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Well I used an old waterbed heater for a heat mat whitout any problems.
It worked well and I had no safety problems. Just keep the plug away from dampness. I put my containers with holes in some bigger containers without holes to prevent the water touching the heat mat.
I stopped using it because growing tropicals here after germination is a problem.
I used the old mattress of the water bed for a small pond. Still works perfect after 3 years. No leakage at all.
Dana, you must have a big greenhouse to have room for a water-bed! I use the same theory in my cold frames real early in the season. I have several old laundry detergent bottles that are made of dark blue plastic. I fill them with water and leave them in the sun on the south side of a shed and at night I move them into the cold frames. I also put a couple of sheets or a blanket on the cold frames to help hold in the heat. Doing this I can get an almost 15 degree buffer on those nights that get really cold. I've had outside temps as low as 20 degrees without any damage to plants in the cold frame. The water bed should do the same thing on a different scale. The heat exchange probably won't be very efficient, but it should give you a lot of buffering capacity on cold nights.
I have 2 12x 20 full my thoughts are put waterbed mattress in there build a frame around it so you can still use the space. heat it during the day and have it on a timer so it doesn't heat all night. Then in the morning when sun comes up heaters back on so electric shouldn't be to terrible.
I think your on to something. If you build around the mattress which will be heavy duty enough to hold real cold sensitive plant's or even potting mix .The actual top of the mattress would have to be in contact with the bottom of the wood or what ever you used for the shelf,so the heat could transfer to the potting mix and then radiate up and out.I'm not sure how much that would help heat the ambient temp of the enclosure .I've had many waterbeds in the past and they don't heat the room up at all,just what ever comes in contact with the mattress. Dana if I remember correctly they were real cheap to run, like pennies a day if the were covered with a heavy comforter. Don
Think about it people use gallon jugs and 5 gallon buckets to heat up during the day. As much water as the mattress holds I think it would work cover mattress pad held heat in. Nothing between it and air I think this will work shelves would be slotted to allow warmth to come thru. My green houses are temp. car ports that I buy green house grade plastic. And close up Christmas I went home to Ct. while I was gone weather took a turn low 20's. I about died I heat with propane while I'm here I figured everything would be dead. But I could not believe it lose of leaves and very little tip damage to brugs, plumeria was fine.