I have a Harbor Freight 10'x12' greenhouse that we heavily fortified for our midwest winds and storms. We added self-tapping screws, silicone, extra panel clips, etc. to ensure strength.
Now...we need heat for our winters.
We've tried basic heaters with fans in order to heat, but they do not even come CLOSE to doing the trick.

I need some ideas on how to heat this size of a greenhouse in the cheapest manner possible. There's no way for us to pull everything out to run floor heat, water drums, etc., so I need something that will be as energy efficient as possible with probably propane or natural gas.

Ideas? PLEASE!?!?

I've purchased a Williams propane heater for my 8x8 foot greenhouse. I hope to get it installed this weekend and have the gas connection done sometime later this month. The advantage of the Williams is that it is vented, the greenhouse can be completely sealed and it should still operate. I'll post pictures once I have it in the greenhouse.

We purchased a small propane heater for our 15 ft dome greenhouse, and it was not outside vented. It would have worked well except that it had an Oxygen safety sensor that made it shut down at our altitude of 6500 ft. It was under $100 and I think it was 20,000 btu. If you're at a low altitude you could just use a free standing small propane or Kerosene heater. I now use a small electric heater, and it keeps it at 60 F which is what I wanted for a minimum temp for some of my tropicals. Later on we may install an outside vented propane heater and mount it high to keep our floor space clear.

Welcome to NGA, @lilnemo2 .

Where in the midwest are you located, and what are you planning to grow over the winter in your greenhouse?

Where I am, in the Michigan Upper Peninsula, I've never felt it would be at all feasible to try to heat a GH throughout the winter, unless possibly with a wood stove of some sort. And, there are other issues, such as day length, which make it difficult to impossible to grow some things during the winter here in the north. So -- more info, please!

have you considered a pellet stove?

Welcome to NGA, @dubiousjay .

Assuming your post was in response to my statement about a wood stove... a pellet stove would cost a lot more than a "normal" wood stove to begin with, plus we'd have to buy pellets - when we could cut firewood on our own property.

But, the hours of daylight also limit what we could grow here in the winter, even in a heated GH, unless we also had artificial lighting; I think it's much more feasible to preserve what I produce during the spring, summer and fall and save winter for planning next year's garden (and shoveling snow off the deck).

pellets stoves can run for atleast 24 hours on 40lb bag..... many hoppers can hold 120lbs or more. you can leave the house for a few days and not worry.... back up electric heaters in case of a failure.

A 40lb bag of pellets is about $3.50-$5.

Most modern pellet stoves have thermostats and the stove turns off and auto re ignites when needed.

If you are in Michigan bulk pellets or cherry pits by the ton are very cheap.

My next greenhouse will have corn/pellet boiler to heat the floors

What do you grow in your greenhouse? and where are you located?

Northern IL. Tropicals mainly. I also use metal halides to supplement light.

I have a natural gas wall-mounted heater in my lean-to. Started out with propane but a pain to change tanks in the winter. (The tanks were outside of the GH.) It was easy for DH to run natural gas to the little lean-to but I'm sure it would be tough to do in a stand-alone. If you really want to heat during the winter (I do), insulate around the bottom of the walls to help hopefully hold the heating cost down. And yes, supplemental light for sure on dreary winter days.

Can you use a ventless gas heater in a GH? We are considering running gas line to the GH. Pros/Cons? Recommendations? What size for 687 feet surface area? web links? Zone 5; lows can get to -15, want to have average temp in GH 65 F, for berry plants.

You may want to check out the Pro-Com line of vent-less heaters. I've had one for a few years that works pretty well. Running the gas line was easy because the greenhouse is attached to the south wall of my garage. I'm in NW IN so zone is pretty similar. There are charts on various websites for calculating the BTU needs.

question, what is wrong with a infra red fan blown heater. have one in a metal shed, 10x24x8 that i use in the winter to keep the neighborhood cats from freezing. keeps it around 60 , and the electric bill only went up about ten dollars a month
why want this work in a greenhouse. ?

I see no reason that it wouldn't work.

I use a propane heater only because there is no nearby electrical connection.

I agree ; no reason to think it wouldn't work.

I use propane especially important to maintain correct temps for cuttings and seeds. Also old heating pads placed under the growing trays to nhelp seeds .

We don't have near as low temps as you nor as long of a winter season here in South Texas, but I use a heat dish (Costco) in combination with a box fan to move the heat. Actually, the box fan goes 24/7, 365 days a year I just move it from place to place where it works the best from season to season. I have a thermostat (Ebay) where the fan plugs into that turns the fan on and off automatically. We had some odd 19 degree temps for 3 days last winter. This system kept the Harbor Freight 10x12 greenhouse at about 40 degrees. I think I need to insulate the cracks more with backer rod. Some had fallen out. I also put up bubble wrap sprayed on with water/dish soap mix on the walls. There is a Mylar foam blanket that was used to wrap and protect our new TV on the interior north wall and Mylar foam insulation boards on the outside. I have a mister system (Amazon) that is a small patio umbrella mister set of 6 nozzles. It runs down the center of the GH and the nozzles point every which way.

More importantly is the placement of my plants which are mostly Tropicals. The plumeria and adenium for example are in the back where they won't be bothered while they sleep. While those who need more water and some attention i.e. ferns are up front where I can manage their water. I have succulents up against the Mylar where they can get the reflection of the sun. Portulaca and bougainvillea hang high up where they can continue getting as full sun as possible. It's hard, but I try not to forget anyone's winter needs.

During the colder months I use an oil filled electric heater and two wall mounted oscillating fans to keep the heat evenly distributed. I also insulated my north and east walls with Reflectix, a small bubble wrap insulation with foil on both sides. What has made the biggest difference for me though is the solar powered hot water storage system I made that collects heated water in 4 - 55 gallon drums during the day. This has greatly reduced my use of the electric heater on winter nights.

This is the system I built.

Very cool @moonshadows Jim! Or should I say very warm? Lol. I'd have to add an extension on to my GH even if I was able to figure out how to build a solar collector! My plants fill up nearly every part of mine floor to ceiling! Every year I say somehing's gotta give. Every year something goes into the garage for warmth. I built the GH to get plants out of the unheated garage!

It did take up room I really didn't want to spare at other times of the year, but it allowed me to grow in cold months without having to remortgage the house to pay for greenhouse heat.

I also built two 3 x 6 raised cold frames last October for a bit of added insulation during cold months.