ERV Efficiencies
I recently installed a Broan AI series ERV in my house. Very happy with the system. After starting with an ASHRAE 62.2 as a starting place, I then monitored the CO2 levels in our home to determine that about 35CFM of continuous ventilation is a good place for us. (Relatively small home, just two of us, keeps CO2 levels under about 800 even at night.)
I was curious about how much power the ERV was using so I plotted its watt draw vs CFM (attached). I found that our ERV, which is rated up to 130 CFM, really does not want to run at only 35 CFM which uses 0.77 watts/cfm.
The most efficient setting is right in the middle of it range at 70 CFM which gets you 0.43 watt/cfm.
Because I want the most efficient settings possible, I changed the fan to run at 70 CFM, turning off and on every half hour. By my calculations, this is a saving of about 100 kWh/yr. I found this to get a great amount of savings for something than can be easily programmed at the controller. I also don’t notice it turning off and on. It’s pretty quiet.
I feel like I usually see everyone setting ERV fans to run continuously but in some case it might be worth investigating the cfm/watt curve a little. Some might find 100 kWh/yr insignificant but I’m sure some of you might appreciate it.
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Replies
Good post. Thank you.
Steve, kudos on taking the time to investigate and optimise both your CFM levels AND power use as 99.9% of the population would just set the ERV at what was recommended and leave it. Your ERV power use numbers work out in my calcs as well. There is a lot of low hanging fruit with respect to energy efficiency in a typical home. You can take this a level further and optimise for when you are not home, and reduce run times even further.
What is your climate zone?
I wanted to point out one thing that are missing in your calculation, and it has to do with ERV efficiency vs CFM settings. Broan's line of ERVs are in the HVI database and look to be around 68% efficient (sensible) in the 65 CFM range, and yep, use about 30 watts to do it. That core looks to be about 9" square on each face, which means it does not have the physical size to get up into the high 80s for sensible efficiency.
In all of my testing, lower flow rates equate to higher efficiencies and in your case is likely around 8-10% between 35 CFM and 70 CFM. I am completely ignoring moisture transfer efficiencies as I have not tested these related to flow rates, however I'd be confident in saying they also change with flow rates. Moisture efficiency particularly in cooling modes, skews the total energy change even further.
Let's assume that ambient outside temp is 32 F, and your house is set at 70 F.
At 78% efficiency (35 CFM) the air is entering your home at 62 F, and will be warmed to 70 F. You'll use 88 wH of energy to warm that air running for 1 hour. Assuming it stayed 0 F outside for 24 hours, that's 2.11 Kwh.
At 68% efficiency (70CFM) the air is entering your house at 58 F and you're using about 132 wH to warm that to 70 F (assuming 50% runtime). Assuming it stayed 0 F outside for 24 hours, that's 3.12 KwH used in heat energy.
So again, assuming a 32 F day, and no accounting for moisture transfer, it may actually cost you around 300 wH more to run the unit at 70 CFM. If you are using a heat pump for home heating with a COP of 3, that's still 100 wH difference.
On a hot and humid day, the ERV running at a lower speed will have better sensible efficiency, and do a better job of moisture transfer as well. If incoming air is a bit drier on humid days, then comfort levels and possibly cooling set points can be adjusted for savings.
I just threw this out as ERV/HRV performance has a few other variables to consider. For those of us in cold climates, another overlooked (or ignored) performance factor are the heat inputs required from the building to defrost an ERV/HRV, although more and more research is being done of late in this department.
Bravo! I hadn't even considered this aspect of it!
Quickly - yes, I do fully utilize the "away" mode on the ERV. It turns the ERV off but still runs it I believe on low mode for 10 or so minutes per hour to get some air changes in the home.
Your numbers check out by my math too. I'll add to them.
Running the ERV, with your parameters above, at 35 CFM results in a 2.11 kWh/day heating penalty. Running it at 70 CFM (halftime) results in 3.12 kWh/day. So in effect, your taking a penalty of 3.12-2.11 = 1.01 kWh/day. Assuming a COP of 3 for home heating, that's a real penalty of 1.01/3 = 0.337 kWh/day.
I had previously stated that the fan power savings for running at 70 CFM (halftime) is about 100 kWh/yr. That's the same as 0.274 kWh/day.
A 0.337 kWh/day heating penalty is greater than 0.274 kWh/day fan savings so in this scenario you are better off running the fan continuously at 35 CFM (at least when the outside temperature is 32F). The difference is small enough that we are all much better off worrying about saving energy elsewhere but that was a great exercise.
For what it's worth, I'm in climate zone 3C (central California coast). Even HAVING an ERV is questionable. Ha! The low temperature this coming week is 32F and it is newsworthy. Our typical low is around 40F in winter.
In that temperate of a climate, I’d agree…the temp delta makes the higher CFM rate a moot point.
I do really appreciate the Broan “AI” approach using co2 and humidity sensing to minimize energy use. I’m at a measured 17 watts for 50 CFM delivery so I’d guess the ECM motors in your unit are ramping a bit at 35 CFM to make up for ducting losses/filtration in your installation.
From looking at the documentation of RenewAire Premium ERVs and their HVI submissions, they operate 2 or even 3x more power efficiently at low CFM than at their highest CFM.