An indirect system is a heating system where the heat emitters are not in direct connection with a boiler or district heating system. The separation takes place by placing a heat exchanger in the heating system.
Certain elements of the heating system can be direct while others indirect, depending on the utility company. The district heating or boiler side of the exchanger is called the primary system. The consumer or heat emitter side is called the secondary system.
Placing the exchanger makes it possible to minimise any damages caused by leakages from the district heating system. It also prevents difficulties that may arise from pressure variations in the district heating system.
Disadvantages include operational losses, lower flow temperatures, and periodic excessively high return temperatures.