Combi boilers

Combi is short for combination: one appliance that combines space heating and domestic hot water. Inside a single wall-hung cabinet there is a sealed condensing gas boiler for the heating system and a second, domestic hot-water path. When a tap opens, cold water flows through a plate heat exchanger and is heated directly by the boiler's burner, so hot water is produced on demand rather than drawn from a storage tank. No tank means no stored-water standby losses and no separate water heater in the mechanical room, which is why combis are a natural fit for townhouses, condos and smaller homes where space is tight.

The hot-water path is kept separate from the heating water. Boiler water circulates through one side of the plate heat exchanger while fresh domestic water flows through the other, so the water in the radiators never mixes with the water from the tap. The boiler fills, pressurizes and treats only its closed heating loop, while the domestic side connects straight to the home's potable plumbing.

When a hot-water tap opens, a flow sensor tells the boiler to switch from heating duty to hot-water duty, and the burner fires to hold the outgoing water at the set temperature. Delivery is continuous, but it is not unlimited: the amount of hot water a combi can supply per minute depends on how much gas input the burner can put into the water and how much the water must be warmed. The same boiler that heats a two-bath townhouse comfortably will deliver noticeably less hot water on a cold January day, because the incoming water starts colder and every gallon needs more heat.

The practical limit shows up as simultaneous demand. One shower is generally within reach of a correctly sized combi, while two showers plus a dishwasher running at once can exceed what the appliance can deliver, and the water temperature at the fixtures starts to drop. Manufacturers publish continuous delivery figures so the limits can be checked: IBC, for example, documents its SFC combi range on its Canadian product page, with continuous domestic delivery of roughly three to five US gallons per minute depending on the model and the temperature rise, numbers that appear on the model pages on this site.

Because hot water is made as it is used, there is no reserve: a combi cannot serve a large soaker-tub fill at full temperature while the rest of the household draws water at the same time. Homes with high simultaneous demand, or owners who simply want a large stored reserve, usually do better with a heating-only boiler and an indirect tank, which is the system-boiler arrangement described elsewhere on this site.

Combi boilers are common in the Lower Mainland for the same reason they are common everywhere: most households draw hot water in short, sequential events, and a combi covers those without the footprint of a tank. Townhouses and condos with a modest number of fixtures, secondary suites, and detached homes where the bathroom and kitchen are close to the boiler all suit a combi. The boiler sits on a wall in a utility closet or kitchen cabinet and vents through the wall, which keeps the whole system out of the living space.

The mild BC climate helps in a second way: the space-heating load is small, so a combi sized for the home's hot-water demand usually has more than enough heating capacity left over, and the long shoulder seasons are where its modulating condensing burner runs most comfortably. The fit question is not the climate but the hot-water pattern, which is why sizing a combi starts from the fixtures and the number of people, not from the floor area.

A combi is still a condensing gas boiler, so the installation rules from the condensing page apply: certified ULC S636 plastic venting to an approved outside terminal, a condensate drain with neutralization where required, a sealed heating system, and annual servicing. What is different is the gas side. Firing the burner hard during hot-water draws means the gas supply pipe and the meter have to carry the boiler's peak input, and an undersized line starves the burner during the exact moments hot water is wanted. A licensed contractor verifies the supply when the boiler is selected, not after installation.

The heating loop itself is compact: most combis include their own pump, expansion vessel and pressure gauge, which is why the mechanical footprint is so small. That built-in equipment also means the installer must confirm the appliance's built-in expansion volume and pump match the home's loop, and that zoning, if any, is done in a way the boiler's controls support. As with any gas appliance in BC, installation requires a gas permit, homeowner permits apply only to single-family owners in the cases Technical Safety BC describes, and strata owners must use a licensed contractor, who also confirms municipal permitting where the city issues its own permits.

Ask what continuous hot-water delivery the proposed combi achieves at the incoming water temperature this home actually sees in winter, and compare that with the largest simultaneous draws in the household. Ask whether the gas line and meter can carry the boiler's peak input, and ask where the condensate will drain. Ask how the space-heating side is sized, because a combi chosen purely for hot water can still be the wrong heating boiler for the radiators.

Ask what the boiler does during a long hot-water draw, and whether the control's behaviour matches how the household lives. Ask about service access, since combis concentrate a lot of machinery in a small cabinet and the annual service is a real requirement, not a suggestion. The consultation service on this site works through hot-water demand and boiler type before any equipment is proposed, and the instant estimator on this site can give a ballpark range for a combi installation in the meantime.