Using an ERV to Collect Attic Heat
I’m curious for people’s thoughts on a little experiment I just installed as part of a DIY pandemic energy retrofit of my 1950’s little condo in east SF Bay area. It’s a little to early to give a definitive estimate of its impact, but I think it’s looking good from an energy standpoint! 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, keeping the air streams separated to avoid any contamination from the attic. 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.
Now whether it is code, or even considered in the code, I have no idea…
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Proposterous,
I'm a little confused, how is your interior space cooling to 65f, but your attic is still 85f? It seems to me that your unconditioned attic would cool quickly and heat up quickly, so the only time this would work would be for a short time in the early morning when the sun heats it up....maybe 1-2 hours per day. This seems like a really neat idea, not sure about code safety....but interesting.
How many hours per day are you gathering heat?
Shawn
@Shawn, not sure why you think it's preposterous. My attic stays very warm all day when the sun is shining.
Hi Brad, his name is proposterous.
That's an interesting configuration of an ERV, using it as an air-to-air heat exchanger, but without the ventilation function.
Why an ERV and not an HRV since you don't need enthalpy recovery in the Bay Area climate? I'm also in the East Bay and have an HRV.
Given your numbers, the idea seems to make sense. What I don't understand is the numbers themselves. If the outdoor temperature is 67F, why would you need any heating at all? Even in a code minimum house, it's assumed that waste electrical and body heat will maintain desired inside temperatures down to 65F outdoors. This is why they use 65F when calculating heating degree days. In a well insulated house, that outdoor temp cutoff drops even more.
I also echo Shawn's confusion as to where this attic heat is coming from. It's got to be either solar gain, or coming from the house. If it's the latter, I strongly suspect increasing the attic insulation would be far more efficient a solution than the ERV. If it's solar gain, I really can't imagine it's going to be useful much of the time. If the sun is beating down that hard on the roof, it's also giving a ton of heat to the house through the windows (and some through the walls). In my house, when it's sunny outside, the heating system doesn't run. Like, ever. It can be -20C outside, but the heating stops from around 10AM until sundown. If the house itself isn't getting that much solar gain due to clouds or whatever, neither is the attic. At night time, when the house has the largest demand for heat, the attic is going to be colder than the desired house temperature, aside from a very short transition period at sunset.
hi all,
I will try to clarify. I actually have some numbers to back this all up, and I'm kind of excited about it. :)
(To answer Dan's: question, I chose an ERV mostly because the EVPremium L had the size and efficiency I wanted for this application, at a not too outrageous price, and I didn't want to even think about installing a drain from the attic. I have recorded RH of the attic, and it is not much different from the conditioned space. So far no problems with excessive dryness or humidity in the home.)
First, in the mild winter here, outdoor temps frequently range from around 40 at night, to mid-high 60's in the day, on a sunny day. So, the uninsulated/unconditioned attic will cool off to about 40 by morning. Meanwhile, the apartment, which I have insulated and sealed as well as I can, will slowly drift down to somewhere in the 65-68 range. (The attic floor is sealed and insulated to about R50)
The sun comes up and starts heating the roof. The attic warms to a useful temp of 72 by about 11am on a clear day. The ERV starts running, and pumps down progressively more heat as the attic heats up from the sun, and the temperature differential between the attic and the conditioned space increases. At the peak around 3pm, with the ERV running, the attic temp will reach anywhere from 80 on a cooler day, to high 90's on a warmer day. The heated return air to the apartment will peak around 76-88, all depending on weather. As the attic slowly cools into the evening, the ERV's heat transfer tapers down again, until it shuts off either when the attic falls below 72, or by timer at 7pm. Or, if the room temp rises above 74, which hasn't happened yet.
Sooo..... Yesterday I tried to calculate what I was actually gaining in Btu's. It was a sunny day, with an outdoor low of 40 and high of 68. I have SensorPush bluetooth sensors hanging from the intake and return vents in the conditioned space, that record once per minute. The ERV moves about 280 CFM when it is on, so using the equation:
Btu/hr = 1.08 x CFM x deltaT
...I am able to estimate Btu's on a minute-by-minute basis by comparing the temps of the intake vs the heated return. I could post a spreadsheet if anyone wants to see, but I came up with a total gain of 26,408 Btu yesterday, from 11am to 7pm. (It heated the space from 68 to 73.) The ERV uses 175 watts and runs for 8 hours, which totals 1.4 kwh. So comparing that to resistance heat at 3412 Btu/kwh, I get an overall COP of 5.47 for that day. At the peak, the COP was up to 8. This seems very good to me. During the current shoulder season, on a moderately sunny day, it is all that is required to keep the apartment between 68-73. (I have previously installed a separate HRV in the apartment, to ventilate at about 30CFM.)
I think this is only likely to work this well in a mild climate, with a fair amount of sun, with an attic that is large relative to the conditioned space. But I suspect there are a lot of older homes with similar characteristics around here. Since my little apartment only has 3 windows, and is now pretty well insulated, it actually doesn't get much direct solar gain. It can stay cold inside when it is warm outside. The attic ERV efficiently bumps up the solar gain of the conditioned space in a controlled fashion.
Even in colder climates, if you had a big enough roof/attic, and a big enough HRV/ERV, you could probably see meaningful supplemental heating. What do you think?
I hope I've explained it well. Please post further questions & corrections, and I can post the actual data if anyone wants. For now I'll post a screenshot of the temp curve for the heated return air. The big green parabola in the middle is when the ERV is running. The down slopes before and after it are room air temps when the ERV is off.
best,
Silas