all.  About the only places I hear it are downstairs near the mechanical room where the unit lives or on the main floor when I’m standing above the mechanical room.

In  terms of indoor air quality, the first thing I noticed was the drop in carbon dioxide levels.  In the den, we averaged 622 parts per million (ppm) of CO2 in the two months before I started the ERV and had a peak of 1,239 ppm.  In the month since I started it, we’ve averaged 522 ppm with a peak of 664 ppm.  Our VOC levels are better, too.  We averaged 162 parts per billion (ppb) in the two months before, with a peak of 1,284 ppb.  In the month since, they’ve averaged 151 ppb with a high of 923 ppb.  We really don’t have enough data yet to draw any firm conclusions, though.

Another thing I’ve noticed in the data from the Zehnder app is that the supply side is more resistive than the exhaust side.  And that’s been the case from the very beginning.  The screenshot from the app shows the numbers, with the supply fan operating at 68% of its capacity while the exhaust fan is operating at 52%.  The exterior grille and filter get dirtier on the side bringing in outdoor air than on the side exhausting indoor air, so it’s gotten a little worse since startup.

I really love having the extract vents the bathrooms, especially in winter.  It’s been colder and drier this winter than many previous ones here in Atlanta, so using the ERV helps preserve a little of our indoor moisture.  I wrote a while back that you may not need to run your bath fans in winter if you can get the humidity out of the bathroom quickly enough.  And last summer I wrote an article saying that running the bath fan in summer may make your indoor humidity worse, too.  I still have my bath fans on the main floor, but I didn’t install any in the basement bathrooms.

Finally, the air in the house just seems a littler fresher than it did before.  There could be a little confirmation bias going on here, but I’ll keep an eye on the data from my various IAQ monitors and see if they can reduce the bias and get more confirmation.

 

Disclosure:  Zehnder provided the ComfoAir Q600 ERV to me for no charge. 

 

Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia.  He has a doctorate in physics and is the author of a bestselling book on building science.  He also writes the Energy Vanguard Blog.  For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.

 

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