Priority control temporarily favours an indirect water heater's call for heat over space heating, and it only works well when the boiler is sized to handle both loads.
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Quick answer
Priority control is the logic in a boiler system that, when an indirect water heater calls for heat, temporarily pauses or reduces space heating so the boiler's full output goes toward reheating the tank quickly, since hot water demand tends to be short and urgent while a brief pause in space heating is usually unnoticeable. This only works well if the boiler itself has enough spare capacity to serve both loads without leaving either one seriously underserved. A boiler sized only for space heating, with an indirect tank added on afterward without reassessing capacity, can end up cycling awkwardly or delivering weak performance on one side or the other. Sizing this correctly is a job for a licensed technician working from real household demand figures.
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When an indirect tank's aquastat senses the stored water has cooled below its set point, it sends a call for heat to the boiler, just as a thermostat calls for heat from a room. Because a shower running while the tank is mid-recovery is a common and urgent scenario, many systems are configured so that call takes priority over any active space-heating call, at least for the several minutes typically needed to reheat the tank. Space heating resumes automatically once the tank is satisfied. This is a deliberate design choice, not a flaw, since a brief pause in space heating is rarely noticed while a slow trickle of hot water during a shower is noticed immediately.
A boiler serving both space heating and an indirect tank needs enough output that, even while heavily prioritizing the tank, the home does not get uncomfortably cold during a longer reheat cycle, and even while running space heating, the tank can still recover promptly when called on. If a boiler is sized right at the edge of what the home's heating load requires with no meaningful spare capacity, adding an indirect tank onto it can result in either slow tank recovery, prolonged space-heating interruptions during cold weather, or both.
| Symptom | Likely cause |
|---|---|
| House feels noticeably cooler right after someone showers | Priority control pausing heating too long relative to boiler capacity |
| Indirect tank takes a long time to recover after a big draw | Boiler undersized for combined demand, or priority not engaging properly |
| Both heating and hot water seem underwhelming most of the time | Boiler likely undersized for the home's total combined load |
| System performs well except during the coldest weather | Marginal sizing that only shows its limits at peak heating demand |
If a household's hot water demand or living space has changed substantially since the boiler was installed, such as adding a bathroom, finishing a basement suite, or a growing family's shower schedule shifting, it is worth having a technician reassess whether the original sizing still holds. This is also a natural conversation to have during a boiler replacement, since replacement is the point where sizing can be corrected rather than carried forward from an installation that may have been marginal from the start.
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.
A well-sized system should only show a brief, usually unnoticeable dip in heating during tank recovery; a noticeable cold spell points to a sizing or control issue worth checking.
It can sometimes be adjusted, but disabling it generally trades away quick hot water recovery for uninterrupted space heating, which is rarely the better choice.
Persistent slow recovery, cooling of the home during reheat cycles, or generally underwhelming performance on both sides are signs worth having assessed.
Not necessarily; oversizing brings its own inefficiencies, so correct sizing based on actual load calculations is better than simply choosing the largest available unit.
Yes, added living space or a secondary suite changes both heating and hot water demand and is worth a reassessment.
No, priority control and boiler sizing are system-level settings and equipment decisions that should be handled by a licensed technician, not adjusted through a thermostat.
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