This is an old thread, but here is my solution to this problem:
After air sealing and insulating the 530 ft apartment, I continued to be irked by times in the heating season when the attic would reach 80-90deg, while the apartment was at 65. I finally ran some back-of-napkin numbers that showed that an ERV could be used to efficiently transfer attic heat into the conditioned space.
So I bit the bullet and gave it a try. I installed a RenewAire EV Premium L, (largest most efficient core) and ducted it so that the attic air and the conditioned air run separately thru the ERV. Basically, the ERV pulls room air up, heats it with warm attic air, and returns it to the room. MERV 13 filters are installed in both streams, and the attic insulation is covered with plywood/plastic which prevents it from kicking up dust. The ERV is controlled by two thermostatic switches in series, so that it comes on whenever the room is below 72, and the attic is above 72. In practice, this results in an starting temp differential of about 5 degrees minimum, since I let the apartment cool to 67 overnight. That differential quickly climbs as the sun shines.
Right now, for instance, at 1pm on a partly cloudy day, outdoor temp 67, attic temp 88, the system is intaking room air at 69 and returning it at 81, at a rate somewhere around 280 CFM. That comes to about 2400Btu/hr, and a “COP” of 4+, given a fan consumption of 170W. In the peak of a sunny day like yesterday, when the attic got up to 95, theoretical yield could be 5300Btu/hr, and “COP” close to 9.
This sometimes won’t be a sufficient heat source. But for the shoulder seasons in this temperate sunny climate, in a small well-insulated/sealed space, I’m guessing it will be all that is necessary. In fact, my “primary” heat source may become the supplemental one. I’m not sure how scalable this idea is, but for older buildings with an accessible clean attic and a well insulated attic floor, it seems possibly worth considering. Obviously it is not cost efficient at today’s energy prices. But it is extremely energy efficient, and taking a longer view of the future costs of GHG emissions, it seems an interesting way to partly solarize an older building. Based on my temp records, I think it will also be able do some cooling at night in the summer. Total materials were around $2500, plus my labor.
