Losing hot water entirely, rather than just running short of it, usually points to an ignition or safety-switch fault rather than a sizing problem.
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Quick answer
A complete loss of hot water in a gas system, as opposed to just running out partway through a shower, usually means the burner isn't igniting, a safety switch has tripped, or the pilot has gone out, rather than the tank simply being too small. A licensed technician checks ignition, gas supply, and the safety controls in that order before deciding whether it's a repair or a replacement situation.
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Running out of hot water partway through use is a sizing or recovery issue — the tank simply can't keep up with demand. Having none at all, right from a cold tap, points somewhere else entirely: the burner isn't lighting, the pilot has gone out, or a safety device has shut the unit down. Knowing which of these you're dealing with changes where a technician starts looking.
If the water was fine yesterday and is stone cold today with no gradual decline, that's a strong signal it's an ignition or control fault rather than a sizing problem that would show up as a slow worsening over weeks.
A tankless unit that's gone completely cold often has an error condition on its control board, a blocked or frosted vent termination in cold weather, insufficient gas flow reaching the unit, or a flow sensor that isn't detecting water movement correctly. Many tankless units display a fault code on a small screen or remote control, which a technician reads as a starting point — though the underlying cause still needs to be diagnosed rather than assumed from the code alone.
| Unit age | What it suggests |
|---|---|
| Under 8 years | Usually a repairable fault — ignition, control, or sediment |
| 8–12 years | Repair often still reasonable, but ask about remaining expected life |
| Over 12–15 years on a tank | Replacement is often more sensible than continued repair |
| Any age with a corroded tank or cracked heat exchanger | Not repairable — replace |
Confirm the gas supply to the home is on and that other gas appliances are working, check the breaker if your unit uses electronic ignition or a powered pilot, and if it's a standing-pilot tank, check whether the pilot light is actually lit through the sight glass. Beyond that, relighting a pilot or opening the unit further is where it's worth calling a licensed technician rather than continuing to troubleshoot gas components yourself.
Safety note
Diagnosing a no-hot-water fault on gas equipment can involve gas valves, ignition components, and safety controls, and should be handled by a Technical Safety BC licensed gas fitter. If you smell gas at any point, leave the home first and call FortisBC at 1-800-663-9911.
Many manufacturers include pilot relighting instructions on a label attached to the tank, and it's a reasonable homeowner task if the pilot has simply gone out. If it won't stay lit, that points to a thermocouple or gas valve fault a technician should look at.
A sudden, complete loss usually means ignition or a safety shutoff rather than a gradual sizing problem, which tends to show up as a slow decline in performance instead.
The code tells you the category of fault, but the underlying cause — venting, gas supply, flow sensor — generally needs a licensed technician to properly diagnose and correct.
Yes, a frosted or blocked vent termination in cold weather can cause a tankless unit to shut down as a safety response, which is worth checking before assuming a component has failed.
It depends on the unit's age, the specific fault, and its overall condition. A technician can give you a repair-versus-replace recommendation once the actual cause is diagnosed.
It's a significant inconvenience, and if you smell gas at any point it becomes a genuine safety issue where you should leave the home and call FortisBC. Otherwise, it's worth having looked at promptly but doesn't itself indicate a hazard.
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