Pilot burners need to be cleaned because there is a air intake just like on the main burner that indoor air with dust and contaminates goes through to mix with the gas. If you have indoor animals, it should be routine to clean seasonally. Same as main burner. You won’t notice the pilot flame decrease in size over time. It is more noticeable when the air intake gets partially clogged with dust. The pilot flame goes from all blue to having a yellow tip. The BTU of pilot burner has to be enough to heat the thermopile to the correct temperature to generate the correct voltage. I assume the pilot flame in picture is correct for the portion of flame that lights the main burner. It is not yellow. Can’t tell in the pic if this pilot is the type that also burns with a full ring of blue around the entire thermopile. Older pilot models were like that. Some have flame tray only cuts across the upper 1/3 of the thermopile, others have little slots with small flame all the way around it. Depends on pilot burner style and type. You would see little slots all around the base of thermopile for gas to come out, if that is the type. There is no visible flame around base if it is that type.
The white deposits that may form on the thermopile / generator is from dust and contaminates in the air going through the pilot burner. Only if excessive would they reduce the heat to the generator. They can be cleaned off with scotch bright or small wire brush. It should come off like a powder.
The red and white wires in the silver shielded cable comes from thermopile. They need to be very clean, as well as screws on valve. I always spray with WD-40 to clean and that puts a protective coating on them from oxygen and moisture in the air to prevent corrosion and poor contact. Same as thermostat wires on valve and at thermostat. Each connection is a little loss resulting in a total dropping voltage by the time it makes the loop to thermostat and back. This is a very low voltage and you need almost all the voltage thermopile generates to open the valve.
Moisture in propane vapor; (providing this is LP and not Natural Gas)
Any moisture in the cylinder or tank will stay at the bottom. Water is heavier than petroleum products. Propane boils at extremely cold temperatures well below zero. Since the fluid in tank is always above its boiling point, it boils in the tank and only the fuel vapor exits. Any water at the bottom is not turning to vapor since it is not above its boiling point like the liquid propane. If there is any moisture escaping, it can freeze at the needle orifice in the regulator outside. This is when extra methanol is injected into fuel. There should be a drip leg on the supply line at gas valve to prevent any contaminates from entering valve. There should be no contaminating fluid in the fuel anyway to prevent cylinder or tank damage. There is methanol added to propane that boils off any water in the fuel like dry gas added to gasoline in the winter. You can get an oil residue from the odorant in the line, but this fluid would drop into the drip leg before entering valve. The drip leg is required by code.
There are many old millivolt gas valves from the 50’s still in service. In the 25 years I serviced this equipment, I may have replaced 3 or 4. And that was probably from abuse, floods, or leak around control knob when pressed in. Newer 12 or 25 volt valves with double windings and double shut off is another story. These old ones just work.
To test generator, remove red and white wires. The red is normally positive. Light pilot and hold safety knob in to keep pilot lit. Read voltage between red and white wires. That is full generated voltage with no load. When connected to valve, test voltage with pilot only lit. You will notice a slightly lower voltage. This is due to power going through the electromagnet winding for the safety valve to keep fuel open into gas valve. Then turn on thermostat and watch power drop since voltage is now also going through the main burner valve electromagnet windings. When voltage is low it may not open valve immediately. When the thermostat is turned on high resistance in the circuit can cause a delay until voltage builds up enough to pull the steel rod into the magnet coil to open valve. That is when you tap on it and this may help it open. You aren’t loosening anything in valve, you are vibrating the valve and the weak power making the magnet not as strong as it should be helps it open. (Like an old stuck solenoid on a starter that a good tap makes it work) Inside the valve is the electromagnet with coil winding also called a solenoid. Same principal, only with millivoltage.
If a electromagnet winding is shorted it will drop current to zero and safety will shut off without the power to hold the safety valve open. That is a shorted failure. When winding is open, no power travels through winding and there is no magnetic force to open valve, so valve fails in closed position. You can test winding with an ohm meter. It should read resistance through winding. Not zero, shorted; Not infinity, open.
Notice how a gas man has to be well versed in electrical circuits. Most troubleshooting is actually electrical, not always pipes, tubing, pressure and gas flow.
Next time this fails, put a jumper wire from the white wire terminal to the outer red wire. These are the thermostat wires. If it lights main burner, it is not a gas valve problem, it is low voltage losing voltage in the thermostat loop. When you take jumper off, don’t be surprised if the burner stays on. You just gave it a surge of current to open valve, and it has enough through thermostat loop to keep valve open.
(Notice the TP marking on valve is the center terminal for Thermo Pile (this powers safety valve winding) and the two outer terminals are marked TH for THermostat going to the main burner winding)
