Hi! Preparing my first greenhouse for starting seeds in March-ish. Greenhouse is an 8×8 in Catskills NY. What kind of heater is really needed for one this size? Also not there on a regular basis, so may need some kind of app control. No idea what kind of heater or how much it’ll cost to run, so any thoughts appreciated!
by DWMinNYC
7 Comments
Really depends on how warm you need to keep it. If just above freezing, you can use an electric oil filled radiator type that’s 1500W. Many of them also come with built-in thermostats for control. They are by far safest vs gas or wood powered heaters.
Getting a few 55 gallon water barrels painted black inside as thermal mass would also help. I would insulate the north wall with rigid foam panels too.
Insulation for sure. I use 2″ R13 foam board.
I pair my 1500W radiator heater with an Inkbird thermostat, but there are other thermostats available.
I use a BioGreen Palma in my 8×12. I’ve used it with a AC Infinity Pro controller and programmable outlet in the past but I switched back to the digital thermostat that came with it. I can still monitor the temperature and humidity in the greenhouse with the AC equipment, but have to manually set the trigger temp with the digital thermostat. I’m in zone 8a, and about two weeks ago we had 4-6 inches of ice and snow and several days when it didn’t get above freezing, but the lowest it got in the greenhouse was 48° F.
Don’t forget to insulate really well. I’ve covered the inside walls and created a lower ceiling with bubble wrap, and lined the walls beneath the benches with R board. It does no good to pump heat in there if it leaks right out.
I’ve been running a small milk house heater in our 6×10 for the semi-feral cats we help care for. Set the dial between low and medium, and even when it got down to 7 degrees F here the heater was able to keep the greenhouse at 39 degrees. There is no additional insulation, either, and I monitor but do not control the temp with an inkbird. The cats have houses with heated beds within the greenhouse, so that’s why I haven’t had it set higher than that. I’ve been very happy with how warm it’s been in there on such a low setting (as have the cats), and thankfully it hasn’t spiked the electric bill too badly. Next year I’ll probably overwinter my hibiscus and elephant ears in there rather than struggling to get them in the basement.
In my old 10×12 greenhouse I use to use a propane RV heater back in the day.
For a greenhouse that size, the heater capacity matters more than the brand. A simple way to estimate what you *might* need is the heat loss formula growers use:
**Surface Area × Temperature Difference × 1.1 = BTUs needed**
Since we don’t know the exact height or glazing, here’s just an example so you can see how it works:
Say a small greenhouse ends up around 240 sq ft of total surface area (walls + roof combined).
If it’s 10°F outside and you want to keep seedlings around 55°F at night:
Temperature difference:
55 − 10 = 45°F
240 × 45 × 1.1 ≈ **11,800 BTU/hr**
That’s not your exact requirement, just a realistic example showing why even small greenhouses need a dedicated heater.
Since you won’t be there regularly, an electric greenhouse heater is usually the easiest route:
* designed for damp spaces
* thermostat controlled
* safe to leave running
* easy to pair with a WiFi thermostat or temp sensor alerts
Adding some additional winter-time insulation helps a lot too. Bubble insulation is a popular favorite for this. Insulating the north wall can noticeably reduce heater runtime and operating cost.
If you want to run your own numbers, this explains the formula step by step:
[https://charleysgreenhouses.com/news/how-to-choose-a-greenhouse-heater/](https://charleysgreenhouses.com/news/how-to-choose-a-greenhouse-heater/)
This looks like our landscape too. North Dakota. Zone 3.
We have a similar style GH. 20×13 but on a knee wall so it’s 13 feet tall. We use a 50,000 BTU gas heater mounted and it keeps our space 60 degrees above the outside air temp.
So when it was -30 recently, it wasn’t so pretty. BUT, we installed a very small supplemental electric heater, for that exact reason, and for when the pilot light on the gas blows out in the winter storms. This keeps it 50 degrees in there, even when it is -30 or -40 outside.
If I had to go back, I would’ve installed a bigger gas heater for our handful of extreme winter days. The heat only runs at night, never during the day, not even in the winter, as long as the sun is shining you’re good. Our gas bill is about 100 bucks a month for only a couple months a year.
You can insulate and throw heat at it easily, the real pain/ problem is cooling it down. 😂 We do not use any thermal mass. We insulated underneath the deck perimeter, and within the wall that it sits on. Otherwise the structure is sealed with gaskets and is really tight.
https://preview.redd.it/o55ztf98cqjg1.jpeg?width=4032&format=pjpg&auto=webp&s=79136c5b6d07cc8a06a8eaac271421e31b978cfa
Hope that helps even in the slightest! Good luck with your project.!