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I'm putting together a greenhouse that will have 2 X 1000L water tanks (IBC tanks). I've painted them black to absorb heat from the sun during the day. The goal is a 4-season greenhouse that won't freeze in winter.

However, in the winter, I am concerned that the tanks might freeze over. The greenhouse area is 10' X 13' and its height is 8'. The walls are polycarbonate (6mm thick), and assume that it is completely sealed without air-gaps.

The goal is to minimize electricity usage. Aquarium heaters unfortunately heat to 20C at minimum so this would be inefficient. I only need to prevent freezing (but can't add salt as the water will be used for plant irrigation).

Given my latitude of 43°N (Niagara region), will I need active heating, and what do you recommend (type and wattage)?

What I've looked at :

  1. aquarium heaters (submersible type)
  2. this type of heater : https://www.amazon.ca/gp/product/B000HHSI1E

Also, the 2 tanks are connected to each other on the bottom via hoses -- will the hoses need to be insulated?

+++ Addendum : the tanks are INSIDE the greenhouse.

Appreciate any advice! Hopefully not an overly-expensive solution.

Thanks, Gene

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  • The purpose of a greenhouse is to provide an enclosed warm environment for plants. If it is cold enough to freeze your water tanks then what about the plants? Are the tanks on the outside, can they be moved inside? – Alaska Man Sep 18 '20 at 18:32
  • Is the greenhouse attached to your house? How is your house heated? Electric resistance is NOT how you heat a greenhouse unless you want to go broke or need it only 2-3 nights per year. Natural gas is usually the best choice if available. If you want to go broke, stock tank heaters are designed to prevent freezing and not much more. – Ecnerwal Sep 18 '20 at 18:36
  • fertilizer you can add will lower the freeze point. A few years back we were messing with hydroponics, using run off water in tanks, the in ground 1000 gal & above Gnd tanks did not freeze at 10F the tank that did not have fertilizer froze at 22f blue tanks day time temps in the high 40’s F the 2 tanks that recirculated with chemicals did not freeze all 3 inside “hot house” made of clear plastic attached 2 1 wall of the house. They were food grade 55 gallon drums the fertilizer made a difference. The tank that froze had about an inch Of ice on top. the plants and water troughs did not freeze – Ed Beal Sep 18 '20 at 19:40
  • @AlaskaMan the tanks will be inside the greenhouse. If the water tanks are not freezing then it means they are releasing ambient heat to the rest of the inside of the greenhouse to prevent the plants from freezing, right? Or do I also need to heat the air in the greenhouse? – Jinren Sep 19 '20 at 0:10
  • @Ecnerwal I was thinking that since water is a more efficient heat store than air, that I would heat the water. Even though it's electrical heat I was thinking it might be more efficient than heating the air. Open to correction if I'm mistaken. – Jinren Sep 19 '20 at 0:13
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Another option; use low wattage aquarium thermostat/heater like 25 watt ( maybe more for your location , my coldest weather is only about +20 F). Only turn on power when a few days of freezing weather is expected. If the power is left on a few days extra, the low power usage will not be high. The hoses need to be insulated as they will be the first things to freeze. The main concern is the surface of the water will freeze and build up pressure; For pools an inflatable bladder ( such as a tire inner-tube ) is anchored at the surface. Then the surface cannot make a pressure tight barrier permitting a pressure build up in the water.

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  • Thanks, most heaters will have a built-in thermostat and only shut off when the temperature drops below the minimum level it needs to maintain. – Jinren Sep 19 '20 at 0:08
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If you are keeping the greenhouse above freezing, there's no need to insulate the water tanks. Their function is to serve as heat buffers to help heat the greenhouse and they can't do that if they are insulated from it - nor can they collect heat from it when it heats up.

If you are letting the greenhouse freeze, then draining the water tanks during that time is the most reasonable option.

The actual performance of the greenhouse as built in your region will determine whether it freezes or not without supplementary heat. Consulting with other greenhouse owners or suppliers in the immediate area would be more fruitful than asking the world about it, but at a guess if you expect it to work through the winter you will need additional heat (and probably lights) in cloudy/snowy/cold western NY state - it's not 330 days of sun per year like the desert southwest.

Side note - Completely sealed (no ventilation) greenhouses have a nasty habit of overheating and cooking plants (it's either impossible or impractical to store the excess heat when the sun shines fast enough to prevent overheating.)

6mm polycarbonate "twinwall" greenhouse glazing has an R value of 1.54 to 1.62 according to various web sources. The generally more useful for direct calculation U-valve is 1/R - 0.64 to 0.62.

Ignoring roof slope, a 10x13x8 greenhouse would have 130 + 368 = 498 square feet (since you have not clarified, this is the freestanding, all glazed case.) Using the more favorable value, that means for each degree (F) below the desired temperature the outside is, you need 309 BTU/hr to maintain the difference. So, when it's -12F outside you need (45*309) = 13,905 BTU/hr to maintain the greenhouse at 33F. 33F may still be too cold form some plants, but "above freezing" is what you asked for. -12 F is the lowest temperature in the last 5 years at Buffalo airport - -20 F if you go back a bit further. That's a 4 kilowatt heater, within tolerances. 4.2 KW if we use the more pessimistic number for the glazing R-value. So you'd appear to need 3 of those 1.5KW stock tank heaters to deal with a cold night and not enough stored heat in your tanks to carry through the night.

How much stored heat could you have? You've got 4,400 lbs of water, and 1 BTU is the heat need to change the temperature of 1 lb of water 1 degree F. To provide 13,905 BTUs/hr the water drops 3.16 degrees F per hour. The water can't get too hot or your plants will die, and you don't want it to freeze, so there's a limited amount of time you can provide that for. However, there's also a "gotcha" hiding here - the heat loss calculation was done for 33F in the greenhouse - the actual temperature in the greenhouse will be higher when the water is warm, so the heat loss will also be higher, so your stored heat (water temperature) will go away faster than that until the greenhouse is down to the point that you need supplemental heating.

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  • Thanks for your comment. No it won't be "completely sealed" all the time, it will have an auto-venting window that will open when it gets too hot. – Jinren Sep 19 '20 at 0:04
  • Sorry I wasn't clear in my question -- wasn't asking about insulating the tanks but whether or not to add active heating to the tanks so they can act as heat stores that will naturally release heat to the rest of the inside of the greenhouse. – Jinren Sep 19 '20 at 0:05
  • Thanks, so do you think it's even feasible to heat the water via electricity? – Jinren Sep 20 '20 at 1:01
  • Feasible, sure. Affordable? That's quite possibly another matter. Electric resistance heat is cheap to install, expensive to run. Other forms of heat cost more to install but less to run, in most cases - can vary if local utility rates are wildly skewed for some reason, though a cold climate heat pump will pretty much always cost less to run (but a good chunk of change up front to install) since it puts out 2-3 times the heat that it consumes as electricity, by harvesting heat from the air. Gas, oil, wood pellets or wood stove may cost a great deal less to run. – Ecnerwal Sep 20 '20 at 2:16

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