When the thermostat clearly shows a call for heat but the boiler never responds, the break is almost always in the signal path, not the boiler itself.
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Quick answer
A gas boiler that does not respond to its thermostat, even though the thermostat display shows it calling for heat, usually has a broken signal path rather than a boiler fault: dead batteries or a lost wireless pairing on a smart or programmable thermostat, a loose or corroded low-voltage wire at the thermostat or the boiler's control terminals, a blown low-voltage fuse, or a thermostat that is incompatible with the boiler's control system after a recent replacement. A licensed technician traces the signal from thermostat to boiler control board to find the break.
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A thermostat's own screen only tells you what that thermostat thinks it is doing, not whether that signal is actually reaching the boiler. A thermostat can show a call for heat, a rising setpoint, and even a heating icon while the wire, wireless link, or low-voltage circuit carrying that signal to the boiler has failed somewhere along the way, leaving the boiler with no idea a call ever happened. This is why a boiler not responding to a seemingly correctly-set thermostat is treated as a signal path problem first, rather than assuming either the thermostat or the boiler is simply broken.
Battery-powered and smart thermostats can display a normal-looking screen even as batteries run low enough to prevent reliable communication with a wireless receiver at the boiler, and a lost wireless pairing after a power outage or a receiver reset can have the same effect. Loose or corroded low-voltage wiring at either the thermostat's wall plate or the boiler's control terminals is a very common physical cause, particularly in older systems where connections have never been checked since the original installation. A blown low-voltage fuse on the boiler's control board, often a small automotive-style fuse, will cut off communication entirely even though the thermostat itself functions perfectly.
A recently installed thermostat, especially a smart thermostat swapped in without confirming compatibility with an older boiler's control system, can also fail to communicate correctly if it requires a wiring configuration, a common wire, or a control protocol the boiler's terminals were not set up for.
Replace the thermostat's batteries if it uses them, even if the display still looks normal, since a screen can continue showing information on nearly depleted batteries that are no longer sufficient for reliable wireless communication. Check whether the thermostat is a recent installation or if anything nearby was recently worked on, such as an electrical panel or other wiring, since accidental disturbance of low-voltage wiring is a common coincidental cause. If you are comfortable and the system is de-energized, a visual check of the wire connections at the thermostat's wall plate for anything visibly loose or disconnected is a reasonable homeowner-level check before booking a technician.
Most causes here, a battery, a wiring connection, a blown fuse, or a compatibility mismatch after a thermostat swap, are quick, inexpensive fixes that do not involve the boiler's burner, heat exchanger, or gas components at all. If a technician traces the fault all the way to the boiler's control board itself failing to respond even when directly tested, that is a more significant repair involving board replacement, but this is a less common finding than a simple signal path issue.
Safety note
Low-voltage thermostat wiring is generally safe to inspect visually, but any work inside the boiler's control panel should be left to a licensed technician. If you smell gas near the boiler while investigating, leave the home and call FortisBC at 1-800-663-9911.
If batteries were recently replaced, the issue is more likely a wiring connection, a blown low-voltage fuse, or a compatibility issue rather than batteries, but weak or incorrectly installed batteries are still worth double-checking first since they are the easiest thing to rule out.
This often points to a wiring configuration or compatibility mismatch between the new thermostat and an older boiler's control terminals, which a technician can usually resolve by adjusting the wiring or thermostat settings.
A basic visual check for loose or disconnected wires at the thermostat wall plate is reasonable, but testing continuity and diagnosing the boiler's control board response is best left to a licensed technician with the right tools.
If you can safely identify the correct low-voltage fuse and its rating, replacing it is sometimes a simple task, but if it blows again immediately, that indicates a short elsewhere that needs a technician to find before just replacing fuses repeatedly.
An intermittent connection, whether from a loose wire or a weakening wireless signal, can cause the fault to appear only some of the time, which is actually a useful clue that points toward a connection issue rather than a fully failed component.
Most fixes here are minor, contained repairs; cost depends on what is found and is best confirmed after a technician traces the signal path.
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