I put a cat flap from the house into the greenhouse so the cats could sit in there any time. They seem to really like it , and I don't have to let them outside at night, saving the wildlife from them and them from cars.

Good idea... since I have 6 cats. What's your thoughts on water tanks with aquarium heaters, versus electrical costs?

Electrical heaters are (as far as we're concerned), all equally efficient (with the notable exception of heat pumps, which are about 3 times as efficient). So if you need to use 8kWh to heat your greenhouse, you'll need 8kWh whether you heat the water or heat the air. However, if you need the water to be warmer than the air, you will be better off heating the water and letting the waste heat heat the greenhouse.

In the UK it is common to use DHW (domestic hot water) boilers/furnaces to heat a loop of water that runs under the benches and through the tanks. These can run on natural gas and the waste CO2 used to feed the water plants. Hydronic heaters will work as well. A solar hot water heater mounted outside the greenhouse(in the roof) is also practical, you can get an evacuated tube heater for around $700US which should be enough for a small/medium house attached greenhouse.

One downside of heating the water rather than the air is that you'll keep the humidity at close to 100% unless your tanks have a cover. If you grow nepenthes or true tropical orchids this may be a good thing, if you grow desert plants (as I do), a bad thing.

Humidity would be a good thing for me... I grow mostly tropicals.

I was reading something earlier about running corrogated piping underneath composting, thus heating the water which in turn would run back through registers throughout the greenhouse. It was sooooo over my head, as you can probably tell. *lol*

If you are going to have that much humidity in a tight space, you are going to have a lot of mildew. Regular watering of tropicals will provide more humidity than you will need.

That's an interesting point, Tsmith. Most days the front would be rolled up. Even still, there's the mildew to consider. More to ponder over. Thank you.

If you have the front rolled up you're going to have trouble heating it...

I'm wondering if I -would- have trouble heating because my enclosed porch, which is also south facing gets pretty hot on many winter days... except during freezing days, and we don't have alot of them. I'm thinking that the temps would rise very high sometimes. What do you guys think?

See, this is what scares me... the unknown!

Here are my average temps... I'd guess we have ten nights below freezing, and 10 days at around freezing per winter.

Highs Lows
Dec 60 39
Jan 55 37
Feb 58 38

If you have lots of water in there, and some kind of radiator/heat exchanger to move heat in and out of the water, you should be able to keep the temps fairly constant. If you're getting hot during the day and cold at night, lots of water can be quite beneficial.

I'm thinking of adding a dog flap.

I have two water troughs that I keep my tropical water lilies in. I use 300w electrical aquarium heater to keep it at 65°F. We have fish in there all year round and my kids love to put tropical fish from the pet store in there. It has black liner that helps absorb the heat and also puts plenty of humidity in the GH!

I think the large body of water helps keep it warm during the night so the heater does not run too much. Once it is up to temp the heaters do not run very long.