Gas wall heaters can run on natural gas or propane and can be vented or ventless. Vented heaters can be surface mounted or installed between the wall studs, the latter type being known as wall furnaces, and ventless heaters can be infrared or blue flame and can be either surface mounted or freestanding. No matter which type it is, though, a gas heater needs an ignition source, and it's almost always a standing pilot. To light the heater, you have to light the pilot.
Most contemporary wall heaters come with a piezoelectric spark ignitor for lighting the pilot, but if you have an older heater, you may have to light the pilot with a match. Either way, the starting procedure is more or less the same. After turning the gas control to "pilot," you press it in while you ignite the pilot flame, and you have to hold it in for a period of time after the flame ignites to allow the thermocouple to warm up. The thermocouple is a safety device that sends a signal to the gas valve to stay open once the pilot flame is burning, and if it's worn out and needs replacing, the pilot won't stay lit.
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How to Light the Pilot on a Gas Heater
Step 1: Locate the Gas Control and the Pilot
If you have a surface-mount or freestanding ventless heater, note the gas control, which is prominently placed on the front or top of the heater, and the pilot, which is at the bottom of the heater element, usually in the middle.
If you have a vented wall heater or wall furnace, open the cover at the base of the heater to locate the control and pilot. The control knob is right in front of you, but you may have to search a bit for the pilot orifice because it's hidden inside the burner assembly. Follow the smaller-diameter copper tube from the gas valve to find it. You may need a flashlight to see it clearly.
Step 2: Open the Pilot Gas Supply
For safety, turn the gas control knob all the way off (if it isn't off already) and wait a minute for any gas that may have leaked out to dissipate. Turn the knob to the "pilot" position, push it in and hold it.
Step 3: Ignite the Pilot Flame
Look for a button (usually red) next to the gas valve. If you see one, that's a piezoelectric spark igniter. Push it in repeatedly until the pilot starts. If you don't see a button, light the pilot by holding a lit match or a long-reach lighter next to the pilot orifice and keep it there until the pilot starts.
Step 4: Hold In the Gas Control
Keep the gas control knob depressed for anywhere from five to 20 seconds depending on the heater and then release it. If the pilot flame goes out, repeat the starting procedure but this time, hold in the knob a little longer. If you can't get the pilot to stay lit, have an HVAC service technician test the thermocouple and replace it if necessary.