I have a 20 x 25 foot greenhouse / sun room that is normal, insulated 2 x 6 construction on 3 sides and one roof, and on the south facing side of the structure, its double-walled greenhouse poly-carbonate roof, and a wall of double glazed windows. I have two of those "Vornado" electric heaters.

All this worked fine, until I got the first electricity bill, which was triple. The lowest heat setting on the heater was 60º. I purchased two thermal electrical plugs, which go down to 40º. This makes a huge amount of difference, as they will only turn on the heaters when it gets that cold, which is only when the outside temps are below 10º. I have a couple of electronic max-min thermometers around, and the lowest it has gotten, over by the windows, is 35º

I don't have cactus, but I have all kinds of frost sensitive plants, like pepper and basil and some succulents, and they do just fine under these conditions. You may, in fact, be like me and find that the biggest problem is that the sun room will easily get over 90º, and the plants will start to wilt from the heat in January.

Thank you for the respoonse, David. My room is quite a bit smaller than yours so I am hoping that heating it with an electric heater won't be inordinately expensive. It will also be on the east side of my house, which should keep it cooler in the summer and on other warm days. The roof will be solid, with windows - made of "Champion Comfort 365" glass - on three sides. These windows supposedly keep in the heat in the winter and keep out the heat in the summer. Guess I'll find out if the advertising is accurate.

I will look into the Vornado heaters that you use. I won't really know what I need until the room is actually built, but I am doing my research into options now.

Holly

Here is a link that might be useful: Champion

Holly, another thing that you might want to think about is condensation. You'll get that on any glass surface, and it can be a surprising amount. This leads to mold, and rotting window sills, and this kind of thing. So caulk heavily around the windows, and use a high quality paint, and I even place cotton diapers in strategic places.

With my polycarbonate roof, and a single - digit night, it can seem as if its raining. I ended up putting a strips of tape along the inside of the roof to channel the condensation so it would fall where it was safe, and not down the wall or something.

If it is still under construction, you also might look into those automatic vent / window openers. They aren't that expensive. It is surprising, even on the east side, how much a bit of sun will heat things up.

Condensation may indeed be a concern, but it will be an all-season vinyl room, no wood to rot, and all the aluminum is covered with vinyl so - supposedly - this leads to less condensation. All the windows are sliding windows so the automatic vent/window openers won't work. These are all things to keep in mind, though. We will try to monitor the temps closely to see what adjustments need to be made - I plan to get one of those "maximum/minimum" thermometers to put in there so we can see what temp extremes we have to deal with before putting any plants in there.

All that has been completed so far is the door cut-out and installation for the door leading from the house to the area where the sunroom will be built, so there is a long way to go!

Holly

Holly, the biggest mistake I made when doing my greenhouse was under-estimating the amount of solar heating.

In the summer, I can't do much in there, it's just too hot. During construction, I put in some large exhaust fans, the kind designed for chicken houses. They are largely worthless, because at best, they use a lot of electricity to get the room to the outside ambient temperature, which can be 100ºF. I now use a shade cloth that I put on every May and take off every Sept, that knocks 15º off. If I were to do it again, or if I can ever scrape together the will, the cash, and the time, I'll put in a swamp cooler in place of one fan, which would cool by adding evaporated, moist air, and in the other hole in the wall, a regular room air conditioner which would cool by removing the moisture the swamp cooler is putting in. Theoretically all run on solar power. A day dream, to be sure.

In the fall and spring, I have to go out and physically open windows and doors, or it gets too hot. Like over 100º. In the coldest part of the winter, Dec / Jan / Feb it can easily get into the 80's.

When I have to replace the plastic roof, I will install some automatic openers, which can be set to a range of temps and openings.

Here is a link that might be useful: assortment of automatic vents

I'll second the Vornado heater recommendation (although I just use mine in a basement room). It has a little microprocessor that adjusts both the heater element temperature and the fan speed upward or downward, automatically and gradually, in response to the ambient temperature in the room. It keeps on circulating the air gently even when it's not heating.

But then again, that feature is lost if an external thermostatic device is needed to set the temperature low enough. I wouldn't want to keep a greenhouse at 60 degrees all night either, so I would want to find one that went down to at least 50. (I hate the thought of running a heater in there at night, only to have to vent heat out somehow the very next day when the sun has been hitting all those windows. Maybe those special-glass windows will prevent that.)

For myself, I want a design that does let some solar heat in during the day, with some significant thermal mass inside to absorb it and give it back off at night. The 'artificial heat' would only kick in at 45 to 50 degrees at most. So in order to let winter solar heat in, I'll be willing to drape the shade cloth over the windows during warmer times too. I wonder if I could find an automatic device for that? Probably not on my budget.

Anyway, I guess one question to consider is whether you want a little more air circulation out of your heater. The oil filled radiator type will only give you passive convection.

Hi Holly,

I've been using oil-filled heaters for years now and I love them. They're inexpensive, easy to operate, easy to move, and will easily heat any normal size room quite nicely. When I was commuting to San Fran I had a medium size bedroom in someone's house and had one in there that worked too well! California people have furnaces in their houses, but I'm not sure why because they NEVER turn them on! It was usually too cold in the room with no heat at all, but I had to keep the oil-filled radiator turned on the lowest setting or it got too hot---I could have turned the room into a sauna if I had set it on high! Obviously a glass-walled room will take more heat, but in that size room I think you could easily keep it warm enough with one heater. I do agree with shudavies, though, that the one problem they have is that there's no circulation system. When houses used to be heated with steam radiators, though, there was the same problem, but they worked---the heat seems to "radiate" quite well, but I think it would possibly be more of a problem in a glass room.

Keep in mind that it will be colder against the glass than in the middle of the room, and you'll need to keep the plants from actually touching the glass or some of the foliage--especially on the more tender plants--could freeze when the temps get really cold. Unless you're going to have tropicals, 50 degrees, or even 40, should be plenty warm overnight.

Let us know what you decide and how it's working out once you have it finished.

Skybird

why not start by heating the water in the fountain and see if that keeps the room at temp?