Percentage Timer on ECM ERV
pico_project
| Posted in General Questions on
Anyone have their ERV on a percentage timer?
Having a Renewaire EV Premium L installed and in addition to the two boost buttons in the bathrooms they added a percentage based timer in the closet.
My understanding is this needs to run 24/7. The owner of the HVAC company says I definitely don’t want to do that on extreme days.
Any thoughts on this?
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Replies
I say run it only enough to keep your indoor humidity under 50% in the heating season. Unless this house is very tight new construction that is likely to be zero % required.
Yes, someone will be along to tell you to run it at 100% 365 or you will die from lack of oxygen. I say BS.
Walta
Not BS.
If I shut down my ERV, the house air turns stale in about six hours. I'm not talking about high CO2, it just smells wrong.
For best efficiency, you want the ERV running 24/7 on the lowest airflow setting that will give you the indoor air quality you like. This is very subjective, so I won't get into it but the code numbers are in the ballpark, maybe on the high side especially if your place is large with only a few people living in it.
I also find keeping the ERV on 24/7 is a nice background white noise which really helps in place with radiant heating where every small noise in the house would otherwise be noticeable.
Walta,
if the only reason you are running it is to reduce humidity, why would you install an ERV in the first place?
I don't think many people are going to suggest you'll die. You will have poor air quality, even in a modestly tight house.
A few questions:
What climate zone?
How tight is the home?
What is the square footage?
How many occupants?
Zone 6. Traverse City, MI. We do have the lake here which moderates the hot/cold quite a bit. Doesn't get below 0F that much here.
Pretty tight. It's a 1955 ranch but just remodeled with new windows, mavjest 500 sa wrap, new plywood sheathing, encapsulated/sealed crawl, air sealed attic floor. Did my best to get it tight. Very pleased to be only house in the neighborhood with no icicles. :)
1,600 sq/ft
4 (2 adults, 2 kids)
Ok, so it makes sense that you need an ERV in there :-)
Renewaire suggests that their ERVs don't need defrost cycles (and they are correct if your interior humidity is at 20%), however the suggested ASHRAE curve at 40% interior humidity is to start defrost cycles at -10 C (14 F). Interesting read on their defrost strategy: https://www.renewaire.com/wp-content/uploads/2022/03/MAR-LIT-165-RGB-WhitePaper-FrostControl-web.pdf
Panasonic starts defrost cycles at -10 C (14 F) with their IB 100 and 200 ERV units
With respect to extreme temps therefore, you're likely quite fine to run your ERV continuously with interior RH at 35-40%, until temps dip below 15 F. See the attached chart's orange line. (page 26.21 ,from the document below)
https://www.ashrae.org/file%20library/technical%20resources/covid-19/i-p_s20_ch26.pdf
Running an ERV on humidity does not make a lot of sense if you understand how they work, so the correct approach would be look at occupants and ASHRAE's recommendations. If you want to experiment a bit, pick up a Co2 sensor. I have a fully instrumented setup here with Co2 monitoring, temp sensors on all four air streams etc. and have been deep diving on air quality for a while now.
What I can tell you based on a similar home size (but zone 7A, colder) and retrofit status with 2 adults, 2 kids, is that you will want somewhere between 75 to 90 CFM (which magically corresponds to the ASHRAE recommendation) if everyone is home to keep C02 levels under 900 ppm. Outside air is around 400 ppm C02. You'll also find that magically any complaints of stuffy air or smell will likely disappear around that level. If you do run the unit on percentage basis, you'll want to reach that overall level over an hour. If your target is 75 CFM but you only run for 50% of the time, then the delivery would need to be 150 CFM.
However, efficiency on pretty much every HRV/ERV out there drops a lot as you increase air flow. On your unit, efficiency peaks at 88%, at 60 CFM, only using 11 watts of power. At 120 CFM, you're at 81% efficiency and using 37 watts. At 200 CFM efficiency has dropped to 74%, and now you're using 114 watts.
It makes more sense if you look at these numbers to run it continuously at the lowest CFM you can. This will be around 80 CFM for your house. How your ERV is installed (standalone or connected to existing HVAC) does affect the math here, particularly if you have an older furnace with an older (not an ECM) fan.
In my test setup here, CFM profiles are pulled from the HRV based on Co2 levels, and interior air quality is graphed each day with PM 2.5, PM10, Co2 levels and interior/exterior RH. I won't get into details (cuz no one wants to hear them...ha) but here's a few observations:
Zone 7a, family of four, 2 cats (they breath too!), heavily retrofitted 1800 sq/ft home (about 100 years old).
HRV runs continuously, however defrost times are programmed in, and vary continuously with outside temps. So in defrost mode, there is no air exchange. At 25 F, defrost is about 5 minutes, every 30 minutes.
With all family members home, the system is calling for the 75 and 90 CFM profiles more or less all the time. It will fairly quickly work itself down to 60 CFM, 50 CFM and then off if we all leave for 3-4 hours. With one family member home, the system will stabilise after a few hours around 50 CFM.
You can ignore all of this information and just go with ASHRAE recommendations as they are based on a lot of research evidently :-) Having the system off, or at least running minimally when you're way makes sense too.
DennisWood,
In a similar discussion last spring, Jon pointed out something I hadn't th0ught of. Humidity is obviously a poor proxy for overall IAQ, but so in some circumstances can Co2 be. In a house with poor ventilation and no occupants home during the day, Co2 would test very low, while other contaminants were building up. It's a good practice to ventilate to make sure Co2 doesn't get too high, but I think it also makes sense to maintain a baseline ventilation rate all the time.
Malcolm, I'd agree 100%. As more research is done on C02 (even the upper limits vs health) I'm sure our thoughts on what is acceptable will change. Co2 is a very good indicator of occupancy and I've found it very effective "managing" reducing ventilation rates when outside winds increase, or occupancy is reduced. I'm using an Ecowitt WH45 which logs Co2, PM2.5 PM10 , temp and RH (but no VOC) and it is very interesting to look at the various levels bounce around with ventilation rates.
Where the Co2 sensor is located is also important. In our main living area (where the WH45 sensor lives), levels drop a lot at night, while increasing in the three upstairs bedrooms. I've added some automation code that uses a different (more aggressive) CFM map at night for this reason. If we had the system ducted to each bedroom (again, current wisdom suggests this) and another sensor in place, we could fine tune even further to reduce resource use.
What remains an accurate instrument of the other potential pollutants though is my wife's nose! Since increasing ventilation rates and doing active Co2 monitoring, there have been precisely zero complaints of air quality. I also don't randomly find our inner acrylic/magnetic window panels removed and random windows open at -20 C anymore.
Dennis_Wood,
" I also don't randomly find our inner acrylic/magnetic window panels removed and random windows open at -20 C anymore."
Which must be very satisfying. Happy New Year!
Malcolm, and to you as well :-) I think the more we celebrate our small successes, the less we'll need to convince anyone of their worth.
Cheers,
Dennis.
Dennis,
I hope that's true. Increasingly I find small successes are all I manage to achieve.