Building the heatsink

I’ve now started building the heatsink. This consists of a loop of pipe at the bottom of hole. I’ve drilled holes in the pipe and this will let air out:

I’ve filled the hole with a mixture of stone and broken glass. The thermal properties of both should mean that the warm air permeating through should warm the fragments. They will keep the heat during the day and release it slowly over night (that’s the plan anyway):

Having filled the hole with stone and glass I then fitted an old drain surround in to act as a chimney and then sealed the heatsink with brickwork: