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One or two HRVs?

I recently purchased a ~3000 sq. ft 2 story colonial built circa 2001 in the seacoast New Hampshire area.

It has an unfinished/unconditioned basement. It has a unfinished walk up full height attic, with only the front roof requiring a knee wall.

I plan on converting the attic to conditioned living space. That will add ~5-600 additional sq. ft.

The house has hydronic baseboards. There is no A/C nor HRV ductwork.

There are currently 6 occupants, three adults, three children 10 to 4. They are home schooled, so the house is occupied (and active) most hours of the day. It's also more common than not to have a half dozen more fellow moms and children. In other words, a relatively high occupant load.

I wish to add an HRV/ERV because (1) I already feel like many of the rooms need fresh air (2) I am air sealing as I room-by-room remodel and (3) I live in the radon prone granite state (tested less than 4pCi/L a few months ago) and given I'm making the house tighter, adding ventilation seems wise to prevent any higher levels.

I am planning to open walls as necessary as I remodel to install both A/C ducts and plan a dedicated duct system for the HRV/ERV.

My question is this - with three, and potentially four stories if I bring the basement into conditioned space in the future, is it practical for a single HRV/ERV to distribute air across all those stories when situated in either the basement or attic?

My concern is that on its face pulling/pushing air up those tall vertical stacks will compromise air delivery. Also, it's easier from a retrofit standpoint to drop the ducts for the 2nd floor from above as I finish the attic, rather than bringing ductwork for all three floors up through the walls from the basement. I am sensitive to the fact that two exchangers must be small or I run the risk of over ventilation, to say nothing of the additional cost.

Thanks for your insight and feedback!

- Matt

Asked by Matt Zahorik
Posted Wed, 11/28/2012 - 18:53
Edited Wed, 11/28/2012 - 19:03

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1 Answer

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Matt,
There are at least two issues here:

1. Can one HRV or ERV handle the air flow requirements for your house?

2. Will it be possible to install the necessary ductwork to all the required locations?

First, you have to calculate your air flow requirements using the ASHRAE 62.2 formula. (To learn more about ASHRAE 62.2, see Designing a Good Ventilation System.)

A house measuring 3,600 square feet with 6 occupants needs 81 cfm.

If you want to increase the capacity of the ventilation unit because the house often has more occupants, you can perform the calculations for a 3,600 sq. ft. house with 12 occupants; in that case, you'll need 126 cfm.

So, are there any HRVs or ERVs out there with that capacity? Sure. Check out the HVI products directory to look up dozens of HRVs and ERVs that would meet your needs.

The second question -- ease of ducting -- depends on site conditions. Only you can decide whether it's easier to run ductwork from one unit or from two units.

Two units will cost more than one unit. But if purchasing two units simplifies the installation of the ductwork, you may prefer that option.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 11/29/2012 - 06:48

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