ERV and flex duct
We’re planning to install a Renewaire EV90P ERV with dedicated ductwork in our new 1900 sq ft home. It’s going to be a budgetary strain to install hard pipe for all the entire system. So I’m hoping we can get away with a rigid duct for a trunk (running parallel to floor trusses) and then 6 inch flex duct going out to the exhaust and supply registers (through the webs of the floor trusses) – this is what the HVAC contractor is recommending, but I’m concerned about this small unit being able to handle the additional resistance to air flow from the flex duct. We will probably have three return air grilles (in 2 bathrooms and the laundry/utility area) and five supply registers going to a great room downstairs, a kitchen area, and to each of 3 bedrooms upstairs.
Ventilation performance for the EV90P ranges from 108 CFM at 25 Pa ESP to 42 CFM at 150 Pa ESP. I have no idea personally how to determine if the increased resistance from using flex pipe is something we need to be concerned about on this scale. Our HVAC company has a great rep for tight ductwork but I’m not sure about how they design the ductowork for the ventilation systems. Should they be running a Manual D for this, just as for the main forced air system?
The long and short of this is … can I assume we’ll have a decent, workable system with a rigid trunk and flex duct for the branches. If it’s somewhat less efficient than the ideal all-hardpipe system, I can accept that. But I don’t want to spend all this money and end up with a system that’s not ventilating as promised.
Thanks in advance.
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