Sizing an indirect water heater correctly depends on both tank volume and the boiler's ability to recover that volume quickly enough for the household's peak demand.
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Quick answer
Correct sizing for an indirect water heater balances two things: the tank's stored volume, which determines how much hot water is available before it needs to reheat, and the boiler's output, which determines how quickly that volume recovers once depleted. A larger household with multiple bathrooms used close together generally needs either a larger tank, a boiler with more spare recovery capacity, or both. Undersizing either element leads to running short of hot water during peak demand, while oversizing adds unnecessary cost and mechanical room space. A licensed technician sizes both together based on actual household fixture count and usage patterns, not a generic per-bedroom formula.
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Homeowners often think of water heater sizing as a single number, like tank gallons, but an indirect tank's real capacity to serve a household depends on the interaction between stored volume and boiler recovery rate. A smaller tank paired with a boiler that has plenty of spare output can recover quickly enough to feel like a much larger tank in practice, while a large tank paired with a boiler that has little spare capacity can still run short if demand is sustained, such as back-to-back showers.
This is why sizing an indirect tank correctly is not just picking a tank size off a chart; it requires knowing the boiler's actual spare capacity after accounting for the home's space-heating load.
| Symptom | Likely sizing issue |
|---|---|
| Hot water runs out during a single long shower | Tank volume likely too small for demand |
| Hot water is fine for one shower but weak on the second one shortly after | Boiler recovery capacity likely too low relative to tank |
| Everything is fine except during the coldest weeks of winter | Marginal boiler capacity that only shows limits under peak heating load combined with hot water demand |
| Household grew or was renovated after installation | Original sizing may no longer reflect current demand |
Oversizing an indirect tank or the boiler behind it adds unnecessary upfront cost and mechanical room space, and an oversized boiler can also short-cycle more than a well-matched one, which is its own form of inefficiency and added wear. The goal is a system matched to the household's actual demand with a sensible margin for peak moments and future changes, not the largest available equipment. This is exactly the kind of judgment call that benefits from an in-home assessment rather than a generic online sizing chart.
Safety note
If you ever smell gas near a boiler, water heater, or vent, do not investigate further: leave the home first, then call FortisBC at 1-800-663-9911. Combustion appliances should only be serviced by a Technical Safety BC licensed gas fitter.
No, oversizing adds cost and space without proportional benefit, and can contribute to inefficient boiler cycling; matching size to actual demand is the goal.
Both matter together; a large tank with a boiler that recovers slowly can still run short during sustained demand, so they need to be sized as a pair.
A technician assesses your specific fixture count, usage patterns, and any planned changes like a secondary suite, rather than applying a generic formula.
Yes, added hot water demand from a secondary suite should be factored into sizing at the time it is added, not assumed to be covered by the original system.
Sometimes a boiler capacity or priority control adjustment helps, but if the tank itself is genuinely too small for demand, replacement with a larger tank may be the practical fix.
An appropriately sized system generally runs more efficiently than either an undersized system working hard to keep up or an oversized one short-cycling frequently.
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