Although both can heat your home and provide hot water, there are key differences. Traditional boilers are designed mainly for space heating. Hot water from the boiler is circulated through radiators or a hydronic heating system to warm the ambient air. The traditional boiler water may also be used for domestic hot water by directing the water through a separate, integrated water heating unit before routing it to a fixture, such as a faucet or shower. Some traditional boilers are compatible with potable water, but others require additional water treatment.

Combi boilers are designed to accomplish both space and water heating with a single appliance. Like a tankless water heater, a combi boiler provides hot water on demand. It can also seamlessly switch between water heating and space heating to provide the best possible experience.