Forced air Furnace fails to restart

AlfaMatt

I have a thirty year old forced air furnace which uses a White-Rodgers 36C84 gas valve and a mercury flame sensor. I had a variety of problems before last winter which I successfully solved. I'll give a quick overview before I get to my current problem:



I replaced the mercury flame sensor due to a broken capillary tube, but found that the furnace worked erratically--pilot would start up fine, but furnace would sometimes drop out when transitioning to the main burner, and start the pilot cycle all over again.



Furnace repair guy discovered that wiggling the mercury flame sensor where it electrically plugs into the valve caused the valve to operate correctly, and diagnosed a bad valve, which he declined to replace, and suggested that I do it myself. I diagnosed a poor connection between the infamous White-Rodgers resistor board and the flame sensor pins. I took care of that and the furnace worked fine all winter.



Fast forward to this winter: Pilot valve intermittent, but a slight tap with a hammer cause furnace to act normally. Thinking the furnace repair guy might have been on to something, I order a new 36C84 gas valve. While I wait for the arrival of the new valve and the time to install it, the furnace exhibits a new problem: Starts and runs fine, but when pilot valve is energized, it will not move unless about 20 minutes have passed since last shutdown.



I have now installed the new valve without the resistor board as White-Rodgers says the new valve (36C84-921) does not need the resistor. Same problem as before, but here's some interesting parts:



1. If I call for heat and then shut down before the fan starts, it can successfully restart.

2. If I call for heat and let the fan start, and then shut down and again call for heat, the pilot will energize, but not move.

3. BUT, if I unplug the mercury flame sensor, and wait a second for an audible click, pilot will operate normally when I call for heat.



Other random clues or red herrings:



A. ignitor seems to click about once every 10-20 seconds even when thermostat is

open.

B. I swapped in a honeywell ignitor board for the W-R 5059, and saw quicker recovery when unplugging the flame sensor, but still not normal operation.

C. When troubleshooting before I bought the valve, I soldered the resistor board to the flame sensor pins. I since pulled the resistor from the board.

D. Local utility installed a new gas meter at the house mid summer.



So, what do the experts think?



Thanks,



Matt

Empire_2

Couple things...

See if you can give us an actual incoming and manifold gas pressure.  Disconnect your t-stat at furnace and jump the system out.  You are basically the t-stat.  I'm looking to see if voltage drop exists.  Also check you 24 volt side of the t-former for at least 24 and also 24 to ground on what ever side is hot.



See if this helps.



Mike T.

Tim McElwain

What you have is a White Rodgers

Cycle Pilot system which you say you replaced the 5059 control with a Honeywell Igniter, what is the number of the Honeywell control? The 5059 is simply a pilot relight control which its job is too simply spark and light the pilot, when the pilot is lit it should shut off. Once the pilot is lit in about 30 to 45 seconds the SPDT mercury switch should walk over and bring the gas valve on and run until the call for heat ends.



That valve has a built in pressure switch which in the past had problems that was why they sent the resister plug in kit to overcome the problem with the pressure switch. If after the mercury switch is walking over you have problems then it is the pressure switch in the valve causing the problem.



Make sure the tip of the mercury sensor glows cherry red as the mercury in the bulb needs to get to 400 degrees in order to put pressure on the switch. It is always a good idea to pull the pilot and clean the orifice.



I do have concern about what you replaced the 5059 control with as most modules need a rectification signal in order to operate and this system does not deal in rectification

AlfaMatt

Reply to Mike and Tim

Thanks Mike and Tim:



25vac across the transformer outputs, and 23.5 hot to ground. I don;t have a manometer, so can't check gas pressure.



Also, I was probably not clear enough about the problem:



All runs normally unless the furnace gets a signal to restart after running.



In that case, closing the thermostat (and I use a toggle switch for troubleshooting), instead of the pilot openning, it hums as if it's getting a voltage. And, if I unplug the mercury switch, I can hear a very faint spark, and when I plug it back in, furnace works fine.



Incidentally, if I shut down and restart BEFORE the fan has started, restart is fine.



Incidentally, I swapped the 5059 back in to eliminate the replacement as a contributor to the problem. Incidentally, I mispoke as the one I swapped in was a Robertshaw 785 which was listed as a direct replacement.



Any thoughts?



Thanks,



Matt

AlfaMatt

Reply to Mike and Tim

Thanks Mike and Tim:



25vac across the transformer outputs, and 23.5 hot to ground. I don;t have a manometer, so can't check gas pressure.



Also, I was probably not clear enough about the problem:



All runs normally unless the furnace gets a signal to restart after running.



In that case, closing the thermostat (and I use a toggle switch for troubleshooting), instead of the pilot openning, it hums as if it's getting a voltage. And, if I unplug the mercury switch, I can hear a very faint spark, and when I plug it back in, furnace works fine.



Incidentally, if I shut down and restart BEFORE the fan has started, restart is fine.



Incidentally, I swapped the 5059 back in to eliminate the replacement as a contributor to the problem. Incidentally, I mispoke as the one I swapped in was a Robertshaw 785 which was listed as a direct replacement.



Any thoughts?



Thanks,



Matt

Tim McElwain

Here is what should be happening

When the thermostat calls it is wired in series with the 24 volt secondary of the transformer and power is sent to the 5059 control and spark should start. It also sends 24 volts to the pilot valve which completes the circuit back to the transformer through the NC (normally closed) contacts on the pressure switch. Pilot should light and the spark signal is now shorted to ground through the pilot flame and spark should stop. Within 30 to 45 seconds the SPDT mercury switch should walk over and make C (common) to NO (normally open). The pilot valve stays energized at this time through the pressure switch. The pressure switch will not make with less than 3.5" W.C. pressure. This should be checked with a  manometer. If less than 3.5" the pressure switch will not make. When the mercury switch walks over it completes the circuit to the main valve coil. The burner should be on and running.



If this is not occurring all the time then I would suspect gas pressure or a problem with the pressure switch in the gas valve.

AlfaMatt

Reply to Tim

Thanks, TIm. Do yo mean to check the gas pressure at the test port on the valve, or can I check at the line coming into the valve?



Matt

Tim McElwain

If I remember correctly

that valve has inlet and outlet test ports. You should always test both inlet and outlet at the same time to make sure you pin down the real problem. It should also be tested with a full gas load in the house up and running.

AlfaMatt

Resolution!

Turns out it was a problem with the new mercury flame sensor, which worked OK all last winter, but was acting up this winter. As Tim said in an earlier post, in the cold position, the SPDT switch opens the pilot valve, and in the hot position, it opens the main burner valve.



Problem was that even when cold, the switch would sometimes stick in the hot position and instead of trying to open the pilot valve, it would try (but not succeed) to open the main valve--the humming I referred to in my earlier post. When I wiggled the mercury switch connector, the switch would move to the cold position, where it belonged.



Here's what I did to fix it: The top of the switch is held together with three tiny screws and nuts. If you take that apart and separate the brass diaphragm from the switch body, you will find a plunger that activates the switch, and a spring. The plunger is adjustable in length--it's plastic with a flattened copper rod screwed into it, with the return spring surrounding it. The return spring is what pushes the switch back to cold.



I figured I had two choices--either stretch the spring so it would more forcefully push the switch back to cold, or shorten the plunger so that the neutral position of the plunger was biased more toward cold. I opted to shorten the plunger slightly because I figured I could more easily return the plunger to the factory length than I could return the spring to the factory length, and also, lengthening the spring would make it harder to reassemble the whole thing.



After a little trial and error--my first time I shortened it too much so that the main valve was never energized--I now have an approximate 45 second turnover time from pilot to main, and I can audibly hear the switch return to its cold setting within about 3 seconds of shutdown.



I've cycled it a few times and all seems good. I'll likely wait to see the turnover time from a true cold start to see if I want to lengthen the plunger a little bit more and reduce turnover time.



I'll post more if anything changes.



Happy Thanksgiving everyone!



Matt

Tim McElwain

NEVER-NEVER-NEVER

Do what you did, that is the pilot safety on the system and you could end up with a worse situation in that the system would not shut off. Replace the mercury sensor and while you are at it clean the pilot. You have a potential bomb!

AlfaMatt

Tim, please explain.

Hi Tim:



Thanks for the warning. I agree it's a good idea to put in an order for a new mercury switch. I hope that it will not be flaky from new like this one was.



I understand that it's never a good idea to change a factory adjustment, especially for anything relating to pilot safety, but would you please explain the how mis-adjusting the switch can cause an explosion?



My thought is that if the switch is adjusted so that power to the main is never made, then the pilot will continue to run and the main will never open. Is that what you meant when you said the system would not shut down?



If the switch is adjusted so that it is always trying to power the main, then it will be in the same condition it was in when I wrote in, that the main will be energized, but won't open.



I really would like to understand how these valves work, so if you would tell me what prevented the main valve from opening when the Mercury switch stuck in the hot position, I would very much appreciate it.



Thanks very much for your help.

icesailor

Failure to start:

What you did is why some here hesitate to give any suggestions for fear that someone might blow themselves and or others up. Just because I can "fix" something, doesn't mean that I should. Nor should anyone else.

If you did something like what you did and there was an injury, there are tort lawyers drooling to take on a case where someone stated that they got help on Heatinghelp,com and they did what was suggested and that is why the accident occurred. Nowhere does the manufacturer of the equipment say that you can modify their equipment. In fact, a lot is designed so that if you futz with it, it is rendered inoperable. So folks like you can't fix it.

Do us all a favor and buy a new valve so you are safe.

And I didn't see anyone here write about how you could do what you did and advise you to do it..

Tim McElwain

Alpha Matt

From day one this system from White Rodgers was trouble, thousands of defective pressure switches prompted the resistor solution which the gas company I worked for would not allow on our lines it is nothing more than a resistive jumper. I actually was able to cause delayed ignitions with the system in our lab.



If you contact me with your postal address I will make a copy and mail to your free of charge how this system works with wiring diagrams. I will do that only if you in writing agree to never tamper with the internals of another gas control system as long as you live. That will protect me legally.



It is not important to me to explain any of what you did here on this forum as others who are curious are lurking. Enough said replace the controls needed and be done with it. If you have defective controls being sold to you I can connect you with the powers that be at White Rodgers and deal with them about the defective controls.

Tim McElwain

Alfa Matt I just checked

and found that you are new to Heating Help. We welcome you and are here to help you in any "safe" way we can. It is often better to involve a professional when dealing with these problems as they can be complicated and sometimes dangerous. We look forward to serving you.

Slimpickins

replace it

All of this over a 30 year old furnace that should have been replaced 10 years ago??? Besides dealing with mercury you have a inefficient furnace compared to today's standards. Don't waste anymore money on that beast and get something that can give you way more comfort with better safety devices and save money in the long run.



Happy Thanksgiving  : ) 

RichPasco

I am not alone

So overjoyed to find that I am not alone.  I too have a furnace with a White-Rodgers mercury flame sensor with exactly the same problem.  Several years ago I replaced the mercury flame sensor--expensive--but it did not solve the problem.  I am hoping there is a way that I can replace it with something non-mercury, as well as more reliable.