December 11, 2023

Greenhouse Ventilation

In case you missed it, here's the last greenhouse photo I shared.

Photo from Garden Notes: November 2023

At the top of the greenhouse roofline, a vent covers a solar attic ventilation fan.

We finally assembled all the pieces needed to hook it up to solar, and now have functional ventilation in the greenhouse.

It's powered by a 12-volt 100-watt solar panel on the roof above the greenhouse. No battery, so it runs when there's sun and doesn't when there isn't. Fan speed is regulated by the amount of sun hitting the panel. This is okay, because I need ventilation when the sun is bright and heating up the greenhouse more than I want! This is ultimately why my hoophouse failed for winter growing. It's not uncommon to get 60°F (16°C) days or warmer during winter here, which meant the hoophouse got too hot for cool weather plants. I had no means of ventilation other than opening the end flaps. This wan't effective, and everything bolted! I'm hoping the vent fan will help with that.

If it's cold out and we don't want to lose warmth, there's a switch to turn it off.

The solar panel is wired to the switch,
and the switch is wired to the fan.

The fan will be on during warm winter days and probably run all summer. Not because I plan on using the greenhouse for plants then, but to help vent hot air stacked up right next to the house and warming up my studio/sewing room!

The jalousie window on the backside of the greenhouse provides cooler outside air for the draft.

There's more shade and hence cooler air on this side of the greenhouse, so it makes sense to draw from this side and vent out on the hotter sunny side.

The big question was, will it work?!? The next day we had a chance to test it out. After lunch, the outside thermometer read 52°F (11°C). The greenhouse thermometer read 80°F (27°C). I turned on the fan and opened the jalousie window. Two hours later the outside temp was up to 55°F (13°C), but the greenhouse temp had dropped to 75°F (24°C). That's promising! The following day was another warm one in the greenhouse. When the temp got up to 80°F (27°C), I turned on the fan but left the door open instead of the window, to experiment. Two hours later it was down to 70°F (21°C). My conclusion is that the 80-watt fan has a good air draw.

Of our overnight lows, the coldest night we've had so far was 22°F (-5.5°C). The greenhouse got down to 32°F (0°C), with no frost or freeze damage to the cherry tomato (my only warm weather plant).

Summer cooling may be another story, but that remains to be seen. I'm not planning on growing anything in the greenhouse in summer anyway, but it would be nice to keep it as cool as possible to keep the heat from transferring to the house.

I'm calling this first year our test year. I'll keep track of temps and we'll experiment. Our coldest weather is yet to come, but we have a few ideas to try.  Hopefully, we'll learn some things about how to regulate greenhouse temperatures (both cold and hot).


Poppypatchwork said...

A while ago I got a roll of the silver stuff you pop behind radiators and used some in my greenhouse, and you asked would it work. I popped mine at the back to stop the heat through the back glass, it has worked, it attracts all the heat, I still have summer flowers blooming.

Leigh said...

Poppypatchwork, that's a good tip, thank you!

Ed said...

My parents ended up putting a cheap thermostat in the loop to control their ventilation fan and it worked really well. The problems I could see doing your setup this far north is that it would cool things way too much on clear winter days. But for your latitudes, it might work quite well. I can't wait to read your updates.

For the greenhouse I built behind our house, I just made it nearly all windows that could be open on both sides so it gets a fair cross breeze through that. Though the main purpose is to just start plants in the early spring, my MIL was able to grow plants in there all summer long these last two years. They are all heat loving plants though and not things that bolt when it gets to warm.

Toirdhealbheach Beucail said...

This is fantastic news Leigh. Very heartened that it is working so well.

I have played with the idea (but obviously never done anything about it) about getting a whole house fan in New Home. It might make some sense, but would make more sense where I grew up, where things actually cooled down in the evening and you could draw in cooler air.

Leigh said...

Ed, that's why we installed a switch; to keep the greenhouse from cooling off too much in winter. The thermostat is a good idea. As it is now, I have to monitor it manually, which is okay because I like keeping an eye on what's growing in there.

Nice that you get good summer breezes! That would help a lot. We have the second jalousie window in front, and I'm curious if that will help on spring and autumn days.

TB. we talked about a whole house fan too. Never got one installed, although even in the south they help a lot. Just getting that stacked up heat out of the house feels good.

daisy g said...

Sounds like it's working beautifully!

Quinn said...

Your greenhouse already looks like it was always there! congratulations to you and Dan on another successful (major!) project. Thanks for my birthday wishes, Leigh - and for entering the giveaway giveaway. Good luck :)

Rosalea said...

It will be interesting to hear if this ventilation system will be sufficient for the heat of summer. Your greenhouse melds into the house like it was always there.

Leigh said...

Daisy, so far so good!

Quinn, thank you!

Rosalea, summer will be the real test, but I'm not holding my breath. I figure it's better not to rely on hopeful expectations and rather just take it as it comes. I'm thinking it should help some, although probably not as much as I'd like.

Nina said...

Looks like you've planned it out well. It should be easy enough to tweak the system for efficiency once you get it up and running. When I first started using my greenhouse, I found a dual thermometer which measured both the highest temperatures and the lowest temperatures. I found this incredibly useful for seeing how my set up was working, and if it was working a I thought it was.

Leigh said...

Nina, that's a good idea. Was it a digital type thermometer? My last one died.

Nina said...

Not digital - old fashioned one metal one with 2 marker hands. Very simple and nothing to break down. Lee Valley has one called a Min-Max thermometer.

Leigh said...

Nina that's exactly the kind I want! Thank you! I'm so tired of having to replace batteries in my clocks and digital thermometers. This is perfect!