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Factoring in Duct Length in ERV Selection

MSSUSR9501 | Posted in Mechanicals on

How do I take duct length or equivalent duct length into account when looking at ERV flow rates and static pressure tables?
Looking at worst case duct run from north basement bath to ERV in attic on south end, the length is roughly 220 equivalent feet of duct.
This assumes 6 inch round, one short run of 8″ oval with taper boots on each end, a 6″ tee,  a couple of 6″ elbows and a 6″ 45 bend leading into the ERV.  An online calculator  using 75CFM flow rate suggests my 220 foot equivalent run represents a little over .1 inch of water gage friction loss.

The Panasonic FV-10VEC20 indicates exhaust rate of 96CFM at .1 inch static pressure but they don’t indicate whether this is .1 inch of mercury gage or .1 inch of water gage.  At 0.4 inches static pressure the flow drops to 95 CFM so this unit is relatively insensitive to static pressure at those levels.
Some other units seem to be way more sensitive with meaningful declines in flow as static pressure goes up.

The exhaust runs are obviously much shorter, perhaps 20 feet plus 3 elbows.  Are the ratings assuming pusher fans and the numbers indicate air flow when pushing against the given static pressure?

Am I overthinking again?

Thanks for thoughts and comments.

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