Replacing a water heater: size it to your use, not to the old tank

Most water heaters are replaced the day the old one leaks, which leaves no time to think. The result is often the same tank size and type as before, installed in the same spot. NRCan's Keeping the Heat In suggests the questions worth asking, ideally before the old tank fails.

Why it matters

NRCan says domestic water heaters use about 20 percent of a home's energy, roughly equal to all the lights and appliances combined, and that in efficient homes water heating can exceed space heating demand. After insulation and air sealing upgrades, the water heater can become one of the largest energy users in the house.

Size it to your demand

NRCan advises choosing a high-efficiency unit, such as a tank-type, instantaneous or combination model, and sizing it to demand, because oversized tanks are less efficient in low-use homes. ENERGY STAR certified units and solar water heaters are options. On-demand heaters may increase wait times at fixtures. A household that has shrunk since the old tank was installed may not need the same capacity.

Distance to fixtures

NRCan suggests locating the heater close to fixtures or using small-diameter pipe, and installing a demand-type recirculation system if a fixture is 9 m (30 ft) or more from the heater. Long runs mean waiting for hot water, which wastes both water and the energy in the pipe. A renovation that moves a bathroom far from the mechanical room is the time to think about this.

Insulate and recover

NRCan recommends insulating water lines to at least RSI 0.7 (R-4), and insulating electric tank heaters in basements with an insulated base and an insulating kit. Drain water heat recovery on shower stacks and solar pre-heating are worth considering where practical. Its everyday tips are to fix leaks, wash clothes in cold water and use low-flow showerheads and aerators.

Combustion safety

For fuel-fired water heaters, NRCan lists signs of combustion air problems: back-puffing or soot near appliances, melted plastic fittings on water heaters, odours, and nuisance CO alarms. Replacing spillage-prone equipment with direct-vent or sealed-combustion units, or a heat pump-based electric system, is one of its solutions. Gas water heater work needs a permit and a qualified gas fitter in most provinces; see ArtBuild's provincial articles on gas work.

Prevention Checklist

• Replace before failure when the tank is old, so there is time to choose.
• Size the new unit to current household demand.
• Consider direct-vent or sealed-combustion units, or a heat pump water heater.
• Add recirculation if a fixture is 9 m or more from the heater.
• Insulate hot water lines to at least RSI 0.7 (R-4).
• Use a licensed gas fitter and permit for gas units where required.

References

1. Natural Resources Canada. Keeping the Heat In - Section 9: Operating your house, Web page, 9.2 Domestic hot water; 9.4.6 combustion air. Accessed 2026-11-03.

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