How could we harness attic heat?

I'm doing some work in my attic this week and noticed an opportunity. It's 50 degrees outside and a borderline chilly 63 degrees in the house with the wood stove not running. But in the attic it is a toasty 80 since the sun is out. I would guess that for 3 months of the year (spring and fall) we have heat in the attic that could be of use in the dwelling underneath. I posted this in "passive solar" but if there's a better forum, please move it.

Is there a way to harness this warmth to heat the rooms below while meeting normal fire and building codes?

The one idea that comes to mind would be to install a blower and run 200-500 feet of uninsulated metal flex pipe back and forth in the attic. Suck air from the ceiling of one room and blow it through the pipe and down through the ceiling of another room. Only conditioned air would travel through the pipe. Since it would be warmer in the attic, I don't think condensation would be an issue.

Anyone have better ideas? Or issues with this? Maybe the electricity to run the fan wouldn't harness enough heat to be worth it?

You could do it the way you mentioned, but unless your attic is very dusty or something, you could just run a duct from the attic to wherever you want supplemental heat, and you could just use a small duct fan to circulate the heat from the attic to that room. It would be easy to add a dust filter to the end of the duct in the attic if need be. That way you're only running a duct for 10 or 20 feet possibly. The fan could be a much smaller and less expensive one. Since the attic is warmest when the sun is out, a small Harbor Freight (or the like) solar panel could run the fan, or charge a battery to run the fan. If it was set to run with a thermostat, you could have the fan shut off at 70 degrees or so.

There's a guy at Iwilltry.Org who's doing something like this. I found him through Makeitsolar.com and they have similar projects there.

I wonder if you could just run a whole house fan in reverse. Special louvers might be needed, or perhaps just a jig of some sort that keeps them open when the heat from the attic is being blow into the house.

I was trying to avoid blowing attic air directly into the house. I don't know exactly why I'm trying to avoid that but I have a hunch there's a code reason or something. My attic smells a bit funky so that's one tangible reason. Although maybe Todd's filter would do the trick. I don't have critters up there now but if there ever were, I wouldn't want a good path for them to get downstairs.

I read the posts by the Iwilltry.org guy and his system is pretty neat. I hadn't thought of the cooling results of the chimney effect but I have felt that air rising as I come down through the hatch. It didn't sound like he came up with a way to heat the house (other than the temporary box fan in the attic hatch).

I don't have a whole house fan but I'd think running one in reverse would be a great option IF blowing attic air into the house is a good idea.

Luckily I have cellulose insulation in my attic so I don't have to worry about asbestos...

Code varies, hurricane code had much tighter rules for attic penetration as do the new airtight rules. Your pipe idea should pass either one if any joints are taped correctly.

Thanks R Scott! Luckily no hurricanes to worry about where I am. We do have tornadoes but that's a different kind of wind. I also asked a couple builders and insulators I know and when I hear back from them I'll post their thoughts.

Step 1) Put box fan in window
Step 2) Turn it on high
Step 3) Open attic door

I do this in the spring and fall when the attic temperature is >10 degrees over the house temperature. Works like a charm.

Once you start pulling a lot of air through the attic regularly, the funky smells dissipate.

my feeling is no, any way to harvest heat from the attic will involve some kind of pump or fan. I think I would build a solar chimney, that way you can heat or cool the house with just one moving part.

How about prewarming the water going into your water heater. The higher the temp going in the less energy needed.

Builditsolar has some plans like that. A coil of PEX in a Florida attic means almost never having to pay for heating water.
In northern states it could work as well, but you would need a way to divert from and and drain water out of the system in the freezing weather.


Concerning the need for fans,I have plans for a "green house" on the south wall of my house. Really it would be a low mass solar thermal collector, but the point is, "the attic" of the structure would be below the second floor of my house, thus hot air will be free to flow up from there int o the master bedroom.
So harvesting attic heat without a fan works fine if your attic is below the space to be heated!

That sort of set-up is easy to create if the house is built with the South Side pointing down hill. On a steep enough slope, even the basement of the heated portion of the house, could receive eccess greenhouse heat.

In areas that don't experience freezing temperatures, a tank of water can be placed in the attic. Once it is sufficiently warm, it can be drained down for domestic use. In areas that do experience freezing, a tank could be mounted on the finished  side , at the highest point of the ceiling. The room that I'm in right now , experiences up to a 15 degree temperature stratification.

 On hot days this works like an air conditioner. Instead of venting that heat, use it for water heating.
 
Condensation must be gathered.

I like the idea of using the attic as a passive solar water heater. But condensation control would be a critical item to pay attention to. And in my case, a drain system for the winter. I'll have to see how hot my attic gets in the summer. I'm not sure it's hot enough to take the place of a water heater. Even in that case though it could be a preheater for the main water heater.

In my particular case I'm planning on building a summer use batch solar water heater on the S side of the house. But I'll think about if the attic would be a better option.

The attic heater could be a pre heater for the rooftop heater. A long pipe with fins on it mounted along the highest ridge inside the attic, could be the feed line for your rooftop heater.

I like the idea of recovering solar heat. Flexible vent tubing can be run the length of the attic and back to maximize heat gain. On the intake side a small in- line fan (solar or 12 v.) will push air through at a slow speed and exhaust into a room.

Ok, I got a response from my builder/Focus on Energy expert. He is interested in the idea but he doesn't think it is terribly likely to succeed. He had some math to work with though

200' of 4" duct contains 18 ft2 of air. The hot air in the duct needs to be 110F or higher to provide much benefit. Below 110 would provide heat to the dwelling but it would feel like a not-warm breeze. He also said that the system would have to be very well sealed from the dwelling in summer and winter to avoid air moving in or out due to thermal expansion in the duct system.

The unknown variable is how quickly the cool dwelling air entering the duct would heat up to 110. (my thermodynamics professor is rolling in his grave) If the fan moves 36 cfm, it would move the air through every 30 seconds which seems like it's not enough dwell time to warm the cool air up.

My house is 17,000 cubic feet. So if 36 cfm was possible, I'd be heating 1/472th of the house's volume each minute, or 1/8th per hour. If that air was coming in at 110 degrees, maybe it would make a difference.

My attic is small (hip roof) but I could probably run the ducts in the rafter bays (up one slope and down the other). He mentioned that there would be a lot of resistance from the flex duct and any elbows. I'm imagining a radon fan could do the trick (Fantech 4" 122 CFM). This one would move 4x the air I need but the restrictions may slow down the airflow. I could split the air between multiple duct runs but it still would likely require many hundreds of feet of ducting...

I'll have to think about a way to rig up a test to see how quickly the duct air gets heated.

He also mentioned that he thought the best way to get heat into the house from the sun would be directly with a solar space heating box. Then you're picking up the direct radiant heat instead of the conductive heat from the warm attic.

I'll noodle further on a test to see if this is likely to work....

That's funny because I was just talking to my apprentice about this the other day.

I've installed 3 systems like this and have 3 very happy clients.

We used 5" and 6" fantech fans with cloth bags on the inlets and short duct runs into ceiling diffusers. Make sure to have a spring damper inline so air won't go out of the house into the attic.

The control is 2 thermostats(I save old ones for this) run in series; one in the house in heat mode and one in the attic in cooling mode. I use a 24v transformer and a relay to control the fan so we just run a stat wire inside.

150 cfm X 60 min = 9000 cubic feet of warm air per hour which is most of the air in an average house. Yep, that helps a lot.

All Blessings,
Bill

Thanks Bill, that's very cool to hear. From the way you talk it sounds like you're an HVAC pro? Does the spring damper resolve any fire blocking issues? How does the cloth bag work if it's on the inlet side? I've seen them on the output side of fans to act as dust collection bags. On the inlet is the bag just covering the 5" opening of the fan or do you have it stretched around a wire contraption to give it more surface area?

Do the customers have to open a window in the house to let out air as the fan tries to push it into the house? My house is decently "tight".

Thanks!!!

Bill Bradbury wrote:

We used 5" and 6" fantech fans with cloth bags on the inlets and short duct runs into ceiling diffusers. Make sure to have a spring damper inline so air won't go out of the house into the attic.

Bill

I think that is the key to the whole thing. You need to use short ducts and pump the air directly from the attic into the space you want to heat. Otherwise you will run into the issues you mentioned: If the air passes thru 200' of duct at a speed high enough to pump any significant amount of air into the other areas of the house, it isn't in the duct long enough to warm up.

From the way you talk it sounds like you're an HVAC pro?

Yes, HVAC pro in a previous life, now I'm into historic restorations and natural building.

Does the spring damper resolve any fire blocking issues?

Yes, but the backdraft issue is more important

How does the cloth bag work if it's on the inlet side? I've seen them on the output side of fans to act as dust collection bags. On the inlet is the bag just covering the 5" opening of the fan or do you have it stretched around a wire contraption to give it more surface area?

The cloth bag is just a filter.

The system should be wired in series so the fan comes on when the temp indoors is below the heat setpoint and the temp in the attic is above the cooling setpoint.

All Blessings,
Bill

I agree that directly pumping filtered attic air into the house would give the most heat with the least amount of effort. My Focus on Energy guy reminded me of the toxic gick that can be in attics so that has me reconsidering. We have cellulose but apparently there are flame retardants and sometimes rodenticides in it. Plus I'm not sure what the air quality of 110 degree attic infrastructure is. Thinking about the gasses that would come off of the osb, insulation, etc and then get blown into the house. Maybe the filter would take care of it but I'm not so sure. I'm not saying it's bad for certain, I'm just getting leery.

I did check the attic temp today and it peaked at 115 on a sunny 63 degree day. So there's plenty of heat available.

I think due to indoor air quality misgivings, I'll probably still lean towards the contained duct. I'll have to run an experiment with a fan to see if that has a hope of working...

Thanks for all the input folks!!!

With the controls that Bill describes, air in the ducts can be heated in "batches".
Once one the air in the ducting is too cool to help heat the house, the system would shut off, allowing the air remaining in the ducting to heat up to useful temperatures again.
A similar control system could work for underground thermal storage in green houses, or for a forced air rocket stove located outside of the main building.

Bill, I have wanted to make a system like you described, but finding the right contactors (?) has been beyond my know how. Can you offer any direction?

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.

Silas, Welcome to Permies, great first post! Some real world experience is nice to see. I also thought it might be able to do some cooling on summer nights, even if only during the wee-hours, it would be great to have that automated, versus manually opening windows before bedtime (when it might not be cool yet, or be noisy).
Our attic needs attention, squirrels have gotten in, insulation needs replacing and upgrading, and storage up there needs going through.
Now I know what I'm going to do with the HRV unit that I salvaged from the transfer station last year...

Depending where the fan motor is located within the HRV/ERV is it might also contribute it's waste heat to the effort...

A "solar" retrofit of this type is also great because of its "stealth" factor. Hidden from view in the attic is great in an HOA situation. It is also protected from the weather and UV, and accessible from within the house, not via ladders to the rooftop. AND, it doesn't inhibit use of the the rooftop for other solar collectors!

I know this thread is old, but here is another Idea:   pumping the hot air from the attic into the crawlspace.

What we have done is this:

- Its a california, bungalow-style house ~1000sqft.
- we connected the attic to the crawlspace with 2x 6" ducts + 40W fans.
- made sure, the attic-side pulls the air from the highest point of the attic, to make sure we get the hottest air.
- we have a vented crawlspace which gets pretty cold during winter (5-10°C).
- I am constantly measuring humidity & temp in crawlspace & attic.
- max temp diff in winter between attic & crawlspace is about 30°C.

We simply push the air into the crawlspace while not even closing the vents. When temp diff is above threshold. The pressure from the attic is
enough to keep cold air out from our 4 crawlspace vents.

This heats up our crawlspace by about 5°C. This is noticeable inside and basically feels like floor heating in the entire house. Its really nice .

I haven't done any calculations on it or anything. Its mostly about how it feels like for us right now.
I am sure a lot of improvements could be made on this. E.g. closing some of the vents in this situation.
But first need to make sure that doesn't cause crawlspace air to flow into the house through cracks in the floor.
The advantage of it is that its really low effort to do this. and you don't get in touch with the air from the attic. I have checked,
if there is any increase in humidity in the crawlspace when doing this. But the humidity is decreased,
because of the heated air from the attic. And the air from the attic is really dry as it is the
same outside air as the one from the crawlspace.