Hi -

We have a passive solar house with a sunroom built on the entire south face of the house. (Plans are at http://babel.hathitrust.org/cgi/pt?view=image;size=100;id=umn.31951p007429573;page=root;seq=156;num=147 if you're curious.) Between the sunroom and the main living space are three sets of French doors.

On the (admittedly rare) sunny days in a Michigan winter, the sunroom does get to a nice 65-75 degrees. Temps peak around 3pm then drop off pretty rapidly, especially in winters like this one where the thermal mass is quite cold. What would be awesome is if there were a way to open a door/vent/window into the living space when temps get over, say, 65, and then close it when it dropped back below 65.

Anyone have any ideas?

Major requirement: this can't let a ton of air through when it's theoretically closed. That's the problem I've seen with most greenhouse-type vents; they just don't really get airtight (or anywhere close) when they are closed. And, of course, you have to be able to shut off the auto venting in the summer when you're trying to keep the house interior cool.

Things that can change: The French doors could be replaced with some combination of operable windows/doors. I'd like to keep a similar amount of glass, though; the house is fairly dark to begin with and all the glazing is on the south side (into the sunroom), so I don't want to lose much of it.

Thanks for your ideas!
Emily

Howdy Emily, welcome to permies.
I do not have much experience with this but one of my uncles cut a hole in his wall and installed a solar powered fan that would go on and off with a temperature switch. Would something like that work?

Do you know of any fans that can truly be sealed off when not in use? The ones I've seen for greenhouse ventilation let the wind blow right through, even if they have louvers.

Emily,

Two years ago I added a south facing hoop house to our house. Last winter we harvested greens all winter and often had temperatures in the HH over 80, so I spent this summer designing and building a system to automatically bring the warm air in when the sun was shining. I used two fans (VenTech DF8 8" Duct Fan 400 CFM): one to bring the warm air in and one to take cool air from the floor of our basement back to the HH. At the end of the 8" inlet pipe and the end of the outlet pipe I used backdraft dampers (Fantech RSK 8 Backdraft Damper 8" Duct). The fans were controlled by a thermostat that my electrical engineer son purchased on ebay for $10 and installed for me. It all worked great in the fall. However, this winter in Northern MI is very different! All the greens in the HH are limp and close to death. Temperatures in the HH while 10 degrees or so warmer than outdoor temps are rarely above freezing. No greens this winter. The backdraft dampers are not absolutely positive and do allow some air through when the HH is pressurized by strong winds. I ended up sealing the entry until things start warming up. In my research I did find thermostatically controlled backdraft dampers, but they were beyond my budget.

Good luck - Curt

Hi, Curt! Thanks for the info - I will have to look into those backdraft dampers and thermostats.

Emily

Greetings and welcome Emily, thanks for sharing your project and question. Same to you Curt, would love to see pics if they are out there. There is probably no easy solution here. I would advise to be very careful about using grid-tied circulation as the fan's energy consumption often exceeds the heating energy saved with most systems like these. PV powered fans are a natural fit for projects like that as the free, power is used in a passive fashion when you need it most. Curts fans suggestions look good for AC current and there are many exciting, ultra efficient fans coming onto the market. Anybody know of some similar DC powered fans that could be hooked right up to a small PV panel?

This passive solar design is very unconventional by today's standards. I think the problems you describe are typical for homes that ignore modern passive solar design in favor of greenhouse or sunroom integrated homes. They gain a tremendous amount of heat in the day and then quickly lose it as the sun goes down and can also have the effect of bringing the rest of the house down with it. Modern passive solar design keeps a more modest south glazing ratio to square footage and focuses on air sealing and insulation to keep the heat in as long as possible.

I suspect that your house is ripe for some very cost-effective weatherization efforts. Namely, air-sealing everywhere and adding insulation in the attic. Folks looking to build new should see Emily's problems as an opportunity to improve on. Blower door directed air sealing could have made a world of difference on the performance of this home. Its not too late of course but its much harder and less effective once things are built.

I like Curts suggestion but be careful about the energy loads of the circulation devices. I suspect that going through the trouble to try to actively circulate the air above 65 will not result in measurable savings. I suppose tearing off the sunroom/greenhouse and doing a more modern south facing wall is out of the question?

Brian,

I never considered adding in the cost to run the fans. I was mostly trying to keep temperatures in the HH comfortable for the cool season greens. The house seemed like a good place to put the extra heat. Because I know very little about Kwh and other electrical issues I emailed this question to my engineer son: "I've been in a forum discussion about the cost of using fans to move heat from hoop houses and greenhouses into homes. Basically, the question is: Is the cost of the electricity worth the value of the heat moved? I have no idea how to calculate the value of the heat, but I tried to calculate the cost to run those two 40 watt fans for 8 hours. Here's how I did it:

2 fans at 40 watts each for 8 hours would use 640 watts. 640 watts would be .64 of a Kwh. Our rate is 0.08640 per Kwh so .64 x .08640 = .055 or 5-1/2 cents for the 8 hours. Seems pretty cheap to me and well worth any 80 degree air that I can move into the house. Are my calculations correct or have I made some huge error?

Thanks for once again being my backup! Dad"

Here is his response: "I think your math is mostly right, though for more significant consumption calculations, you should use the TOTAL you pay for electricity instead of the listed rate, as all the taxes and tariffs are an appreciable addition, despite how tiny the numbers look on your bill.

In this case, you can also keep in mind that the a bunch of the energy used by the intake fan turns into heat that you get to keep, as well. Additionally, the 40W figure is probably on the high side for a relatively unconstrained situation like you have (as the power used by a fan is a function of actual work done.)

I did a little napkin math, and assuming 200CFM and a 10degree F temperature delta for the air, and it worked out around .5kwh of "heating" per hour vs .08kwh of fan use per hour.

Moving 80 degree air into your house with a fan should be a damn good deal."

The other reason I never gave photovoltaics serious consideration is because this part of Northern Michigan gets very little sun through much of the winter. In Jan. 2014 we have had less than 8 total hours of direct sun all month.

I'll to post some pictures of my attached HH in a separate post.

Hey Curt I agree that the kwh math looks correct but is 40w what you measured for power usage? I found these fans and the listed watts are 112-150 http://www.iaqsource.com/product.php?product=111070

The tougher math is the CFM, Delta T, and BTU contributions. I doubt there would be a 10 degree Delta T or 80 degree air temperature to use for 6 hours even on sunny days but it sounds like thats how you have it rigged up with the Tstat control? Is 6 hours a pretty regular fan run time for sunny days? Sounds like a great setup!

Got to point out that the cost we pay for grid electricity is a fraction of the true cost of our dirty energy usage. The $ per kwh does not account for mtn top removal, coal ash spills, poisoned aquifers from fracking, acidification of forests and streams etc..

Curt Regentin wrote:Brian,
The other reason I never gave photovoltaics serious consideration is because this part of Northern Michigan gets very little sun through much of the winter. In Jan. 2014 we have had less than 8 total hours of direct sun all month.

Here in central maine, I bet we beat you out by 4 whole sunshine hours! November and December, though, were another story

I'm rocking a 5th wheel camper with an attached hoop house - lost our pea crop in the raised bed along the exterior and the spinach/chard never made it past sprouting. Of course, this was all acidic clayey muck with little prep. We added a RMH along the interior wall and have been successful in keeping MOST of the hoop house above freezing MOST of the time, but have had the raised bed freeze up a bit several times when temps dropped below -10*F for a couple nights. We "insulated" the hoophouse with bubblewrap as well. The RMH keeps the interior wall A LOT warmer though - definitely was a smart move. We've used less than 300lbs of propane over the winter so far - about $200 worth - and have kept it generally 65-75*F at all times indoors.

Back in the day, I set up a similar system using 12V computer fans on styrofoam board, mounted in the windows with makeshift backdraft dampers, and an old battery powered programmable AC thermostat (when temps in the solar collector got over 80*, fans would kick on to blow air inside the house). The collectors were simple boards painted black hung under the windows on the exterior with plastic glazing and a 1-2" air space at the bottom. System worked beautifully except when the winds were kicking hard. Just add sun!

We've been thinking about setting a similar system up again coming up on warmer/sunnier weather but haven't gotten around to it. On a good sunny day, we can often see 85-90* air temps out there, but they're so rare that it doesn't seem worth the effort this year

The venting systems aside you might want to consider building some insulated thermal mass into the room to help carry that heat through the night. Night time is probably where you are seeing the most loss.