I installed an ERV on my last job with the intake and exhaust utilizing 6" round hard duct.  I have no problem locating an exhaust vent cover for the exterior of the house with a backflow preventer.  However, I cannot locate an exterior cover with a backflow preventer for the INTAKE side of the system.  Does anything like this exist?

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Hi Elizabeth, Zero experience with installing H/ERVs, but what you are looking for is probably a powered damper or powered latch.  Between the changing seasons where stack effect changes direction and the wind a passive solution would be challenging. 

What is the duty cycle on your ERV?  Is the home owner complaining or are you just anticipating this to be a problem?


Bud pretty much nailed it - only a powered damper would really help do what you are envisioning - I take it this unit only runs for part of the time? If it runs 24x7 then it is completely unnecessary & one could even argue probably unnecessary even for the short periods of time the unit is off unless connected to an HVAC unit which in that case per codes requires a mechanical damper

I could see flipping a damper and placing it in the duct work inside but the only catch with that is how much would it affect the amount of air being pulled in & are you creating any moisture / icing problems  

I would use an in-line backdraft damper such as Artis Industries or the ones made by Fantech and other ERV/HRV manufacturers.  The Fantech ones are mildly spring loaded but have a low pressure drop that I have measured at less than 0.04 inches water column. The Artis brand are gravity operated and even when installed level, the center post hinge axis is tipped back so gravity does the job.  If the duct runs uphill a bit, all the better. Remember these are positioned in direction of flow. You want to install them as close to the exterior as possible and insulate the ductwork from the building exit including past the damper. Being outside air ductwork, you want to insulate it all anyway, but on the exhaust, at least from the damper to the exterior.



Bud and Sean both asked a good question though, what is the duty cycle and on vs. off hours?  Typically we know that an ERV/HRV to be cost-effective, requires higher hours of operation. If manual cycle duty (e.g. a bath fan), I would use a direct exhaust fan for what might be a half hour a day.

I prefer simple and install these regardless and have not had reported problems, but then again, the systems operate for more hours than not. Not a lot of off-hours to show up back-drafting.

But yes too, you can install motorized dampers. Adds some complexity, interlocking and the question of if the damper is powered open/spring closed, which is what I think you would want. In a power failure the damper would fail closed.


Talk to Paul Raymer , he is contributor to this website.


I have made my own but some work better than others.    You can take sail skrap and fold in a way to make a one way damper.   Jump up to 7" - 8" so the presser drop is not to great.   Eric Kjelshus Energy HVAC.

First,  What is the 24/7 CFM requirement for your house?  

Second,  What is the CFM rating for the unit?

The most efficient ERVs run closest to 60 minutes out of each hour.  Buying a unit that moves 70 CFM is less capital outlay then one the moves 150 or 220 CFM.  45 - 60 CFM is a good range for many new homes.  If you are getting above that as a requirement, then does the house have 2 HVAC systems?  You should then consider going to two ERVs, if you plan to use the HVAC ductwork to distribute the air.

A better design strategy in larger homes would be to separate the systems. If you have a high performing building enclosure (below 5 ACH @ 50 blower door and above code insulation)  the shoulder seasons benefit greatly from an effective ERV distribution and collection.  Keeping the house comfortable when the inside temperature swings from 67 - 74 degrees for a week and the HVAC does not run is very energy efficient.  I see that when we have outside temperature swings from 50 - 80 degrees during the 24 hour period.

Over-kill unless you are in an arctic environment. 

Back flow preventer is a device used to protect potable water from the "back-flow" of non-potable fluids that may contaminate a water supply. 

Damper is the term you are searching for. 


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