District heating is the dominant heat source in Danish cities and a central HVAC work area. Here we cover direct and indirect connection, the district heating unit with heat exchanger, why large temperature drop (low return temperature) is incentivised, and the supply/return temperatures you work with — plus the district heating certificate in the education.
Introduction
In district heating, heat is produced centrally and pumped out to consumers as hot water in an insulated pipe network. At the individual customer, the heat must be 'exchanged' into the house's own system — heat and hot water — and the district heating water is sent colder back to the plant. The plumber's task is to connect, dimension and balance this transition so the system is safe, efficient and cools the district heating water well.
Two ways to connect
There are two basic principles for how the building's system is connected to the district heating network: direct and indirect connection. The difference lies in whether the district heating water itself flows into the building's radiators, or whether it is kept separate by a heat exchanger.
- 01Direct connection: district heating water circulates directly in the building's heating system (possibly via a mixing loop/shunt that lowers the temperature). Simple and inexpensive, and it provides the best cooling — but the building's system is exposed to the district heating network's pressure and water quality.
- 02Indirect connection: a heat exchanger separates the district heating network's primary side from the house's secondary side, so the two water circuits are never mixed. This protects the house's system from network pressure and is used e.g. in tall buildings and where network pressure is high — at a small temperature penalty.
The district heating unit
The connection itself is nowadays typically assembled in a prefabricated district heating unit — a compact module with heat exchanger(s), pump, controls, safety equipment and meters. A unit typically contains two functions: heating the building's heating circuit and producing hot water.
- 01Heat exchangers: a plate heat exchanger for the heating circuit and — with flow-through water heaters — one for the hot domestic water heated 'on demand', so there is no large tank losing heat.
- 02Control: a thermostatic or electronic controller governs the flow temperature (often weather-compensated) and typically prioritises hot water over space heating.
- 03Safety: safety valve, pressure gauge and thermometer protect and make operation readable.
- 04Meter: the energy meter records both energy and cooling — and is the basis for billing.
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