Heat exchangers in the home: what they do

How does a heat exchanger work?

Heat flows from the hotter fluid to the cooler one through a wall of metal. The wall keeps the two streams apart, and the design aims to give as much surface area as possible, so heat has more chances to cross. The fluids can be hot gas and air, hot water and cold water, or exhaust air and fresh air. The temperature difference between them and the contact area set how much heat moves. More area, such as extra tubes, plates or fins, means more heat moved in the same space.

What does the heat exchanger do in a furnace?

In a gas furnace, burner flames heat the inside of a metal chamber, and the blower pushes house air across the outside. Combustion gases stay inside and go to the vent, while the air is warmed and sent through the ducts. A high-efficiency condensing furnace has a second exchanger that cools the exhaust until water vapour condenses, which captures more heat and is why those units need a drain and plastic venting.

A crack or hole in the primary exchanger can let combustion gases reach the house air, which is a risk for carbon monoxide. It is a common reason to replace a furnace rather than repair it, a point covered in furnace repair or replace and in the furnace replacement guide. A yearly check is part of the furnace annual tune-up.

Where else will you find heat exchangers at home?

• Boilers. Hot combustion gases heat water that is pumped to radiators or in-floor loops. See boiler versus furnace.
• Heat recovery ventilators. A core warms incoming fresh air with outgoing stale air. See the heat recovery ventilator guide.
• Heat pumps. The indoor and outdoor coils are heat exchangers that move heat between refrigerant and air or water.
• Water heaters. Indirect and tankless models use one to pass heat to potable water.
• Solar thermal systems. A heat exchanger keeps the collector fluid, which often contains antifreeze, apart from the water in the storage tank. See how solar water heaters work and drainback versus glycol.

Why were heat exchangers so important at Drake Landing?

Drake Landing Solar Community used the same principle on a larger scale. Heat from the solar collector loop passed through a heat exchanger into the water of the storage tanks, and the district loop then carried heat to each house, where it passed through a coil in the air handler. Each hand-off kept the circuits separate, so the system could use different fluids and pressures in each part. The how it works page follows that path, and what is a district loop explains the piping link.

What problems affect heat exchangers?

Fouling is the main one: dust, scale or deposits on the surface cut heat transfer. Corrosion, cracks from repeated heating and cooling, and blocked passages cause the rest. Regular filter changes, annual service by a licensed technician and good water quality in hydronic systems reduce the risk. If you notice soot, a burning smell, a carbon monoxide alarm or uneven heat, call a professional and see heating emergency in winter. Farm uses are covered in livestock barn heat recovery.

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