Unbalanced HRV/ERVs

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I have been disturbed to find that many HRV/ERVs are installed without any concern about balancing the system.  (Click on HRV Survey for a CMHC study of system installations.)  When I first got started with these things, it was a mandatory step in the process.  They also had to be ducted independently from the HVAC system ductwork.  That is seemingly a rare installation these days.  But if you think about it, in the extreme situation when there is no airflow coming into an HRV/ERV from the outside, the system is working as an exhaust fan.  There is no heat or enthalpy recovery.  In the opposite extreme, when there is no airflow coming into an HRV/ERV from the house, the system is working as a supply fan and there is zero heat or enthalpy recovery.

The efficiency numbers that are provided through Home Ventilating Institute (HVI) testing are at the balanced condition – same amount of airflow in each direction.  If the efficiency of the heat or enthalpy recovery matters, then it should be operated in a balanced condition.  The vast majority of these units have internal fans.  If the HRV/ERV output to the house is connected to the return side of the air handler, when the air handler turns on, it will depressurize the return duct and suck the air from the HRV/ERV increasing the supply flow through the unit.  It turns out that this unbalanced condition has more impact on the house than it does on the efficiency of the HRV or ERV.  If the house is in a heating dominated climate, it may not be advisable to operate it in a higher supply volume configuration because it may force humidity in the house into the building system components.

It is certainly more expensive to provide independent ductwork for HRV/ERVs.  It takes extra work to actually design the ductwork and calculate the resistance.  It takes extra work to measure the flows and balance the system.  It takes extra work to commission the system.  But if houses, especially tight houses, are going to depend on their mechanical ventilation system for good indoor air quality, these are steps that should be taken.

Duct Design and Static Pressure

As the static pressure of the system increases, the fan/blower has to work harder, and the airflow decreases.  For example, blowing 100 cfm through 100 feet of 4” diameter duct has a static pressure of Image0.7 iwg or 175 Pascals.  Increasing the duct diameter to 6” drops the pressure to 0.082 or 20.5 Pascals.  The Effective Length of the ducting is the sum of the Actual Length and the Equivalent Length of the fittings like the elbows and grilles and exterior hoods.

The exchanger unit itself has a high static pressure because of the resistance of the core and the filters.  But that is the pressure that it was designed for and tested at.  If that resistance is much higher than the resistance of the duct work, then the resistance of the duct work won’t make much difference in the performance of the system.  In the case of simple bath fans, for example, if the duct run is so bad getting to the hood, it really doesn’t have a great deal of impact on the flow if it is a restrictive hood!   The damage, as they say, has already been done.

For a “back-of-the-envelope” calculation you can figure that a 90 degree elbow has an equivalent length of 10 feet, a wye fitting with equal takeoffs is also about 10 feet, a tee fitting is about 50 feet, a tapered increaser about 4 feet, a typical exterior supply hood with no back draft damper about 35 feet, and an exhaust hood with a damper about 60 feet.  So if you have an installation with 30 feet of actual length, 4 elbows, and an exterior exhaust hood, you would have an effective length of 130 feet or approximately 0.11 iwg for 6” duct work.  If you have about the same run on the intake side, the hood is less restrictive, so the run is very close to 100 feet.  And then you have the connections to the rooms.  Here is a quick duct calculator from Hart & Cooley, the grille manufacturers: Duct Calculator.  (There is more information on this in my book Residential Ventilation Handbook.)

The effective length can add up quickly, so the installation needs to be thought through for performance not just for convenience.  Since the system needs to be maintained – filter and core cleaning, at least – the exchanger should be located someplace very accessible and not buried in an attic or crawl space.  If you’re going to pay this kind of money for a ventilation system, you want it to work right.  You want it to work as expected.

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2 Responses to “Unbalanced HRV/ERVs”

  1. John Semmelhack Says:

    The PHIUS+ Passive House certification program is the only one I know of that actually requires testing/balancing of ERV/HRV airflows. For PHIUS+, the total supply and total exhaust airflows must be within 10% of one another. We also require testing/balancing of the individual branches, but that’s a separate topic…

    I’ve never known an HVAC company that tested/balanced an ERV/HRV.

    • praymer Says:

      The ASHRAE 62.2-2010 standard actually requires flow testing although it doesn’t require balancing. CMHC has a document that demonstrates to a homeowner how to maintain and check the balance on their HRV/ERV using their garbage bag approach.

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