Troubleshooting older furnace with White-Rodgers 36C84 gas valve.

CalBoy101

I have an older Rheem RGUD-07EA gas furnace that uses a White-Rodgers 36C84 Type 220 gas valve and a 3098-136 mercury flame switch, it also uses a 5059 pilot igniter. The furnace lights the pilot and the spark stops and after about a minute it sounds like the 3098 switch is trying to turn on the main gains (a big click happens and I can feel it on the valve body) but the main gas valve apparently never opens as the main burners don't light. When this happens it then shuts off the pilot also, the spark starts again and the process starts all over again.

I took the 3098 out and connected an ohmmeter to the Hot pins (2 and 4) and heated the tip with a propane torch. A minute or 2 after it gets cherry red it is switching the Hot pins on and also switches the cold pins off.

Does this mean the 36C84 valve is definitely bad or could there still be another problem causing the main gas valve not to open?

This furnace has something else I haven't seen before, it uses a Honeywell model L4064T1103 fan switch. This has the standard fan on and limit terminals but also has a 300 ohm heater element in it, when its entergized it apparently also closes the fan switch. I tested this and it appears to be working but is this heater element somehow also involved in teh startup phase?

Tim McElwain

The fact that heating up the 3098 caused to to break from NC to NO (SPDT Switch) tells me it is okay. The pilot needs to be cleaned as the tip of the sensor on the 3098 has to get "cherry" red. The mercury will not boil until it gets to 400 degrees so a good sharp pilot flame is needed. Try that and see if it works.

The resistor was placed on the 4064T Fan and Limit control to trick it out due to Honeywell putting a function on all Fan/Limit controls which should have only been on special applications. The resistor makes it work like a regular 4064 Fan and Limit.

CalBoy101

Tim, thanks for the input. The best I can tell the 3098 is working correctly and is switching on the hot pins but the main gas valve just isn't opening. Does this mean the 36C84 valve is probably bad?

CalBoy101

By "hot pins" I meant the pins that turn on the main gas valve.

Tinman

Are you getting a steady (continuous) 24 volts at the gas valve once 4 & 2 close?

CalBoy101

Its hard to measure that since the 3098 covers its pins up once its plugged in but now that you mention it I think there is another terminal on the valve that goes to 24vac when the main valve is actuated, I'll check that pin and see if its going to 24vac. If it is it pretty much have to be a bad valve, correct?

JUGHNE

If this is what I think it is, there might be a gas pressure switch in the valve. Have you checked the gas pressure going into the valve?

Tim McElwain

That pressure switch was a problem when this White Rodgers Cycle Pilot System first came out. It was resolved with a new gas valve or an EQSO resister placed between the plug in mercury pilot and the gas valve. JUGHNE is correct if your incoming gas pressure is too low the pressure switch will not make. That would be one of the things I would check before ordering a gas valve. By the way I hope you can still get those valves.

CalBoy101

Now that you mention it I guess gas pressure in the house could be an issue. We are on a city natural gas feed but the gas dryer has also been acting up (not getting fully hot), what could have caused our gas pressure to drop and how do I measure it, can I just stick a compressed air gauge on it and what range should I expect to see?

If I do end up needed the valve White-Rodgers claims the 36C84-921 is a replacement for it and it apparently fixes the issue that required the resistor fix you mentioned.

Thanks for the help fellas, I really appreciate it, money it tight for us right now and avoiding a big repair bill will really help us.

Tim McElwain

Try this for a replacement
http://www.bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=GSV36C84421&Category_Code=GV

JUGHNE

You might call your gas company to report low pressure.

Your furnace works with 1/4 of a pound of pressure.
That would be 4 ounces or 7 inches of water column.
This is right at the gas valve inlet.

The tire gauge will not do it. Call them or someone with the right tools

BobC

i had the gas company out here last year because my boiler was acting strange. They came out and found the gas pressure was dropping under load. They had to replace the pipe from the street to the gas meter to correct the problem.

Bob

CalBoy101

Thanks for the suggestions, I called the gas company and they sent a guy out. He said the pressure "was just a little low" and replaced our regulator and meter. But I hit another issue. I had taken the mercury flame sensor out to test it and used a small propane torch to heat the tip until it was cherry red and it flipped the switch correctly, let it cool and did it again and it worked again. The sensor did look a little dirty so I sanded it lightly with 220 grit sandpaper and put it back in the furnace. Now when the pilot goes on there is a good sized flame heating the sensor but its not glowing cherry red and never flips the switch. I let it sit for almost 10 minutes and tried adjusting its position in the flame but it never turned cherry red or switched.
Did I ruin it with the sanding, ie is their a heat conduction coating on it that I might have sanded off?

JUGHNE

That cap tube that old is very fragile and can snap and lose the mercury charge. That is possible......but don't know why the bulb would not get cherry red without the mercury. Looks for little beads of mercury laying around......deadly stuff.....I used to play with it 50 years ago.

With the gas service valve off you could try the propane torch with the bulb installed in place to see if it flips the switch.

CalBoy101

It took a week to get it shipped in but I got a new valve and mercury sensor installed and the furnace is now working correctly, thanks again for the help fellas.

A couple questions-

After the main gas ignites the pilot stays on also, is that what is supposed to happen? The mercury sensor continues to glow red with this happens.

Whoever had installed the furnace had used teflon tape on the pipe fittings on the valve, which I expected.

But they had also put some teflon tape on the brass gas hose adapter fitting end that has a tapered sealing surface, both on threads and on the tapered sealing surface.

I'm assuming that was a mistake and cleaned off the old tape and didn't put any back on it, that was correct, right?

Abracadabra

CalBoy101 said:

After the main gas ignites the pilot stays on also, is that what is supposed to happen? The mercury sensor continues to glow red with this happens.

Yes

CalBoy101 said:

But they had also put some teflon tape on the brass gas hose adapter fitting end that has a tapered sealing surface, both on threads and on the tapered sealing surface.

I'm assuming that was a mistake and cleaned off the old tape and didn't put any back on it, that was correct, right?

Yes.. no teflon on flared connections... shouldn't be flex gas line anyways though... should be black iron pipe to the valve

CalBoy101

Abracadabra said:

CalBoy101 said:

But they had also put some teflon tape on the brass gas hose adapter fitting end that has a tapered sealing surface, both on threads and on the tapered sealing surface.

I'm assuming that was a mistake and cleaned off the old tape and didn't put any back on it, that was correct, right?

Yes.. no teflon on flared connections... shouldn't be flex gas line anyways though... should be black iron pipe to the valve

Thanks for the input.

The furnace is in the attic and I'll look at it tonight but I think there was black pipe up to the attic and then a flex line about 3' long to the furnace.

Is it asking for trouble to leave that flex line in place or should I replace it with real pipe?