Heating

I knew that I'd have to heat the greenhouse if I wanted to use it year round. My goal wasn't to keep it at a balmy 80 degrees, but make sure that it didn't freeze, or better, lows around the 50 degree range. I figured that plants do fine in the spring with cooler temperatures, so it's OK to let the greenhouse dip a bit at night. So how does one go about sizing a greenhouse heater and determining cost?

ACF Greenhouse provides a very useful heating calculator. I had to figure out some of the variables for my area, such as the average low temperature and cost per kilowatt-hour, and the external surface area of the greenhouse. I also assumed that I'd be a bit lower than the maximum estimates on heater size and operation cost, as I have an insulated slab floor (see earlier posts) and I was planning on insulating with bubble-wrap (more on this later).

If I recall the heating calculator estimated a maximum heating capacity of about 20,000 BTU (approximately 5800 watts). For comparison - small indoor space heaters are rated at a maximum of 1500 watts, so the 11'x15' greenhouse would require four heaters during the coldest winter nights. Ouch!

Space heaters convert electricity into heat (duh). Household space heaters are limited by the voltage and current supplied by the 110v / 15 amp circuit - roughly 1500 watts. For the greenhouse I planned on a 220v / 30 amp circuit.  I selected the the King Pic-A-Watt heater.

Originally, I had it mounted about six feet above the floor. Here's a picture of the bracket, now used as a plant hanger. You can also see the 220v socket in the back.

And here's a picture of the heater resting about a foot above floor level.

I also have a fan up near the ridge of the greenhouse which helps circulate warm air back down to floor level.

Have you noticed the bubble wrap? One of the tricks I've picked up along the way is to put bubble wrap on the inside of the glass during the colder months. I bought 2' wide rolls from Amazon - I think I have 1" bubbles but I can only find 1/2" bubble examples on Amazon. Installation is very easy - simply cut to the height of the window. Mist the window with water and place bubble side against the windows. I figure this will give me an r=1 insulation value, similar to using double-pane windows in the greenhouse. At the end of the winter I simply roll up and save for the next season.

I also researched water barrels for storing and releasing heat into the greenhouse. You can Google to find out information about the amount of heat stored per gallon - if I recall, by definition, a BTU is the amount of heat required to raise one gallon of water one degree F. So, a 55 gallon water take will absorb or release 5500 BTUs for every degree difference in air temp. When I calculated for my greenhouse it turned out that I would have needed about 12 55 gallon barrels - too much. But I do have one barrel, mostly so that I don't have to cart water to/from the greenhouse in the winter (the greenhouse water runs on our sprinkler system, which is drained during the winter months).

Next up - geeking out with an old laptop, a USB temperature monitor, and some free software...