Compost heater

Hi there, I was doing some gardening today raking up grass into piles and it occured to me that the decomp is making a LOT of heat that is wasted. So as a little project I got a 225l compost bin, coiled a 80mm slotted drain pipe inside It and filled it with all the grass. Leaving one end of the pipe low down sticking out as an intake, the other end of the pipe goes through a hole into my greenhouse. After a few hours it's now a noticable draft of hot air coming out of it and my greenhouse is a good 5 degrees warmer at least! 

I have a huge volume of grass cuttings and composting matter so thought I'd expand on this for a long term heating solution for my plants. I've got a 3m x 5m enclosure that I'm hoping to place another 80mm drainage pipe coiled through, leaving more room compost material tho, and fill it so it will take months to rot down. 

My question is, will the pipe dissipate the heat enough to prevent it setting on fire? What sort of threshold for a sustainable volume is there? It will be composed of layers of fresh grass and dried leaves. The pipe will supply constant cool air as convection circulates it.