Heating a greenhouse

I am on my second make-shift greenhouse, gradually working my way up to something that I don’t have to MacGyver, however, I am not there yet. The last 3 years I’ve managed to keep a zippered vinyl 3’x5’ “greenhouse” from freezing in a northern Washington winter, but I now have an actual twin-walled 6’x8’ structure and need some knowhow on the best way to heat it.

For the 3’x5’ I used a 5 gallon pail of water with a pail heater. The electrical ran under the vinyl wall and plugged in to an outlet adapter that turned on at 35* and off at 45*. This kept the ambient temperature in the small structure from dropping below freezing, although we did not have extremely cold winters in those years.

Can anyone make suggestions for my slightly upgraded circumstances? This greenhouse is for those trees that do need some cold for dormancy, but are not quite up to snow, and freezing temperatures, even with root protection.


I live near Spokane, WA and my greenhouse is about 12 x 20 ft. with 6 mil (0.15 mm) white poly sheet on the outside and 0.7 mil poly stapled to the inside to give a 1.5" (4 cm) air gap. I have primarily used a 1500 W box fan connected to a line voltage thermostat from the big box store that I typically set so it comes on around 25F (-3C). When it get really cold, I supplement with a portable propane heater with a max output of about 12,000 BTU/hr (3,500 W) and typically takes it up to freezing and has to be manually started and shut off.

I am looking to upgrade myself to control the temperature around 34F (1C) automatically. There are heat pump units that will do the job, but cost nearly $1000. Being a DIY type, I may get a craigs list freezer, take it apart and put the eavaporator inside and condenser outside and hook it up the lien power thermostat.

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I used 2 heat mats last winter, they don’t cost as much to run as a 1500 watt heater. It kept the temps at a reasonable temp.
This year I was going to do hydronic heat system. Being that I have installed these before it’s not a big deal for me. The upfront cost is what can steer people from this type of heat, but in the long run a lot cheaper than electricity. :evergreen_tree::metal:t2::grinning:

With the mats, did the air temps (asuming in a small greenhouse, etc.) stay above freezing?

It got down to the low 20’s outside and the greenhouse was 28 inside.

I truly appreciate the DIY approach! I used bubble wrap to insulate little vinyl enclosure that got me through the last two winters!

This small upgrade I’m working on began with a Craigslist deal for $200 for a used hobby greenhouse - similar to one that can be found online for about $600. I’ve read enough reviews to know that the aluminum frame would not be able to stand any sort of snow load, so what made this deal into a steal, was the welded steel reinforcement that the prior owner had added.

In addition, he had installed a fan which is controlled by a thermostat that he wired into the unit. Even though I will replace the panels, I think that the frame alone makes it worth what I paid.

But that brings me to the next issue… This greenhouse was used for spring and summer propagation and early vegetable starts, so it was far more concerned with staying cool. The fan does not have louvers to block warm air from escaping, and there is a vent near the floor (which can easily be closed off in the winter) to allow air in.

I think I will definitely need to replace that fan before I put anything in there that I want to keep above outside temps. Can you tell me what I need to know about ventilation that does what we need for a climate that experiences temps below freezing?


This is what I use in my sub-tropilcal greenhouse.

I have 4 zones (not USDA climate zones) when I winter my trees. There are those that stay outside with little except a mulch cover over the pots, the outdoor greenhouse, for those trees that need to have some cold in order to grow dormant, but aren’t happy with frost-level temps. This is the greenhouse I am currently working on.

Then I have another small vinyl zippered unit in my garage for those that will winter best as long as they stay above 50*. *the enclosure helps with to maintain humidity.) My heater and water heater are in my garage, and the walls are finished, so except when winter outside is below freezing, it stays pretty constant at about 52 - 60. On those days when it does get cold enough to drop the temps in the garage, I use the heating mats.

The fourth zone, of course, is for my tropicals which come into the house. But that is a whole different topic!



I have normally only been concerned with minimizing freeze-thaw and keeping it from getting below about 22F so I leave the door open quite a bit. When combined with the white poly and shade cloth covering the top half I don’t have overheating issues. I have even had frost on the inside walls on days when the outside temperature reaches the upper 30s. With the door open quite a bit I don’t worry about ventilation. I normally end up watering once or twice a year if I don’t have some good fluffy snow to throw inside.

I use a line voltage thermostat like the one in the picture to control the box heater. I mounted it in a plastic outlet box in the middle of a short extension cord. It goes in-line between the primary extension cord and box heater. In previous years I have laid it on the ground – last year I mounted it about a foot above the ground.

If I try to keep it above freezing this year, I will need to make some adjustments. I will add a small vent that I can open and close rather than the large door. Ideally it would be powered with good louvers, but I won’t go there yet.


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