I’m almost halfway through the video and the suspense is killing me but I’ve gotta get the kids to bed so I’ll have to find out how it ends later. I’m thinking I’ll have my kids do an assembly line making wicking tubs once I’ve figured out how they work. 
yeah so maybe you’ll want to look at esphome for greenhouse controls. I’ll do a post later but I have an esphome theromstat, a light timer and a separate irrigation controller. they’re all programmable and report sensor values over wifi to home assistant which runs on any kind of server. the only place I’m using the programmability to do something neat is the mist controller, I wanted to adjust the mist schedule to only take place during daytime and adjust the interval based on temperature or maybe an evaporation rate formula. you can get that in an off-the-shelf controller to approximate this but it’s $$$. I get graphs of temperature, humidity, and I have soil moisture and temperature sensors too
so, it’s all very cool but the esphome/home assistant programmers aren’t great about stability or bug fixes. so you’d be signing up for some fiddling with it every so often. so I’m not recommending this unless you want a 2nd hobby of playing with greenhouse controls alongside your 1st hobby, playing with plants
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I think for now I’ll just start with a pi that logs temperature, and hook it into one of these to power a heater and grow lights:
I’m guessing that a few lines of python should be enough to get me started at least. Pretty cheap, too, so if it doesn’t end up working I’ll try something more involved like home assistant later. Thanks for the info and inspiration to hack this instead of buying fancy stuff!
That’s a great size! what zone are you in?
Beautiful greenhouse! Wow! A tube of indoor/outdoor caulk and some scraps of the twin-wall poly will take care of all those small gaps.
For heat, based on my experience I’d stay far away from electronics. The heat and humidity in a greenhouse are hell on electronics. I’ve had way too many gizmos fail and I’m back to solid state especially for heat. If you lose your heat mid-winter you can lose everything in your greenhouse overnight. There are a million ways to do it using electronics but there are easier ways.
The easiest thing to do is buy a Thermacube. Just plug it in, plug a space heater into it and you are done. If you want more control get a Peco. They work for either heating or cooling (buy two if you want to do both heating and cooling) and mine has been foolproof. To switch between heating or cooling you are wiring it in two different ways. It is very simple if you have any basic knowledge of wiring. I run these to an outlet then plug my heaters into that outlet.
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I’m in Seattle, so I don’t need to deal with very low temperatures (rarely below 20° and this last winter lowest temp was 25°), but days are very short and overcast all winter, so I’m guessing I’ll still struggle to keep it warm enough for anything more tropical than the hardy avocados, citrus, and garden starts.
I had enough scraps to add a layer around the entire exterior, and I’ll be going around with silicone or caulk from the inside next. Here’s what I’ve done so far to cover up the lower gaps:
Thanks! The thermacube looks like a great idea, too bad they aren’t adjustable (35° is a pretty low to start heat, that’s our ambient low temperature for most of the winter!). The Peco looks a lot like the Dramm I had linked above, but 1/4 the price. I would prefer something where no wiring is required, but I can probably manage it. Thanks for the suggestion!
In my experience, raspberry pi computers can withstand quite a bit of abuse without any failures, including outdoors or in humid environments, but I do like the idea of simple solid state controls with the pi just acting as monitor rather than controller.
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The Thermacubes come with several different temp ranges. I think you can see them all at the link. But yeah each one is what it is and can’t be changed. They are a great back-up. A cheapo resistance heater plugged into one of these can save you if the other system fails.
Just follow the directions that come with the Peco. I think it’s neutral to blue for it to turn a heater on, neutral to green for AC. Something like that. If you can wire an outlet this is childs play.
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Already seeing a big improvement and I’m not finished sealing it up yet. A couple days ago it was similarly overcast and cool and it was only about 8° over ambient, now it’s over 20° (take the inside humidity reading with a grain of salt because it’s sitting directly on the soil):
If it’s doing that well on a cloudy day without any heater, I’m hopeful I’ll be able to keep it OK in winter without excessive heating costs. 
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It looks like we’re going to be below freezing tonight, so I borrowed a small marine heater from a neighbor to see how well it does at the 900w setting overnight:
I’ll try it on the 1500w setting tomorrow night if this doesn’t do the trick.
To help monitor temperatures, I’m testing out one of the I2C temperature sensors wired into a raspberry pi:
I’ve got that uploading every minute to an SQL database, which for the moment is just viewable as a table, but I’ll probably mess around with that part more once the list of temperatures gets unwieldy in a few days.
As I am writing this (~10pm), the temperature in the greenhouse is about 52°F and falling, while the outside temperature is 39°F and falling.
EDIT: Astute observers of the data linked above may notice that shortly after I posted this originally there was a rapid loss of a few degrees when I looked out the window and saw all the plants on the lawn that I had meant to put in the greenhouse for the night. I’m going to leave a few of the larger grafted avocados out, though. Maybe a cold shock will get them flowering.
It takes me over 2,000 watts in Z7 MD to keep above 50 in winter. Winter nights are long and cold!
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A greenhouse heat pump would be so much cheaper than electric resistance, natural gas, oil, or propane.
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I wonder how expensive a heat pump, or whatever is most efficient, would be for cooling in summer?
I don’t think I’ll be trying to keep mine quite that warm. 40°F should be plenty for most of what I’ll have in there, I think. Last night the outside temperature was 29.3°F (a typical middle-of-winter low for Seattle and very atypical April low), my lowest temperature in the greenhouse was 43.5°… not terrible for 900w, especially since I haven’t sealed the cracks around the doors or the bottom of the south wall yet. I think 1500w might do the trick even for our coldest nights, once I do a better job sealing it up.
I haven’t crunched the numbers yet, but at about $0.10/kWh it might take quite a few years for a heat pump system to make up the difference in initial cost (1.5kW electric resistance heater is only ~$100). Do you know of any DIY heat pump systems that aren’t thousands of dollars in upfront costs?
We have a heat pump mini-split system in our house and rarely ever use it in summer (but it runs on a low setting all winter), mostly opening windows is enough here in Seattle during our cool summers. It looks like we’re in for a sunny stretch with highs above 70°F in the next week, so I’ll get to see how the greenhouse handles that.
I need to come up with an enclosure for my sensor to protect it from direct sunlight, though, if I want accurate daytime readings. Direct sunlight caused a false spike this morning:
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Besides maybe the little fan that is in the heater,was there another on last night?
I’ve put my wifi sensors in a pvc tube with the bottom open and that helps some but isn’t perfect. Maybe painting the tube silver (or wrapping in foil if you don’t have to worry about wind) would be an improvement.
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The heater has a pretty good fan and it was on high, but no other fan yet. I’ve ordered a circulation fan, but hasn’t arrived yet.
Thanks for the suggestion! I can probably scrounge up a suitable piece to try. I have some mylar I could tape onto it, that should do the trick.
So far the auto vent is doing a good job even with no fan or open doors/windows (which might be a sign I really do need to seal it up better):
Pretty expensive compared to evap cooling if it isnt at 40% load on the heat pump. In general here a evap cooler will cost you 900 watts/hr and a extremely efficient 3 ton mini split heat pump will be around 2200 watts/hr however the average (18 SEER) model is around 4800 watts for 36k btu cooling. Usually these can pump up to 130% output but lose even more efficiency when doing so.
Heat pumps just shine and can create 4 btus per 1 spent when they are ran on low in moderate climates and do a really good job and are extremely efficient at heating (yet not cooling) a greenhouse
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To cool my greenhouse at 1725 sqft requires two 1/4 hp motors for the exhaust fans. Plus a sump pump which is probably less than 1/4 hp. I suspect I can cool it for no more than an equal sized but 10-15x insulated home with a heat pump. But that’s just a guess.
In a greenhouse what I’m fighting is the solar gain. In a house you’re fighting some solar gain but also temperature differential of maybe 20F.
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Yeah that was my estimate for your area was about 3/4 hp to cool and sized that to a 2k sq ft home here.
My aunt is about to put in the swamp cooler this year and change around her ventilation like we talked about and i am going to do my greenhouse addition i picked your brain about last year hopefully in the next 18 months! Thanks for the help as always
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Been a busy day in the greenhouse. Moved in the two rain barrels which will hopefully help moderate temperatures a bit, though I wasn’t able to get them in during the hottest time of the day so they could warm up today. I realized my greenhouse was far enough downhill from my house that I could transfer rain water from my existing rain barrels just by siphoning, so I was able to fill them even though I haven’t hooked them into the greenhouse downspouts yet.
Also used up one more 10oz silicone tube, still only about halfway done with the sealing up, though.
And of course, I made the permanent home for my raspberry pi temperature monitor:
It was powered off for a few hours, but I had the doors open and it was quite pleasant. The current high temperature was from sitting in the sun this afternoon right before I turned it off, I’m pretty sure it never actually got above ~90° today:
I think I’m done for the day, but tonight I’m going to try the setting on the heater where it only turns on when it hits 38°, to see how long it takes to get that low without heat and whether the heater can successfully push it back warmer from that point. I don’t think it will be quite as cold as last night but maybe close.
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