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Old 10-24-2010, 11:47 AM   #1
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


I am trying to diagnose a problem with an old furnace. I have some pictures to help clarify what I have, a ThermoPILE system.

The problem is that the PILOT flame is on, but the main burner does not always light.

I cleaned all electrical contacts today with sandpaper, and made sure they were all tight.

I also made sure the PILOT line and BLEED line were not blocked. (Took them off and blew through them).

I also took out the THERMOPILE sensor, and cleaned it off with steel wool.











And this is a repeat , but I am trying to show that there is a steel connection(steel plate) between RED(Thermopile,pos) to TH1(thermostat,pos).

Also a steel connection(steel plate) between TH2(thermostat,neg) to L2(LIMIT switch, pos):


I was trying to use the "MILLIVOLT TROUBLESHOOTING CHART", on page 2 of this diagram:
http://www.robertshawtstats.com/spaw...oltSystems.pdf

But I'm not sure what connections to put my meter across.

For example, the chart says in TEST-A, to test across TP and TH for more than 100mV, with the thermostat contacts CLOSED. (On my gas valve, would that be RED to TH1, or RED to TH2, or something else?)

For example, the chart also says in TEST-B to test from TH/TP to TP for greater than 325mV, with the thermostat contacts OPEN. (On my gas valve, would that be TH1 to WHITE, or TH1 to RED, or something else?)

Also, when it says thermostat contacts CLOSED - does that mean with the thermostat temperature set to higher than the room temperature?

I attempted to do TEST-A, with thermostat set to turn furnace on(I assume that what what they meant by thermostat connections CLOSED?) and measured:
RED to TH2 = 123mV
WHITE to TH1 = 387mV
TH1 to TH2 = 20mV
RED to WHITE = 400mV
L1 to L2 = 26mV

For some reason I did not measure RED to TH1, or WHITE to TH2. Should I have?

Also, those readings were what actually displayed on my meter, when the dial was on the 2000mV range, and the 200mV range.

So I think I'm reporting the voltages correctly? That is, if the meter displayed 123 on either of the ranges(2000mV, or 200mV) - that means I measured 123 Millivolts?

If the gas valve is sticking, can this be disassembled and cleaned? Can it just be replaced?

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Old 10-24-2010, 12:41 PM   #2
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


Take the 2 leads from the thermopile/aka pilot generator off the valve and connect them with alligator clips to your meter. Do not hold them in your hands or juice will flow thru you and give wrong reading. Should have at least 500 mv for the system to work. A healthy generator will produce over 600 and up to 750 mv. A furnace that old has a HUGE chance of a cracked heat exchanger and is not safe to use IMO.

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Old 10-24-2010, 02:06 PM   #3
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


As Yuri said. A good burning pilot & generator will produce between 600-750 mv. That's an old furnace. Back when they use real steel to make the heat exchangers. It still could be cracked.. If that's an old generator assy. I would buy a new 1. Any corrosion in the wiring ( like the green stuff on the limit wires) will eat up your mv's..You need good clean connections.

How old is that pilot assy.?

Last edited by kenmac; 10-24-2010 at 02:10 PM.
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Old 10-24-2010, 02:30 PM   #4
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


ken,

There was very little dirt or anything on the leads when I cleaned them. I was just doing it for completeness.

I'd think the pilot tube is original, the pile generator is maybe 4 -5 years old.

As I recall the limit switch is still original, and I had it out about 4 years ago and it looked a bit cruddy. But that would only affect when the main burners are turned off - right?
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Old 10-24-2010, 02:33 PM   #5
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


Yuri,

I was looking over my notes, and I did disconnect the pilot leads with the pilot light on the pile generator. And I think that was around 700 on the 2000mV scale. So if I had 400 with the leads attached, what does that mean?
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Old 10-24-2010, 02:40 PM   #6
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


I forgot to mention that I got the main burner to light on 2 occassions by tapping on either the gas pipe before the main shutoff valve, or on the honeywell gas millivolt valve itself.

Does that indicate more a gas valve problem or a thermopile generator/pilot flame problem?
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Old 10-24-2010, 02:52 PM   #7
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


The gas valve ( which came over with Christopher Columbus ) is finally seizing up/kaput. It has been obsolete for over 30 yrs now. Gas valves nowadays are one piece with 100% shutoff for the pilot flame gas and shutoff valve etc all in one valve.
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Old 10-24-2010, 05:17 PM   #8
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


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Originally Posted by mnalep View Post
ken,

But that would only affect when the main burners are turned off - right?


the millivolts should still travel through the limit wires back to the valve. I see green (corrosion ?)on 1 of the limit wires in the pic. Take off the connections & clean then see if the valve opens. You can also loose mv's at the t.stat. Jump those wires out & see if valve will open
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Old 10-24-2010, 05:54 PM   #9
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


Yuri,

I searched for that gas valve, and found this one as the replacement:

http://americanhvacparts.com/Merchan...tegory_Code=GV

How would I convert the 6 wires I have now, to that (looks like 3 terminals) gas valve?
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Old 10-24-2010, 05:55 PM   #10
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


kenmac, those pictures are from befor I cleaned up the connections, and what looks green really was not.
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Old 10-24-2010, 06:14 PM   #11
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


I don't give hands on advice how to alter gas lines, manifolds etc. Have to be a licensed gas fitter to do that here. Pilot tubing and gas line fittings need altering and I doubt you can access those fittings. Seriously, that furnace is unsafe to use. Doubt that a 40 yr old limit control will be accurate or work safely when required. You have to live there so it is your call. VERY inefficient unit. Less than 5% of those units left here and mostly in slummy rental properties where the owner doesn't care.
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Old 10-25-2010, 01:49 PM   #12
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


I found a place that has a replacement limit control. I'm going to go see if it is the same, but the guy says it is. Around $30.
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Old 10-25-2010, 05:34 PM   #13
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


Well there's thirty loonies down the crapper....could have gone for a new furnace.

I have not seen one of those in twenty years...
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Old 10-26-2010, 10:31 AM   #14
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


hvaclover, Thanks for your opinion, but that $30 would not even cover the taxes on a new furnace. Probably not even the permit.

I may end up with a new furnace, eventually. But if I do, I will make my choice - based on my needs.
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Old 10-26-2010, 11:53 AM   #15
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How do I test Millivolt valve system when Pilot is on, but main burner not lighting


Quote:
Originally Posted by mnalep View Post
hvaclover, Thanks for your opinion, but that $30 would not even cover the taxes on a new furnace. Probably not even the permit.

I may end up with a new furnace, eventually. But if I do, I will make my choice - based on my needs.

You are obviously an intelligent person. But I think you fail to understand the nature of that ticking time bomb of a killer that heats your home.
You came to this board for advice which we give freely. But what we also do is point out raging unsafe conditions when we come a cross them.

Brother yours is one of them.

I have nothing to gain whether you buy a furnace or not.

It's obvious from your pics that the furnace burners are flashing back into the vestibule due to the scorching on the burner cover. Fire hazard there.

The age of your furnace makes it a candidate for replacement on age alone. Carbon Monoxide level there. You will probably think " My monoxide detector will sound if the furnace starts pouring monoxide into my home,". Wrong again, by the time your detector sounds (which I bet even if you have one is over five years and the battery is dead or the sensor is no longer effective).

You do not maintain the furnace wall at at all hence the discoloration of the burners.

Penny wise and Pound foolish....you must be very frugal not to have changed that unit by now.

If you have family leaving with you than you are a very selfish person for not taking a more proactive approach to ensuring their safety from monoxide poisoning.

I don't know what the state of your finances are nor is it my business. But i do know that since there are so many aid programs that can arrange a new furnace for you you have no excuse not to replace it. So you cannot plead poverty as an excuse for poor safety judgment.

I hope noting happens to you or yours but I don't think you will be swayed to action by my words either. Folks who hold on to stuff a old as your furnace are a stubborn lot and if a tech came to your home and tested your furnace and found a monoxide you would still keep using it.

It happened to me on the same model of furnace you have.

I monoxide tested the furnace for a gentleman who lived in my town and found a dangerous leak. When I attempted to shut down the unit the customer became belligerent and would have struck me with a hammer he picked up. I backed off on shutting it down. I did not stick around to collect my service call but did manage to get a sign off on the leak.
I filed a police report to cover my self.


The next day in the late after noon they carried the customer and his three kids out of the home to the coroner's

You want to risk that, then I can't prevent it. But at least this post has your attention. Maybe you'll make the smart choice.

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