Circulating heat in cathedral ceiling to bedroom in basement
For clients, I have built an addition on a 1960 A-Frame (Vermont, zone-6) which includes a basement master bedroom and main floor dining room and mudroom. The existing house has a forced air heating system and wood stove. The later is the primary heat source. The cathedral space of the A-Frame collects a significant amount of heat. The addition is reasonably well air sealed and insulated, R35 walls, but the existing is a wild-card. Currently, the kitchen has an exhaust only range hood and the bathrooms rely on window opening and have no other ventilation. Could I expect an ERV with a stale air exhaust placed in the cathedral space to transfer enough of the available heat to the basement master bedroom fresh air supply?
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Hey Philip,
I'm not sure what you are planning to do. ERVs exhaust indoor and and replace it with an equal amount of outdoor air, exchanging energy (heat and moisture) in the process. Some people install anti-stratification systems to send hot air that has risen to the top of a house down to the lower levels, but these are usually closed indoor systems. Can you describe what you are planning to do so we can offer and information or experience that may be helpful?
Brian, thanks for the leading question. I am not familiar with anti-stratification systems but that sounds like what I truly need. Without information from a blower door test (owners have been resistant to the expense/invasion of a deep energy retrofit) I do not now if an air exchanger is appropriate.
An anti-stratification system, as I have seen them installed, is a pretty simple thing--a dedicated duct with a blower to move the air. Take a look at this FHB article:
Antistratification system
I think Philip is thinking of getting double duty out of the ERV. If he pulls the warm exhaust from the top of the cathedral ceiling, it will transfer its heat to the incoming supply air that he's sending to the new master bedroom. If he needs an ERV, that approach would probably work but he'd have to do some math to see if it would be enough heat, or try it and see. A bit of supplemental heat later might not be too difficult to install.
Philip - you should also be aware that this system will also pull heat out of the top of the cathedral ceiling and deliver it to the bedroom in summer - potentially overheating the bedroom unless the ERV isn't used in summer.
I think you’d be best off with a simple air circulator. An intake vent located high in the heated room, a simple fan like a duct booster, and an exhaust vent located low in the room needing to be heated. Allow for crossflow by putting an “out” vent on the opposite side of the room you’re trying to heat, with that “out” vent connecting back to the upper floor heated area. This way you’re taking the hottest air near the ceiling of the heated room, circulating that air through the lowest level, and returning it to the cooler floor area of the heated area. That’s probably your best option for a passive system moving warm air around. You could even use a thermostat rated for AC power use (these are common in industrial buildings to run unit heaters, you can get them grainger) to turn the duct booster on and off to regulate the temperature in the lower level room.
Bill
Unless the difference in air temperature is something like 40F or higher, the efficiency efficiency of moving air in ducts is pretty low, but even the amount of heat moved at the cfm of an ERV/HRV is pretty tiny.
At 20 cfm and a 30F temperature difference you're talking only...:
20 cfm x 60 min./hr x 30F x 0.018 BTU/cubic foot = 648 BTU/hr ....
....which is about the amount of heat emitted by two non-copulating conscious human adults sitting on a couch watching a K-drama, or two PASSED OUT adults lying down on the couch, plus one sleeping Cocker Spaniel.