Gas pressure switch

If you smell gas, leave the building and call FortisBC at 1-800-663-9911 or 911 from outside. Do not reset the boiler, and do not touch the gas train. A gas pressure switch is a safety device, and the only safe response to a gas smell is to leave and call.

In British Columbia, gas pressure switches sit on the gas side of the appliance and are serviced as licensed gas work under Technical Safety BC. Homeowners should never adjust, jumper or bypass a pressure switch, because bypassing it removes the protection that keeps the burner from firing under unsafe supply conditions.

A switch that trips is doing its job: it has detected gas pressure outside the range the boiler was designed for. The correct response is to find out why, not to force the boiler back on.

A gas pressure switch is a small diaphragm device wired into the boiler's safety chain. It monitors pressure at a point in the gas train, usually the inlet, and its contacts stay closed only while pressure is within the set range. If pressure falls below the setting or, on some designs, rises above it, the contacts open, the safety chain breaks, and the burner cannot fire or is shut down.

Viessmann's Vitocrossal 300 CA3B service instructions document this arrangement in detail: the boiler carries an inlet gas pressure switch, GDW1, and a second switch, GDW2, that monitors pressure at the first valve seat. The manual states that the gas pressure switch used for the inlet pressure check is factory set to 4 inches water column and warns plainly: never alter this setting. The control displays both switches in its process-information menu and reports their states as 0 or 1.

On some equipment the switch is optional or added with a package. Laars' Mighty Therm2 manual shows how 24-volt power travels through optional gas pressure switches, along with the auto-reset high limit, before it reaches the block flue switch and manual-reset high limit on the way to the temperature control, which is how these devices are stacked into one safety chain.

The switch itself can fail open or closed. A failed-open switch looks like a boiler that will not fire at all, with a fault code naming gas pressure or a lockout that returns after reset. A failed-closed switch is more serious: it reports safe pressure even when the supply is inadequate, letting the burner attempt to fire under conditions the switch was installed to prevent.

Most tripped switches are not failed switches. Low supply pressure from the utility, a partly closed manual valve, undersized or blocked piping, or a regulator problem upstream will open the switch legitimately. Viessmann's CA3B fault table, for example, lists a closed gas shut-off valve and gas supply problems among the causes of its gas-pressure-switch faults, and directs the technician to check the switch setting and the gas supply before condemning a part.

A switch can also drift or be disturbed during other work. That is why the factory setting is protected and why a technician verifies the setting against the manual rather than trusting whatever position the adjustment happens to be in.

Write down the exact fault text or code and whether the boiler fires at all. Check whether other gas appliances in the home are working normally; if the range or water heater is also weak, the problem may be at the meter or in the building's gas piping rather than the boiler.

Do not open the boiler, do not touch the gas valve or switch, and do not reset repeatedly. If you smell gas, leave the building and call FortisBC at 1-800-663-9911 or 911 from outside.

Keep the boiler area clear for service access, and tell the technician about any recent gas work, utility activity or changes in other appliances, because those details shorten the diagnosis.

A technician starts with pressure measurement, not parts. Using a manometer at the gas valve's test ports, they record static and running supply pressure and compare it with the manual's required range, such as the 4 to 14 inches water column Viessmann lists for natural gas. If supply pressure is genuinely low, the cause is found upstream before the boiler is returned to service.

Where a switch has failed or drifted, it is replaced or restored to the manufacturer's specified setting, and the gas train is leak-tested afterwards. Viessmann's instructions are explicit that the factory setting on its inlet switch is not to be altered, which tells the technician the switch is a fixed reference, not an adjustable preference.

A boiler that will not fire because of a gas pressure fault is a safety-system event, and the diagnosis belongs with a licensed contractor. Call ROMA Heating & Cooling to have the gas train measured and the switch tested.