Combi Boiler

A combi boiler is a wall-mounted or floor-standing appliance that combines a water heater and heating system into one device. When a hot water tap opens, the boiler fires and heats incoming cold water to the setpoint before delivery—no storage tank, no thermal mass sitting idle. The same burner handles radiator loops in winter and DHW in summer, or both simultaneously if the unit is sized appropriately.

In the field, combi units save floor space and eliminate standby losses from traditional tank storage, making them popular in retrofit work and tight residential jobs. They respond quickly to demand but can't supply multiple high-flow outlets at once without a noticeable dip in temperature. Flow rate is the limiting factor; most domestic combis top out at 10–15 GPM for hot water. If a job calls for high simultaneous demand—say a shower and a fill-and-flush—you'll either need a larger combi, a buffer tank, or a different system altogether.

Combis require good water pressure (typically 20 PSI minimum) and adequate flow from the main to modulate correctly. Low-pressure supply causes hunting or lockout. They're also less forgiving of scale and mineral buildup than simple tanks, so water quality matters. Common in Europe for decades, they've become mainstream in North America since the 2000s, especially in condensing and tankless hybrid applications.