Boilers - Steam and Hot Water Systems - Boiler won't light pilot

I have a 20+ yr old Weil-Mclain HE 2 natural gas steam boiler. It uses a White Rogers 5059-21 24V Pilot Relite module to provide spark. It is experiencing the following symptoms:

A) the spark is too weak to ignite the pilot gas

B) if I light the pilot by match the flame is sensed, the main burners come on and the spark stops. About a minute later, the Pilot Relite module starts to spark again though the burners are already lit.

What I've tried so far:

1 - cleaned the spark electrode and flame sensor element
2 - checked spark electrode gap
3 - verified 24V across Pilot Relite terminals and checked for good ground
4 - checked wiring for loose connections
5 - attached a test wire to the high voltage output terminal and verified weak spark to ground

If I unplug the 3-prong flame sensor, I get a stronger spark but no pilot since the switch inside the sensor controls the pilot gas valve also.

What can be wrong? Bad Pilot Relite, bad flame sensor or can it be something else? What other tests can I try? :confused:

Thanks

It seems the 5059-21 has been replaced by the 5059-23.
Here are the instructions on it: http://www.white-rodgers.com/wrdhom/pdfs/instruction_sheets/0037-3616.pdf

After what you've checked, I would suspect the sensor but honestly I don't know how to check it. Possibly one of the other guys will jump in here.

Thanks for the 5059-23 info. I found that Robertshaw 785-001 is also supposed to work.

I noticed that it is within a second or two of the main burners coming on that the electrode starts sparking again.

As I understand it, the pilot relite checks for pilot flame by detecting current from the spark electrode through the flame itself to ground in between sparks. The flame conducts electricty. The electrode must be within the blue area of the flame. The mercury flame sensor is only used to control the gas valves. Does that sound right?

I guess I'll try replacing the pilot assembly first and, if that doesn't work, the relite. Since this boiler is so old, almost all parts for it have been obsoleted. Apart from this pilot problem, it works fine.

Replaced pilot relite module with White Rodgers 5059-23. Getting a strong spark and no more sparking when the pilot is lit or when the main burners come on.

But... it is still not igniting the pilot gas on its own unless it is initially lit by match. If I light the pilot and then blow it out, it relights by itself after the first couple sparks.

What could be the reason for this?

I cleaned the pilot orifice hole with a thin wire and blasted it and the pilot tubing with carb cleaner. Made no difference. I read some orifices may have an additional air hole but I did not see one on this one.

Could it be the pilot gas pressure? The pilot gas pressure should be the same as when the boiler was installed since I've never adjusted it before.

I repositioned the electrode tip so that it sparks toward the flame sensor bulb instead of the pilot burner assembly. It now lights the pilot on the first spark or two at all times. I think the gas wasn't mixing with the air well enough toward the assembly so it wouldn't ignite. The gas bouncing off the flame sensor probably created small air currents to help mix it up. The new 5059-23 module sparks about 3 times per second. The old 5059-21 sparked about once a second. I may have been able to get this pilot to light without replacing the module if I had thought to position the electrode toward the flame sensor earlier even with the weaker spark. The old electrode is riveted to the pilot assembly so it is still there but it is not connected. The new electrode has a universal mounting strap to hold it in position. The tip is long enough such that if the tip wears, the gap can be adjusted easily. I contacted Weil-McLain for a suggested replacement for the obsolete pilot burner assembly. They came back with universal conversion kit part number 510-811-458. This kit includes a pilot burner, a gas valve and a control module. Most places sell this kit for over $600. The cheapest I could find it was $377. Patriot Supply sells a replacement pilot burner assembly alone... part #511-330-112-B for around $80. I paid $20 for the electrode and $26 for the relite module through eBay. I hope this thread helps other people with a similar problem.

I hope this thread helps other people with a similar problem.

I do too! It should... if they can figure out how the SEARCH function works! :)

Good job!