How do I test Millivolt valve system - Pilot is on, but main burner not lighting

Discussion in 'HVAC Heating & Cooling' started by mnalep, Oct 24, 2010.

  1. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    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.

    [​IMG]

    [​IMG]

    [​IMG]


    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):
    [​IMG]

    I was trying to use the "MILLIVOLT TROUBLESHOOTING CHART", on page 2 of this diagram:
    http://www.robertshawtstats.com/spaw2/SiteContent/Files/faq/ControlTips-MillivoltSystems.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?
  2. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    2 more pertinent pictures:

    [​IMG]

    [​IMG]
  3. Thatguy

    Thatguy Homeowner

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    1,459
    Location:
    MD
    There must be two terminals in this connector block that powers the gas valve coil. I suspect they are the "white" terminal and TH1.

    This wiring should reduce to a simple series circuit: a gas valve coil in series with a thermopile in series with a t'stat with normally open contacts in series with a limit switch with normally closed contacts.
    The next time the valve opens, the current measured anywhere in this series loop gives you the current through the valve coil, limit switch and t'stat. Current specs are hard to get but it might be approx. 100 mA.

    The next time the valve fails to open you need to measure the voltage across this valve coil, or, better, the current through it.

    When they replaced my gas valve it was $400, and I did the troubleshooting ahead of time.

    My interpretation

    TH1 to TH2 = 20mV - voltage drop across closed switch contacts plus voltage drop across several yards of wire of some gauge number.
    RED to WHITE = 400mV-voltage output of t'couple while under load assuming you measured it while the valve was open and working. It should be close to 750 mV when the valve is closed and off.
    L1 to L2 = 26mV-voltage drop across closed switch contacts plus voltage drop across a yard or so of wire of some gauge number.
    WHITE to TH1 = 387mV-?-I'd think the coil voltage should read 400 - 20 - 26 = 354 mV. [1- (387/354)] x 100 = a +9% error, which is kind of high for a digital meter.
    RED to TH2 = 123mV-?

    What gauge number wire and what length do you use to your t'stat?
    http://en.wikipedia.org/wiki/American_wire_gauge
    Usually #18 is used for physical strength and not so much to limit the voltage drop.

    BTW, "test A" implies that the valve will open on as little as 100 mV, and "test C" gives 80 mV max. system voltage drop for whatever current is flowing. For 100 mA this max. resistance value would be 0.8 ohm.
    Last edited: Oct 24, 2010
  4. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    Thatguy, ther are 6 terminals, 2 from pile generator, 2 from limit switch, and 2 from t'stat.

    - So I should test for milliamps also?

    - You again mean in milliamps?

    Not sure of the T'stat wire. I'd guess it is 14gauge? The wire off the limit switches are 14, as I just had to strip one of them. The t'stat wire runs from a 2nd story flat into the basement. Could be 30 feet maybe a little more.
  5. Thatguy

    Thatguy Homeowner

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    1,459
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    the robertshaw link gives max lengths vs. gauge

    The voltage "drops" in a series circuit must add and be equal to the source voltage. I guess they call them "drops" because the source voltage represents a "rise" in voltage.
    Lengths of wire and closed switch contacts have small voltage drops across them when passing current.
    The t'pile has an internal resistance so the voltage it supplies when open circuited will be more than when it is under load.
    Last edited: Oct 24, 2010
  6. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    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?
  7. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    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?
    Last edited: Oct 24, 2010
  8. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Draw yourself the circuit. It looks like this: the thermopile is a generator...puts out up to 400 MV. The gas valve is the load....need to have current flow from one leg of the thermopile...through the limit switch ( zero ohms ) througn the thermostat switch ( approx zero ohms on call for heat) through the coil of the valve...and back to the other terminal of the generator. If either of the switches is bad, no go. If the switches are good, that sort of defaults to a bad gas valve. 400 millivolts across the generator loaded...current flowing through the switches and valves...is about right.
  9. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    And we already know that the t'stat + terminal is jumpered to the t'couple + terminal, and that the t'stat - terminal is jumpered to the limit switch + terminal. Except for the 123 mV this circuit is pretty well obvious.

    700 mV open circuit and 400 mV closed circuit with an internal t'couple resistance of 3 ohms means the current draw is (0.7-0.4) volts /3 ohms = 0.1 A. Maximum power is transferred when the closed circuit voltage is half that of the open circuit voltage so these numbers are entirely plausible.
    For a 20 mV drop, a 60' loop distance, #14 AWG and 100 mA you should have 15 mV drop on the wires, leaving 5 mV across the closed t'stat contacts.

    Almost certainly, with these readings and the tapping, your valve is sticking closed.
    If it sticks open, there could be a big BOOM.

    What would a person without a death wish do next? :D
    Last edited: Oct 24, 2010
  10. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    Your system is quite old and somewhat unsafe, in that the pilot comes off ahead of the safety valve. I would not work on it, without completely replaceing the gas control system, and the furnace may not be worth that expense.
  11. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan

    I had the 400mV reading while the main burner was off. Would that count as being 'under load'

    So if I put a test wire across the LIMIT switch contacts, does that remove the LIMIT switch from the circuit? (ie: eliminate that as a problem).

    Could I also do that for the stat, or do I need power there? How would I do that?
  12. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
  13. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    I would seriously consider what Jimbo said, but, yes, if you bypass the t'stat and the limit switch with jumper wires and the pilot is on, the valve should open if the schematic is what I think it is.

    Yes, the load for the t-pile is the valve coil. With the 400 mV on the valve the main burner should soon turn on.

    I'd think a gas valve should just have one terminal, coil + and the valve body is ground,
    or two terminals, coil + and coil -.
    Maybe a third terminal would be the ground terminal.

    Your present valve seems to use the valve body terminals as tiepoints for the circuitry and I'd think several wirenuts could do the same job.
    Without internal schematics for both valves I can't be sure, but with an ohmmeter I guess you can figure out some of the inside wiring.

    This whole project seems risky. If I lived by myself in a house some distance from the neighbors I might consider repairing this furnace if it were under 25 years old.

    Years ago I heard about residential gas pressure becoming excessively high somehow. Something failed and the house filled with gas, and then a girl coming home turned on a light switch to investigate. There was not much left of her.
    Last edited: Oct 25, 2010
  14. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    thatguy, I hear you. But it has been reliable for a long time.

    Here is a pic of the schematic. This is the lower part, which is the Millivolt gas valve:
    [​IMG]


    This show 2 schematics, the upper is a 24 volt gas valve, which I am assuming was optional on this furnace:
    [​IMG]
    Last edited: Oct 25, 2010
  15. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    Yes, this shows the pilot gen. in series with the limit switch in series with the t'stat. The internal valve coil must then be connected to the topmost and bottommost of the four terminals.
    It's a wired AND function with 3 inputs: the t'stat contacts must be closed AND the limit switch must be closed AND the pilot gen. must be putting out voltage. Only then does the valve open.

    I couldn't open your link; you have specs and whatnot on the 3 terminal valve? If you connect the gas pipes to the new valve make sure you use a soapy solution to check for leaks.

    Let's hope your repaired furnace fails gracefully and piecemeal rather than catastrophically. It has a 1% chance of making it to 80 years.
    BTW, having three high sensitivity CO detectors will give you aircraft level reliability [so called "six nines"] since the chance of all three failing is miniscule.
    Last edited: Oct 25, 2010
  16. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    Let's try that link again:

    http://americanhvacparts.com/Merchant2/merchant.mvc?Screen=PROD&Product_Code=GSV820A1047A&Category_Code=GV

    I do not have a schematic for that replacement gas valve(GSV820A1047A), but you can see it lists my current one (VS87A9X4X2) as one it replaces.

    When you state "The internal valve coil must then be connected to the topmost and bottommost of the four terminals." (SO THE VALVE COIL is CONNECTED TO WHICH 2 terminals in my picture)?

    Also, you made me think that my physical valve actually has 6 screw in terminals? I have never learned to read a schematic, but it seems to indicate 4 terminals?

    Also, the two steel plates on the physical valve terminals are 'jumpered 'from a pile generator(RED) to (thermostat)TH1, and another from the other thermostat terminal(TH2) to a Limit terminal(L2).

    But the schematic seems to me to show one connection from one of the limit terminals (Lx) to a Thermostat terminal(T1 or T2), and another connection from the other LIMIT Ly) to a PILE GENERATOR terminal (either RED or WHITE).

    Am I interpreting the schematic wrong?

    Thanks for that heads up on the CO detectors!
    Last edited: Oct 25, 2010
  17. jimbo

    jimbo Plumber

    Messages:
    8,997
    Location:
    San Diego
    You should NOT short out the t'stat terminals to check it. There is a heat anticipator coil in there which limits current to the valve, and shorting that out can damage the valve.
  18. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
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    6 terminals + 2 jumpers = 4 effective terminals = 4 nodes
    http://en.wikipedia.org/wiki/Node_(circuits)

    You need to find out which terminals on the replacement valve go to the coil. I couldn't make it out from the link but if you can get a photo of the terminals their labelling might tell you, or if their cust. service rep has one handy, he/she can read it to you.
    Last edited: Oct 25, 2010
  19. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    Jimbo, Thanks for that tip. I tried that once already, but I won't again.
  20. mnalep

    mnalep New Member

    Messages:
    107
    Location:
    Michigan
    thatguy,

    Here is a white-rodgers replacement millivolt gas valve installation instructions:
    http://www.emersonclimate.com/en-US/products/valves/Documents/0037-6264.pdf

    Page 3, FIGURE-3, has diagram of the wires and seems to show where all 6 wires would go. ;-)

    now I would just need to verify if the input is 3/4" (which I think it is), or 1/2". I think the output is 3/4" also. Besides me taking it apart, is there any way to tell from the picture, or somewhere else online?
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