I still think the only way this would work (aside from having the hot tub in the green house, would be to capture lost heat. Now my hot tub is very well insulated, especialy above the tub. My hot tub is a self contained tub, the heater is contained in the tub enclosure. But I think this method would work even better with an inground hot tub. Idealy, if the hot tub is not installed yet, you could build an inground hot tub (the heater is seperate with an inground hot tub). Install the just the heater inside the greenhouse. Otherwise, the place I have found the most Waste heat with my Tub, is above the heater. So hputting some sort of heat exchanger above the heater to capture the heat lost from the heater would work very well.

I'd like to tag a question in here too. Have to start seriously looking for a new tub- think I want an air tub, but have never tried one. But being comfortable while soaking is the FIRST priority. Haven't had a bath for months during this remodel, but DH rigged it up so I could have a soak in our old tub (standard 5 ft length enamelled steel?), and boy was it hard on the neck! I felt like something was digging into my back between my shoulder blades. Guess I'm used to the hot tub and climbing in there and having nothing digging in. So, my question is, what shape of back is best for soaking? Should it be sloped? Guess I'll have to wear comfortable clothes and try and find a showroom that I can climb into tubs in. Glad I had the 'crink in the neck' experience though before shelling out $$$$ for a new tub!

I just tried out my brand new Aquatic Hotsoak 74 for the first time last night. So far so good. It wasn't hard to clean at all, but we installed a sprayer which made the process easier. The recirculating pump makes only a very slight noise and seems to function well at maintaining the water temp. I also chose this unit sight unseen for the same reasons that you mentioned and I think I made the right choice.

I insulated a soaking tub before I installed it and I love it! I actually insulated the tub surround and the tub itself with pieces cut from 4' X 8' X 4" foam and used foam-in-a-can to fill in gaps and to stick it all together. It's a wonder that tubs aren't made pre-insulated.