Pilot goes off and Burner starts with a delay

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Old 03-17-12, 07:49 AM
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Pilot goes off and Burner starts with a delay

I would like to troubleshoot/diagnose why my pilot flame tends to go off, and why, when I restart it in the morning, it takes a while before the burner turns full on.

The furnace is a Utica 125AGB (installed after 1981 according to the rev.date on the plate), the gas valve is a White-Rodgers 36C03U Type 319, the thermocouple is a Honeywell Q313A118 (it was replaced last September, but I'm not sure it needed to be). The controller, according to the diagram sticker on the side of the furnace is White-Rodgers.

Last November, my plumber replaced the thermocouple, because the pilot would not stay on. He actually didn't verify that the old thermocouple was broken, but he said that was likely and it made sense replacing before anything else. But now after the pilot going off every 3-4 days, and the furnace taking its time to kick in I think I need to troubleshoot the controller. (the delay happens even if I switch the override ON). The recirculating pumps turn on correctly when the thermostat give the signal. There are three thermostat zones, and three pumps.

Once it is on the burner seems to correctly go off at 210 F and 20 psi. When the furnace is off and cooled down the water pressure dial reads 15 psi. I have an electric meter, and the thermocouple, when the burner is fully give out 98 mV. The plumber mounted the thermocouple slightly below where it was supposed to be because he had crimped the ring that is supposed to keep the thermocouple in place, before inserting it in the right place.

Any help would be appreciated.
 
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Old 03-17-12, 08:39 AM
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A couple questions first:

(the delay happens even if I switch the override ON)
What is this 'override' you mention?

the burner seems to correctly go off at 210 F and 20 psi
I dunno 'correctly'... 210 is a pretty high temp setting for the high limit. Why is it set that high?

now after the pilot going off every 3-4 days
Is this a 'new' development? or has this been occurring ever since the thermocouple has been replaced?

Did anyone check the gas pressure?
 
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Old 03-17-12, 08:52 AM
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Your gas valve is a MILLIVOLT valve, and you do not have a THERMOCOUPLE but rather a THERMOPILE. There is a distinct difference.

If you are correctly measuring the output of the thermoPILE, then 98 millivolts is too low and could be due to the fact that you say the guy installed the part improperly. If it is too far below the flame it won't get hot enough to produce enough voltage. I'm surprised the thing is working at all with 98 mv... you should be seeing at LEAST 150, and even that is lower than should be.

Open circuit (not connected to the valve, but can't be tested that way) voltage on a thermopile can be as high as 750 mv. Connected to the valve and fully hot should probably be minimum of 250 mv.

How are you measuring the voltage? Across the two leads from the thermopile?

What is the 'controller' that you mention? model number?
 
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Old 03-17-12, 09:07 AM
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Override. I think it's a thermostat override. It's the side of the controller, and it's a slide-type switch with two positions Off and Override. The plumber explained I was supposed to use it if there is no electricity to power the thermostat. I used it for instance in November when after the storm we were without power for a week.

Temperature setting. I think I wrote an incorrect temperature. It was about 200 F yesterday when I observed it going off. There is screwdriver-adjustable dial on a smaller component inside the controller box, it seems to be pointing between 190 and 200.

Pilot going off. It seems to be increasing in frequency. It happened 2 weeks ago once. Then again one week ago. Then 3 times this week, and this morning again, after I posted for the first time.

I was there when the pilot went off. The gas valve had turned off the burner, even though the thermostat was asking for heat. The boiler thermometer had cooled down to 100 F, and the pressure gage was down to 15 psi. The thermocouple was showing 141 mV (I think that's low but my meter is a RadioShack which even without touching the terminals would show anywhere between 10-90 mV). Then as I was touching probing around the boiler controller, I heard a click and the pilot went off.

I don't think anybody checked the gas pressure, but I thought it is alright, otherwise the stove would not work well either.
 
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Old 03-17-12, 09:34 AM
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Yes, thank you for correcting me, it's a thermopile. and the controller is a MILLIVOLT. I disconnected the thermopile terminals and measured:
  • no flame 0-1 mV
  • pilot on it goes almost immediately to 120 mV
  • it climbs steadily and after 1 minute it gives 200 mV

I will post later pictures for the Millivolt and its diagram on the side of the furnace; the number stamped outside reads 21D32-1; inside 8304.
Thanks for directing me through this troubleshooting.
 
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Old 03-17-12, 09:46 AM
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Old 03-17-12, 10:12 AM
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Override.
The 'override' is something unique to millivolt systems. Since the power needed to operate the gas valve is genertated by the thermopile itself, it is possible to bypass the normal controls with a manual switch and get some heat into the home even without normal power supply.

Temperature setting.
I would turn that down to 180F. There is very little possibility that you won't be able to heat your home with the lower temperature water and you could possibly save a little fuel.

Pilot going off.
I can almost read the diagram. There's a little glare and it's bit on the small side... can't find a way to blow it up on screen.

The voltages you are posting now seem like they should be high enough to operate the system.

One thing about millivolt valves though is that since they operate on such low voltages, any stray resistances in the system can cause dropouts even if the pile is functioning properly. You may find that if you were able to get in and clean the contacts on the relay in the controller box that you might 'fix' the problem. It doesn't look like the contacts are easy to get to though.

You could pick up a spray can of 'DeOxit Gold' at Radio Shack and give the contacts a little spritz and see what happens.

See if you can get a slightly better pic of the diagram and I might be able to suggest a further test with the meter that could diagnose if you are losing voltage across a dirty relay contact.

A close-up of the wiring to the gas valve would help this cause also.
 
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Old 03-17-12, 10:18 AM
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I was able to snag the photo and open in my photoshop... no problem seeing now... don't need new pic.
 
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Old 03-17-12, 10:27 AM
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With the pilot lit and the tp hot, measure the voltage at the ' PG ' terminals on the gas valve. This is straight off the thermopile.

Next, measure the voltage at the ' TH ' terminals with the thermostat calling for heat. Alternately, you could measure the ' V1 ' and ' V3 ' terminals in the controller (aquastat) . When calling for heat, there should be ZERO, or very close to zero volts on the V1 and V3 (which are same points as the TH on the gas valve). If you see significant voltage at this point, it indicates dirty contacts.

BE EXTREMELY CAREFUL! the wiring on the lower left inside the aquastat is 120VAC and it CAN KILL YOU!
 
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Old 03-17-12, 11:41 AM
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measure the voltage
I first turned off and on the electric general switch to the furnace to verify: switch off 0 V on the lower left side, 120 V with switch on.
  • PG hot: 180 mV de-energized, 100 mV energized
  • TH: 180 mV de-energized, 25 mV energized with thermostat on
  • V1-V3: de-energized 200 mV, energized 1-2 mV
I guess that means as you explained, there are dirty contacts in between. But where do I have to clean? The wires that are twisted together seem short, and I will lengthen the contact, while cleaning with sandpaper. Anything else?

P.S.
I also turned down the aquastat to 180 F as you suggested.
 

Last edited by gciriani; 03-17-12 at 12:55 PM.
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Old 03-17-12, 01:02 PM
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Override = Manual, Gas and Relay

I stated incorrectly before that the Millivolt controller has an override control. It actually says Automatic and Manual, and the manual bypasses the thermostat. Also the gas fired water heater sits next to the furnace and is working perfectly so the gas pressure should be OK.

After doing all the thermopile voltage controls and trying to clean the contacts the furnace has not started even after waiting for a long time. I haven't sprayed the contacts of the relay though, but when I checked for continuity on the relay, it seems to have tripped the pilot gas off.
 
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Old 03-17-12, 04:07 PM
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NO... do not use sandpaper! the contacts are PLATED with either silver or silver nickel and sandpaper will remove the plating... don't do it! I hope that you haven't already.

The test with the thermostat calling for heat, you are only losing 1-2mv across the contacts and that is fine, there is no problem with the control.

Can you see the thermopile? Is the tip of it in the flame, or did your guy mount it so badly that it is not fully engulfed in the flame?
 
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Old 03-17-12, 06:13 PM
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I only touched the contacts with sandpaper, I also sprayed a Radioshack product for oxidized contacts, but the situation didn't change. The thermopile has only the tip in the flame, that's why, I think, the thermopile is not giving a higher voltage. It would seem plausible that there is enough voltage to keep the pilot on, but not enough to open the rest of the gas valve. Is there another test to see if the gas valve is gone?
 
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Old 03-18-12, 06:08 AM
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The thermopile specs say it should produce 600 mV. Does it mean I'm dealing with a thermopile that has deteriorated? And what are the possible causes of a thermopile deteriorating in five months?
 
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Old 03-18-12, 08:25 AM
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You'll only get something in the order of 600 mv in an OPEN circuit. Once a load is on it the voltage will drop, but it shouldn't be as low as you describe.

Before stretching out too far the first thing to do is get the thermopile properly mounted. You've said it was done improperly, so that's gotta be fixed first.
 
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Old 03-18-12, 08:48 AM
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Let's see a pic of your gas valve.....
 
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Old 03-18-12, 01:19 PM
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Success! - Thank you NJ Trooper

Success, and thank you for putting me through the hoops NJ Trooper!

I think the malfunction was caused, as you first imagined, by the thermopile functioning at the limit of what was needed to open the gas valve. When something changed recently, like more heat than usual, additional thermal expansion caused it to slightly move beyond the working point.

I took the thermopile down, and tested it on the stove: 200 mV in 30 seconds, 300 mV in 1 minute. The only parameter that seems off is the resistance, which I measured as 9 Ohms; the specs say 2.89 Ohms. I compared to the old one, and noticed it would fit in the holding ring 1/4" lower, because of an internal thread. The newer thermopile supposedly locks in place with a retaining spring, but it didn't get to the point, so the plumber who replaced it in October, stuck there somehow, but a little lower. So I forced the threading by turning it for a few turns against the retaining ring, and gaining those precious 1/4". I did some damage though to the pilot tube, because my pliers slipped, and squeezed it almost flat. I tested it and it seems to convey enough gas pressure flame for it to work correctly. Please take a look at the pictures of the damage and let me know if you think it's dangerous.

https://docs.google.com/file/d/0B887...zRwandXdw/edit

https://docs.google.com/open?id=0B88...c3RrbjJ1S0tCUQ

https://docs.google.com/file/d/0B887...WJQM29aUQ/edit
 

Last edited by NJT; 03-18-12 at 02:14 PM.
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Old 03-18-12, 02:57 PM
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I did a little more studying on thermopiles myself... I saw something in the Honeywell info that made me want to understand a bit more how they work.

Remember the old McD's thing... or was it Wendy's ... where they wanted to keep the 'cold side cold and the hot side hot' ? Thermopiles work on the temperature DIFFERENCE between the 'hot junction' and the 'cold junction'. The greater the difference, the higher the voltage.

I think it was in that brochure where I also read that they recommended a 'radiation shield' be constructed if the burner design was such that the cold side were exposed to heat. So, perhaps with yours being mounted low, the cold junction was being heated somehow... ?

There was a post on here recently where the fellow reported that the pile voltage started dropping after the main burner ignited... and this seems a likely cause for that.

Luckily, the HOLDING current of the gas valve once open is much lower than the OPENING current, so I bet in most cases this isn't a problem. But if it is, we know a possible fix now!

The only parameter that seems off is the resistance, which I measured as 9 Ohms
About resistance measurements: Meters work on the principle of 'voltage drop'. The meter applies a current from the internal batteries to the resistance in question and measures the drop across that resistance. If there is ANY EXTERNAL voltage present when making a measurement it will throw off the reading. If you ever do this again, REVERSE the meter leads and don't be surprised if you get a completely different, and even NEGATIVE, resistance reading. Bottom line: Fuggedaboutit.

Please take a look at the pictures of the damage and let me know if you think it's dangerous.
It don't look purty... but I would think that as long as the pilot gas flow is OK, and IT's NOT LEAKING, that you will be OK. You might look into replacing the part though.

In the photo that shows the burner and mounting bracket, it appears that bracket is a bit bent. It looks like the TP is too close to the pilot tube. I would try bending it back a wee bit. You want the burning part of the flame to completely engulf the tip of the TP. With it that close to the opening it may not get as hot as if it were a bit further away.
 
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Old 03-18-12, 03:45 PM
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If there is ANY EXTERNAL voltage present when making a measurement it will throw off the reading.
Makes sense. Thanks again
 
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