When we are called to a job, it will probably be because the equipment is not working properly. What I want to do in this article is go over each component of the system, explain what can go wrong with it, and how to fix it.

We will talk about 24-V standing pilot systems. For now we will just be concerned with natural-draft boilers, water heaters, and furnaces.

The first thing I will talk about is the pilot safety circuit. We need three things for combustion, air + fuel + heat. The air is supplied from the venting section, and the fuel from the gas piping section. The heat comes in the form of a pilot light, intermittent ignition device, direct spark ignition, or hot surface igniter. As I said before, in this article we will discuss standing pilot lights only.

Let’s go back and take a look at how we used to light pilots; then we’ll talk about the progression of safety devices and the state of technology today.

Let’s use a stove as an example first; it’s a gas appliance and it’s easier to illustrate.

Development of the Standing Pilot

On the old stoves, people used to turn on the gas and light the main burner with a match. As time went on, someone invented the pilot light. The pilot always stayed lit, and when the main burner was turned on, this component ignited it — nice and simple. But what happened if the pilot went out?

A small amount of gas was released to the atmosphere from the pilot burner, and when you went to light the stove, you realized the pilot was out because the stove wouldn’t light. You relit the pilot and you were set.

Now let’s apply this to a boiler. The original gas boilers had what we called wild pilots. There were two gas cocks on the old boilers, an “A” cock and a “B” cock. The “A” cock ran full size to the main gas valve and the “B” cock ran off a 1/8-in. tapping on the “A” cock. The outlet side was a 1/4-in. compression fitting.

From this fitting, 1/4-in. aluminum tubing ran to a pilot assembly. The pilot assembly sat above the main burner, and when the gas valve opened, the pilot lit the main burner. This was known as a “Wild Pilot.” There was no pilot safety control, and if the pilot went out, the main gas would still turn on when there was a call for heat.

I guess you can see the problem here: no pilot, main gas, big boom. This did lead to explosions, fires, and lots of burned hair and eyebrows.

The Safety Switch

Once this problem was recognized, the next step was the development of the electric pilot safety switch. With this device, a thermocouple was put in the pilot flame. The flame should cover 3/8 to 1/2-in. of the thermocouple tip.

The thermocouple is a device made of two strips or wires of dissimilar materials. (See photo.) They are welded together at one end; this is called the hot junction. The pilot flame then heats the hot junction, generating a dc voltage on the other end of the two strips or wires. This is called a thermoelectric reaction.

The output of a single thermocouple is 20 to 30 millivolts dc (1,000 millivolts = 1 V). The voltage at the cold junction energizes an electromagnet, which closes a circuit on the electric pilot safety switch. The way it works is, you open the “B” cock and light the pilot. There is a button, usually red, on the pilot safety switch. You hold it down for 30 sec, which gives the thermocouple a chance to heat up and send the necessary voltage to the electromagnet to close the circuit. If the pilot goes out, the circuit opens.

This switch would get wired in series with the other safety and limit controls on the gas valve; nice and simple, no pilot, no main gas. Don’t forget, back in the old days all the gas valve was, was an electric solenoid valve.

Most valves you run into now are combination gas valves. The solenoid valve, regulator, and pilot safety are all built into one valve. On a standing pilot model we still use a thermocouple, but in a different way. The pilot valve is also built into the combination gas valve, so there’s no more “B” cock.

To light the pilot, you hold down the button on the gas valve to start the flow of gas to the pilot burner. By holding the button down, you are allowing gas into the valve and, at the same time, blocking the flow of gas to the main burner outlet port while allowing it to flow to the pilot burner through a pilot tube coming from the gas valve body. You light the pilot and hold the button down for 30 sec, and when the thermocouple sends the voltage to the gas valve, an electromagnet holds the main safety shutoff in the open position. In this position, pilot gas will always flow and main gas will flow on a call for heat. If the pilot goes out, no gas will flow into the valve.

The only difference between the pilot safety switch and the combination gas valve is that the pilot safety switch is shutting the gas off electrically, and the thermocouple in the gas valve is shutting the gas off mechanically.

Lock Out Test

A properly operating safety system should lock out within 3 min of pilot outage. This test should be performed after gas pressure has been adjusted to manufacturer specifications.

Shut the gas to the appliance using the gas cock. After 3 min, turn up the thermostat to call for heat and turn on the gas; no gas should flow to the main burners or pilot burners. If gas is flowing, you must replace the gas valve. If you replace the pilot assembly, you should also do a turndown test.

It is an ANSI requirement that the pilot flame lights the main burner within 4 sec of gas reaching the main burner. This is done with the pilot gas at the minimum necessary to hold the pilot valve open.

Turndown test:

1.Turn the thermostat below room temperature or turn the gas cock on the gas valve to the pilot position.

2.Locate the pilot adjusting screw and start turning it clockwise until you notice the pilot flame start to decrease. Turn the screw one-quarter turn clockwise. Wait 1 min and turn clockwise. Keep doing this until the pilot safety circuit drops out, shutting the pilot.

3.Turn the pilot adjusting screw one-quarter turn counterclockwise and relight the pilot. The flame should now hold.

4.Now turn up the thermostat to call for heat and turn the gas cock back to the “on” position. The main burners should ignite smoothly without any rollout within 4 sec. If not, relocate the pilot burner and repeat steps 1 through 4.

5.Readjust the pilot flame to cover 3¼8 to 1¼2 in. of the thermocouple tip.

Note: Electronic ignition requires different methods to prove the pilot flame; these will be covered in another article.

Let’s Go On a Service Call

You get a call from Mrs. Jones, who has no heat. You look at the boiler and find that the pilot light is out. The heating unit (boiler or furnace) has a standing pilot with a thermocouple.

The first step is to make sure there is gas. Shut the gas cock at the appliance and open a union or drip cap. Turn on the gas cock. If there is gas, shut the gas cock and retighten the gas union or drip cap.

There are now a series of things that can be wrong. Let’s check for them step by step.

Depress the button and relight pilot:

  • The pilot will not light. Remove the pilot tube at the gas valve and depress the button. If gas is present at the gas valve and not at the pilot, check the pilot tube and pilot assembly orifice for blockages; clean or replace as necessary.
  • The pilot stays lit.
  • Check the thermocouple for lint or pet hair; clean if necessary. Check the gas pressure at the inlet and outlet of the gas valve with a manometer. It should be within the parameters set by the manufacturer (which can be found on the rating plate or in the installation and operating manual). If the gas pressure is good, turn on other appliances; does pressure drop? If yes, check line sizing. If no, check the pilot flame; is it covering 3/8 to 1/2 in. of the thermocouple tip? If not, adjust. Check the condition of the draft diverter. If damaged, replace it.

  • If all the above items are within spec, check the thermocouple with a thermocouple tester or a millivolt meter. If the readings are not within the manufacturer’s specified range, replace the thermocouple. If it’s within the manufacturer’s specified range, replace the pilot safety unit or combination gas valve.
  • Note:
  • When replacing a pilot safety unit or a combination gas valve, always replace the thermocouple. If you do not have a thermocouple tester or millivolt meter, replace the thermocouple; if the pilot still will not stay lit, replace the pilot safety unit or combination gas valve.

    This is the only situation where I would ever recommend replacing parts before testing, since the thermocouple would be changed with the gas valve.

    This article was excerpted from “The Complete Guide to Residential Gas Heating” by Richard Bruno, an instructor in Nassau County BOCES, Nassau County, NY, and at the Suffolk County BOCES, Suffolk County, NY. Among his professional affiliations, he is a member of the National Old Timers Association of the Oil Heat Industry, and a Long Island Lighting Co. Contractor Advisory Committee Member. He can be reached at 718-668-0180.

    Publication date: 10/09/2000