I had read that heat was the main trigger of seed germination. Soil has to be loose and light, so there is room for the roots between the soil particles. Water is important too but not so wet that it fills the room for the roots. Enough so the seed knows there is water around to get started. So heat, good soil and some water. Some seeds need certain daylight hours to trigger germination, like the longer hours of spring. Mainly the light becomes important as the plant grows.
So I put together the structure in the photo above. This has a board about an inch think. I put some nails in it to guide an old garden hose around the base. the hose is synched off at the end in the middle to stop the water and the hose to the left leads off to a funnel. Into this funnel you pour hot water. The seed trays sit on top of this hose, which heats the soil. I haven't got a thermometer but it is meant to be about 23 degrees for most seeds. To change the water I unsynch the hose in the middle and blow the water out. Close it off again and fill it with water from the kettle.
The photo above shows the seed germination system covered. It has simple heavy gauge wire wound abound the base and some clear plastic laid over the top. This keep moisture and heat in and amplifies the heat from the outside.
So this is the results after 6 days. Compared to 3 weeks when we just put seed trays on the veranda. Average outside temperature during this period has been 19 degrees. There has been lots of rain too so not much sunlight. I changed the water twice a day, morning and afternoon. From left to right is lettuce, spinach and beans. I am getting my winter crops ready. I will be transplanting some of these beans this afternoon. If I was to do this again, I would try to make emptying the water easier. I would have the hose in the middle, feed through the board to a tap or something.
A U-Beaut system would have a thermometer in the soil linked to a release valve on a hot water supply like an urn. The dream continues, but this works well, and gets us healthy seedlings quickly.
See version two here - Next Stage Seed Germination