A tunnel has three temperatures: the air above the compost, the compost itself, and the plenum below. DMR says the difference should be as small as possible and must not exceed 3 °C. A wider gap means air is not passing evenly through the mass.
During peak heating the compost rises at roughly 1 °C per hour and holds at 58 to 59 °C for 4 to 6 hours. Above 60 °C the process turns: DMR warns this kills the thermophilic fungi Phase II depends on, and lists it as a trigger for olive green mould.