# Thermopile/millivolt generator



## Mike-B (Feb 11, 2015)

Im working on a old Wise furnace, circa 1962. Natural gas 82K BTU. Its an old power pile system. The gas valve is a honeywell vs820a1047. It is in my mothers house.

The symptom is the pilot stays lit but the gas valve wont open and light the burner.

I disconnected the thermopile wires from the gas valve and lit the pilot. After about a minute it was producing just over 600mV with no load.

I reconnected the TP to the gas valve and checked voltage. I saw around 140mV.

The t-stat and limit switch are wired in series. I removed those wires from the gas valve and put a jumper from the TP terminal to the T-stat terminal on the gas valve. It worked just fine. Gas valve opens, burner lights.

I reconnect the limit switch and t-stat to gas valve. The valve will not open. I checked voltage and there is only around 80mV under load. Not enough to operate the valve.

I checked resistance thru the limit switch and t-stat. It was just over 2 ohms.

So the questions are:
1. Is the thermopile just too weak to provide the needed voltage? It looks original
2. Is the thermopile OK and the resistance of the limit switch and T-stat too high at 2 ohms
3. If it needs a thermopile, does anyone know where i can find one like in the photos? 

One of our direct employees is an old HVAC/furnace man. He is stumped. 

Our local supplier wasnt much help. They kind of looked at me funny when i started talking about a mV gas valve. One guy knew what i was talking about and said he might be able to cross something if i bring the parts in.

Ive got an account at Lennox but those folks dont seem very helpful. All they know are part numbers.














Sent from my SM-G930V using Tapatalk


----------



## hdavis (Feb 14, 2012)

Those resistances should be well below 1 ohm.

Power from the TP is V^2/R. Get R too big, and there isn't enough power.

I'd start by cleaning the connections.

You also need to clarify where the 80 and 140 mv readings were taken


----------



## Mike-B (Feb 11, 2015)

The 140mv reading was taken at the 2 terminals on the gas valve that the ThermoPile connects to. The T-stat and limit switch were also connected to their respective terminals but t-stat was turned off.

With the t-stat on and calling for heat, the mV dropped down to 80. I was measuring this at the gas valve. One lead on the terminal that the t-stat connects to. One lead on the + terminal that the t-pile and limit switch share. 

There are only 3 terminals on the gas valve. Two are for the thermopile. The + side of the TP has a 2nd wire on the terminal which sends power up to the limit switch, thru t-stat, then back down to the 3rd terminal which operates the gas valve.

Total resistance was about 2ohm in the t-stat/limit switch circuit. Up and back thru T-stat alone was about 0.2ohms so that's good. Up and back thru limit switch was about 1.9. I pulled the wires out of the limit switch. They were shiny copper. I made sure they were well connected on both ends. No difference.

Guess i should have bypassed the limit switch to see if eliminating that resistance fixes the problem. Seems to make the most sense at this point.

The existing TP/pilot setup is a honeywell cs82A. Basically extinct. If eliminating the limit doesnt fix it, i think i will order a new pilot/TP setup. I'll throw the new TP in temporarily just to see if it fixes the problem. If so, we will retrofit a new setup in place if the old "matchbook"



Sent from my SM-G930V using Tapatalk


----------



## Mike-B (Feb 11, 2015)

FYI for anyone working on this old shtuff.

Honeywell CS82A is the pilot assembly including thermopile.

Honeywell Q302A 1025 is the thermopile/powerpile/millivolt generator only.

I was able to dig up a Q302A on ebay.

Will probably buy a limit switch too and see what fixes it...hopefully.



Sent from my SM-G930V using Tapatalk


----------



## Inner10 (Mar 12, 2009)

Sand the probe, she will put out a few more volts.


----------



## Fouthgeneration (Jan 7, 2014)

Buy your Mom a New furnace or at least a 90%+ efficiency used one.:thumbsup:

Seriously, that furnace is older than I am...

ALL parts have past their design lifetimes, especially the firebox plenum/ heat exchanger= CO and CO2 poisoning = No Momma.:sad:

Lower gas bills = More inherited money....:jester:

If you are stubborn, try a couple of added ground straps to make sure the electrons can complete the circuit.

A little low pressure canned air & a gentle vacumning wouldn't hurt.

Make sure the thermocouple well is wholely in the hot spot, maybe try turning up the Pilot gas flow for one more year's heat if you can't replace it now.

Your limit switch is out of tolerance According to web consensus...Replace and post again please.


----------



## Mike-B (Feb 11, 2015)

We have ignition!
New limit switch and new millivolt generator installed today. 
She runs like a champ. 







Sent from my SM-G930V using Tapatalk


----------



## daffysplumbing (Oct 16, 2017)

Funny!!! I've been doing heating for more than 50 years and never called a thermocouple a thermopile. I had to look it up.

Regardless, a good thermocouple generated 750 millivolts (3/4 of a volt) and the only place I can find a 750 mv thermocouple is at Johnstone supply. Every thermocouple we purchase at supply houses generates about 350 millivolts or only 1/3 of a volt.

Test the thermopile with an volt meter and set to millivolts. If you are not getting 320 millivolts then that is the first thing you need to change.

We also test valves by wiring a 1.5 volt battery across the thermostat wires and this will tell you whether or not the valve is functioning. If the valve does not work flip the positive to the opposite wire.

I always disconnect the thermostat wires to test the voltage. This helps to tell when wires are corroded, or are twisted together somewhere and have bad connections.


----------



## beenthere (Mar 5, 2009)

daffysplumbing said:


> Funny!!! I've been doing heating for more than 50 years and never called a thermocouple a thermopile.


Well thats good. Cause a thermocouple is not a thermopile.


----------

