Air circulation with Ductless minisplit and Spot ERV
I’m in Seattle and will be tightening up and insulating (REMOTE method) the existing 112 year old house that we just added on to. We have a ductless minisplit in the main room that has worked okay for the whole house since the addition. In anticipation of a tighter house, I plan on installing a Panasonic Spot ERV or whole house ERV. I’d rather stick with just the spot unit for simplicity and ease of install. If I do a ducted ERV/HRV, all insulated ducts must go through the vented attic and I don’t have an ideal placement for the main unit in conditioned space.
Question: without mechanical ventilation to the three bedrooms, will there be enough air movement to get fresh air where it needs to be, especially at night? Would an exhaust fan from the main heated and ventilated space to the bedrooms be worthwhile? We generally keep doors open but I can see the kid’s room sometimes having the door closed at night. We have hardwood floors so there are gaps under each door around an inch (not sure how much air moves under doors).
Thanks!
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Replies
Pat,
People have different sensitivities. The usual worries are too much condensation on cold window glass in winter, and odors. Use those indicators to guide you, unless you have other issues that worry you.
The traditional way to address a bedroom that feels a little stuffy is to open a window a crack when you go to bed. Some people do that every night -- others, never.
If you are sensitive to these issues and reluctant to open a window, you may want to install a ducted HRV or ERV system that delivers fresh air to your bedroom. But it's up to you.
In Seattle window condensation isn't very likely. Even at Seatlle's ~25F 99% outside design temp a code-min low-E window would need 75% RH to have copious window condensation.
An inch of door undercut on a 28" wide door has more than twice the area of a 4" round duct, and thus more than sufficient return path for ventilation air flows using small & efficient bath fans for moving air around, if needed.
Pat,
Note that Dana's suggestion -- a door undercut -- won't help much with air circulation unless there is a fresh air register in the bedroom and an exhaust grille in a room on the other side of the door.
The question about installing small exhaust fans to supply air to the rooms was followed by asking how much air moves through a 1" door cut.
I assumed (perhaps erroneously) that the question about the door cut was whether it was a sufficient return path for the postulated exhaust fan, but it's good to be explicit. Air doesn't move much on it's own, without a driving force such as convection or fans. An open door can move a lot of air with even minimal convection drive- a door cut not so much. But it's PLENTY of path for an energy sipping ~2-watt 10 cfm fan like the Panasonic FV-01WS2, or a 20/40 cfm like the FV-04WS2 (or even a 200 cfm ventilation fan.)
Thanks very much for your inputs, Martin and Dana.
I think I'll go with the spot ERV (already have one uninstalled) for fresh air and plan to keep doors open. If we find the need to keep doors closed for some reason, I'll install a fan to move the air into those spaces.