Troubleshooting millivolt gas furnace help request.
Hello. I am new here. I am hoping for some assistance.
I have millivolt gas furnace (Luxaire SH-125), and the main burner is not lighting consistently. I am able to FORCE the main burners on by jumping a wire between the th to th-pg (th to th-pp) terminals on the gas valve. I cleaned of the old thermopile, and put in a new thermostat, and it has been working for a few days (at least for now). (The new thermopile from ACE Hardware put out less voltage than my old one after I cleaned the old thermopile!)
Anyways, I am not sure if the gas valve (White Rodgers 36C03U-433) is going bad, or if the thermopile is too weak, or maybe the circuit has too much resistance?
I only have a few of the suggested voltage readings. ( I did not see this guide before these measurements).
I measured across the th to th-pg (th to th-pp) terminals, I have 633mv burners off, and 187mv with burners on. (my gas valve uses pg for pilot generator instead of pp for power pile).
I measured across the pg to th-pg (pp to th-pp) I have 636mv with burners off, and 388mv with burners on.
The troubleshooting guide says the minimum closed circuit voltage (with controls in the circuit), from pp to th-pp should be 280mv. Is that 280mv the minimum - with the BURNERS OFF, OR BURNERS ON?
(Since I am seeing 636mv with the burners off and 388mv with burners on - then, if I am applying the guide correctly, means the thermopile is ok since either way it is above 280mv. Is this correct?
When I was able to FORCE the main burners on by jumping a wire between the th to th-pg (th to th-pp) terminals on the gas valve, my first thought was the thermostat circuit was the reason why the main burners was not firing up, since the jumper wire bypasses the thermostat circuit and started the main burner, implying that the powerpile was ok, and probably the gas valve was also. So, maybe that is why the burners are firing up, since I put the new tstat in?
Also, why doesn't the guide indicate meausring, and a minimum closed circuit voltage, from th to th-pg? Wouldn't that measure the voltage coming from the entire thermostat circuit to the gas valve to see if it is adequate?
Thank you for reading this. And hopefully responding.
I found the well detailed "Procedure for Troubleshooting Powerpile Systems" in this forum:
and the chart for interpreting the testing results here:
Comments
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how well is the flame enveloping the thermopile? are you measuring the closed circuit voltage across the solenoid in the gas valve? There will be voltage drop over the thermostat wire which can be significant at millivolt voltages. A bad connection anywhere in the thermostat wiring will make the voltage drop worse.
2 -
Agree with @mattmia.
Millivolt system work on low voltage and any resistance in the thermostat circuit will be an issue. They used to have special thermostats to work on a millivolt system. In the meantime, check all connections and scrape off any corrosion and make all connections tight.
Thermopile readings sound ok.
One work around is to use a relay like a Honeywell RA89A at the boiler. This relay uses a 24v volt transformer for the thermostat circuit and the relay contacts switch the millivolt circuit. This shortens the millivolt circuit a lot (like putting your jumper wire on the gas valve)
But if it worked before it should work now without the relay.
Take the thermostat off the wall and jump the wires there and see if it runs then.
1 -
My pilot looks like it is enveloping the thermopile well (see picture 1).
I am measuring mv across screws from both TH to TH-PG and from PG to TH-PG. I think the TH to TH-PG is the main solenoid in the gas valve, because when I put a green jumper wire between those terminal screws that is when I can force the main burner on. (see picture 2 for my gas valve).
The mv reading I get at TH to TH-PG is 187mv when main burners are lit (closed circuit), and 633mv with burners off (closed circuit).
The "millivolt troubleshooting guide" says the minimum closed circuit voltage, from PG to TH-PG (pp to th-pp) should be 280mv. But I don't see in the troubleshooting guide if that 280mv minimum - is with the BURNERS OFF, OR BURNERS ON? If 280mv is with burners on, then my 187mv is low, but if that 280mv minimum is with burners off - then my 633mv, when burners are on, would be good.
My tstat wire runs from the 2nd floor, and there is more than one place where it's been cut then spliced back together.
PICTURE 1: PILOT FLAME ON THERMOPILE:
PICTURE 2: THE GREEN WIRE JUMPED FROM TH TO TH-PG WILL FORCE THE MAIN BURNER ON:
PiCTURE 3: MY GAS VALVE WITHOUT THE GREEN JUMPER WIRE IN THE PICTURE:
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"Closed circuit" means with the stat calling, so gas valve open for normal operation; meaning yes, burners on.
Has anything changed, aside from the new thermostat?
Contact failure is a real thing.
I'd consider running new wire if there are lots of splices in it. A relay like @EBEBRATT-Ed mentioned might save wiring that's too marginal to reliably operate a millivolt system; or perhaps a wireless stat (my favorites are Honeywell's RedLINK wireless thermostats).
1 -
Hi, I'll add that if the thermostat wire run is long, voltage drop may be playing a part. So, I'd up-size the wire by one gauge also.
Yours, Larry
3 -
ratio, EBEBRATT-Ed, Larry,
My tstat was calling for heat, but the burners were NOT lighting. That is why I am confused about applying the mv troubleshooting guide I am looking at. I get 633 mv, with my tsat calling for heat - but no burners on. When burners on, drops to 187 mv. (I was guessing that the 190mv minimum was probably with the main burners on - just wanted to be sure there was an issue for me to keep looking for - in the circuit, and not the valve, since the furnace is currently coming on).
I did swap out the limit switch, before putting in the new tstat, but that did not get the burners on, but I just left the limit on the furnace. And, I did splice out two inches of some tsat wire that looked a bit 'singed'. The furnace was working for almost a month this season, before the burners failed to light.
The existing wire is on a 2nd floor tstat of two story home. I was thinking about replacing it, and went to HD and bought thermostat wire, then got home and realized it was 18awg, and I think the old wire is bigger, maybe 16, or even 14awg. I can't find my wire stripper to measure the existing tsat wiring. I googled bigger tstat wire, but couldn't find any, although I did find some wire called low volt for LED lights, that says it can withstand heat in plenums. I think snaking new wire is maybe too much for me, unless I could drop it straight down the duct work. (and even then it would be visible on the walls for a short distance) which I would do if I could find and buy suitable 16/2 thermostat or even 14/2 thermostat wiring? Or, can I just use any solid wire I find at Home Depot?
I don't understand how that Honeywell RA89A works exactly. For one thing I have furnace, not a boiler, if that even matters. Does the RA89A hook up at the gas valve, then send 24v thru the circuit (limit, tstat, and wires) then change the 24v back to the correct mv before entering the gas valve?
I googled Redlink wireless tsat and got all kind of hits. I'm not sure that are all even wireless. Can you point me to what you are referring to please?
Are these (RA89A, and redLINK wireless tsat) simple to wire up?
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You need to measure the dc millivolts across the two-wires coming from the thermopile with the pilot burning you need a minimum of 250ish volts for it to work. If the voltage is below that either the thermopile is bad or its not in the flame enough.
But my question is did this work B4 and if it did and doesn't work now what has changed? No sense putting on relays and running wire until you know where the problem is.
Did you try jumping the wires at the stat to see if the thermostat is the problem?
RA 89A relay needs constant 120 volt power hot and neutral Hot on 1 and neutral on 2. Your thermostat wire connect to T & T. The milli volt wires that used to go to the stat go on 3 & 4.
But you need to find the problem before you go through all that especially if it used to work
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EBEBRATT-Ed, When you say "250ish to work" - is that with the burners ON, or OFF?
Furnace is currently working. It had stopped a few days ago. But, I am worried I am 'right on the edge' of it stopping again.
Before it started working I was inspecting the circuit, and then noticed if I jumpered a wire from TH to TH-PG the burners came on. I thought this meant the gas valve was ok, and that I just had a voltage issue in the circuit. That is when I tried things like replacing the limit, then replacing the tsat, and then splicing the tstat wires to remove a singed area.
I just don't know if I am 'on the edge of failure' or in the clear. I read somewhere else that 190mv is the minimum, and the guide here says 280mv minimum.
Do I compare my reading of 187mv - burners ON, against the 190mv to 280mv? (meaning I'm 'on the edge').
Or, do I compare my reading of 633mv - burners OFF against the 190mv to 280mv? (mening I am worried for o good reason) (PS: the burners were OFF, due to limit temporarily cycling the burners OFF for a while, when I measured 633mv.)
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Yes, when it says "closed circuit" it means with the burner on.
63x mV is your open circuit voltage, that's above the 540 mV recommendation, so should be good. (you took it twice, but they agree very closely.) 388 mV is your closed circuit voltage, it's well above the recommended 280 mV, so also good. 187 mV indicates the resistance of the control circuit, that's towards the lower end of the 175-205 mV recommendation, so more good.
Were these taken with the old stat, or the new one? Were any of these taken with the thernostat calling but the gas valve failing to open? Everything looks fine right now It could have been something as simple as a flaky connection on the thermostat, changing the stat would have fixed it if that were it. The contacts in the old thermostat may have been getting unreliable.
If the problem goes away, it was one of those two. If it still happens, it's more flaky wiring connections (possibly in the splices you mentioned), or the gas valve is failing.
IDK if a power pile could be intermittently failing, I suppose it could still be that. Need to catch it when it happens to know for sure.
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BTW, cycling on the limit is indicative of a problem, most likely low airflow. How's the filter look?
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If i were going to re-run the t-stat wire i'd use like 16 awg especially if the run is like 50 ft.
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Hello mnalep,
" I measured across the th to th-pg (th to th-pp) terminals, I have 633mv burners off, and 187mv with burners on. (my gas valve uses pg for pilot generator instead of pp for power pile). "
This measurement was across there the thermostat wires connect ? Since the green jumper works, it sounds like the thermostat circuit has excessive resistance.
As far as the wire size you can use 18 Ga. 4 or 5 conductor and double it up (parallel 2 wires) or run two runs of 2 conductor and parallel them.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System
1 -
I am not going to repeat anything the other guys have recommended since they are "spot-on". I will ask however "is the new thermostat millivolt compatible"? If not, it may be the source of your problem. Anything that adds resistance to the wiring circuit including old thermostat wire, any wrong control including the thermostat will affect the operation of that gas valve. Any control on a millivolt circuit should be "millivolt compatible".
1 -
The two procedure that you noted in your opening statement were contributed by me to Heating Help. They should at least 95% of the time find the problem. The other 5% could be wiring or gas related (poor pressure etc). Call me if you want at 401-437-0557 and I will see if an old 85 year old guy can help you.
5 -
ratio, OK, I think maybe I understand now? I was thinking that the 187mv (TH to TH-PG, burners on) was what I needed to compare against the "closed circuit, 280mv minimum, controls in the circuit" as stated in the Guide.
But, if I understand you, the 388mv (PG to TH-PG, burners on) is the correct comparison to the "closed circuit, 280mv minimum, controls in the circuit" as stated in the Guide?
I did just noticed the Guide was measuring that 280mv min - from PP to TH-PP. And then, per you, my 187mv (TH to TH-PG, burners on) is the resistance. (I do now see that 175mv to 205mv mentioned in the Guide, under Test #4, and the Guide labels that as TH to TH - that is what I was calling TH to TH-PG, right?)
What I was thinking of as TH to TH-PG (in my comments above, is labeled TH to TH in Guide - is that correct? The Guide is calling the TH terminal TH, and also calling the TH-PG terminal - TH (in test #4)?
So, just to complete my understanding, when I see 633mv from TH to TH-PG with the burners off - is that also resistance? Is it 633mv, instead of 187mv, because the gas valve is not being held open (The valve is "closed", the burners are off)?
The readings above were taken with the new stat, new limit, and singed tstat wire spliced. (When the old stat, and old limit were in the circuit, the burners were not coming on, even with tstat calling - unless I jumped TH to TH-PG. The readings were a bit higher at 638mv across PG to TH-PG, and TH to TH-PG, with the burners not firing up (tstat calling), and before I did the replacements mentioned).
Furnace is running for 4 1/2 days now. I hope one of those things I did fixed it. I guess I'm just not sure! That is why I was so worried about the 187mv across TH to TH-PG when the burners are on - worried that low voltage caused the initial failure of burners to light, and thinking that maybe the new tstat just has less resistance, or faulty connections that got better when I wired the new tstat in - which could mean I was 'on the edge' of failing again.
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ratio, I said cycling, but i do not think it is 'short cycling'. I meant the Limit seemed to be doing it's job - letting the furnace run for a while, getting hot, and then turning the burner off. The filters are clean.
0 -
There appears to be a VERY fundamental misunderstanding here. The OP is referencing millivolts as "resistance" — which it is not.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Yes, those measures were on the tstat wires on the gas valve. (White Rodgers valve labels them TH and TH-PG).
If I need new wire - If I were to double up the 18awg tsat wiring - that would reduce resistance? How would I reduce the two wires at the tsat connection - twist them and then crimp on larger 14awg spade connector? I am thinking solid core wire, not stranded wire?
Also I just tested the old tstat that I removed 5 days ago, and at the R and W terminals, saw from 50 - 170 ohms resistance! (50 with alligator clips on short run of wire, 170 with meter probe ends held onto the tsat "pin contacts" on the back of the tstat. I have another model, of that same tstat, not new, and it is only reading 1.5 ohms! The tstat I replaced had a lot of resistance! Maybe that was my issue?
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The tsat is both 24v and mv compatible. I will admit it is from Ace hardware.
Also I just tested the old tstat that I removed 5 days ago, and at the R and W terminals, saw from 50 - 170 ohms resistance! (50 with alligator clips on short run of wire, 170 with meter probe ends held onto the tsat "pin contacts" on the back of the tstat. I have another model, of that same tstat, not new, and it is only reading 1.5 ohms! The tstat I replaced had a lot of resistance! Maybe that was my issue?
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I appreciate the offer. I may call you later, or tomorrow…
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The 187 mV measurement that you took measured the voltage drop of the control circuit (illustration #4, correct?), which is directly related to the total resistance of the control circuit (wiring, stat, limits, etc.). It's literally the difference in voltage at the start of that part of the circuit and the end of that part of the circuit. The range mentioned in the PDF (175-205) is given for a particular thermostat, length & gauge of wire, and limit; but the mechanics of where the voltage drop comes from doesn't matter to us. You're on the better side of in the ballpark.
In any case, if it didn't work on the old stat but worked with a jumper, that sure sounds like a control circuit problem; and as you've replaced most of it and reworked the rest, and it's been working fine since then, I think you've got it licked. Maybe get a replacement valve ordered Just In Case. (They're not making any more of them like that, so they're not going to get any more available—or cheaper—than they are now.)
A limit is just that, a limit. It should never exceed it. You're low on air, or overfired, or both. That said, it's worked for how long now? Unless it just started happening, don't worry about it, the old equipment could take that kind of stuff. Your next furnace won't be able to that however, most/all limits are 3 strikes you're out now.
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I was just commenting on ratio's use earlier, telling me that "187 mV indicates the resistance of the control circuit, that's towards the lower end of the 175-205 mV recommendation, so more good.". I did say, after that comment "the 187 mv "is the resistance", but I should have used the more precise wording that ratio did "indicates the resistance". English is my primary language, but I still manage to abuse it!
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Yes, my 187mv reading was as depicted in illustration #4. (TH to TH-PG on my valve).
I did not realize that the 187mv was the difference from the start to the end of the circuit. (I was thinking 187mv was either the voltage available after holding the solenoid (main valve) open, or, the voltage available to hold the solenoid open … sounds like neither? That is what still confuses me. Can the voltage required to hold the solenoid open be determined from the measurements I took? What would it typically be?
Is the solenoid included in what you described as the 'beginning to the end of the circuit', or is the circuit just measuring from the terminal screws on the gas valve body?
So, using the two readings I was looking at, both at TH to TH-PG, (633mv with burners off, and 187mv with the burners on), would it be correct to say that the circuit 'starts' with either the 636mv (PG to TH-PG) when burners are off, or, the 388mv (PG to TH-PG) when burners are on, and then either ends with 633mv when the burners are off, or, ends with 187mv when the burners are on? Does the 187mv voltage reduction (voltage drop) in the 'circuit' include what is consumed holding the solenoid open? Sound like it does not?
I was hoping that the control circuit was the culprit also! I did order a new valve, just in case - but i guess that new valve would only be needed if using the jumper wire on the gas valve faild to start the burners, and, the thermopile remains relatively strong - like over 554mv - as what I think the Guide indicates?The furnace has been working for the many years that I've had it, close to 20 years I think. It's cycled off after several minutes for that whole time. It does have a temp setting on it. It is set at 200 F.Did you see my comment that the old tstat was showing between 50-170 ohms resistance? I wonder how much mv were lost due to that resistance?0 -
Thank you! I may call.
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" saw from 50 - 170 ohms resistance! (50 with alligator clips on short run of wire, 170 with meter probe ends held onto the tsat "pin contacts" on the back of the tstat. I have another model, of that same tstat, not new, and it is only reading 1.5 ohms! "
With low voltages and low resistances the quality of your multimeter and the probes has to be good. If it is not the readings will just drive you nuts.
Page 14
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
The 187 mV is the voltage drop of just the control part of the circuit (the whole circuit encompasses the power pile, solenoid, etc.). Knowing it by itself is not enough to figure out what the holding voltage is, but that is simple to find empirically—check illustration #6. With the meter on the PP & TH-PP terminals, (and the burners operating), blow out the pilot. The voltage will decay as the power pile cools, and the gas valve will close once the output drops below the holding voltage.
I missed your post where you noted the 170 Ohm resistance. That's almost certainly your issue, like @109A_5 says above it's two orders of magnitude larger than the expected value of 1.68 Ohms (bottom of page 14).
Interestingly, that 1.5 Ohm measurement corresponds quite well with the calculated resistance of 30' of 18 gauge wire (according to it's 0.192 Ohms) which totals up to 1.692 Ohms, only a hair over our expected 1.68 Ohms.
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So, the voltage drop is the 187mv reading (with furnace's burners on), the the 388mv reading across same PG to TH-PG is the closed circuit voltage. And these are both acceptable voltages (as you indicated earlier when you stated: "388 mV is your closed circuit voltage, it's well above the recommended 280 mV, so also good. 187 mV indicates the resistance of the control circuit, that's towards the lower end of the 175-205 mV recommendation, so more good.")
Speaking of resistance - if I disconnected the tstat wires from the gas valve - would that be a good place to measure the entire control circuit's resistance? Of yes, what would be an acceptable reading?
How would I blow out the pilot? When I first read that, I was like, so it's about 24" into the furnace, behind the pilot assembly, and in between two of the furnaces lit burners! I'm dying to know how this is done? Some kind of steel tube? And wouldn't it just relight anyways as the pilot valve is still open and the main burners still flaming would ignite it?
In the Guide's test #6 it says the drop out is typically around 95mv. So, I suppose if I try to guess - that since my thermopile is reading at 636mv across PG to TH-PG, would that mean that 636 - 95 = 541mv (roughly) to hold main solenoid open?
Yes, that resistance on the old tsat was 50 ohms one night, then a few days later was 170 ohms. (That is why I had stated it was 50-170 ohms). But when I saw 170 ohms that 2nd day, I measured it several times, along with another old tstat I had in my bag which was the same model, and where the resistance was only 1.5 ohms on the 2nd tstat. (BTW, that 1.5 ohms reading is not the new tstat that I have currently controlling the furnace. I did not measure it before installing it, and now I wish I had. Right now I'm reluctant to even touch it at this point , as the furnace is working, and out of fear of ruining the tstat wire's contacts when I reconnect, or something similar, lol.) So, that 1.5 ohms reading was across only about 10" of wires, as I held the tstat in my lap, and also measured on the back plate of the tstat where the 'prongs' are that the pins that are in the board of the face of the tstat 'squeeze' into.
0 -
1095_A,
That resistance on the old tsat was 50 ohms one night, then a few days later was 170 ohms. (That is why I had stated it was 50-170 ohms). But when I saw 170 ohms that 2nd day, I measured it several times, along with another old tstat I had in my bag which was the same model, and where the resistance was only 1.5 ohms on the 2nd tstat. Also the meter showed 0.1 ohms across the meter's leads. My 2nd meter is broken, and I have not yet replaced it. Would you know of a decent back up meter that is reasonable?
0 -
" Speaking of resistance - if I disconnected the tstat wires from the gas valve - would that be a good place to measure the entire control circuit's resistance? Of yes, what would be an acceptable reading? "
To me in this scenario the anticipator (if the thermostat actually has one) is the most significant part of the total resistance of the control circuit. The wires and contacts should be in the milliOhms area.
Page #14
" How would I blow out the pilot? " Shut the gas off to the furnace only.
With multimeters you basically get what you pay for. Personally I like better quality meters since I use them every day and want reliability and consistency. Nothing worse than your test equipment deceiving you with erroneous readings. Reading and understanding the specifications also helps, you need to understand the capabilities of your test equipment.
Many good brands with various features, maybe this will help
Better meters usually do better with low Ohms (with less than an Ohm resolution) measurements. Some meters you can zero or null out the shorted probes. I guess it all depends on your needs, knowledge and budget.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
If you disconnect the thermostat wires from the gas valve, you should be able to measure the resistance between them, it should be in the 1.68 Ohms range with the thermostat closed (calling for heat) and OC/inf/whatever indicated an open circuit with the stat not calling for heat. I might measure from each one to ground too, to check for a ground fault. In that case, OC is a passing grade, anything else is failing.
It sounds like the 95 mV is the actual drop-out voltage you'll see at the PP & TH-PP terminals when the gas valve closes. If you can't get to the pilot, just close the gas cock (manual shutoff valve) that feeds the furnace while the gas valve is open (heating). Put your hand on the body of the gas valve and listen carefully, you should hear and/or feel when the valve closes—whatever the meter is reading at that moment is the drop-out voltage. It takes at least that much voltage to keep the gas valve open.
The pull-in voltage is the complement to that, it takes at least the pull-in voltage to open the valve. If your control circuit (thermostat, safeties, wiring) resistance is too high to allow the output of the power pile to reach the pull-in point, the gas valve won't open. That's probably what was happening in your case.
1 -
Thanks for that list of multimeters.
So, just shut off the gas supply. OK.
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Blowing out the flame is probably the best way, most real world test, maybe use a short hose, a pilot can blow out (down draft) and since the gas pressure is still there interacting with the solenoid valve. The manual gas shut off valve usually does not shut itself off. Since your pilot is well protected shutting the gas off may be the next best thing.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0 -
It does sound like it my tstat had resistance that was way too high. ( From 50 - 170 ohms). I am going to buy a backup tstat to have on hand. I'll ohm it as soon as I get it home it also. The last 2-3 have been an Ace hardware brand. Maybe I should look for a better brand?
My meter shows OL when measuring ohms and with the leads separated and not touching any circuits. Is OC for Open Circuit. I thought that OL was open circuit.
I'm not familiar with ground faults, that is when a hot wire touches ground (like the furnace cabinet)? If I measured ohms in that case, wouldn't the voltage ruin my meter? Why, if the cabinet was energized, would my ohms meter show OC (Or OL)?
Ok, So if the pull in voltage is what turns on the burners, by opening the gas valve's main solenoid, I see that test #7 in the Guide shows measuring from PG to TH-PG - and says "with the controls in the circuit should pull in from 179 to 320". That is where I am now seeing either 388mv with burners on, or, 636mv with burners off. So, with burners off - I have 636mv available to pull the main valve open, right?
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MiliVolt systems are very sensitive to parasitic Voltage drops caused by various reasons, there just isn't much Voltage to waste, extra (unwanted) series resistance (a Voltage drop) lowers the circuit Current.
The Voltage is limited by the Power Pile, the Current that the Power Pile can source is limited by its internal resistance. So as you draw more Current (adding the solenoid's coil resistance as a load) the Voltage of the Power Pile drops, this is normal. All about Ohm's law.
The solenoid really works on DC current flow. Since the solenoid's coil has a fixed resistance it is usually just easier to measure the Voltage for troubleshooting purposes. Voltages are usually just easier for folks to deal with when looking for parasitic Voltage drops.
In Ohmmeter mode OC, OL, etc. (Open Circuit, Over Load, Open Line) or whatever the meter manufacture dreamed up just means the probes are not connected to each other or the circuit being measured is over the maximum resistance that the meter can measure, typically 20 Meg Ohms or 20,000,000 Ohms, the maximum resistance varies with different meters.
To the best of my knowledge a miliVolt system is floating, no connection to ground or anything else. If it is connected to ground at one point that probably won't be a problem. If it is grounded at one point and then becomes grounded elsewhere the circuit probably will not work correctly.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System1 -
A word about that variable resistance on the thermostat. That is a sign that the contacts in the thermostat are very worn. Ideally a mechanical contact thermostat will be less than 1 ohm closed, perhaps much less. A mercury thermostat will be in the milliohm range — and not measurable on most meters. A solid state thermostat may, or may not, give a usable ohm reading; most likely not.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England1 -
I think my thermostat is digital. It's an Ace Hardware brand.
I am going to get a backup tstat, since the old seems to have caused my issue with the burners not coming on. Does brand matter, or do they all degrade after a few years?
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If my burners stop lighting again, I guess the first thing I'll do is check the mv at the gas valve connections. If the readings are ok (I'm thinking above 500mv?) then I will suspect the control circuit.
How would you go about testing if the problem recurs?
If mv reading is low, I'll then try a new thermopile. I have an new backup thermopile, that I bought a few days ago.0 -
You need to get a thermostat that will work on a millivolt system. Not all will.
If it does not say millivolt on the package you can't use it. Millivolt stats have to be low resistance not sure digital stats will work with millivolt.
If you want to use a digital stats you need to use an RA89A relay as I posted earlier.
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If the old tstat has open contacts i bet running a business card over the contacts a few times would have fixed it
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If that thermostat is in fact miliVolt system compatible and you are ambitious you could change the bad relay. When taking the board out be careful of the LCD, some are connected by a delicate method. Can't quite make out the part number of the relay (Red box) but I bet Digikey.com, Mouser.com or eBay may have them.
Battery terminal looks kind of crusty.
National - U.S. Gas Boiler 45+ Years Old
Steam 300 SQ. FT. - EDR 347
One Pipe System0
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