Before we start, you should really consider having a real professional do this job for you.
I understand the cost problem, so I wont go into that.
Here are some of the reasons you should not try to do this yourself.
1- Gas is explosive.
2- You may have a Carbon Monoxide problem, that requires a whole boiler replacement.
3- You may not use the correct gas valve.
4- Gas valves need to be setup with a water column gauge, that you don't have.
5- The boiler may need many upgrades, because it is so old, and not having a professional look at the boiler, will leave you in the bark about real safety issues, that are very likely with an old boiler like this.
There are other reasons, so please think about it.
This is the old Burnham Holiday boiler I will be working on.
This boiler had many problems that I fixed, but this page is about the gas valve only.
I checked the Carbon Monoxide levels before I started doing any work, because if the CO was high, the boiler would need to be replaced, not repaired.
The reason I replaced this gas valve is, it got very wet from a leak above, once a gas valve gets that wet or if it gets submerged under water, it must be replaced, just letting it dry is very dangerous.
First thing to do is, turn the power off, turn the gas off.
Make a note of the wires connected to the old gas valve before you remove them from the gas valve.
Get your new gas valve here.
Next remove the thermocouple from the gas valve, this would be a good time to go ahead and replace the thermocouple.
It turns counter clockwise to remove it, next remove the pilot lite tubing from the gas valve.
Now that you have the thermocouple and the pilot tubing removed, and the wires marked and removed, its time to start taking the pipe apart.
The picture below is of the black iron pipe union, this union needs to be opened by loosening the large nut, while holding back on the other part of the union, not holding back will cause the whole union to turn.
The above image is me reconnecting after the gas valve is replaced, so the wrenches are reversed from opening the union.
Once the union is separated, back off the nipples and fittings going to the old gas valve.
Remove the old gas valve, it all turns counterclockwise.
Now before you put the new gas valve on to the pipe you need to check the directional arrow on the gas valve to be sure you are installing it in the right direction.
You need to put pipe compound on the male threads of the pipe and nipples, DO NOT use Teflon tape or any other type of tape or lamp wick on the threads.
You can use Teflon paste, Mega lock or other pipe compounds.
When screwing the new gas valve on to the nipple, make it tight but be careful, the gas valve is made of a soft metal and can crack if you make it too tight.
Once the new gas valve is installed and the nipples are replaced, Note sometimes the new gas valve is not the same size as the old gas valve, so you may need to change the size of the nipples to get the union to line up again.
Very important, do not put any pipe compound on the union seat, and make sure you tighten the union.
Now that the new gas valve is installed, connect the thermocouple.
Next reconnect the pilot lite tubing, do not put pipe compound on the threads.
Reconnect the wires to the new gas valve.
Make a soap and water mix with dish soap and water, turn the gas on, test every pipe joint by putting soapy water on them, if there is a leak, it will make bubbles.
Now you are ready to light the pilot, turn the knob on the gas control valve to the pilot position, push it in, lite the pilot, hold for 30 seconds, let button go, if pilot stays lit, turn the gas control knob to the on position, turn the power back on.
This is where you need to use a water column gauge to adjust the gas valve.
Once that is done you are good to go,
Retest for Carbon Monoxide.
Need Help? Use the 24/7 Help Line.