When an indirect water heater stops producing hot water, the cause is almost always somewhere in the boiler side of the system, since the tank itself has no heat source.
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Quick answer
An indirect water heater has no burner of its own, so when it stops heating, the cause lies in the boiler, the circulator that moves water to the tank's coil, the priority control that calls for hot water, or the aquastat that senses tank temperature, rather than in the tank itself. Common culprits include the boiler being in lockout, a failed circulator or zone valve dedicated to the indirect loop, or a faulty aquastat that never signals a call for heat. Because diagnosing this involves the boiler's controls and gas-fired components, it is a job for a licensed technician rather than a homeowner repair.
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An indirect water heater is a passive storage tank with a heat exchanger coil; it has no burner, no igniter, and no combustion components that can fail on their own. When hot water stops, the tank has almost certainly not lost the ability to hold heat; instead, something upstream has stopped delivering heat to it. That means the useful diagnostic questions are about the boiler and its controls, not about the tank, which narrows the troubleshooting path considerably.
| Symptom | Likely area |
|---|---|
| No hot water but space heating works fine | Indirect loop specific: circulator, zone valve, or aquastat |
| No hot water and no space heating | Boiler itself is likely in lockout or has failed entirely |
| Hot water is lukewarm but not cold | Possible partial coil scale, weak circulation, or boiler running at reduced output |
| Hot water was fine yesterday, stopped suddenly today | Points toward a control or component failure rather than gradual wear |
Homeowners should not open the boiler cabinet, reset a lockout repeatedly, or attempt to test circulators or valves directly, since these sit within the gas appliance's controls and wiring.
A technician typically starts by checking the boiler for any active fault, then traces the priority circuit for the indirect tank, testing the aquastat, the zone or priority valve, and the dedicated circulator in sequence to find where the call for heat is being lost. Because this involves both electrical controls and the gas-fired boiler itself, it is not a task to attempt without the right diagnostic tools and training, and a misdiagnosis can lead to replacing a working part while missing the actual fault.
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.
Rarely, since the tank has no heat source of its own; the more likely cause is somewhere in the boiler, its controls, or the circulator serving the indirect loop.
This points toward a fault specific to the indirect loop, such as a failed circulator, stuck zone valve, or faulty aquastat, rather than a boiler-wide failure.
A single reset attempt following the manual is sometimes reasonable, but repeated resets without diagnosis can mask a developing fault and should be left to a technician if it recurs.
Yes, a zone or priority valve that fails to open when hot water is called for is one of the more common causes of this specific symptom.
Loss of hot water is inconvenient but rarely urgent from a safety standpoint on its own; however, if it comes with any lockout fault or unusual smell, it should be treated as more urgent.
Sometimes, depending on which component needs replacing; a technician will determine this once the fault is located.
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