Heat Recovery Ventilation Recommendations

Brad White_9

of specific manufacturers to others in this forum. I have both a Lifebreath 200 and a Deschamps unit (second hand but of 300 cfm capacity), sensible-only units that work fine. There are others of which I am eager to learn more especially reports from the field over time.

But I do want to address your concerns about the 70 foot duct run. It is not the length but the total pressure drop that matters.

IOW: The duct size can be increased to compensate for what might be excessive when sized conventionally. What you have to work with is a given ESP (external static pressure; that which is left for your use after the internal components of plenums, filters and the exchanger are taken into account). A little up-sizing of the duct goes a long way because the pressure drop changes by the square of the flow.

And forget elbows taken as so many equivalent feet of duct; that is a rough rule of thumb method. I use the coefficient C x velocity pressure. Fitting losses are dynamic primarily, (wholesale changes in direction of mass flow) not friction (surface rubbing along the duct walls).

I would encourage this so that you can avoid installing the unit in the attic.

Another trick is, if you are distributing widely in the house, you can install a "Ring Duct" which can significantly reduce system pressure requirements. This is specific to your layout of which I know nothing so may not apply to you.


BTW: 70 feet with a few fittings is not that long a duct if you size it at say 0.05" per 100 feet and velocities below 500-600 FPM for air flows less than 300 cfm. 10-inch round or 10x8 or 14x6 would be typical sizes with these characteristics. A run such as you describe, not including the grilles, might total 0.10 inches. With grilles perhaps 0.15".

Word of advice though: Seal the duct thoroughly, every seam, joint and point of leakage, with brush-on duct sealant. (Hardcast Iron Grip I like). Leakage losses are not recoverable in terms of delivery to your spaces. I am bullish on duct sealing. Airtight is the goal, not "well enough". When you are dealing with the generally small airflows of HRV's, every cfm is precious.

Greg Sarber

Heat Recovery Ventilation Recommendations

I am building a house in Alaska and am looking for recommendations for a heat recovery ventilation system. Does anybody have any suggestions? The unit will be installed in a 5500 sq-ft, 4 bedroom house. The unit will probably be installed in the crawl space, although the bedrooms being on the 2nd floor will require a 70' duct run for the supplied air. Does this sound reasonable? If the runs are too long then the unit will have to be installed in the attic. I am a little leary about the attic installation as I am concerned about the cold attic space in the winter shortening the life of the HRV unit. Any suggestions you have will be appreciated.
Thanks,
Greg