The district loop: how heat travels between buildings

What does a district loop consist of?

The essential pieces are few. A central plant or energy centre produces or stores the heat. Insulated supply pipes carry hot water out, and return pipes bring the cooler water back, usually buried so they stay protected from frost. Circulation pumps keep the water moving. At every building, a heat exchanger or a coil passes heat from the loop to the building's own heating system without the two waters mixing. Controls and valves regulate the flow. The wider concept is covered in district heating in Canada explained, and the plant side is shown on the energy centre page.

Why does the loop temperature matter?

Heat is lost from the pipes along the way, and the hotter the water, the more is lost. A lower loop temperature also lets the plant use gentler heat sources such as solar collectors, waste heat or heat pumps, and it lets those sources run more efficiently. The trade-off is that buildings need heating equipment that can work with lower-temperature water, such as large coils or in-floor systems. The topic is developed in low-temperature district heating in Canada and hydronic heating with low temperatures.

A useful rule of thumb is that a loop is described by its supply and return temperatures. The gap between them shows how much heat each litre of water carries.

How did the loop work at Drake Landing?

In the Drake Landing Solar Community in Okotoks, Alberta, the district loop connected the Energy Centre to 52 homes. Solar collectors on garage roofs gathered heat, which went through two short-term water tanks of 120 cubic metres each and into the borehole thermal energy storage field. The loop then carried heat from storage and tanks to each house, where it passed through a coil in the house's air handler. The loop typically ran at about 40 to 50 degrees Celsius. The pages on district heating and the air handler describe the details.

The central idea is that one shared loop lets collectors, storage and houses be sized and managed as a system. The borehole side is explained in borehole thermal energy storage.

What are the strengths and limits of a loop?

A shared loop makes it possible to use a large central heat source and to store heat for a group of buildings, which is hard to do for one house. Maintenance is also centralized. The limits are the cost of trenching and piping, heat loss from the pipes, and the need for enough customers per length of pipe to make it worthwhile. Dense areas suit it best, and long runs to scattered houses do not. The comparison is the subject of district energy versus individual heating.

Is a district loop the same as a geothermal loop?

No, though the names cause confusion. A ground loop connects one building's heat pump to the earth, as explained in ground loop types. A district loop moves heated water between a plant and many buildings. Some systems combine both, for example a district network that is supplied by borehole storage. To see how solar fits, read solar district heating in Canada.

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