Running ERV in humid summer
EliciaD
| Posted in Mechanicals on
How do I combine my mini split and ERV, in my tight house in the humid southern summer? The mini split struggles to dehumidify with my high ceilings, even with all the doors and windows closed and ERV not running, but I still need fresh air. Should I let the ERV run on the lowest setting, and just be okay with the mini split only being able to dehumidify to like 60%? Or should I just run the ERV once a day, ideally at the least humid time, to get some fresh air, and run the mini split on dehumidify mode the rest of the time? Or is ok to let the humidity be over 60% sometimes and then run it? Thank you!
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Some mini splits have a dehumidify mode where the fan runs on a low speed while still keeping the coil cold enough to condense the moisture out of the air. If that doesn’t work, it may be time to consider a dedicated dehumidifier.
Thanks. It works on dehumifidy mode.. just fairly slowly. And that's with the ERV not running. Im waiting for new filters. So I'm wondering .. how often should I run the ERV when I'm dehumidifying?
It would be helpful to know your indoor CO2 levels. You can purchase an ndir CO2 sensor for a out $60(I use the qingping one from Amazon and it's as accurate as my more expensive aranet4)
Co2 allows you to know how well ventilated your house is. If it's above 1000 ppm you could turn on erv, and below 1000 ppm you could turn it off. This is an arbitrary number. Use what you're comfortable with.
60% rh is not terrible. It's almost as good as 40% in controlling mold.
If you don't find it comfortable, lower the air temperature of your mini split.
Before I ventilated my house would be 74f at 37% humidity. With ventilation I like it at 68f at 60-62% humidity
Everyone thinks their kid is cute and their house is tight. The question is have you had a blower door test and how low was its ACH-50 number?
The next question is what percentage of each hour does the AC run on the hottest day of the year?
From the sound of it your AC unit is hugely oversized and, in your climate, running the ERV adds humidity to your home. My opinion ERVs in cooling dominated climates are a silly idea.
Short of replacing the oversized AC system you are stuck running some dehumidifiers to convert the latent heat into sensible heat the AC can easily deal with.
Walta
Thanks. But I need to run the ERV because otherwise there is no fresh air. Right?
Maybe.
Case-by-case. Is this a new home constructed to PH standards, or an old leaky one? How many people live in the house? Are you broiling steaks? Or using a gas range? Do you have carpeting? Vented independent bath fans? Pets? Are you in and out often?
Even then IAQ is somewhat subjective.
Does your ERV have a core that can be replaced with one that does not transfer moisture? 60% is far too high.
Agree with Walt- where is your location? ERVs are most appropriate in dry climates- the American west, desert and mountain zones where arid air is predominant.
“60% is far too high.”
You are misunderstanding how an ERV works. If you switch to an HRV then the outdoor air would be brought into the house untempered, with the full outdoor humidity hitting the house. An ERV is more appropriate in nearly all cases since it mixes and tempers the air so that outdoor air is brought in with less humidity.
An HRV does not exchange moisture, an ERV does. Both exchange tempered air, whether that’s heated or cooled. In climate zones where cooling is dominant, as mentioned above, an HRV is more appropriate.
60% RH is nearly twice desirable levels.
Climbing carpenter, and hrv will almost always bring in more moisture than an erv in a cooling climate, at the hotter air will contain more moisture, while your cooled interior will have a lower moisture content as your ac extracts moisture as a byproduct of cooling the air. The only time a hrv will reduce interior humidity is in a heating climate, when you are bringing in cold air with lower moisture content than the interior air (which is warmer).
An erv works by essentially equalizing the exterior air moisture content to whatever interior air moisture content is. An hrv has no ability to do this and instead the small amount of moisture that condenses out is extracted via condensate line.
You need to know how tight your house is. I bet you don't need the ERV on.
My understanding of the humidity exchange in an ERV was that the more moist air will transfer some of its moisture to the less moist air. In a cooling climate, the air pulled in from outside will (usually) be more moist than the outgoing air so it will transfer some of its moisture to the outgoing air. Therefore what's brought in will be less moist than it would have been with an HRV. (In a heating climate, the outgoing air will transfer some moisture to the incoming air making the incoming air less dry than it otherwise would have been).
Is this understanding not correct?
Yes this is correct.
"EliciaD | Jun 25, 2025 10:41am | #4
Thanks. But I need to run the ERV because otherwise there is no fresh air. Right?"
The way I see it no one has suffocated in a home because it was so tight. Humans have been living in confined spaces since we were cave men. We are born with the sensors to tell us when the air is unhealthy no gadgets required. I think you know how to open a window when it gets too stuffy.
The way I see it HRVs and ERVs were invented to solve a real problem of high indoor winter humidity in small, crowded and nearly air tight homes in very cold climates. The walls were growing mold and rotting because they had 70% humidity and surfaces would fall below the dew point.
Then the sales department got started and sold the world on the idea that your brain stopped working if the CO2 got over a number measured by a gadget. Even better let’s make it a code requirement that everyone must buy one.
The manufactures claim 90% recovery but I have yet to see numbers from the real world that would support that claim. It is my belief that when the temperature differential is low say under 20°F the recovery is under 10%.
Walta