Heated raised beds

I was wondering how long the heating lasts? it obviously must run out of steam eventualy. We used to have stand pipes all round our poultry farm and piled manure round them in the winter to keep them frost free.

I've just made a hotbed, although its only got wooden sides so its not particularly well insulated. I started off with a layer of soaked cardboard, then 1ft of manure, topped a week later with 4 inches of compost. I've used horse manure, fresh from the stable, but with wood shavings as bedding, not the more favoured straw. The manure has been in for 2 weeks, and the compost layer on top for a week. I covered the manure and then the compost with bubble wrap to keep the heat in. The hottest it got was 30 degrees C, and its now dropped to 20. I planted some french bean seeds and a courgette seed in it yesterday, putting the bubble wrap back over and also a sheet of rigid clear plastic about 4 inches above the surface. I planted some similar seeds at the same time in the greenhouse for comparison.

As I understand it, the bed should gradually lose heat over the next couple of months, by which time the weather should be warm enough for it not to be needed. the surface level of the bed has already dropped an inch or so since I filled it, and I did tread it down as I put the manure in.

You need to watch it to get the full details.
But I think the main fuel was fresh damp hay.
The clip showed the hay giving off steam.
Jimmy

Update on the hotbed after a very cold night. The soil temperature dropped from 20 degrees yesterday to 10 this morning with the outside temperature jsut below freezing, but it soon heated up again (the thermometer is about 2 inches in from the edge of the bed, so it will be hotter in the middle).

The comparison between the hotbed and the greenhouse is proving interesting. 4 of the 5 french beans I sowed in the hotbed are showing above the surface now, as is the courgette. None of the seeds sown in pots in the greenhouse are showing any sign of life yet.