Instructions on seed packets often suggest starting them off on a sunny windowsill or some other such warm place. Now, warm places are in rather short supply round here (bread dough takes ages to rise at this time of year), so I thought it might be worth providing a dedicated heat source for my tomato seeds. More to the point, I had an idea for how to do it.
1. Take one drawer from an old wardrobe.
2. Line with polystyrene then put a heater, from the same old wardrobe (yes, there were heaters in the wardrobes when we moved in. That tells you something about how damp this house gets), in the bottom. I thought about taking it off the board it was attached to, then decided it wasn't worth the effort, so the heater is a bit off centre. The plug socket is a bit unnecessary, but I couldn't be bothered to remove that, either.
3. Add bricks. These are mainly to help support the top, but they'll add thermal mass too, for what it's worth.
4. Cover with a sheet of aluminium that used to go between a gas fire and the fireplace behind (hence the hole).
5. Add seed trays and a couple of sheets of greenhouse glass on top. Just the right size for eight seed trays.
6. I pinched a thermometer from the central heating so I could keep an eye on it.
I spent quite a lot of time with Google, trying to figure out how long to leave the heater on for, and whether I should try to incorporate a time switch. This heater is 60W, the same as a not-particularly-bright old-fashioned light bulb. I saw descriptions of propagators with 8W heating elements, but they only heated one seed tray at a time, so scaling it up, 60W would be about right for 8 trays. I also came across one DIY system that was 150W for 20 trays (I'm assuming all seed trays are the same size), which would also be about right. All these were on all the time, but only for a few days until seeds germinated. The temperature should ideally be in the low 20s (Celsius), so I'll leave it on constantly and check quite frequently to make sure it doesn't get too hot and cook the seeds.
EDIT: It had got to 20°C by morning, so will certainly need turning off for some time during the day, even if the sun doesn't come out.