Greenhouse heater (emergency use)

Looking at the weather, it’s apparent that it’s going to get pretty cold here Wednesday night. We’ve had several days forecast in the mid-upper 30’s but have had only spotty light frost once that I’ve seen this May. Wednesday though, the forecast temp has crept steadily down from 33 now down to (most recently) 28. Since we have all of our tomatoes and other tender things in the ground in our high tunnel, I’m contemplating a heat source. We have and use remay, and that’s an option, though the tomatoes are already clipped and trained partway up their wires I have a propane construction type heater- not a salamander, it doesn’t plug in or have forced induction. I’m concerned it’s too much too fast though. I was contemplating a kerosene wick heater. The output seems more reasonable- 25,000 btu/hr or so, but I’ve been in spaces where they’re burning and the fumes can make you’re eyes water. I gather there is extra clean kerosene you can burn, though it appears to be nearly $15/ gallon. I should add that we don’t have electricity available in the high tunnel. Thoughts? Anybody have experience to share?

Of course I’m thinking about all of the fruits in bloom and already set. It was looking like it might be a crazy good apple year, at least based on the shear number of blossoms. We’ll see what the mercury does, but it’s not looking promising at the moment.

Your 25,000 BTU is probably a convection type unit, where the heat will rise quickly from of the top going upward. I would be concerned about the possibility of heat melting the plastic (polyethylene?) tunnel wall.

To understand how many BTUs you need, you can try this online calculator, but it will be only as good as your known values you put in. The R value you put in will be critical and may be difficult to figure out, since it depends on thickness of the plastic sheet you used for the tunnel walls and probably more importantly whether you double walled or not. You might try an experiment to determine what what the temperature different is by adding a given heat source. The same temperature difference should roughly apply when it will be near freezing.

Few other things about Kerosene…
K1 kerosene shouldn’t be $15/gallon if bought in larger quantities, you may find special K1 kerosene pumps at some gas stations at much more affordable price than Walmart.

Kerosene wick heaters work great and it should burn clean if your wick is clean, you have good fuel, and you burn at optimal design BTU. There’s not much wiggle room between low-med-high before you get soot and fumes.

Overall I think you might be better off with a 20lb propane tank and something like this. You can adjust the propane heater low-medium-high to your liking and if one is insufficient, you can stack multiple units. Obviously don’t point heating side towards the tunnel plastic walls.

Good luck.

Also: consider adding a fan, as you will probably have a big delta temp between ceiling and floor of the Polytunnel. You can make the temperature near the ground warmer with a fan given the same BTU heating source.

Thanks. Yeah, I have a sense of what 25,000 btu/hr is since I heat my house with a jotul 602 of roughly that output. That seems about right to me. My construction heater is probably more like 80,000 btu/hr, though it’s pretty old and I’m not sure there’s even a tag at this point. I’ve not used it a lot but found it touchy at lower settings. The thermocouple is probably to blame. As you said, melting is a concern. Our high tunnel is covered in solawrap. I’m pondering my options. Probably propane makes sense as you suggest.

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I’ve used a turkey fryer base in an emergency to heat my greenhouse. Turn it on just enough to maintain temps above 45 degrees. Mine is rated at 50,000 BTU’s but I used only about 8000 BTU’s to keep the greenhouse warm.

I also had a fan which is just about a necessity for your conditions. Can you run an extension cord to get electricity out to the hoop house? A fan should be hung from overhead arches at least 6 feet high so air circulates from the top down to the ground.


Yeah, I’m sure I’ll see some pretty good stratification. It’s a 32’ w gothic, so maybe 14’ or more to the peak. Unfortunately, it’s 300 or so ft and across the driveway (shared with neighbors further up) to the high tunnel, so pretty much out of range for most practical purposes.

Remay helps a lot, we’ve found, though I’d hesitate to put it on with a combustion type heater running. Your figure of 8,000 btu/hr is heartening. I’d think convection is more of a factor than conduction, but our Solawrap ostensibly provides r-2 or so, about like double wall poly. The bead in extrusion attachment style makes for lots of leakage paths though. If you’re not familiar, it comes in a roll (ours is 4’) and slides into plastic extrusions that you screw down to every frame. I’d considered caulking these grooves with Lexel sealant or something similar, though didn’t because of hassle factor and in consideration of potential replacement of a section.

I’d think a modest heat source would/could raise the temperature ten or more degrees, I just want to do it safely and thought I’d mine the wisdom here a bit.

Sounds like you know exactly what you’re doing already. :slight_smile: If you have one of those newfangled “solar generators” it would come in handy here to power a fan without a wired power connection.

12V fan + car battery could work in a pinch too.

That’s a good idea. Not sure it’s move that much air, but pretty low barrier to entry and worth a shot.

One other thing I’m trying to remind myself of is that we noticed in the fall (our first running the high tunnel) we had nary a bit of frost on tender things well below the thermometer reading 32. I don’t remember the number, exactly, but cucumber and tomatoes were alive, if reluctantly through about the end of November. Inside temps seem to track outside pretty closely. We may have a 2 degree bump or so above normal nighttime temps. I don’t know what temp it was when we actually saw visible frost and evidence of frost damage, but we were both surprised at how low it could go before seeing that.

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There is a skin effect that may be a concern. This occurs when a fairly strong wind is blowing outside. The greenhouse skin layer cools down and forms a layer of cold air that rolls down the inside next to the covering and hits ground level with air temperatures that can be several degrees colder than elsewhere in the tunnel. The purpose of a fan is to disrupt that air circulation. I’ve lost plants that were right beside the sidewall of my greenhouse due to this effect. It was a “life lesson” to learn that a greenhouse needs air circulation to prevent such from occurring. Also, I heated my greenhouse for about 15 years with an old Ashley wood heater. Fire in the heater with a fan above the heater directing air flow to the far corner kept everything toasty warm.

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