We spent an hour or so last night putting together our new system for providing bottom heat to our germinating seeds. I wrote a little about the cost of the set up in this post; it's the cheapest design we could find. The method is taken from this site, which I came across while searching "diy seed heat". Our set up is a little different than the link, but the concept is the same--using rope lights to warm a container of sand which sits below a container of seeds/plants. The basic idea is that the warm sand provides heat to speed the germination of the seeds.
I won a lot of 4 18' rope lights (not LED!) off of ebay for $20, after shipping. We bought three bags of play sand from the hardware store and Mason picked up a box, measuring 24x24x12, from Budget Box in OKC on his way home from work one day. Last night, we gathered our supplies and set to work. First, Mason cut the box in half horizontally, leaving us with two lidless boxes measuring 24x24x6. He also taped up the corners and cracks to prevent any leakage. The two boxes fit snuggly on one of our 24x48 shelves.
Then, we coiled one 18' rope light in each box. The lights connect together and only one cord needs to run to the power supply. We probably could have done a better job of measuring and coiling it evenly across the bottom of the box, but we just spread it out to cover most of the box and started scooping sand on top to hold it in place. We covered each light with about three inches of sand, smoothing and leveling with our hands as we went.
With only three inches of sand needed to cover the lights, our seed flats fit perfectly into the box, with plenty of room to adjust our overhead fixtures as the seedlings grow. We cut a sheet of black plastic to cover the sand and set the cell trays on top of it. Mason plugged the rope lights in and we left them to go watch Big Love. An hour later, the cold sand was a little warmer than room temperature. When I checked this morning, it was noticeably warmer--probably warmer than necessary, but no where near burning hot. We'll need to get a thermometer and figure out how long the lights need to be on to maintain a good temperature for germination. If we can find the right increment of time, we can use a cheap lighting timer to turn them on and off automatically.
It's hard to get a decent photo, but it looks like this design is going to work for us. There are potential problems--rope lights burning out, sand getting moist, etc--that we'll have to deal with as we go along. But, I read numerous accounts of problems with heating mats, so I think we'd probably have had to deal with burnouts one way or the other. At least this way we can replace one string of lights for five or ten dollars without having to throw out the entire system.