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Greenhouse heating alternatives

PostPosted: Jul 18, 2010 10:27 am
by eastwood2007
I posted in the other thread, but would like all you experts ideas.

I have a 10x16 greenhouse that I like to keep going all winter. I'm in zone 5, so we have extended periods of cold weather.

For years I've been heating with up to 3 1500w electric heaters. They aren't on a thermostat - I've just turned them on 1, 2 or 3 depending on how cold it is. In the past, the electric bill would run up to $80 per month if it was really cold.

Last winter we had extended periods of extremely cold weather and one month the electric bill was probably more than $200. (I say probably because it is just all lumped with the household electric.) I can't do that anymore.

I'm looking for ideas. Either a thermostat that I can plug my heaters into, or a completely different heat source that would be MUCH LESS EXPENSIVE.

Any ideas?

Re: Greenhouse heating alternatives

PostPosted: Jul 18, 2010 2:54 pm
by Gardentoad
What temperature do you want to keep it in the winter? I am in zone 5 and I just use my greenhouse to overwinter plants that are not hardy here, but I don't keep it warm enough to keep them growing. I just don't want the pots to freeze.
My greenhouse is a leanto against the south side of my garage. It is 20 feet long and 9 feet wide.
It is insulated with double walls and windows.

There is 2" thick styrafoam about 2 feet deep all buried around the perimeter.

I have caulked all the cracks and holes I can find.

The second year we added a door 3 feet from the original one so I can shut the outside one before opening the inside one.

There is an extension, looks kinda like an awning over the fan on the outside. It is like a box that has vents on the bottom that were designed to vent crawlspaces and open automaticly when it is warm enough for the fan to kick on, This keeps cold wind from blowing in through the fan opening.

I have several 20 gallon plastic storage bins full of water under the benches on the south wall to store solar heat. The water in them is cold in the winter but it doesnt freeze. From what I have read, as long as water isn't freezing it is still holding some solar heat. I tried bags of composting leaves once but I didn't see where they made much difference.

I have just a small electric heater that I have on a timer. It kicks on for a half hour then off for 2 hours. During November, December, March and April I only have it kick on between 6 PM and 9 AM.

When building it I was planning to keep all my houseplants out there for the winter but decided that it was just too expensive to keep it that warm so I still bring tropicals in the house to crowd around the windows. I am active in the greenhouse starting in late February with cleaning it up, organizing and filling my soil bins to be ready for starting my seed in March. Otherwise I only go out about once a week to water the dormant plants or to snip a sprig of rosemary.

Re: Greenhouse heating alternatives

PostPosted: Jul 18, 2010 3:22 pm
by eastwood2007
thanks for the reply, Gardentoad.

I think I would like to keep the temp at 40- 45 at the lowest. I, too, don't need plants to grow, but just live thru the winter. I find that even tropicals will do okay and just need a sprucing up in the spring.

The greenhouse is freestanding with a wood frame and poly covering. It is pretty air tight, except a little around the door.

If I had a thermostat that would work, I think even keeping the heaters from warming things up too much would help alot. Since I am working again, now, it is harder to monitor it. It needs to take care of itself.

Re: Greenhouse heating alternatives

PostPosted: Jul 19, 2010 6:42 am
by Gardentoad
You can plug your heaters into outlet thermostats. I thought about buying one of these: ... pd_cp_hi_1
There are also cheaper ones that only kick on at 35 or 38 degrees and off around 45 degrees.
It probably would save me money to go that rout instead of a timer. I just stayed with the timer because it worked.

The biggest problem I have in the winter is that with the windows all shut up on cold sunny winter days the greenhouse gets up to the 90's which brings some plants out of dormancy and then gets almost freezing at night. Keeping it above freezing is easy, but keeping it within a range is not. I do lower the thermostat on my fan but I don't have automatic windows other than the vents outside of the fan.

I didn't know that tropicals could get that low. I thought they had to stay above 60 degrees. Is that all of them or just some? I have several peace lilies, jade plants, a large aloe, 4 potted citrus trees, a banana plant and an olive tree. It would be nice if I could keep them out in the greenhouse. I know the citrus could get close to freezing but I worried about them going through the hot then cold all in one day cycles.

Re: Greenhouse heating alternatives

PostPosted: Jul 19, 2010 5:19 pm
by eastwood2007
Some tropicals are more sensitive - bouganvilla for one - anything below 38 and you are really taking a chance.

I have a small lemon tree, bird of paradise, mandevilla, boston fern, sago palm, aloes, agaves....and they all do fine.

I keep a few annuals - geranium, impatiens, black eyed susan vines, elephant ears, etc.

I have some plants that I have had for 10 years or more that I really want to keep and some that bloom better when root bound (agapanthus, bird of paradise), and they have to be older to get that way, or it may be cheaper to just buy them each year. :lol:

Thanks for the link on the thermostat....I'm definitely going to check that out. I haven't been able to find anything on any of my searches.

Yes, the heat can certainly be a problem. On days when it is going to warm, I just prop the door open a bit, but I may research a vent opener. (I'm sure that's not the technical name for it. :lol: )

Thanks again!

Re: Greenhouse heating alternatives

PostPosted: Sep 21, 2010 11:46 am
by kHT
We have learned not to put all the heaters on the same breaker in case one is tripped the others will keep running.
We are also keeping barrels of water inside to help keep the heat in there. What fun!! We try to keep ours at at least
50 due to the tender tropicals we grow. :wink:

Re: Greenhouse heating alternatives

PostPosted: Mar 20, 2011 11:43 am
by eastwood2007
Thought I'd post an update to this.

Gardentoad, I took your advice and ordered the plug-in thermostats from Amazon. That was the only place I could find them, BTW.

They worked beautifully! I set them on 50 on 2 of my heaters. (One heater on each end of the greenhouse to keep the temps more even.)

They kept the greenhouse between 49 and 50 all the time, so that kept my electric bill low (or lower - it's never LOW when trying to heat in the winter), and my plants faired better as there weren't the temperature extremes that I have had in the past when I was just turning heaters on and off as best I could. It was a time saver and a stress saver as I wasn't always worrying about whether it would get too cold overnight, or too hot while I was gone during the day. I kept a close eye on them the first week to be sure it was all working properly, then it was virtually maintenance free.

Thanks so much for the tip.

I have benefited so much from advice I have gotten from generous folks on this forum.

Thanks again!