# ERV duct sizing

| Posted in Mechanicals on

I am deciding whether to use 4″ or 6″ ductwork for my fresh air inlet and stale air exhaust lines running between an ERV and the outdoors. The inlet line is a straight 8′ run and the exhaust is a 12′ run withÂ  two 45 degree elbows. Using a 6″ line would mean less air resistance in the duct, but it also means a much bigger hole in my thermal envelope (roughly 28 square inches per 6″ hole rather than 12 square inches per 4″ hole.) With such short duct runs, I am leaning towards going with 4″ ductwork. Or would there be any benefit to using 6″ ductwork and reducing it down to 4″ just before it exits the building?

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### Replies

1. | | #1

Your heat loss is not due to the size of the hole in the envelope. It's the surface area of those ducts running between the ERV and the wall. It's outside-temperature air inside those ducts. I'd be inclined to insulate them really well, and use 6", but you could also look at the static pressure specs for the unit and do the appropriate calculations.

2. | | #2

Thank you, Charlie, for your response. That makes sense that the surface area of the ducts is where the heat loss takes place. Do you have a resource to recommend for calculating static pressure in ducts?

3. Expert Member
| | #3

Really depends on the CFM of the unit, especially in boost mode.

I run a 100cfm ERV through short runs of 4" duct to the outside, in my case it works but just barely. I get 95% of rated CFM under boost mode but the ducting causes 0.4" of pressure drop. This is about 1/2 the max pressure the ERV is capable of.

For most ventilation for smaller 1 or 2 bed house you are looking at min 5" duct, for larger 4 bed 6" . Short run or not, there is only so many CFM you can pull through a small duct.

4. | | #4

Locating outdoor air ducts in interior space is like locating indoor air ducts in unconditioned space (eg an attic). Consider moving the ERV to the wall.

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