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Weirdest failure I have ever seen

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delta T
delta T Member Posts: 884
Some back story first.

Weil Mclain GV Series 2 feeding 9 zones of radiant (some staple up, some in slab) and an 80 gallon side arm. I get the call for no heat unless these two valves are turned off, and even then its not great.

Get there and I find that the boiler is not piped correctly, though it might work. Basically one of those "I don't really understand primary secondary piping, so I'll just make a bunch of loops" set ups. I didn't take pics, because the room is too small to really get a good picture, and the piping is so hard to follow it wouldn't really matter in any case. We will be re-piping it in the summer. Two 1" Caleffi 521 mixing valves (one for staple up zones, one for slab zones) installed up side down and both completely failed. Someone had replaced the pump to the staple up zones with a 0011 (did the math, a 007 is more than adequate, a 005 would probably be fine) because, well, "heat doesn't want to go, I'll make it go gosh darnit!!!". Valves that the HO was turning off feed the slab zones, if that bank of ZV's is turned off, main zones (staple up), heat just fine, until recently. Primary pump was seized up.

So, repairs start.

Replace the mixing valves, replace the over-sized secondary pump, replace the primary pump (internal in the boiler taco 007), try and do some balancing to get the improper piping to work correctly, and I'm now getting 140 going to the staple up, and 110 going to the slab. All is well!

Well, NO.

Get the call the next morning, house is at 62, nothing seems to be working.

Well, ****.

Go back out start to look for other things that could be bad, all secondary pumps are working, but primary loop just doesn't seem to be pumping as much as it should. It IS circulating however (I later realized this was simply becuase of the way the improper Pirmary loop was installed and how it was connected to the secondary mixed loops). Well am I sure there is power to the primary circ? Check again, 120 V at the board, continuity through the wiring harness, connect the wiring harness to the board, 120 volts steady as a rock at the end of the wires. Okay, dud pump? Get another pump (Grundfos 15-58 this time) install it in the primary pump spot, again check voltages, all good, hook the wiring up to the pump and still no circulation. Now I'm confused and frustrated, so I grab an extension cord, cut the end off, wire it up to the primary pump and It works like a charm.

So to sum up, the main problem was the board not allowing current to flow to the pump, even though there was 120 V present, and the wiring was sound (this was verified by me multiple times multiple ways because I couldn't get myself to believe it). Any one else ever see anything like this??!! Would have been a snap to diagnose this problem if it was a simple "pump should be running, no power from the board? need a board." but there WAS power at the board, until you hook a pump up to it. I cut the wires a little long to leave some exposed copper that I could test voltage on, and as soon as the wires are hooked to the pump, there is no potential between them, and no current on the hot wire either. Disconnect the wires and the 120V potential immediately returns. New board and wiring harness are on their way.

What gives? Anyone else ever see this?

Comments

  • ratio
    ratio Member Posts: 3,652
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    Fed through an SCR instead of a relay? Those will float up to supply voltage without a load on 'em.

    A relay is an ON/OFF device, there's a physical gap that interrupts current flow. SCRs are more like low-resistance/high-resistance. There's no physical gap to make it really hard for current to flow, μA leakage will slowly, over the course of nanoseconds, raise the output terminal to the same potential as the input without some way of draining off the electrons. Imagine a leaky valve connecting two tanks. Eventually, the leak will bring the second tank up to the level of the first one.

    delta TkcoppJackmartin
  • delta T
    delta T Member Posts: 884
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    Would make sense, though I do hear discernable clicks when the gas valve lights, so I think at least those are real mechanical relays. Not sure about the pump relays though, could indeed be solid state. Thank you @ratio I will have to keep this in mind for future trouble shooting, sure made this one a pain, gonna start checking and relying more on current than voltage for stuff like this.
  • Zman
    Zman Member Posts: 7,573
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    If it's weird problem, check the neutrals. Usually if you have 120 to ground and the motor won't run, the neutral is the culprit.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    Jackmartin
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
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    I see this type of thing on a fairly regular basis, although not specifically on heating equipment.

    You either have relay contacts that have a layer of oxide/carbon which will conduct enough electrical current to give a digital meter a good voltage reading but with a load the voltage can be anywhere. Basically any high resistance connection can act like this, and sometimes intermittently. It's worse in low voltage where it only takes milliamps to drop a couple of volts.

    My bet it the relay itself or a poor/microcracked solder joint on the PC board.

    Always, whenever possible test voltage with a load on the circuit. I keep a 500W halogen light in the van just for this purpose.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    delta T
  • DZoro
    DZoro Member Posts: 1,048
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    Yes have seen that on the WM's. With that particular board always test voltage with pump inline. I like the light idea! Just be safe doing so. Have also changed out good pumps.......
    delta T
  • Tomato
    Tomato Member Posts: 21
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    A low impedance meter like the old Wiggies or a new Fluke 117 Electrical Multi-meter with Non-Contact voltage will usually catch something like this. I get all of my new techs the Fluke 117 because it is less confusing than a typical DVM which will show voltage from inductance, or even with the leads disconnected. Plus, it is an Auto-volt meter so it is less likely to get fried when not paying attention. The problem with the old Wiggie type is they must not be left connected or they might melt or catch fire.
    Solid_Fuel_Man
  • ratio
    ratio Member Posts: 3,652
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    +1 for the Fluke 117. It's my drag-it-around-by-the-leads meter. Works harder than I do!
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,623
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    I had a flourescent light fixture where the original electrician had not grounded the fixture. If you measured from the hot wire with a meter with the ballast connected and the other meter lead to the metal fixture frame it read 277 volts (it was a 277 lighting circuit) but you could touch the metal fixture frame and there was no power there....just phamtom voltage. Grounding the fixture fixed it
  • BobC
    BobC Member Posts: 5,479
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    The ballast magnetically coupled the voltage into the floating case. Sometimes a lamp with pigtails can be a good tool, if the bulb lights you really do have power there.

    Years ago I put up a florescent valence light in my sisters kitchen, it worked but sometimes took a while to start. Months later I found the hot and neutral were flipped further back in the ckt, when I corrected that the light worked right.

    Bob
    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • Jamie Hall
    Jamie Hall Member Posts: 23,441
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    I had a flourescent light fixture where the original electrician had not grounded the fixture. If you measured from the hot wire with a meter with the ballast connected and the other meter lead to the metal fixture frame it read 277 volts (it was a 277 lighting circuit) but you could touch the metal fixture frame and there was no power there....just phamtom voltage. Grounding the fixture fixed it

    I had one sort of like that, but even stranger -- the light wouldn't turn on unless you touched the metal fixture frame. Then it was just fine. Lovely.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Leonard
    Leonard Member Posts: 903
    edited February 2017
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    As a kid when I got a VOM meter I used it to trouble shoot a car dome light. The voltmeter said 12V but put a bulb in and it would not light. Turned out the connection under the wet carpet was a heavily corroded green. Voltmeters draw so little current that even a high resistance connection read good. From then on I stopped using the meter and went back to using a test light ( it draws current, so it can detect high resistance connections).

    When you get a voltage reading voltage off ungrounded florescent light fixtures it's likely stray capacitance ( not inductance). The meter draws so little current it can work off stray capacitance. Meter and stray capacitance form a voltage divider.

    I had an ungrounded refidgerator, it read 25 VAC to ground which was ~ 20% of meter range . Interesting thing was when I switched meter range downward the meter needle stayed at about same place. ( Simpson analog VOM meter). When I connected a bulb across the meter it would not light, and meter read O volts. That meant the power source had a very high impedence in series with it ( the little bit of stray capacitance)
  • delta T
    delta T Member Posts: 884
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    Thanks for all the good info everyone, love learning new stuff!
  • Jackmartin
    Jackmartin Member Posts: 196
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    Hello from Canada
    I followed your pump problem with great interest. The chap that said you had a problem with your neutral was correct. An SCR does not supply voltage to the pump from the board it is two transistors an npn and pnp in the same overall device. In electronics ( I am an indutrial tech in electronics as well as HVAC) the idea of an SCR is to sink the remaining voltage not used by the pump windings to neutral or ground. Unless you are using a very large SCR the device could not handle the amperage the pump would draw. In electronics transistors sink to ground they do not supply voltage as a rule because of amperage limitations. Now that is not to say in some applications large transistors are not used as supply " switchs" because they are. The SCR is triggered by a small excitation voltage from the board and it turns on the SCR allowing the sinking of voltage to occur. You had 120 volts certainly but your SCR was not sinking to ground so the circuit to your pump windings were not in a complete circuit. A little menomic to remember in this kind of problem. The hose will not pass water unless I take my thumb off the discharge end. My pump will not go unless I let the supply have an exit a neutral. The remarkable thing about SCR circuits are once you have gotten the SCR circuit to conduct it no longer needs the initial excitation voltage :neutral: in other words, once you get it to conduct, it will continue conducting until you remove the sinking voltage or in other words the voltage you are sinking to ground left over from the pumps windings. I strongly encourage you to read Practical Electronics for Inventors easy to read and incredibly interesting. Oh by the way do not ever provide your boatd with an external ground to test it out so to speak ,this causes an unregulated amperage rush , and you will see a puff of very dark smoke. How do I know this trust me, experience is a cruel teacher. All the best ,by the by ,we are having a mild blow they say wind speeds of 80 miles per hour in Churchill tomorrow even the polar bears willl be chilly. Jack Martin
    delta TPaul48
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,623
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    Thank you @Jackmartin good information