How a hot water boiler heats your house

How does a hot water boiler work?

A burner or electric element heats the water inside a sealed vessel. A circulator pump sends it through pipes to the heat emitters in each room, and the cooler water returns to the boiler to be reheated. An expansion tank takes up the swelling as the water heats, a pressure relief valve protects the loop and a thermostat tells the boiler when to fire.

In many homes the loop is split into zones, each with a valve or its own pump and its own thermostat, so the bedrooms and the living area can run at different temperatures. Because water carries heat well, hydronic systems are quiet and steady, and there are no ducts to carry dust. The trade-off is that a boiler does not move air, so cooling and ventilation need to be handled separately. See boiler versus furnace in Canada.

Is a boiler the same as a water heater?

No, though the words overlap. A water heater makes hot water for taps, showers and appliances, and the water it heats flows out of the house. A hot water boiler heats water that stays in a closed heating loop. Some homes connect the two, with an indirect tank fed by the boiler, which is covered in indirect water heaters explained.

A third version, the combination boiler, heats the loop and also heats tap water on demand. It suits small homes with limited space. For tap water on its own, compare options in water heating system options.

Which fuels and efficiency levels are available?

Boilers run on natural gas, propane, oil or electricity, and the choice is usually dictated by what is available at the address. Electric boilers are almost fully efficient at the point of use, but electricity can cost more per unit of heat than gas, depending on the province. Conventional gas boilers are rated in the 80s for AFUE, and condensing models can reach the 90s when return water is cool, as explained in condensing boiler efficiency.

Check the sealed combustion option, which brings outdoor air to the burner, and the sizing method. Boilers should be sized from a heat loss calculation of the house, not from the size of the old unit. A heat pump can also serve a hydronic loop, a path compared in low-temperature hydronic heating.

What upkeep and lifespan should you plan for?

Boilers need an annual service. A technician checks combustion, cleans the heat exchanger, tests safety controls, looks at the pressure and the expansion tank, and bleeds air from the loop. A well-kept boiler can last a long time, often longer than a furnace, and the figures are in heating system lifespans in Canada. A service contract may bundle the yearly visit.

Drake Landing Solar Community shows the hydronic idea at scale. Each home had its own hot water coil in an air handler fed from a district loop. See the air handler page for how that heat reached the rooms.

Quick answers