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Understanding ERV CFM Ratings

user-5946022 | Posted in Green Products and Materials on

The Panasonic WhisperComfort ERV series is labeled like this on the Panasonic Website:

“WhisperComfort ERV FV-04VE1 Low Cost, Easy to Install Balanced Air Solution 40/20 or 20/10 CFM, Ceiling or Wall Mount ERV”

This may lead you to believe there is a setting to get 40 CFM of fresh air.  HOWEVER, if you dig into the specs, the 40 CFM is really only exhaust air, and the supply is only 30 CFM at this setting:

“Air Volume Exhaust (CFM) 40 @ 0.1″ SP, 20 @ 0.1″ SP, 10 @ 0.1″ SP

Air Volume Supply (CFM) 30 @ 0.1″ SP, 20 @ 0.1″ SP, 10 @ 0.1″ SP”

Two questions:

1. Aren’t all of the rules of thumb, codes, etc based on the cfm of supply (fresh) air the ERV/HRV provides?
2. If, at the highest setting this always exhausts 10cfm more than it supplies, do these things make a tight house have negative pressure?

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  1. Trevor_Lambert | | #1

    1. I think the rules of thumb and codes are meant to apply to both balanced and unbalanced mechanical ventilation. So if you have exhaust only, necessarily the exhaust is going to be x amount and the (mechanically) supplied amount is zero. In that case it's assumed the supply is coming in through leaks, or perhaps a passive vent. For an ERV/HRV it's unusual for the supply and exhaust to be unbalanced.
    2. In any situation where your supply doesn't match the exhaust, the house will be under pressure or vacuum, it's just a matter of whether it's significant or negligible. In any reasonable house, 10cfm difference is probably in the negligible zone. Of course if you have multiple units in the house, then it might change the picture.

  2. Expert Member
    Akos | | #2

    The way to look at it is like this:
    -you have 30CFM of fresh air supply
    -you have a net 10CFM of exhaust only ventilation

    Total vent capacity is 30+10=40CFM

    This total 40CFM is what code cares about when it comes to ventilation needs. The extra 10CFM of fresh air is supplied through the cracks in the house.

    I think part of the reason why the unit is unbalanced is because of the small core, thus low efficiency. Without running it slightly unbalanced on high speed, the air supplied would be much colder and also the core would be more susceptible to frosting up in the winter time.

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