Gas heater thermocouples

How do they work? The pilot flame heats a thermocouple which produces a miniscule electric current that allows the main gas valve to open. If the pilot flame goes out, the gas is shut off.

My question is this: The power provided by the thermocouple is tiny and not enough to activate a solenoid directly. There is no other source of power involved… how does this tiny current enable the main gas valve??

I once asked this to a repairman and you should have heard the explanation he gave me. Maybe it made sense to him but to me it just showed you can spend a lifetime repairing gas heaters and not have a clue of the principles involved.

If you’ll look closely, you’ll see a small step-down transformer is part of the mechanism - it supplies 12 volt current for operating the solenoid.

No sir, you are mistaken, my gas water heater has no outside input whatsoever and the furnace has it only for the thermostat but not for the pilot light part.

So solly. My furnace (and thermocouples on most forced-air gas furnaces) has a step down transformer with two leads directly connected to the gas solenoid.

The solenoid on a water heater is different. The short answer is, a thermocouple generates a small DC voltage when the disimilar metals are subjected to different temperatures. The whole story can be had here:

http://www.electronics-cooling.com/Resources/EC_Articles/JAN97/jan97_01.htm

Based upon the assumption that you have a natural gas or propane or other similar type of water heater:

Inside the pilot safety valve is a coil of wire on a soft iron form and a metal plate that is affixed to the pilot valve. The thermocouple generates a small voltage, but at a substantial current, which flows through the coil of wire.

You depress the pilot valve and light the pilot flame, which heats the thermocouple. After about 60 seconds or so the thermocouple is sufficiently hot to maintain current through the coil of wire–holding the pilot valve open. You can now let go of the external pilot button and turn the knob to the burner position.

Among other types of devices, an assembly known as an ether-bulb/capillary tube (operated by expanding/contracting liquid) is used to switch the main burner on and off, via an expanding bellows and mechanical amplifier linkage.

Also used, in some heaters, is a bimetallic element to open the main burner valve. In this case the bimetallic element causes linear thrust or rotational torsion to achieve the desired switching action.

Point is, for most domestic water heaters, the thermocouple is used only to hold open the pilot safety valve, and the main burner is switched on and off thermo-mechanically.

High tech? No, but it works.

Sorry if this is overly brief, but I’m beat and I’m going to bed now.


Kalél
Common ¢ for all ages…
“Well, there was that thing with the Cheese-Wiz…but I’m feeling much better now!” – John Astin, Night Court
“If ignorance is bliss, you must be orgasmic.”

A quick followup after seeing Nick’s post regarding furnaces…

Many furnaces do use step-down transformers to achieve their wizardry. Usually 12 volt or 24 volt secondary windings for that task.

Seeing as water and electricity would pose a hazard (giving the folks at Underwriter’s Labs a serious fit, no doubt); and given that this is a bad mix in the normally unawares domestic setting, most domestic water heaters don’t incorporate step-down transformers in their design for activating main burner valves. However, quite frequently you will see such implementations in industrial settings.

Nite all!


Kalél
Common ¢ for all ages…
“Well, there was that thing with the Cheese-Wiz…but I’m feeling much better now!” – John Astin, Night Court
“If ignorance is bliss, you must be orgasmic.”

Nickrz, EnigmaOne,

while the furnaces do have an external power supply, I believe they do not in any way amplify the thermocouple’s output. A thermocouple’s output is tiny. I know how it works, the output is not enough to move anything, just barely enough to be measured.

Enigma, what you say makes more sense to me except… I checked and it ain’t so. When I replaced my natural gas water heater, a couple years back, I removed the burner, valve etc, precisely because I wanted to study it. I now have it in front of me. The pilot flame heats the thermocouple and nothing more.

It seems like I am going to have to dismantle the darned thing which is going to be quite a task as all the screws are jammed tight.

The capillary tube idea did occur to me as I know them well and how they work because they are the basis of refrigerator thermostats. But the pilot flame has no such thing…

So, my original question remains: how can the tiny power supplied by the thermocouple enable (not activate but enable) the main gas valve. I wonder… it may be that the current supplied by the thermocouple is indeed enough to move a tiny latch which would prevent the main valve from opening… now I am really curious!

Since there are no wires at all leading to my gas hot water heater, I assume that the whole thermocouple process is mechanical, not electrical. To light the pilot, I have to physically hold down a button to release gas to the pilot. I have to hold this button down for a minute or so AFTER the pilot is going. The pilot heats (and then bends) a bi-metal strip. The bent metal opens a valve to allow a normal supply of gas to enter the system. After this valve is open, I can release the button I have been holding. If the pilot goes out, the bi-metal strip mechanically closes the flow of gas to the pilot.

I assume the water tank has a bi-metal strip that measures the temperature of the water. It opens when the water is cold and shuts whent the water is hot.

A gas furnace uses the same mechanics for the pilot, but uses an electrical connection to the house thermostat to control the gas.

Ok, I have partially dismantled the gas valve. One interesting thing is that there is a second thermocouple in the water sensing element and it is connected in series with the first one.

I have isolated the valve that interrupts the gas flow if the pilot flame goes out and it has a plunger that requires at least 1 lb of force to move. There is no way two thermocouples can move this against the spring… and yet…

I cannot open this element without destroying it (not that I mind but I need to do some errands. Maybe I will try putting some current through it and se what happens…

We know the thermocouple has to be hot for a minute or two before this valve opens… what may be the principle at work?

I was thinking maybe the thermocouple heated some gas? but I tried heating this element and nothing moves…

any ideas?

By the way, this valve is what you are pushing against when you hold the button to light the pilot flame, so you can see how much force it requires.

I have posted a photo and diagram at: http://www.geocities.com/alfgonalf/valve01.jpg http://www.geocities.com/alfgonalf/valve01.gif

the thermocouple wires connect to the rear and move the plunger at the other end which is pushed out by the spring

After I posted the previous post I realized the energy from the thermocouples does not need to depress that valve. I do that when I push to light the flame. It only needs to hold it, which is quite a different matter. Moving a small latch would do it. Still I am very curious and next time I have some time I will open (and thereby destroy) the valve mechanism. maybe I will gain the knowledge I am searching…

Of the explanations posted, EnigmaOne’s first post is basically correct.

The only real error is the statement that a thermocouple generates “a substantial current”. To test a thermocouple, you use a micro-amp meter. Not exactly substantial. But that’s a small quibble, he got the basic explanation right.

Sailor, the piece you posted the picture of is a standard thermocouple pilot magnet. The small disk with the rubber-like surface blocks the pilot gas flow until you manually open it by pressing the pilot button. Inside the larger housing is a small electromagnet. The current generated by the thermocouple flows through the electromagnet, and creates a strong enough pull to hold the plunger open against the spring once you manually push the plunger in, but not enough to pull the plunger open by itself. (Think about how fast magnetic field strength falls off over distance.)

Residential gas water heaters don’t normally have any external power source. The thermocouple holds the pilot valve open, and the main sensing element (which extends into the tank) is fluid or gas filled. When heated it moves a small distance, which pushes through a lever linkage and closes the main valve. When it cools off, the linkage is spring loaded to return to the open position.

Gas furnaces usually do use external power. The difference is not due to safety regulations, but to the function required. A furnace frequently has to have a remote temperature control (we call it a thermostat). This is much easier to accomplish with electricity than without. (There were electric thermostats that didn’t require external power. They were called millivolt systems, and used a cluster of thermocouples in a single larger housing (usually about the size of your finger) which were wired in series. This put out a high enough voltage (about half to 3/4 of a volt) to be run short distances to a wall thermostat and back, and power the main gas valve as well. But you almost never see these types anymore.)

In addition, most furnaces (again, with a few exceptions for older or very small units) require a fan to move the heated air through the system and out into the airspace to be heated. This is easiest to accomplish with electricity.

Lastly, the liquid sensor used in water heaters works well in the heat range used for hot water (typically 100-180 degrees). But for furnaces, we don’t want to regulate based on the air temp in the plenum, but on the air temp in the living space. Again, this usually calls for a remote temp sensor.

Put these things together, and it makes sense to use an electrically powered main gas valve in a furnace. These used to be common in both low voltage (12 to 24 volts, with 24 volt the thing used for many years now) and in line voltage (120). However, for many years, I haven’t see anything but 24 volt valves and control systems on any residential gas furnace.

So a domestic gas water heater usually doesn’t have any external power because it works fine without it, and doesn’t have any functions that require power (pumps, fans, remote sensors) so there is no reason to complicate it (which costs money).

All this from the more than you ever wanted to know about the subject, from someone who spent nine years selling, installing, repairing, and testing the equipment in question.

Ugly


Go ahead, be frank with me.
In fact, be anyone you’d like.

{{{while the furnaces do have an external power supply, I believe they do not in any way amplify the thermocouple’s output. A thermocouple’s output is tiny. I know how it works, the output is not enough to move anything, just barely enough to be measured.

Enigma, what you say makes more sense to me except… I checked and it ain’t so. When I replaced my natural gas water heater, a couple years back, I removed the burner, valve etc, precisely because I wanted to study it. I now have it in front of me. The pilot flame heats the thermocouple and nothing more.}}}—sailor

I think you need to re-read my postings. Particularly paragraph 3 of the first posting.

You’re measuring voltage, not current.

The coil of wire I mentioned forms an electromagnet–a current operated device, and it was in those terms that I was speaking (writing). That’s something I should have made evident in my first posting.

You might like to try this:
Hook the thermocouple up to the coil of wire (the electromagnet) and insert a DC Ammeter into the circuit (series). You’ll need to cut a wire someplace and insert the meter into the circuit such that the current will flow THROUGH the meter (alligator clips will male this easier to do). Heat up the thermocouple over a stove and watch your meter readings…you’ll end up with something other than “miniscule” from a good thermocouple assembly.

I’ve yet to disassemble every model of water heater known to man, and I was working from memory of earlier surgical fun.

I did pull a few things from the top shelf of the garage, and I got things right the first time around for the particular unit from which I scavenged parts.

To recap:
Thermocouple is heated by pilot flame to generate CURRENT. Said current operates an electromagnet to hold the pilot safety valve open, thus keeping the flame lit–a closed loop, if you will.
No amplification process here.

A second device is immersed within the water jacket, and is usually on the back of the cast aluminum valve body. This device is generally of the ether bulb/ capillary tube type or a bimetallic assembly.

In the case of an ether bulb/capillary tube/bellows assembly, a MECHANICAL amplifier linkage (perhaps a poor choice of terms, but that is what it is) is used to actuate the main burner valve.

In the case of a bimetallic assembly, either linear motion or torsion is used to actuate the main burner valve.

When I made mention of transformers, it was restricted only to their implementation in furnaces, and presented good reasoning for their omission in domestic water heaters.

I have personally worked on industrial water heating equipment that uses both high voltage and low voltage to accomplish tasks within certain sub-systems BUT, you generally won’t find these implementations in domestic water heaters.

I don’t think I’ve said anything substantially different than I did before. I hope I’ve made myself more easily understood though…I’ll try not to be overly brief next time…my bad.


Kalél
Common ¢ for all ages…
“Well, there was that thing with the Cheese-Wiz…but I’m feeling much better now!” – John Astin, Night Court
“If ignorance is bliss, you must be orgasmic.”

I was called away from the computer in mid-post…I guess I should have checked to see who posted in the interim.

Thanks for catching me on another poor usage of terms, RJKUgly.

It’s been so long since I’ve done in-circuit measurements of thermocouples that my recollection is probably off by an order of magnitude.

I was thinking more in terms of about an Amp when I posted–nothing approaching the filling-rattling proportions that the word “substantial” must conjure up. :o


Kalél
Common ¢ for all ages…
“Well, there was that thing with the Cheese-Wiz…but I’m feeling much better now!” – John Astin, Night Court
“If ignorance is bliss, you must be orgasmic.”

Ok, so that answers my question. My statement that a thermocouple produces a miniscule amount of power is correct and I found it difficult to believe it was enough to activate a solenoid but now I know it is enough.

The reason I found it hard to believe is that even a small relay will consume about 1 or 2 watts which is way more than a thermocouple will provide.

Anyway, question answered and matter resolved

EnigmaOne
[Thanks for catching me on another poor usage of terms, RJKUgly. ]

No sweat, your explanation was a good one, and my quibble was definite nit picking. And hey, if we can’t be off by power of ten here and there, I’m going to be more trouble than some of you guys! :0

I remember when I first found out that thermocouples actually do generate enough current to energize the magnet and hold against those springs. I was surprised it worked!

So in this case, the answer to the question “How substantial is the current generated by a thermocouple?” seems to be “Substantial enough!”

Ugly


Go ahead, be frank with me.
In fact, be anyone you’d like.

Rather than rip the solenoid valve open I conducted some electrical experiments. A current of about 100 mA will hold it but a little less will not. This is more than I would expect a thermoucouple to provide. At 100 mA the voltage across the solenoid is about 4 mV but the error may be high. In any case, the resistance of the solenoid is extremely low. The power to hold the solenoid would be under .5 mW which I find amazing as even a small relay will require more that 100 times as much.

I will test the thermocouples to see what I can get from them…

Great explanation but maybe i missed it can you explain why the DC (2 or 3 wires on gas valve)where that comes into play if Thermo generates millovolts to hold open main valve why is DC wires needed? i am confused

A solenoid valveis designed to minimize the force needed to activate it. They are used in dishwashers and washing machines as well. I assume, but don’t know, that the two-stage type of valve as EnigmaOne described would be in use for many gas heating devices.

I too found the idea that a thermocouple puts out enough juice to operate a valve doubtful. However, I too have taken some apart and found no other possibility. A very soft spring is part of the trick. Water heaters I am familiar with only have one thermocouple and valve. No pilot light, no gas.

Modern furnaces don’t have pilot lights or thermocouples. Not sure when the change came, the one I installed in 1995 didn’t. It has a hot surface ignitor and a flame detector that relies on a signal from the powered control board shorting out through the flame. Flames are conductive.

That also requires proper wiring to run a furnace. Some sloppy DIY work can keep a furnace from running on an emergency generator.