To help control the temperature on the Tube network for customers and staff, there are a series of ventilation shafts that carry hot air from the Tube tunnels up to ground level. One of these ventilation shafts is located at the corner of Moreland Street and Central Street.
Warm air created by trains and machinery in the Underground network goes up the ventilation shaft and is pushed by a fan through a machine called a heat pump. The heat pump captures heat from the warm exhausted air, via a closed loop water circuit in the ventilation shaft. This is used to heat a gas, which is then put through a compressor, converting it into a very hot liquid and making the pipes that hold it very hot as well.
These hot pipes are used to heat the water that runs in the pipes of the Bunhill Heat Network so that they can heat the buildings connected to the network. The pipe network is very well insulated to minimise the amount of heat lost on the way to the buildings.
Taking advantage of the properties of a refrigerant gas, the hot liquid in the heat pump passes through the water in the pipes and in doing so turns back into gas. The heat pump is then ready to be warmed up again by more warm air from the Tube. In addition, the fan in the ventilation shaft has the potential to be reversed in the summer to provide cooling to the Tube network, helping to make journeys more comfortable.