Quick answer
The size of boiler you need equals your home's heat loss on the coldest design day, calculated room by room to CSA F280 and checked against the radiators or tubing it feeds. Do not copy the old nameplate: older cast-iron boilers were commonly oversized. Match the heat loss to the boiler's output rating, not its input, and size a combi by hot water flow.
What size boiler you need is decided by one number: how many BTUs per hour the house loses when it is as cold outside as it normally gets. Everything else, including the rating stamped on the boiler in the basement now, is a distraction from that number. A boiler sized to the real load runs long, quiet cycles and lasts. One sized by guesswork short-cycles for twenty years.
Size a boiler from heat loss, not from the old nameplate
The starting point is a room-by-room heat-loss calculation to CSA F280, the Canadian standard for sizing residential heating. It adds up what each room loses through walls, windows, ceilings and air leakage at the local design temperature. Our glossary entry on the CSA F280 heat-loss calculation explains what goes into it.
The old boiler's rating tells you what someone installed decades ago, not what the house needs. Many cast-iron boilers went into houses that had no wall insulation and single-pane windows, and installers added a generous margin on top. Since then the attic has been topped up and the windows replaced, so the load has dropped while the nameplate has not. Copying it forward bakes the old mistake into a new boiler.
The second check is the connected radiation. A boiler can only deliver what the radiators, baseboards or floor loops can give off, so the technician counts and measures them. Cast-iron radiators are rated in square feet of EDR, and each square foot emits about 150 BTU per hour at an average water temperature near 77 °C. In a pre-war brick house, the radiators are often bigger than the insulated house now needs, which is good news: the system can run cooler water.
Input, DOE output and net rating: which BTU number counts
Compare your heat loss to the boiler's output, never its input. A boiler plate carries up to three numbers, and mixing them up is the most common sizing error we see on quotes.
- Input: the gas the burner consumes. A 100,000 BTU/h input boiler at 95% efficiency delivers about 95,000 BTU/h to the water.
- DOE heating capacity: the heat that reaches the water. Use this figure when the boiler and its piping sit inside the heated part of the house, because pipe losses still warm the building.
- Net rating: the DOE figure divided by 1.15 on a hot-water boiler, an allowance for piping and warm-up losses. Use it when long pipe runs pass through a crawlspace, garage or other unheated space.
Why a modulating condensing boiler forgives sizing error
A modulating condensing boiler tolerates a sizing miss because it can turn its flame down. A cast-iron boiler has one firing rate: full. A condensing boiler with a 10:1 turndown ratio and a 110,000 BTU/h maximum can idle at 11,000 BTU/h, which matters because the house only needs its full design load for a handful of hours each winter.
For that reason the minimum firing rate deserves as much attention as the maximum. If the smallest zone, say a single bathroom loop, cannot absorb the boiler's lowest output, the burner will still short-cycle. Condensing boilers also earn their efficiency only when return water comes back below roughly 55 °C, so oversized radiators and an outdoor reset control are what let the boiler condense for most of the season.
Extra loads that change the boiler size
- Indirect water heater: usually no extra capacity is needed in a house. The control gives the tank priority, the boiler pauses space heating for the few minutes the tank takes to recover, and then goes back to the radiators.
- Combi boiler: domestic hot water sets the size, not the heating load. Lifting cold winter mains water to shower temperature at two fixtures takes far more BTUs than heating the rooms, so a combi is chosen by litres per minute at a stated temperature rise. See combi boiler installation for how that is matched to a household.
- Radiant floor zones: floors run on low-temperature water and are often split into small zones. That favours a boiler with a low minimum fire, or a buffer tank, and a mixing control. More on radiant floor heating.
- Snow melt: a heated driveway slab can demand more than the whole house. It is calculated separately, runs on glycol through a heat exchanger, and is staged so the house keeps priority.
What a boiler sizing visit involves in Burlington and Hamilton homes
Expect the technician to measure rooms and windows, ask about insulation upgrades, count radiator sections and look at the piping, the chimney and the gas line. In the pre-war brick houses of central Hamilton and the older streets of Burlington and Oakville, that usually means big cast-iron radiators on a system first laid out for gravity flow. Halton and Hamilton water is hard, so on a combi the hot water heat exchanger needs scale protection planned from day one.
We do not publish a flat price band for boilers because the number moves with the boiler type, venting, near-boiler piping and how many zones are reworked. Quotes are free and in writing; ask for the heat-loss figure to be shown on yours. For what drives the total, read how much a new boiler costs in Ontario.
If your boiler is failing now, start with a diagnosis: many no-heat calls are a circulator, an expansion tank or a control, not the boiler itself. ZK Mechanical handles boiler repair and replacement across Burlington, Oakville, Hamilton and Mississauga with TSSA-licensed gas technicians. Call (647) 801-1252 or request a boiler sizing visit.