Troubleshooting millivolt gas furnace help request.
Comments
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" How would you go about testing if the problem recurs? "
Document the reading of the working system, analyze them to enhance your understanding of how it works, and compare then to the failed system. Good notes will help if it fails years down the road.
Draw / document it all on a wiring diagram.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
At least this one claims milliVolt compatibility.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
It does say it is "millivolt compatible" on the package.
Perhaps I've used the term "digital thermostat" incorrectly though? I was trying to say that it was not a"mechanical" tstat - which I figured was the correct way to describe it, since it is all circuit board inside. Maybe I used incorrect terminology? But that tstat is "millivolt compatible".
I went out today and bought another one like it from Ace, intending to have a backup, but then I went to Home Depot and bought a Honeywell millivolt thermostat, and may take the one from Ace Hardware back. I'm thinking Honeywell may be better than whomever makes the Ace tstat?
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I checked out the battery contact, and it is ok. But, it does look crusty in the picture. Good eyes.
I looked closer at the relay after you it out, and noticed some tan discoloration on it. Could that be a sign of it overheating?
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This number shows up with the Lux brand too. Probably made in the same factory as other major brands, just sold (marketed) with different names.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
the picture is very out of focus but it looks like marker
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You have too many units to keep track of, the one with the crusty battery terminal is a different unit than the one pictured with the clean terminal.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
The battery terminal is clean, although the picture does look crusty.
I did notice a brown mark on the relay, I wonder if that is a sign of overheating?
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Click refresh, you seem to be missing posts.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
The thermostat is millivolt compatible. Perhaps I misspoke when I called it "digital". I was trying to say that it was not "mechanical", as mentioned above. Maybe that is incorrect also? I just figured since it is a circuit board, with an LED screen - that looked to be digital to me.
I did go out today and bought another Ace as a backup stat, but then went to Home Depot and saw a Honeywell brand, and bought it. I think I'll return the Ace thermostat, as I would hope Honeywell is better brand than Ace?
MY OLD ACE THERMOSTAT INDICATES IT IS MILLIVOLT COMPATIBLE:
NEW HONEYWELL THERMOSTAT FROM HOME DEPOT BOUGHT TODAY:
NEW HONEYWELL IS ALSO MILLIVOLT COMPATIBLE (750mv heating systems):
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OK, any of them should work.
I think the relay is mechanical and not a solid state switch. I understand what you mean, no problem.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System
1 -
Yes, you are correct. The other stat has a discolored battery terminal. This was an even older stat than the one I removed last week, Maybe the first I ever bought?
So, I took another close up picture of the brown streak, on the relay, of the stat I removed last week, see below.
Stat I removed last week's with a close up of the relay brown streak
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I actually posted last night, then after I hit 'refresh' the posts seemed to disappear. So I posted again. This happened twice. But, this morning, two of the 4 posts are visible. System hiccup maybe?
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Ok, so I think I've got a good idea on how to proceed with trouble shooting the millivolt thermopile and gas valve in case my furnace stops again.
One part I am a little uncertain of, is tracing the mv voltage with my meter.
For example, lets say my burners stop lighting, and I still see 633mv from TH to -TH-PG, and 636mv from PG to TH-PG - is there more testing that needs to be done to ensure the voltage is correctly going from TH-PG on the gas valve, to TH on the gas valve. (I believe this was my problem with the burners not coming on last week, and that it was high resistance in the old thermostat. Maybe.)
I just don't want to put meter leads on a circuit and break something.
So, I'm thinking I next would test for mv across the LIIMIT, and if I see 0mv (which is correct if the limit is closed). If I see 636mv with limit closed can I remove the wire from the input side of the limit from TH-PG and test to see if any mv are present there? Or is that not safe to do?
Same questions if I can continue to trace the voltage - lets say next I go to the thermostat upstairs and take the stat's face plate off, and test for mv from the R and W stat terminals, and see 0mv there. Since the face plate is off, hence no stat controls to close the stat's circuit - 0mv could mean no voltage from LIMIT to tsat input side (which I think is the R wire?), or, it could mean all is normal - since no stat controls are present with the stat's face plate removed - to close the stat's R to W circuit. Could I next test the limit's output terminal and the stats input side by taking the wire off the limit's output terminal, and then untwist the splice on my tstat wires (I have a splice on the stat wires in the cabinet) and then put my meter leads between those two wires to see if I am getting close to the 636mv, or, would that 'fry' something, or does the meter in that position act like a jumper wire?
Then, if it looks I have voltage past the thermostat in the test above, could I test the circuit further downstream of the tstat, by removing the stat wire from the TH terminal on the gas valve, and put my meter leads from the removed untwisted tstat wire in the furnace cabinet, to the gas valve TH terminal and see if I am getting near 633mv, or would that 'damage' the circuit or the valve.
PICTURES AGAIN OF MY FURNACE AND THERMOSTAT:
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Sorry @mnalep. Your posts got held in our spam filter. This sometimes happens if a post is edited/submitted too many times within a short time period. Thanks for your patience!
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Ah, I was wondering what happened. Thanks for letting me know.
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" lets say my burners stop lighting, and I still see 633mv from TH to -TH-PG, and 636mv from PG to TH-PG - is there more testing that needs to be done to ensure the voltage is correctly going from TH-PG on the gas valve, to TH on the gas valve. "
633mv from TH to -TH-PG describes things two things;
1 the load of the main valve coil is not applied to the Power Pile,
2 the thermostat / thermostat circuit and/or high limit / high limit circuit is open.
636mv from PG to TH-PG, means one thing, the load of the main valve coil is not applied to the Power Pile, something in the circuit is open.
I believe the simplest way to troubleshoot this type of circuit is; With no call for heat measure the Power Pile Voltage, verify it is 540 mV or greater (within reason) then with a call for heat the Voltage should drop since the Power Pile is being loaded (or should be). Loaded Voltage 175 - 280 mV according to the document. The difference between the loaded Power Pile Voltage and the Voltage measured across the Main Valve (TH to PG), is the control / limit circuit's Voltage drop and should be within spec.
With the circuit closed 'On' the Voltage source (the Power Pile) and the Gas Valve coil (the Load) should have about the same Voltage. The Voltage drop of the control circuit should should be much lower or close to zero (assuming no anticipator resistor). If the control circuit (in this case including the high limit) has too high a Voltage drop the main valve won't open.
Basically for a closed circuit, power sources and loads have the Voltage across them, the switches or contacts in the control part should be near zero Volts. With the circuit open or Off the power sources and the switches or contacts will have the Voltage across them, the load will be zero Volts.
Excessive Voltage across control devices that should closed and should be near zero Volts is usually an indication of a problem.
I would get a quality alligator clip lead to jumper or bypass your thermostat Rh to W for testing purposes.
With a milliVolt system like this you probably can't hurt it much unless you short out the Power Pile, not sure how much abuse they can take.
That mark on the relay, as mattmia2 stated probably just a marker or paint stick mark, heat damage would probably fade to the normal color more gradually. Not sure the circuit (batteries and/or Power Pile) would have enough power to melt a relay.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Thanks for the diagrams, I am trying to understand them.
I know that I was able to get my burners to come on by putting a jumper wire from TH to TH-PG, which completed ('closed') the circuit and opened the main gas valve.
The 2nd diagram shows the coil is going from ('across?') TH to PG, so, that makes me think that if I had put that jumper from TH to PG (instead of from TH to TH-PG) - that would also have opened the main gas valve? (Since PG and TH-PG are connected internally?) But, when I look at the 3rd diagram, I'm not sure what I just said is correct, because on the 3rd diagram it looks like TH and PG are already connected (which I don't see on the 2nd diagram) - so it would seem that closing the limit and stat is the only way to open main valve?
Is the power unit actually the valve for the pilot flame (ie: the pilot valve)? I see a black wire that goes from TH-PG to a opening that goes into the gas valve. The 2nd diagram does not show a coil there, but the 3rd diagram does.
If I jumpered a wire across Rh to W, and the main burners did not come on (while I have 63x mv on both PG to TH-PG, and, TH to TH-PG) - is that telling me that my tstat is not working (not closing that circuit)? Should I also be able to see 63x mv with my meter across the Rh to W wires in my thermostat, unless the limit is not working properly?
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I am currently looking for some good alligator clips that don't keep slipping off my meter probes. I was at Home Depot and saw a set of Klein probes (69410) for sale that had screw on alligator clips. I was going to buy them, then notice a nice Klein MM450 meter for just $40 more that I thought included the screw on alligator clips (and some other nice features like temperature and capacitance). Unfortunately after I brought it home I realized those clips were not the screw on type. So, I think I am taking the meter back and just buying the lead set. Or, do you have a better suggestion for alligator clips?
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" The 2nd diagram shows the coil is going from ('across?') TH to PG, so, that makes me think that if I had put that jumper from TH to PG (instead of from TH to TH-PG) - that would also have opened the main gas valve? "
No. A jumper TH to PG would place a short circuit across the Main Valve coil, that would be bad.
" (Since PG and TH-PG are connected internally?) "
No they are not directly connected internally. The coil of the Power Unit is in-between PG and TH-PG the way yours is wired.
The only three places you want to jumper is;
1 - TH-PG to TH which in your case bypasses the thermostat / thermostat wiring and the high limit and the high limit wiring.
2 - Rh to W which just bypasses or provides a substitutes for the thermostat at the thermostat.
3 - Across the High Limit switch.
" Is the power unit actually the valve for the pilot flame (ie: the pilot valve)? I see a black wire that goes from TH-PG to a opening that goes into the gas valve. The 2nd diagram does not show a coil there, but the 3rd diagram does. "
Yes I believe the Power Unit is the coil that keeps the pilot valve open which is why on the Manufactures wiring diagram the high limit switch is placed in that circuit. That way if there is a high limit event everything gets shut down (locked out). The way yours is wired just the burner would get shut down, then if the high limit switch self resets the burner will turn on again possibly to trip the high limit again and again and again.
" If I jumpered a wire across Rh to W, and the main burners did not come on (while I have 63x mv on both PG to TH-PG, and, TH to TH-PG) - is that telling me that my tstat is not working (not closing that circuit)? "
No, in this case something in the thermostat wiring and/or the high limit and/or the high limit wiring is defective. Since the main valve is not opening, and you have substituted the thermostat with a known good jumper, yet the control circuit is still open (63x millivolts across it).
" Should I also be able to see 63x mv with my meter across the Rh to W wires in my thermostat, unless the limit is not working properly? "
Correct.
In your case I would have a dedicated clip lead with alligator clips on both ends of a wire to bypass the thermostat when it is removed from its mount.
The Klein probes (69410) is a probe replacement set that also has alligator clips that attach to the probes. This can be handy, for me I just use clip leads and slide the probe into one alligator clip and then connect the other alligator clip to the circuit under test.
By a clip lead I mean something like this pictured below, although the ones pictures are usually poor quality.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
Thanks for that feedback that: "The only three places you want to jumper is:; - good to know. Reading your comment reminded me that one time I did accidentally put a jumper wire across either TH to PG, or, TH-PG to PG (I cant recall now which) - and that shut the pilot flame off with an audible click in the gas valve. (I didn't think that was what you called a "short Circuit" though, was it?)
I tried putting "clip leads" that I have on hand across the old thermostat Rh and W screws. It was not easy to get the alligator clips to hold on. I did see a magnetized set on Amazon, and bought it, and it works, although the magnets should be stronger, as the weight of the wire tends to pull the magnets downwards leaving the magnets attached - but precariously.
The clip leads that I had:
The new magnetized jumper wire I just bought on Amazon:
The magnetized leads on the Rh and W screws of the thermostat. They started the furnace, but seem weak, like the magnets should be stronger.
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If jumpering from Rh to W makes the furnace come on, but the thermostat doesn't, it's a thermostat problem. All the thermostat is is a switch.
Whether the magnets holding the jumper to the screw are weak or not is utterly and completely irrelevant.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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I would consider that jumpering either TH to PG, or TH-PG to PG to be a Short Circuit. It creates a path that is not normal to the proper operation of the circuit and in some cases may cause damage. Jumpering TH-PG to TH creates a 'substitute' path for testing if the control and/or limit devices and associated wiring is defective, the rest of the circuit is expect to behave normally. One could argue that Jumpering TH-PG to TH is short since it is bypassing the control and limit devices.
Most of the time you don't want to intentionally create a short circuit, since is can cause damage or minimally blow fuses which can be a nuisance.
Clip leads with screw heads is almost always a struggle. I look for alternative places or add a short piece of wire under the screw to clip to. Also in your case the clip leads may work at the wires (Green arrows) or at the thermostat interconnect contacts as long as you do not deform or damage the contacts (Orange arrows). It is just a matter of finding out what works for you reliably.
The magnetized type is interesting, but maybe frustrating if they often fall off and sometimes the screws are Brass and the wires are Copper.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
Thanks for those tips! I wish the magnets on that jumper wire were either rounded (concave) to fit the screws tighter, or the tstat screws had flat tops to fit the magnets better, as the magnet use is good idea.
On another note, I was thinking more about that 633mv I see when I put my meter leads across TH and TH-PG, with the burners OFF - is this actually indicating that there is 633mv on TH-PG, and 0mv on TH?
And if so, then when the burners are ON, and I read 187mv across TH and TH-PG - is that indicating that there is 633mv at TH-PG, and 446mv at TH (since the multimeter would be reading the difference of 633mv on TH-PG and 446mv on TH = thus reading a 187mv difference on my meter) ?
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" On another note, I was thinking more about that 633mv I see when I put my meter leads across TH and TH-PG, with the burners OFF - is this actually indicating that there is 633mv on TH-PG, and 0mv on TH? "
With a closed, completed, active circuit the power supply voltage is applied to the load, so the power supply and the load voltage is expected to be basically the same.
With a circuit that is Off or open you still have the power supply voltage but since the circuit is Off the voltage that was across the load is now across the switch, which is open (or Off). Your volt meter is reading the power supply voltage through the load.
The meter reads voltage between two points of a circuit. If you conciser the Black lead's position as the reference or 0 volts point, the meter displays the voltage where the Red lead is placed in the circuit.
" And if so, then when the burners are ON, and I read 187mv across TH and TH-PG - "
The voltage drop you are seeing across TH and TH-PG (187mv) is due to the inefficiencies (resistance) in the circuit, wire, connections, contacts, heat anticipator, all have resistance and they add up fast in a Millivolt system. With a 24 VAC system a voltage drop of 187mv is practically meaningless.
" is that indicating that there is 633mv at TH-PG, and 446mv at TH (since the multimeter would be reading the difference of 633mv on TH-PG and 446mv on TH = thus reading a 187mv difference on my meter) ? "
Yes in perfect world, that is true. However do you actually have 633mV PG to TH-PG when the burners are on ? When the load of the Main Valve is applied to the Power Pile, the actual voltage of the Power Pile may drop (due to the internal resistance of the Power Pile). So you may not see 633mV PG to TH-PG or 446mV across PG to TH.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
This style test clip is probably the most versatile type, it has tiny tips than can grip small terminals but it can also grip large things and even pierce insulation
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I'm still kind of confused.
You said: "With a circuit that is Off or open you still have the power supply voltage but since the circuit is Off the voltage that was across the load is now across the switch, which is open (or Off). Your volt meter is reading the power supply voltage through the load."
IF the circuit is open, why do you describe that as voltage 'across the switch'. Wouldn't voltage only be 'across the switch' - if the switch is closed (completing the circuit).
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I made this little graphic to show what my meter reading are now, with the furnace running. (In case I lose my paper notes):
Does this make sense?
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Thanks. I'll look for them.
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I encourage you to study simple electrical circuits more. The power supply voltage is always present. With a closed switch (minimal voltage drop across the switch) the power supply voltage is applied to the load.
When the switch is open (Off) the voltage is no longer applied to the load, so the voltage across the load is zero. But the the power supply voltage is always present, so where is the voltage drop ? Well it is across the open switch. The 63x = 633, TH to TH-PG, across the open switch. The sum of the voltage drop(s) has to equal the power supply voltage, regardless if the circuit is active or not.
The meter across the open switch will see the the power supply voltage that is always present through the load. Since the load is a very low resistance compared the the voltmeter.
Your 2nd graphic makes sense except if the thermostat and the limit are actually 0mV with the burner running the 187 mV drop across the wiring may be excessive IMO.
388 mV = Loaded Power Pile voltage (TH-PG to PG).
187 mV = Control circuit voltage drop TH-PG to TH.
201 mV = Load voltage (TH to PG) which may be a bit on the low side, but may work fine.
To know for sure the circuit current would have to be measured and the total length and gauge of the wire would need to be known to figure it out, Ohm's law and wire resistance chart.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
A basic example. A battery (a power source) on the left side of each circuit, a load (a resistor) on the right side of each circuit.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
I've been doing Christmas stuff the last few days. I'm a bit too tired so I hope this reply makes sense.
It sounds like my current mv readings are 'adequate'. I am trying to figure out at what measurements, would indicate a concern.
I was thinking that my 388mv TH-PG to PG was adequate. The "Guide' indicates the minimum is 280mv "closed circuit with controls in the circuit", so at 388mv I thought I was good. I also was thinking my TH-PG to TH of 187mv was 'tolerable', as the guide indicates under 175mv is good, but can vary from 175mv to 205mv, so as you mentioned earlier I'm at least at the lower (better) part of that range.
The "load voltage" you mentioned of 201mv - is that the voltage 'consumed' holding the main gas valve open?
You mentioned my load voltage is 201mv (that I assume is the 388 - 187 - 201?) - is on the low side. What would be a good mv voltage? At what mv wold likely be too low to open the main gas valve? Is there a minimum in the guide for this load voltage that I have missed seeing?
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I would like to understand circuits better. My problem is I get tripped up on terminology when I read about circuits and electrical functions. Is there material here in this forum, or elsewhere you could point me to?
That diagram you just gave really helps. For example, I never understood before that a read of 0mv across a closed switch was the correct reading for a functioning switch. (I had previously thought 0mv meant no power was even getting to the switch!) I'm just beginning to understand 'voltage drop'. My understanding is hindered because it seems so many ideas are described by multiple words/terminology (closed circuit, completed circuit, active circuit, etc.).
I do appreciate your explanations, and dumbing things down for me enough - to help me comprehend your explanations! Thank you, and everyone else who has helped me. Merry Christmas!
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" The "load voltage" you mentioned of 201mv - is that the voltage 'consumed' holding the main gas valve open? "
Not sure I'd use the word "consumed", in this case it is simply the voltage across the load, a voltage drop. A power source is voltage rise and any loads, switches, connections and wire resistance is a voltage drop. The total of each is equal, total rise equals the total drop.
I'm certainly not a Millivolt system expert, although it is a rather simple circuit, I just think of it as circuit rather than a set of stated values like in the document, the document gives good data and should help someone troubleshoot a Millivolt system, understanding all the circuit details and circuit dynamics may be a bit outside the scope of that document. With the circuit when one item changes other things change, so as much as your system works, there still may be some things that could be improved.
Now-a-days, there are many resources to learn electrical and electronics. I would start with a book on very basic electrical, there are many, maybe at a local library. People learn differently, you need to find the resource that works best for you. There may be adult education classes in your area, Youtube too. Some folks do well with a series of books as they advance, others do better with an instructor, since an instructor should be able to explain any confusion.
One of the problems is that electrical / electronics is so vast there is many directions to go in, but you do need to learn the basics as a foundation.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
Thanks. I will see what I can find online.
Do you have any recommendations about what things could be improved?
I assume this might sound far fetched, but, what would happen if I added either a second thermopile, or low mv thermocouple, into the system? Even if just temporarily - as a test (assuming the burners stopped coming on again). Could an extra 600mv damage anything, like the gas valve? (The tstat is 24, or mv capable, so I assume that, and the limit cannot be damaged).
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Personally I think the " 187 mV = Control circuit voltage drop TH-PG to TH. " seems a bit high if the High Limit and the Thermostat is actually 0 mV as you stated in your diagram above. This leaves 187 mV voltage drop from resistance losses of the wire and possible poor connections.
Without knowing the actual wire gauge and length and the circuit current it is only a guess and may actually be normal for your system.
Knowing the actual wire gauge and length the total wire resistance can be calculated from data from a wire resistance chart. Then with the circuit current the voltage drop of the wire can be calculated, Ohm's Law. If the calculated voltage drop of the wire is a lot less than the actual measured voltage drop than I would be looking for poor connections or a defect in the wire.
The other thing to consider in this situation is your meter, can it accurately and reliably measure 1 mV ? So you know that the reading of 0 mV across the High Limit and the Thermostat is actually 0 mV is actually accurate. Many details to truly get an accurate representation of what is actually going on in the circuit.
Most folks are happy when it just works reliably, however it does not mean that there are not subtle inefficiencies there too that could be improved, but that takes time.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Your comment got me wondering if the two diagrams I posted above were actually correct. So, I went back through the notes I made the day that I was having the problem where the burners were not coming on, and, I think, that the reading I took of 0mv across the thermostat's Rh and W wires, was probably while the burners were off, and before I got the furnace working. Today, I'm not sure I even took a mv reading, at Rh and W, after I got the furnace working again. (If I did, and now I don't recall, I didn't write that down in my notes).
(Also, maybe I should have labeled the 'thermostat' as the 'Rh' and 'W' contacts? - since I had the body/face of the thermostat removed from the wall plate to access the Rh and W screws.)
The thermostat is about 40 feet from the gas valve, and I think it is all 14awg - except for about two inches of 18awg 'pigtails' at the thermostat's Rh and W terminals. I looked at this online resistance calculator , but, I was not sure if I plug in 40 feet, or 80 feet (since 1 wire of 40 feet goes from limit to stat, and, then the other wire of 40 feet goes from stat to gas valve).
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Usually with notes and meter readings you want to record both states active and inactive especially when the circuit is working normally, that way in the future if there is a problem you have documented information of how the circuit works.
For circuit analysis data the 40 feet can be relevant, however with taking measurements, I suspect you meter does not have 40 foot test leads, so 80 feet is the round trip (so to speak) wire length and it is at a place you can measure with your meter. As far as circuit's thermostat wire resistance, the circuit actually has 80 feet of wire so with 14 Ga. wire, 80 feet long the resistance would be about 0.202 Ohms. BTW most inexpensive multimeters don't do a good job showing 0.202 Ohms, it probably just looks like the meter's probes are touching together.
The actual round trip voltage drop of the actual thermostat and the thermostat's wiring should be noted (measurement #2).
1. Power Pile voltage.
2. Voltage drop of the Thermostat and the Thermostat's wiring.
3. Main Valve voltage (Main Valve voltage drop).
4. High Limit voltage (High Limit voltage drop).
5. Thermostat voltage (Thermostat voltage drop).
You can also measure (like you did before) TH-PG to TH, which should be equal to the sum of measurements #2 and #4 which is the voltage across the whole Thermostat, Thermostat wires and the High Limit part of the circuit.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
Earlier you had said: "Knowing the actual wire gauge and length the total wire resistance can be calculated from data from a wire resistance chart. Then with the circuit current the voltage drop of the wire can be calculated, Ohm's Law. If the calculated voltage drop of the wire is a lot less than the actual measured voltage drop than I would be looking for poor connections or a defect in the wire." The calculated wire resistance is 0.202. I used this ohms calculator , and it calculated that for 0.202 resistance and 750mv the current is 3.712871 - but I don't see how that tells me the "calculated voltage drop" of the tstat wire - that I would use to compare to the "actual voltage drop" measured by #2 in your diagram above?
Also, for measurement #3 - am I measuring from TH to PG? (The arrow does not look like it is pointing at PG, but part of the solenoid?)
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Looking at your calculation with some experience you know when something is wrong, seriously wrong. With that circuit current of 3.7 Amps, that is probably impossible, I don't think you can get that amount of current from a Power Pile. And even in normal operation with a 24 VAC system you don't see current that high.
So… if you use;
187 mV = Control circuit voltage drop TH-PG to TH (previously stated).
80 feet of 14 Ga. wire = 0.202 Ohms (Copper wire resistance chart).
No voltage drops across the High Limit and the Thermostat (which may not be actually true).
So a 187 mV voltage drop across a wire that is 0.202 Ohms, (V/R = I), 187 mV / 0.202 Ohms = 0.9257 Amps. That current still seems absurdly high and is probably not actually true or normal (or even achievable) for that type of circuit. So you know something is wrong or missing from the analysis.
Personally would measure the actual circuit current, but if you don't know how and/or don't have the equipment to do that, you can only derive it from the data you have, which may not paint an accurate picture. Actual compete measurements of a working circuit will ultimately make the most sense.
I find when drawing a Schematic or Wiring diagram that minimizing crossing lines makes the image less 'Busy' and easier to read and understand.
In this case the actual wires are short between the Power Pile and the Main Gas Valve coil. PG is probably the only practical point to make a measurement. The Blue wire is going into the burner area to the Power Pile and the Purple wire is internal to the Gas Valve, so neither is practical to put a meter probe to and the wire voltage drop for the length of that wire should be 0 mV for analysis purposes. Measuring at either of the Red arrows, if you could would give you the same measurement.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0
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- 3.5K Oil Heating
- 64 Pipe Deterioration
- 927 Plumbing
- 6.1K Radiant Heating
- 384 Solar
- 15.1K Strictly Steam
- 3.3K Thermostats and Controls
- 54 Water Quality
- 41 Industry Classes
- 48 Job Opportunities
- 17 Recall Announcements