Given the purpose you're using it for (heat production) you probably need a composter twice the size of the one you have - 3 feet square is the minimum size for a 'hot' compost heap, and twice that is better. It'll need turning at least twice a week, and if its still not hot enough, a source of nitrogen added periodically - whether that's from greens or urine is up to you. Heap should be moist, but not sodden, open to the ground at the base, and covered to retain heat. Using this method, any material in the heap should be ready for use on the garden within 3 months, maybe a bit longer in the winter, so that means you'd need to build another heap once that stage is reached.

you dont give the width of your pool, assuming the 15x3 is length & depth. You are dealing with thousands of gallons of water to warm up so in order to get the required amount of heat from compost would require a pretty hefty sized container way beyond the 400 litre size you mention.
Subterranean systems are used for such projects.

I suspect one of those ground heat systems would be much more effective.

thanks each

I have considered the size issue but not the idea of a subterranean system. More info or a link would be apreciated.

Kelvin

PS 15ft Diameter

I don't have a link, but what I'm thinking of is one of those ground heat exchangers they use in eco houses when they build them - they're usually buried pretty deep and feed back into underfloor heating systems, so not sure whether you could find one that would serve your purpose. Nor how expensive a solution it would be to fit, come to that.

Your question reminded me of the pineapple pit at Heligan Gardens in Cornwall. They grow pinepples in a mostly underground sort of greenhouse that is heated by decomposing horse manure.

http://en.wikipedia.org/wiki/Pineapple_pit

I believe that they can't keep using manure for this as they run / ran out of suitable manure and it takes a huge amount of manual labour to keep it running, so later crops have been produced using electricity for heat.

This article describes the mixture used to produce maximum heat -
http://www.heligan.com/press-releases/press-release/pineapples-emerge-at-the-lost-gardens-of-heligan/

Ice energy ground source heat pump link is.....

http://www.iceenergy.co.uk/

My idea was to keep the costs down, 4 years ago it was costing >£400 per year to keep it warm (25/26 deg) between may and sept and that was E7, but then again at least we were using the pool. The cost of a ground pump system would/could be in the same region per year not inclueding the setup! Not too bad though if you are heating your house.

The size of my garden isn't bad but nowhere near large enough to support enough horses to produce 30 tons. Just think how many tennis balls of the stuff that is?

I am now planning how to increase the size of the bin and how to pump air through to avoid too much turning. I am also considering putting a water resovoir in the centre about the size of a football but as yet the mechanics are aluding me!

Kelvin

Sorry to be a wet blanket, Kelvin, but I just don't think it's feasible to get a significant amount of heat out of compost to heat up such a large volume of water. How about passive solar heating?

good luck to you Kelvin, I admire the inventive 'Heath Robinson' spirit. Been there a thousand times & now & again something works eh?

Don't forget it is not from a zero start point, I have just checked and it's 15 Deg c and that is basically as a result of a passive solar system which is seperate from what will be the compost system, they would work in tandom. Last week it was cold but with sunshine and the pool raised 2 deg. A warm 23 with no sun is of little use to the system.

Heath Robinson's got nothing on me. +5 deg now would just make it swimable

Thank you for your input

Kelvin