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Heating · Boilers & Hot WaterCombi Boiler Installation
Combi boiler installation replaces a floor-standing boiler and a separate tank water heater with one wall-hung condensing unit that does both jobs. It suits a house where one or two hot-water fixtures run at a time; a home that regularly runs three showers at once is better served by a boiler with an indirect tank. A typical changeover takes one to two working days, and the price comes as a free written quote after we measure the radiators, the gas line and the vent route.
A combi is sized twice. The heating side is sized to the heat loss of the house, which in most homes here is modest. The hot-water side is sized to flow: how many litres per minute it can raise from winter inlet temperature to shower temperature, and municipal water drawn from Lake Ontario arrives only a few degrees above freezing in February. That is why a house needing a small boiler for heat still gets a combi with a large burner: the input is there for the shower, and the burner modulates down for the radiators.
Domestic hot water takes priority over heating. When a tap opens, the combi diverts its full output to the plate heat exchanger and space heating pauses until the tap closes. In a house with cast-iron radiators that stay warm long after the burner stops, nobody notices. It matters more with low-mass baseboard or a hydronic air handler, and it is the reason we ask how your household uses hot water before recommending a combi over a boiler and separate water heater.
The installation details decide whether the unit lasts. The boiler loop is piped primary/secondary, or through the manufacturer's manifold, so the small internal pump always sees its required flow no matter which zones are open. Venting is ULC S636 plastic pipe run to an outside wall with the clearances the gas code and the manual require. Condensate drains through a neutraliser to a floor drain or pump, because untreated condensate is acidic enough to attack cast-iron drains and concrete.
Gas supply is the item most often missed. A combi's burner is sized for hot water, so with both loads on one appliance the existing branch line is frequently too small; upsizing the run is the same job a tankless water heater needs and adds $400–$1,200. Old radiator systems also carry decades of iron sludge, so we flush the piping and fit a dirt separator ahead of the new heat exchanger. The wider boiler picture, including repairs to the unit you have now, sits on our boiler repair and installation page.
A combi boiler is the right replacement when
- The boiler and the tank water heater are both near the end of their lives and you would rather buy one appliance than two
- One or two hot-water fixtures run at a time, not three showers at once
- You want the floor space back: a combi hangs on the wall where the tank and boiler stood
- The old boiler vents into a masonry chimney you would like to stop relying on
- The radiators or in-floor loops can heat the house at lower water temperatures, which is where a condensing boiler earns its efficiency
How it works
We count and measure radiators or loops, run a heat-loss calculation, list the fixtures that run together, and check the gas meter, the branch line and the vent route to an outside wall. The quote is written and itemised, with a boiler-plus-indirect option beside the combi where the hot-water demand is borderline.
The old boiler and tank come out, the system is flushed, and the combi is hung with its primary/secondary piping, service valves on the hot-water side for future descaling, the neutraliser, the S636 vent and a correctly sized gas line.
We set gas pressure at high and low fire with a combustion analyser, set the outdoor-reset curve to suit the radiators, purge every zone and run a hot tap against the heating to show priority working. You are shown the pressure gauge, the fill valve and what the display codes mean.
Brands: We install condensing combi boilers, with Navien a frequent choice, and service other residential boiler brands including IBC, NTI and Viessmann.
Common questions
How much does a combi boiler installation cost?
No single figure is worth quoting, because four things move it: the size of unit your hot-water flow demands, how far the vent has to run, whether the gas line needs upsizing, and how much old piping has to be reworked or flushed. Our answer on new boiler costs in Ontario gives ranges, and a site visit with a written quote is free: book one here.
Can a combi boiler run two showers at once?
A large combi can, with some drop in flow or temperature in mid-winter when the inlet water is coldest. A small one cannot. If two showers plus a dishwasher is a normal morning in your house, we show you the litres-per-minute figures for each model at a winter temperature rise and, where the numbers are tight, quote a boiler with an indirect water heater instead.
Will a combi work with my old cast-iron radiators?
Yes, and often very well. Cast-iron radiators in pre-war houses were sized generously, so they can usually heat the rooms with cooler water, which keeps a condensing boiler in its efficient range. The risks are sludge and leaks at old valves once the system is disturbed. We flush, fit a dirt separator and pressure-test before commissioning.
Does hard water shorten a combi's life?
It shortens the life of the hot-water side if nobody maintains it. Halton and Hamilton water is hard, and scale builds on the plate heat exchanger where tap water is heated. We install isolation valves so the plate can be descaled without dismantling anything. Signs that a flush is overdue, and what the service involves, are on our combi boiler repair page.
Do I need a permit or a licensed installer for a combi?
The gas connection, venting and commissioning must be done by a TSSA-licensed gas technician working to the CSA B149.1 gas code. Whether a building permit is also required depends on your municipality and how much of the heating system changes, so we confirm with the local building department before work starts and show any permit on the quote.