Positive pressure Zehnder system

Hi, I have a Zehnder Q600 ERV. I never got the whole system commissioned because we installed it during COVID, and the Zehnder rep said they couldn’t cross the border…
Anywho it is set to basic balance all this time, but with the wildfires I realized our in home pm2.5 was getting very similar to the outside, which I thought could be mitigated by having an ERV system.
So I looked and found the imbalance feature. The documentation in Zehnder seems odd, but I essentially set it to having 10% less exhaust fan and intake fan. In my mind, this filters the air in a controlled fashion via the Q600, and keeps diffusion of exterior air into the house but letting the interior air to diffuse out. (oddly Zehnder calls this “under pressure” in the manual).
Was wondering, if others have looked at this setting and if 10% is a reasonable number?
Thanks,
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Replies
guillow,
Not a direct answer to your question, but I found this very interesting:
https://www.greenbuildingadvisor.com/article/fire-related-indoor-air-quality
Thanks. Looks like the q600 has a merv13 equivalent filter which is better than the cracks in my house at merv11, so I'll continue to pressurize for the relative benefit of 50% vs 20% filtration of pm2.5. but will also bust out the COVID HEPAs again.
Regarding the original question though, at 10%, I'm getting 280m³/hr in, and 250m³/hr out. So for an extra 30m³/hr leaking out now through the cracks. Doesn't seem like much... Anyone else have it turned up higher or have heard any other numbers?
I'm sure it depends on the windyness outside as well. And I'll need to remember to turn it back down to balanced for winter.
But any other people try this or have it set to some target m³/hr?
Thanks,
10% unbalance won't do much unless your place is extremely tight. You want to pressurize above stack pressure at least so depending on the number of floors you will probably need more supply airflow.
I run a different ERV and need to drop the exhaust flow by about 1/2 during pollen season for it to work well. This is generally not an issue during warmer months, but not something I would do in the winter as you can risk pushing a lot of moisture into your building envelope.
Also check that your ERV filter is enough. For me a MERV13 didn't do enough (confirmed by measuring PM2.5 on supply VS outdoors) and had to add in a 2nd stage HEPA filter.
Combination of HEPA and unbalance got my PM2.5 down to single digits even on a very smoky day.
Thanks. Akos, that's what I think I'm going to have to get to as well.
I went to try to get to 50% flow difference, but found that the Zehnder Q600 only allows a maximum of 15% off balance.
Interestingly I went back to my post-energy retrofit results to review our leakiness... For background we have a 100year old brick home with rubble foundation on a high water table... We've been doing some renovations and keeping a "passive house" approach with everything, but we are stuck with our starting place.
Our post-retrofit has 7ACH. Our house is ~2100sq ft. with 4-5 people at home, that puts me a target of ~60-100 CFM I think? But with such a leaky house...
Any who, thanks to this conversation it's given me the kick in the butt to get to tweaking my ERV.
1) I'm thinking I'll target CO2 overnight <1000, and find the right duty cycle for overnights, and then drop CFM in the day time when people are milling about or not at home.
2) keep the 15% maximum imbalance, and then try to remember to adjust it back to balanced for when temperatures drop below 5C (maybe link it to my snow tire change timing), and vice verse
3) look to adding a MERV 16 filtration box to the Zehnder unit
4) Continue to work to tightening up the house as best as I can (still have bunch of low hanging fruit to work on)
5) bring out the covid HEPA units for wildfire days going forward.
I am also getting an AirGradient outdoor sensor to be able to reliably compare what's happening indoors with the outdoors. What's nice, is that it all these sensors integrate with Home Assistant, so thinking about creating a few automation/alerts in the future for action (maybe that reminder to adjust the balance...).
Please let me know if you have any further advice to guide my journey for healthy air!
Thanks!
Sorry, also to add, we have a basement and 2.5floors (although the attic with cathedral ceilings does has a max height of 9ft in the middle). So i'm not sure what the stack pressure might be in such a case?
Also, agree, that the MERV 13 + 7ACH leakiness is not sufficient.
With outdoor PM2.5 going to 100, indoor was reaching 30-40. Increasing the Zehnder duty cycle to try to pull in more filtered outdoor air made no impact on indoor PM2.5 at that point. (although I didn't wait long to see if the unbalanced 10% was going to make much of a difference).
What i did find useful was running the air conditioner in fan only mode. It has a MERV 11, so it was kind of whole house low efficiency filter.
Eventually I realized none of this was really working, so pulled out the COVID era HEPAs and we're back into the single digits now during wildfire season.
But am planning on slowing working through the other steps above.