Just wanted to say thank you for this suggestion! My first exhaust fan arrived today (6" inline duct fan, 440 CFM, which in theory replaces the air every 5-6 minutes), so I’ll be testing out the 78°/70° thermo cube for the rest of this afternoon and tomorrow, though it’s already past peak temperature today. EDIT: Pretty remarkable precision, it turned off when my temperature monitor read 69.2°F, which is close enough to 70° for me!
So far no intake fan, but I’ll probably add one soon and plug it in the other side of that thermo cube. I bought the one for heating, too, but that will probably have to wait until fall to test out.
Today was warmer than yesterday, reached a high of 76°F outside, and the greenhouse once again maintained about +11° with windows and one door open + auto roof vent. The last bump is when I installed the exhaust fan and closed the doors and windows, but since temperatures were dropping its hard to read too much into that:
I’m in the Denver area, we get such a mix of weather in April, with very intense sunlight, that I need to cover all of my options. I just put on shade cloth (25%), sides of the greenhouse can be raised. I have a mix of fans, powered automatic louvers and solar powered louvers. Without intervention a 55 degree day can raise the internal greenhouse temperature to 120 degrees. I don’t have the bandwidth to monitor it 24/7 so I use automatic temp monitors to turn the fans and heaters on. I agree with the previous post that the hardest thing to do is manage cooling not heat. The greenhouse is amazing, my goal is to avoid winter kill and get more growing/fruiting time on the front and back of the season.
That’s definitely where I’m hoping to be soon… in about 2 weeks we have a 3-week trip planned, so that’s how long I have to sort out the ventilation/cooling and automated watering (haven’t even started that yet!) if I don’t want to come home to a lot of dead plants. I already have the monitoring figured out, so if something fails and roasts my trees, at least I’ll be able to watch in real-time from thousands of miles away.
Today I tried keeping the doors and windows closed to see how well the new exhaust fan would manage, and it wasn’t long before I had my answer (not well) and opened the doors and windows again. It’s already over 70° outside, but after the morning being closed up it looks like it isn’t going to get below +15° at the moment even now that it’s all opened up again:
I have a 220 CFM intake fan on the way, hopefully that will help some as well. I doubt there is any further risk of frost, so I’ll probably leave the windows open when I’m gone if I can’t get it within my target range (+15° max) with only automatic fans and vents.
It seems a bit complicated when you mix fans, doors, windows, and louvers. At times they can be working against each other. All you need to replace all of that is one exhaust fan up high that’s capable of one air exchange per minute. Then on the opposite end a passive vent down low that allows cool air to enter. That’s easily automated and will hold inside to about 10-15F warmer than outside. If that isn’t cool enough then replace the intake with a wet wall.
The small intake fan seems like a waste. That’s not an efficient way to cool a greenhouse. It mixes the cool air coming in with hot air inside and a mix goes out.
Having an exhaust fan up high pulls out the hottest air and replaces it with the coolest air coming in the opposite end, no mixing.
Either that or forget about fans and just have large openings to exhaust hot air and intake cool air. Like roll up sides and large openings up high as hot air vents.
I also had trouble getting good air intake. I ended up using three louvered vents. I’m curious about the impact of your new intake fan.
If you get into a bind you can always cover a portion of the greenhouse with a tarp to provide shade. That should change your temperature dramatically. My 25% shad cloth drops the temp 20 degrees.
Shade is what most people resort to after having inadequate cooling. And that’s OK if you can stand the reduction in light. My greenhouse has no shade cloth and I’ve never had heat damage in 15 yrs. What I do have is two 36 inch exhaust fans and a wet wall. That only requires about 800 watts max power use to cool 1800 sqft in 100F heat. That’s 10 cents an hour at midday for cooling 1800 sqft. Seem plain cheap to me for that large of a greenhouse.
I’m not ready quite yet to admit that I’ll need to buy (and then pay $$ to run daily) a 2500+ CFM fan, but I will at least admit that it is possible that is going to be necessary.
Today I closed the doors for the hottest time of day (2pm onward), meaning just passive intake via windows and 440 CFM exhaust (replacement every 5+ minutes) + roof vent, and it was +13°F (78° peak outside vs. 91° peak inside), so I’m hopeful that I can at least add auto window openers and replicate this without requiring human intervention. But I keep chickening out and manually opening stuff when I see that mid-morning spike:
I like you methods of monitoring / tinkering with the ventilation.
I agree with fruit nuts comments.
I Like building using natural convection to cool, as opposed to using power to run fans.
This approach requires larger openings, appropriately placed .
If it was me , my next step would be to install more vents up high.
Such as the blue shaded area in photo below , on both ends.
That is where the hot air is , a larger vent there will let it out, likely without a fan.the red shaded areas are also a option.
Could add a vent down low also, below your window area.
May be able to rig them up with ropes / pulleys/ springs to work off of one controller.?
A cheep box fan connected to a thermostat is another option .
I think you would want a “over kill “ability for cooling, for those extreme weather events
At what height is your temperature probe, ? At plant height ?.
That’s an example of what I’m talking about. You are mixing methods of cooling and they are fighting each other. Using that combination the exhaust fan is pulling air into the roof vent. Those two don’t compliment each other, they oppose each other.
With either passive or fans it’s hot air out the top and cool air in the bottom. You don’t want a fan pulling outside air into the roof vent.
Passive will work, if you are there everyday and don’t forget. Fans are much easier to automate. But mixing them together probably isn’t the best choice.
My problem with passive at this point is that it takes a huge open area to move enough air. Like 20% of the top needs to be open to hold temperature rise to 15F. Your greenhouse isn’t set up for a huge open area either at the top or the bottom. Instead you’ve got beautiful solid tight covering. That begs for a fan that can move enough hot air out thru a small opening. That fan can be easily automated with a thermostat. Then an air intake at the opposite end. That needs louvers that close when the fans shut down and open when they come on, passive open if possible, not powered…
I’d keep the small exhaust fan that you have as stage 1 cooling. Then a bigger exhaust fan for stage 2 that comes on at 5 to 10F warmer.
If you do get a larger exhaust fan when you set it up think about what positioning would be best in the event that you add a wet wall later on. In your climate if you don’t get many days over 85-90F that may not be needed.
@fruitnut the small fan was a mistake. It is all new to me, just started the project last November. I thought a 12” fan would be enough to cool my hoophouse/greenhouse (6x30 feet) during a Denver winter. Our winter outside temperatures range from -12 to 75 degrees. I added the 18” fan after my greenhouse temperatures climbed into the upper 90s. My vents and fans don’t fight each other. Air moves very rapidly from east to west and the branches of my honeyberries sway in the gust. No sure if I will pull out the 12” fan or not.
I do agree that the outtake fan should be placed high on one side and the intake vent low on the other side. However, I disagree with the comments about the solar powered louvres and shade cloth. I travel for work and my biggest fear is that the power goes out on a hot Feb-Apr day (already occurred 1x in March and 1x in April) and I’m not home to raise the sides of the hoophouse to vent and everything cooks. The shade cloth cost me 25 percent of my light transmission, but gives me a added insurance. Note that I’m not trying to grow plants during the winter, only prevent winter kill and extend my growing season.
The fans only run about a fourth as much as they use to prior to shade cloth. I will only be using shade cloth March to May (switch to 80 gram netting). My end goal is to move to solar powered cooling after I get everything dialed and some new plants in the ground… LOL I’m already running out of space.
My solar powered louvre vents (one on east side and one on west side) open at 65 degrees and my electric powered fans and electric vent turn on/open at 80 degrees.
During the summer, I will be switching out my greenhouse plastic to 80 gram netting to combat Spotted Wing Drosophila (SWD) and maximize cooling. Hoping not to need to run the fans, but expect I will need to do that so I get adequate air flow.
I’m very interested in your use of a wet wall. Can you include a pic of your set-up?
I wasn’t referring to your setup. It sounds like yours is entirely different but one that would work for many areas. Just screened in summer but poly covered in winter. I like that for crops that won’t be affected much by summer rains: berries, citrus, mango. For stone fruit I’d rather not have any rain on the plants or fruit at any time. That reduces disease and aids in increasing brix.
The wet wall allows me to get 900 chill hrs in 45 days when the average high outside is 60F and inside would hit 100 without cooling. During summer I can hold low 90s during all but the most hot and humid weather. In Denver with even lower temperatures and humidity you could hold it in the 80s all summer. I’ve thought about moving to Arvada where my daughter lives and putting in a greenhouse like mine mainly for stone fruit because that’s the best tasting fruit suited to a greenhouse in that area. Chilling won’t be as much of an issue up there. I’d lengthen the chill period to 90 days or maybe longer. During that time little to no heating would be needed. Just keep the plants above damaging temperature all winter and hold high below 48-54. Make sure there’s 900 hrs between 37-54 during that time. Let it start to warm up in Febr and have an 8-9 month growing season.
I’ll see if I can find pictures of my exhaust fans and wet wall.
Temperature probe is at 8’, affixed to a rafter. The idea was to capture the warmest area where the canopy of my trees might eventually grow, but not up in the gables or peak where it’s the hottest. All my plants are still small now, though, so it’s probably a lot cooler down low (and I’ll add a sensor soon).
I get that idea, but when I stand on a ladder with my head near the roof vent, there is zero cold air coming in unless the windows are closed (haven’t tried this on a windy day though). The air appears to be exiting the vent (or at least not entering) despite the fan pulling. If I close the windows, then I can feel cool air coming in the roof vent, though. I assume the under-powered fan and the long distance between the fan and vent mean that they are mostly pulling from separate ends of the body of warm air, as long as there is sufficient intake of cool air down low (which is why I was going to add an intake fan near the ground on the north wall).
That being said, today I’ll try removing the auto opener and see how well the exhaust fan does as the only way for heat to escape. With windows open, of course, as passive intake. Today is another summer-like day, and we might even close in on 80° this weekend, so it’ll be a good time to test out cooling. I’m always happy to experiment and figure out what works best!
Edit: it’s only been a few minutes since the auto vent would normally have opened, and the fan has kicked in and looks like it’s already doing a lot better than yesterday, so this is the part where I admit you were right @fruitnut!
Second edit:
After an initial dip when the fan turned on, so far it’s been doing worse than yesterday with the vent open too (currently the fan is only keeping it +23°F compared to outside).
I’m resisting the urge to open things up more now, but we’re still almost 10° below the forecast high today, so if it goes too much above 90° in the greenhouse then I’ll open the vent too, to see whether it helps or hurts. That should pretty definitively answer whether the vent + fan is better or worse than the fan alone.
The fan alone won’t be effective without some kind of air inlet. And together they probably are better than either alone. That doesn’t make for an efficient system of cooling.
Since you’re fighting excess heat in April with low sun angle it’s obvious that whatever you have won’t be enough come summer. I’m just trying to help you put together an effective and efficient system.
Also do you want something that works when you are tracking it every hour or something that will work while you’re on vacation. It all factors in.
This morning I opened the far window as a passive inlet. This can easily be automated with the auto-opener from the roof vent if it’s effective. It doesn’t look like it’s effective based on today’s test, though. I opened up the roof vent again when the temperature hit 92°, and it’s back down to 90° for now.
We are having extremely unusual heat for April. Today’s weather is more like a typical August day in Seattle, and I’ve never before seen upper 70s in April here:
Also, our sun angle is never too bad here. Currently it’s 53° at noon, and in mid-July it’s 59°, back down below 53° by the hottest month August. Sun angle is one thing PNW doesn’t have to worry about as much as other locations.
Definitely the latter! But I have two weeks to get it to that point, so I’m tracking it by the minute for now so that I can see which changes are effective and which aren’t. I don’t want to go for overkill if that means higher upfront costs and higher operating costs. I’d rather have it slightly less effective even if it means I’ll need to be there to take extra measures on those hottest summer days, rare as they are.
Just realized I never did a wrapping-up post for this thread. I ended up replacing the 440 CFM fan with a 1700 CFM fan, and have a single 12" passive intake. As soon as the (outside) overnight lows consistently stay above ~50°F I’ll also be swapping out the glass on the far door for the screen panel that the door came with to improve air flow. That setup seems capable of maintaining a delta of about 14°F relative to outside temperatures, which should be sufficient here in Seattle.
As for heating, the 1.5kW heater can maintain an overnight delta of just over 15°F, which should be enough for my purposes (though perhaps costly next winter). It’s been almost a month since the heater had to work full strength, though. Mostly now it turns on for a few minutes every 45 min overnight.
It’s been overcast recently, so the weekly average shows what a typical overcast late spring day looks like, while this morning is only partly cloudy so the fan already turned on a little while ago:
The thread linked in my last post has some recent photos of the avocado trees, but everything is going pretty well. My latest challenge is trying to get an automated drip irrigation system attached to the rain barrels in time for an extended holiday vacation in December. I’m afraid I’ll come back to a mixture of over-watered and under-watered trees since I’m running out of time to test things beforehand …
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