Don't confuse ERV/HRV with make-up or combustion air.
ERV/HRV are ventilation and indoor air quality devices that EXCHANGE air. Some of the units with 2 fans give you the opportunity to create slightly different pressure environments by varying the speed of the supply/relief fans accordingly.
ERV/HRV do not (at least in my opinion) take the place of exhaust fans in bathrooms or kitchens. These exhaust fans are typically low volume that do not run for long periods of time and have little effect on the overall indoor air pressure environment.
If you have a combustion appliance, you really need a direct source of outside combustion air into the plant room.
If you have a fireplace that is used a lot or have a BIG gas range with BIG kitchen hood, you probably need additional make up air. Conditioning this make up air costs money, however.
What you want to avoid is negative pressure environment that backdrafts combustion appliances and sucks in outside air and moisture through the building envelope. I think a slightly positive pressure environment is beneficial, so when I do this in a couple years, I plan to use a 2 fan ERV and dial in more supply than relief.
The upside of the direct duct connection from outside to the return air plenum of the furnace/air conditioner is that it creates a positive pressure indoors, at least while the unit is running. The downside is that if you don't at least run the fan, you get no outside air and there is no heat exchanger or additional filtration.
Bruce |