Combi Boiler Facts

How They Work

• A combi boiler incorporates a tankless water heater and space heating boiler into a single compact unit for installation in homes where space is limited. Combi boilers eliminate the need for a hot water tank. Instead, they have an internal heat exchanger that transfers heat from the boiler water to your kitchen and bath water as it runs through the unit. Some models include a small supplemental heater and small tank to provide instant hot water when the boiler isn’t firing. A combi boiler can heat bath water in about 40 seconds and provide an unending supply of hot water to your kitchen or bath for as long as the boiler is firing.

Installation Considerations

• When selecting a combi boiler, make sure it meets both your heating and domestic hot water needs. Since combi boilers have no domestic hot water tank, they depend on the pressure from your water mains to provide flowing hot water. Make sure your heating needs, water pressure and flow rate match the specifications for your combi boiler unit. Combi boilers don’t do well in areas with hard water. Without water treatment, the calcium salts in hard water eventually will clog the heat exchangers.

Fuel and Heat

• Combi boilers heat rooms with baseboard or radiant hot water heat. They cannot be used where homes are heated with hot air. They usually are fueled by natural gas or propane. Combi boilers are more economical for heating water than a stand-alone tank-type water heater because they don’t use fuel to keep reheating the water in a hot water tank until it’s needed. Combi boilers are common in Great Britain and continental Europe for house and apartment heating.

Combi Drawbacks

• Combi boilers have two main drawbacks. When the outside weather is very cold and demand for space heating is high, a combi boiler can have trouble maintaining a boiler water temperature high enough to keep the water for kitchen and bath piping hot. Also, combi boilers generally don’t have the capacity to handle several simultaneous hot water users. They may not be able to keep up with demand when one user wants to run the dishwasher while another wants a hot shower.