One of my passions is growing food all year round, I especially like to grow and eat a huge salad of leafy greens every day of the year. Using hotbeds makes this much easier and it just makes February and March on the allotment more fun.
There are lots of different ways to make a hot bed, but I've chosen a method that suits the design of my plot. It's fairly expensive though and the payback on materials is about 18 months, the rest of the plot has paid back in a few months. I don't mind that extra investment though because it turns February to May :-).
I made my first hotbed last year and it worked very well, so this year I've squeezed in this second one and filmed the whole process. This video ends with an empty box, which I will fill with horse manure and wood chip bedding, a few leaves, seaweed, rock dust and a few handfuls of chicken manure. On top of that I put 3 to 6 inches of garden compost, which I plant into. All that starts decomposing and generating heat and it's ready to plant in early February.
I add the leaves, seaweed and rock dust because when I empty the hot bed in late Autumn, I have lovely rich compost all ready to use as a mulch on some of my other beds.
If you are considering growing more veg all year round I recommend starting with just a cold frame, then a hoop tunnel and finally a hotbed, ie in order of cost effectiveness. I'd only start with a hot bed like this one if you have a cheap source of wood and really need extra composting space. You can achieve quite a few benefits of a hotbed, just by over-wintering young salads in a hoop tunnel.