Thermocouple Testing Procedure

HeatingHelp

Thermocouple Testing Procedure

A thermocouple is a device used to satisfy pilot safety on many 24-volt gas systems. The thermocouple is a device made up of two dissimilar metals.

Read the full story here

RPK

Thank you Tim McElwain for sharing this! I’ve always thought this was an excellent article.

seattlepioneer

<<DROP OUT - This is the final reading. It requires the pilot to be blown out. It measures the ability of the magnet to hold under reduced MV input. A good unit should drop out below 6 MVs - normal is 1 to 2 MVs. The allowable "drop out" time is 180 seconds yes three minutes. It is more likely to be a minute and half to two minutes. There will be an audible "click" when the magnet shuts down.>>
the actual problem is a bad magnet/gas valve.
This is correct, but doesn't describe the main reason for doing the drop out test.

Suppose the magnet drops out with 6-30 millivolts energizing it? Then you have a bad magnet of the gas valve, and the valve needs to be replaced.

This is an infrequent condition, and bad magnets are usually found on old equipment.

But this is why it's not enough to simply replace the thermocouple and clean the pilot. After you do that and the pilot continues to go out, you will need to return and do the magnet dropout test to verify that the actual problem is a bad magnet/gas valve.

Boyan

Very helpful article. Thank you. I do have a follow up question. The amount of energy required to keep the pilot valve open and the main valve should be different, no? After all one is a bigger coil than the other and that's what I am observing. When the main valve opens I see more of a voltage drop compared to when only the pilot is on - the base line of comparison is open circuit? Thank you

mattmia2

When the main valve opens, assuming it is some sort of millivolt valve, there is twice the load on the thermocouple or thermopile, instead of just the solenoid for the safety valve drawin power, both the solenoid for the safety and the main valve are drawing power so it is at least roughly twice the power so the voltage will fall more with both calling. The main valve also likely draws more power since it has to produce enough force to open the valve, not to just hold it open once you open it with the reset button for the safety valve.

Tim McElwain

The millivolt circuit is separate from the 24 volt system. The millivolt systems is what keeps the magnet energized to hold open the flow of pilot gas and when the call for heat takes place and the 24 coil is energized also the flow of main burner gas. They are two separate systems either one failing will cause a no heat due to no flow of gas. Also keep in mind in the millivolt circuit the source is the millivoltage supplied by the flame hitting the upper portion of the thermocouple. The 24 volt coil gets it power from a transformer which is totally separate from the millivolt circuit.

Reason for "Drop Out" test: Many times we have pilot outage which can be caused by many things. To possibly eliminate one cause we do a drop out test if the drop out is above the required limit it means the magnet could be the reason for the no heat. Years ago we could replace just the magnet which was fairly simple procedure. Due to code changes individual components can no longer be changed so the entire valve must be replaced. This means we want to be careful in our diagnoses that the definite cause of pilot outage is a weak magnet.

Tim McElwain

One of the things that make troubleshooting these systems easier is that Honeywell many years ago came out with a set of charts you can use for troubleshooting these systems. There are also charts for the Honeywell Power[pile systems. Get in touch with me and I will make the charts available to you. My e-mail; address is timmcelwain@gastcri.com. I also have a complete manual Millivolt Systems (Powerpile) (Thermocouples) is the title.

JUGHNE

Tim, just to clarify, is there only one millivolt circuit and only one magnet?

Then does the gas pressure of the pilot chamber in the valve allow the main gas to flow if the 24 volt MV coil is energized?
(The main valve needs pilot pressure to open even if energized?)

If the single magnet drops out then gas to the main valve port is closed off?

I was looking for a cut away view of the old standing pilot gas valve....can't find right now.

Also the term "redundant or combination" is used to describe these valves, could you expound on that?

mattmia2

How does a conventional gas water heater thermostat work? Is the thermostat electromechanical using the power from the thermocouple to open the main valve or is the thermostat mechanical and the thermocouple is only used to hold the safety valve open?

Tim McElwain

Mattmia2

let me answer your questions first. The water heater works off the rod and tube into the water sensing temperature. The pilot system with the thermocouple is tied into a separate circuit through the magnet assembly. The magnet assembly is closed until the pilot is lit (holding down a button with the pilot-on-off knob in the pilot position) when the pilot flame is hitting the upper 1/2 to 3/8 of the thermocouple sufficient millivolts are generated to hold the magnet assembly open. This will allow upon release of the button the pilot to remain lit. Then turning the knob to the on position and adjusting the thermostatic control to a temperature above the water temp in the water heater the burner will come on lighting off the pilot for safe operation. When the water temperature is reached the rod and tube control which is sensing temperature expands and causes the valve seat in the water heater to close. The pilot remains lit waiting for the next call for temperature. The thermostat is therefore thermal/mechanical. When the thermostat is satisfied there will be an audible click letting you know the burner should have shut off leaving the pilot lit.

Tim McElwain

JUGHNE to answer your questions:

Tim, just to clarify, is there only one millivolt circuit and only one magnet?

YES THAT IS CORRECT

Then does the gas pressure of the pilot chamber in the valve allow the main gas to flow if the 24 volt MV coil is energized?

FIRST OF ALL THE COIL IS NOT "MV'S IT IS 24 VOLTS WHEN TALKING ABOUT A HEATING VALVE. THE OPERATION OF THE MAIN VALVE HAS NOTHING TO DO WITH THE PILOT OPERATION ASSUMING WE ARE STRICTLY TALKING ABOUT A MODERN COMBINATION VALVE. AS LONG AS THE PILOT IS LIT AND THE VALVE IS IN THE OPEN POSITION A CALL FOR HEAT 24 VOLTS, OR IN SOME CASES 120 VOLTS SHOULD INIATE THE BURNER COMING ON. KEEPING IN MIND BY THE WAY ON ALL COMBINATION VALVES THE PILOT GAS IS THE SAME AS THE LINE PREESSURE COMING OUT OF THE METER. THE MAIN BURNER GAS IS TYPICALLY REDUCED BY THE REGULATOR BUILT INTO THE GAS VALVE REDUCING THE PRESSURE MOST TIMES TO 3.5" W.C.

(The main valve needs pilot pressure to open even if energized?)

SEE ABOVE

If the single magnet drops out then gas to the main valve port is closed off?

THAT IS CORRECT!

I was looking for a cut away view of the old standing pilot gas valve....can't find right now.

I WILL LOOK FOR ONE AND POST IT HERE LATER. ARE YOU LOOKING FOR THE OLD SINGLE SEATED OR REDUNDANT?

Also the term "redundant or combination" is used to describe these valves, could you expound on that?

PRIOR TO 1979 ALL GAS VALVES WERE SINGLE SEATED. AN INCIDENT A FEW YEARS BEFORE 1979 PROMPTED CODE OFFICIALS TO REQUIRE A REDUNDANT SAFETY SYSTEM IN ALL VALVES AFTER 1979. ALSO ANY SINGLE SEATED VALVES MUST NOW BE REPLACED BY DUAL SEATED WHEN THEY FAIL. THE TERM COMBINATION HAS NOTHING TO DO WITH REDUNDANT IT SIMPLY MEANS THAT ALL THE INDIVIDUAL COMPONENTS ON A MANIFOLD YEARS AGO WHICH WERE SEPERATE CONTROLS SUCH AS AN IN LINE REGULATOR, PILOT SAFETY VALVE ETC ARE NOW COMBINED IN ONE CONTROL. THE TERM REDUNDANT MEANS THAT INSIDE THE VALVE THERE ARE NOW TWO SEPERATE VALVES IN SERIES MECHANICALLY WITH ONE ANOTHER. BOTH VALVES ARE ENERGIZED ON A CALL FOR HEAT AND NORMAL OPERATION OCCURS. THE SECOND VALVE FUNCTIONS IN THAT IT OPENS UPWARD, THE PURPOSE OF THAT IS THAT IF THE GAS PRESSURE TO THE VALVE EXCEEDS 1/2 POUND PRESSURE (28.7"W.C. IS A POUND) SO 14' W.C. IS THE MAXIMUM A VALVE CAN HANDLE. BECAUSE THAT SECOND VALVE OPENS UPWARD AND PRESSURE EXCEEDING 14 " W.C. WILL CAUSE IT TO REMAIN SHUT THEREFORE REDUCING THE POSSIBILTY OF A RUNAWAY SYSTEM. RECENTLY IN MASSACHUSETTS WE EXPERIENCED WHAT HAPPENS WHEN HIGH PRESSURE GETS INTO LOW PRESSURE LINES AND SYSTEM ONLY HAVE THE OLD SINGLE SEATED VALVES. WE HAVE RUNAWAY SYSTEMS AND IN SOME CASES EXPLOSIONS AND FIRES.

Tim McElwain

Here is a Honeywell VR8300 redundant gas valve cutaway:

Tim McElwain

Hope all this helps some of you!

Tim McElwain

By the way do not confuse the operation of some of these older systems with one another. We have 24 volt gas systems which use a thermocouple. We also have an older system which is Powerpile which uses a 750 millivolt pilot generator for both pilot safety and main valve operation.

Tim McElwain

If you are interested I have an entire manual I developed on both thermocouple systems and Powerpile systems. Contact me at 401-437-0557 or timmcelwain@gastcri.com

JUGHNE

Thank you Tim!
My MV abbreviation used above was meant for "Main Valve".

And after posting I remembered what combination valve implied.

Many, many years ago furnace techs would have known more about gas valves as they were repairable and could be opened for part changing.....absolutely not something to do today.

And today with t-couple valves and millivolt valves being pretty rare, they become a mystery to younger techs.

We all would have benefited from your gas school classes.

Tim McElwain

I am still running classes. Next class is May 10 to 14 at my Training Center in Warren, RI Another class scheduled for June 21 to 25.

JUGHNE

I would like to attend even though I am pretty well retired but am over 1500 miles away from you.

Tim McElwain

I have had folks from as far away as Alaska and England distance should not be a factor for good training.

processparameters

I've heard a lot of people here over in the UK query about thermocouples within gas applications also. Surprisingly, there's even a few self-proclaimed 'experts' who don't understand the thermocouple testing procedure properly