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. Thatguy

    Thatguy Homeowner

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    In figure 3, TH and PG should be the coil connections and probably measures about 3 ohms. I don't know what they mean by 'power unit' or how this terminal ties in to the rest of the circuit.

    The Figure 4 wiring is a series circuit and makes perfect sense.
    Last edited: Oct 27, 2010
  2. mnalep

    mnalep New Member

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    Figure 4 is 24 volt. Fig 3 is millivolt, which I have. I went and looked at this valve at the store, and the 'power unit' is a large nut screwed into the valve body with a black wire coming out the middle of it. If you look at the 2 diagrams on the first page, the one on the right shows the wire, with a slide on clip on the end of it. I think that is the 'power unit'. I don't get it either, but what else could it be?

    When you mention 3 ohms, are you saying I should be ohm-ing these?
  3. Thatguy

    Thatguy Homeowner

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    I would use an ohmmeter to confirm what terminals the coil is wired to, and what it is not wired to. You want to know where the current goes and how it goes because fig. 3 is puzzling.
    This circuit has to work on paper first.

    If there is only one black wire coming out of the power unit then the case of the valve is being used as an electrical conductor. If your current valve doesn't do this you may need to add a jumper wire.

    Fig. 3 seems to be having the t'stat alone turning on the valve, so the power unit working with the hi limit can somehow disable the valve turning on, but how it does it is not obvious from this figure.

    There may be something wrong with fig. 4. There doesn't seem to be any way to prevent the valve from opening if there is nothing to ignite the burner, unlike in fig. 3 which uses the pilot gen. to supply current to the valve.

    For testing you can simulate the t'pile with a 1.5 v "D" cell and two 3.6 ohm 1/4 w resistors from Radio Shack.

    The "power unit" could be an 18 mV thermocouple but it is not clear to me what function this serves.
    Last edited: Oct 27, 2010
  4. mnalep

    mnalep New Member

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  5. Thatguy

    Thatguy Homeowner

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    I know figs. 4 through 14 and fig. 1 don't apply. If fig. 3 doesn't physically look like your valve it doesn't apply either.

    It looks like fig. 2 applies for the 36C03U and you should measure the coil resistance between TH and PG. The terminal TH-PG is just a tiepoint and does not connect internally inside the valve housing.

    Ask them what the power unit is and how it works with the limit switch and how come it only has one wire.
    Last edited: Oct 27, 2010
  6. mnalep

    mnalep New Member

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    Fig 2 looks like the 3 screw terminal on the valve. Fig 3 cold have been the 'power unit' lead, but this picture shows 2 wires, and the one I'm looking at only has one.
  7. Thatguy

    Thatguy Homeowner

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    And so this figure doesn't apply.
  8. mnalep

    mnalep New Member

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    Sorry it took me so long to get back here. I've had some other problems that needed attention.

    So I took apart the pilot burner and orifice, and cleaned it out.

    I took a couple of still pictures of the flame before and after. And a video clip of the flame is posted below also.

    I also bought a new limit switch and installed it, as the old one was looking kind of rusty. You can see the two limit controls side by side below.

    The furnace has still been working well, although we have had temps in the 60's this last week. So it's been a light duty test.

    The pilot generator was putting out about 750mV on my last meter reading. I was worried at first because my initial reading was around 600mV. I took it all apart and made sure it was real clean, and after I put it back in the 2nd tome the reading was at the normal 750mV. Not sure why it was low the first time?

    So I am hoping this furnace is set.

    Thank you guys for helping me with this!!!



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  9. mnalep

    mnalep New Member

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  10. mnalep

    mnalep New Member

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    I would like to get a vacuum in there and start cleaning the burners and heat exchange area. you can see in the picture there is a lot of dust that needs cleaned.

    what is the best way for me to clean? vacuum? brush, steel wool, or ? it seems really dusty right by the pilot flame area, and on the cabinet just under the main burners. any advice?
  11. Thatguy

    Thatguy Homeowner

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    I assume 750 mV is open circuit voltage. The voltage for a t'pile is related to flame temperature. There's probably a graph somewhere online for the relationship between temp. and voltage.
    A blue flame indicates more nearly complete combustion than a yellow flame.

    Two things define a voltage source: open circuit voltage and output voltage when current is being drawn.
    Voltages sources have a Thevenin equivalent circuit and it's pretty useful to figure out the two values in this equivalent circuit.
    Open circuit voltage is only half the story.

    You might want to buy a 3 ohm resistor and measure the t'couple voltage output when loaded down with this many ohms. Independent tests of each system component comes in handy.
    If the internal resistance of the t'pile is 3 ohms you will get 750/2 = 380 mV across the resistor.
    Last edited: Nov 10, 2010
  12. jimbo

    jimbo Plumber

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    AH....my old buddy Thevinin, and his right hand man Kirchoff! I believe you are about right on the loaded source measuring about 380,
  13. Thatguy

    Thatguy Homeowner

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    I used to go drinking with these guys. . .:D
  14. mnalep

    mnalep New Member

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    thatguy,

    I'm not sure what the meaning of 'open circuit' is? I measured with the pilot flame on, and the leads unhooked from the gas valve. Is that what you meant?

    how would I physically hook up " a 3 ohm resistor and measure the t'couple voltage output when loaded down"? I've never heard of this, would you educate me please?

    where would i get a 3 ohm resistor?

    when you said "open circuit voltage and output voltage when current is being drawn. " do you mean with vs without the gas valve being hooked up to the leads of the pilot generator?
  15. Thatguy

    Thatguy Homeowner

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    Yes, the t'pile can be loaded down with the valve or a resistor.

    This is assuming somewhere in this thread that the valve coil resistance of a working gas valve has been determined to be 3 ohms.
    Last edited: Nov 11, 2010
  16. mnalep

    mnalep New Member

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    thatguy, i've started reading that site, electronics-for-beginners.com, but McAfee Site Advisor (my antivirus) is saying there are potential security risks with that site. have you used it safely?
  17. mnalep

    mnalep New Member

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    ok, so an open circuit has no load device on it?

    I thought a short circuit was where a device, say, a toaster, was not getting any current? That sounds like the opposite of "maximum current available from a source is flowing" - wouldn't it?

    Isn't a short circuit when, say, a wire between the fuse panel and, for example, a toaster - has a bare spot in the rubber, and the copper wire is touching something metal - short circuiting the electricity away from the toaster?

    So a closed circuit has a load device? and 'some current' would still be enough to run the device properly?

    - I never tested the resistance(ohms) of the valve...it sounds like i'd need to?

    - so if I test for DC mV with a 3ohm resistor attached to the t'pile generator (and assuming the t'pile also had 3ohms resistance) - then I'd measure 380mV DC ?
    Last edited: Nov 13, 2010
  18. Thatguy

    Thatguy Homeowner

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    With one assumption, yes

    0.750 =open circuit t'pile voltage [Voc] in volts
    2.890 =presumed t'pile internal resistance [Rint] in ohms
    3.000 =load resistor [Rload] in ohms

    0.127 =current = Voc/(Rint + Rload) in amps
    0.382 =voltage across load resistor [Vload] in volts = Rload x I

    Try electro tech forum. Those guys seem to know their stuff.

    My Apple doesn't much care about viruses. My other computer, which hasn't been online for more than a year, still warns me about potential viruses.
    Anti-virus s/w is a money machine. What's to keep them from generating new viruses so they can sell you more upgrades?
    Last edited: Nov 14, 2010
  19. mnalep

    mnalep New Member

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    thatguy, Thanks, I am starting to understand. So if I did all this and found .382v DC, would that mean my t'pile is good, or bad, or weak? Sounds like it might be weak - if it is supposed to generate 750mV closed circuit am I saying that correctly? Or is the 750mV stated for this part just the open circuit voltage rating with no load on it (gas valve, or, resistor)?

    By "electro tech forum" - do you mean "ELECTRICAL FORUM" on this website?

    I hear what you say about anti-virus s/w makers. Who knows what they are up to.

    I am surprised to hear its not an issue on Apple! I'd be creamed on PC without it. And I still have not found one I really like. McAfee is seeming effective, but updates daily and stops me from working for about 15 minutes whenever it updates. It's free with my ATT DSL internet though.
  20. mnalep

    mnalep New Member

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    btw, do you ever have a problem replying here - get a message like 'the token has expired'?

    This just happened to me, and even reloading the page did not help. I'vw begun copying my replies, before clicking on POST REPLY, in case this happens - to avoid losing a message i've taken time to type. But sometiem forget to copy firts and lose a message I've typed.
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