Residential Boiler Repair

Note: Due to the dangerous nature of gas, most juristictions regulate the installation and modification of gas appliances and restrict the servicing of them to trained and licensed technicians. Check with your local authority before doing any work on your own gas appliances.

In this Instructable I will change a gas valve in an older (1990 vintage) residential hot water boiler. In an older boiler like this as in an older forced air furnace, there is a standing pilot (it's on all the time). The end of the thermocouple sits in the pilot flame and the miniscule voltage (20-30 milivolts) produced by the thermocouple keeps the pilot gas valve open. If the pilot flame goes out, the thermocouple stops producing voltage and the pilot gas valve closes, shutting off the gas. More modern units, have no standing pilot. They usually use a spark to ignite the gas and some kind of infra-red flame sensor. Hot water boilers like this unit circulate heated water through "zones" which are basically pipes that circulate the hot water through and the heat is radiated to the rooms through radiators. After the hot water is circulated through the zone, it's returned to the boiler. Hot water boilers are generally more complicated than forced air furnaces and therefore have more parts to break down. This small boiler has three zones with zone valves for each zone that are opened when the thermostat in each zone is activated. In each zone valve is a switch to prove that the zone valve is open.

Step 1: Tools and Parts Needed

1) New gas valve with the same ratings as the old one. Can be purchased at an HVAC supplier.

2) Thermocouple. Can be purchased at HVAC supplier or Home Depot.

3) Large Vice Grips. Can be purchased at Home Depot

4) Pipe Wrench (2). Can be purchased at Home Depot or plumbing supply store.

5) Multimeter. Radio Shack or HVAC supplier.

6) Yellow Teflon Tape for gas. Home Depot or HVAC supplier.

7) Multi-bit screwdriver

8) Soapy mixture for detecting gas leaks. HVAC supplier

9) Matches. (Your Favorite Bar)

10) Assortment of pliers. Home Depot

11) Assortment of small to large open ended wrenches. Home Depot.

12) Large Crescent Wrench. Home Depot.

13) Large and medium Slipjoint Pliers. Home Depot or Lowes.

14) Butane lighter or Propane blow torch.

Step 2: Troubleshooting

The boiler wasn't producing heat I was told. The first thing that I checked on the boiler was to see if the pilot had gone out. I knew that this older boiler had a pilot since I had serviced it a couple of times in the past. We had had lots of wind that day so I first suspected that the wind had blown the pilot out. I tried re-lighting it but the flame wouldn't stay on. Experience tells me that when you have this problem, its almost always the thermocouple so I took the old one out and installed a new one. In the picture, you can see that the thermocouple is the long copper-colored device that has one end threaded into the gas valve and the other end into the pilot assembly. The pilot assembly has the thermocouple threaded into it and the pilot gas tube attached to a bracket which is screwed onto the lower body of the boiler. Different brands of thermocouples are slightly different the way they go into the bracket but just follow the directions on the package as to how to mount it. Just make sure that the tip of the thermocouple sticks generously into the body of the flame. The other end of the thermocouple has a standard thread found on most brands of general replacement thermocouples. I like to use the shortest possible thermocouple that will do the job as it will have the lowest voltage drop. You can get them in 24, 36 and 48 inches. I replaced the thermocouple and put everything back together. I made note of the position of the thermocouple bracket by taking a picture. It was in the middle position. If it is too far out, the pilot flame might go out when the main flame goes out.

After assembly, I went through the lighting procedure which involves pushing down the dial on the top and turning it to the pilot position. Continuing to hold it down, a match is struck and holding the match with a pair of needle nose pliers, sticking it into the area of the end of the thermocouple where a little bit of gas is flowing. The match will light the gas stream as you keep pushing down on the dial. After 30 seconds, the flame will heat the thermocouple and the tiny voltage produced by the thermocouple will keep the pilot valve open. In my case, the new thermocouple didn't keep the flame on. The problem was either a bad pilot valve coil or a faulty thermocouple. I would be negligent if I didn't check the thermocouple with a meter, as the thermocouple is $10.00 and the combination gas valve assembly is around $200.00!

I took the thermocouple out of its assembly on both ends and stuck the flame sensor end into a blowtorch flame while checking the voltage on the other end. The multimeter should be put on the lowest DC value and should read around 30 millivolts or .030 volts. You check the voltage from the center of the end that screws into the gas valve and the outside of the thermocouple. The thermocouple was OK so I went and bought a new gas valve. The jobber didn't have a Robertshaw unit so he gave me an equivalent Honeywell unit.

Step 3: How Does the Gas Valve and Thermocouple Work?

The combination gas valve and pilot valve has one gas inlet and two outlets. When the pilot flame is proven, the thermocouple keeps the pilot gas valve open, allowing a small amount of gas to flow to supply the pilot. The pilot valve is a small, very sensitive electromagnetically operated valve. You should never check the coil with an ohmmeter because the voltage and current will burn it out. Only check the coil with a thermocouple known to be good.

The main gas valve is a rugged electromagnetically operated valve that works usually on 24 volts (in my case) but bigger boilers have 120-volt gas valves that switch on and off enormous amounts of gas. When the thermostat completes the circuit and supplies 24 volts to the gas valve, it opens and the pilot flame ignites the burner. Once the thermostat shuts off, the gas valve shuts off extinguishing the main burner.

The thermocouple is an interesting device that is made up of two dissimilar metals that are bonded together. When they are heated, a voltage is produced across this junction, many of these are connected together, the device is called a thermopile. These are used in different types of gas appliances such as gas stoves and fireplaces.

The thermocouple effect has been known for 200 years, having been discovered by Physicist Thomas Johann Seebeck in 1821. One of the most interesting uses for thermocouples has been in the Radioisotope thermoelectric generator used in deep space probes to produce power. The heat developed by the nuclear decay of a radioisotope is harvested by multiple thermocouples connected together. The efficiency is low but they last a long time. RTG installed in the Voyageur spacecraft in 1977 are still operating and producing power albeit at a lower level.

Step 4: Taking Out the Manifold Assembly

As shown in the above picture, the manifold assembly needs to come out. Since the pipes were held tight in the old gas valve with many years of hardened joint compound, the only way to get it apart was to put it in a vice and gradually loosen the joints with a pipe wrench by moving with a back and forth motion. This needs to be done with care as the manifold and gas orifices are easily damaged. Before loosening anything, turn off the gas and power to the boiler!

Step 5: New Gas Valve Installation Is Reverse of Disassembly

The pipes should be installed in the new gas valve with yellow Teflon tape and tightened with a pipe wrench. Since the pipe is being threaded into aluminum, care should be taken not to overtighten. In my case, (the far-right end joint) where it connected to the piping outside the boiler cabinet, needed to be tightened many turns so that the orifices would line up with the holes in the upshot burner. Once this is all connected together tight, turn on the gas and check all the joints for the smell of gas. A soapy substance can be purchased to apply to gas joints where any bubbles can be easily noticed. Allow the gas to build up pressure for 10-20 minutes and check and double check for leaks. Once you are satisfied that there are no leaks, everything can be put back together. Turn off the gas before you do this. The thermocouple and pilot gas line are tightened into the gas valve body with hand tightness and then a one quarter turn with a small wrench. Turn on the gas again and light the pilot. Again check around where the pilot gas line screws into the gas valve body for a leak. Use the soapy solution. Once you're satisfied that there are no leaks, turn on the power to the boiler and try the thermostat for normal operation. Check the joints again for slight gas leaks and tighten if needed.

Note: Teflon tape should be threaded in the same direction as the pipe thread.

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