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Old 01-30-2009, 11:16 AM   #1
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Resource for understanding gas valve?


I recently disassembled a gas valve from a discarded water heater. I found inside an amazing assortment of diaphragms and mechanisms that seem way beyond the basic function of the valve as I understand it.

Is there a resource where I can find out how these inner mechanisms work? I find that understanding the workings of a device is extremely helpful when things don't go smooth while repairing them.
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Old 01-30-2009, 04:58 PM   #2
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Resource for understanding gas valve?


They are not repairable. You only replace them. Do not attempt to repair a gas valve!!!!!!!!
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Old 01-30-2009, 09:33 PM   #3
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Darsunt:

Gas valves are actually very simple to understand. Once you understand how they work, it'll almost seem embarrassing to have to explain how they work cuz they're so simple.

When you look at a gas valve, imagine two valves in series.

The gas port where the tube to the pilot light connects is between the two valves, so that gas can flow to the pilot light as long as the first valve, or "safety" valve is open. If the second valve, or "main" valve is open, it can also flow to the burner trays.

Both valves are operated by electromagnets. The electric power to hold the first valve open, and therefore allow gas flow to the pilot light comes from the thermocouple or thermopile immersed in the pilot light flame. The thermocouple or thermopile generates electricity from the heat of the pilot light flame.

So, if the pilot light flame goes out, then the voltage to hold the safety electromagnetic valve open is lost and the safety valve closes, thereby shutting off gas flow completely, including the gas flow to the pilot light.

The second electromagnetic valve in the gas valve, the "main" valve, allows gas flow through a much larger tube to the burner trays. This second electromagnetic valve gets power from either a 24 VAC transformer or the thermopile. Up until recently flue dampers were not required on water heaters so 120 VAC power wasn't needed to operate hot water heaters. As a result hot water heaters typcially used thermopiles to generate more electricity than a thermocouple could, and used that electricity to operate both electromagnetic valves in the water heater's gas valve.

A thermopile is nothing more than about 25 thermocouples all connected in series, so that a thermopile generates about 750 millivolts instead of the 30 millivolts that a thermocouple produces.

On a boiler or furnace, there will be 120 VAC power available to operate the circulating pump or blower, and so by using a 24 volt transformer this 120 volt power can be converted into the same 24 VAC power that the safety controls on a boiler or furnace operate on. By simply putting the second electromagnetic valve in series with all of the safety controls, the second electromagnetic valve will open and allow gas to flow to the burner trays as long as every safety control in that "control circuit" allows power to flow through it. If any safety control detects a problem, it interrupts the circuit to that second electromagnetic valve, thereby shutting off gas flow to the burner trays (but not the pilot light).

This is where I fly off on a tangent:
Typically, the thermostat in a house won't be connected directly to that 24 VAC control circuit to the main valve in the gas valve. That's because to heat up a house you also need the boiler's circulating pump or furnace blower to come on to deliver the heat to the house. I'm much more familiar with hot water heating systems, so I'll describe the typical arrangement for a boiler. Typically the thermostat will be connected to a double pole 24 volt relay, and when that relay gets energized, it completes two circuits; a 120 VAC circuit to the boiler circulating pump or furnace blower, and a 24 VAC circuit through to the temperature control (or "aquastat") on the boiler. When power flows through that aquastat circuit, it energizes a second relay that completes a 24 VAC circuit through all the safety controls to the "main valve" in the gas valve. So, as long as the thermostat is calling for heat, the aquastat is controlling the boiler temperature setting (from 160 to 190 deg. F) and turning on and shutting off the gas valve to maintain the boiler water temperature at that setting. If the first relay completed the circuit to the gas valve directly, then the boiler would keep firing until it kicked itself out on high limit. By having that first relay control a circuit through the aquastat that controls a second relay which operates the gas valve, then the aquastat controls the gas valve to, in turn, control the water temperature in the heating system. A boiler will typically have a "ladder diagram" which shows the start up sequence as sequence of "loops" (kinda like a ladder).
Newer boilers or furnaces will simply have a "controller" that the thermostat connects to that does all this and more.
Back to the chase...

So, to diagnose a gas valve, first check to see if the pilot light is on. If so, then the thermocouple or thermopile is generating enough voltage to keep the safety valve open. If the appliance is a hot water heater but it's not firing up, the thermopile might be weak. It may be producing enough power to open the safety valve, but not enough to open both the safety and main valves together. If replacing the thermopile doesn't work, then you need a new gas valve.

If the appliance is a boiler or furnace, it'll use 24 VAC to operate the "main" valve. Check to see if you're getting 24 VAC at the terminals on the gas valve. If so, and the boiler or furnace isn't firing up, then you need a new gas valve. If you don't have 24 VAC at the gas valve terminals, check for continuity across all the safety devices in the control circuit of the boiler or furnace as it's likely one of the safety devices is interrupting the circuit to the main valve.

The "button" you have to push on a gas valve when lighting the pilot light simply overrides the first electromagnetic valve to allow gas to flow to the pilot light. Once the thermocouple or thermopile is hot enough, the electricity it generates will be sufficient to hold that electromagnet open and you can release that "button". This can often take a few seconds.

Also, thermocouples gradually lose their ability to generate sufficient voltage to operate that safety electromagnet, and when they do, gas flow to the pilot light stops. So, a pilot light that won't stay lit is often the result of an old and weak thermocouple. Different thermocouples get replaced differently. Most of the time you can just pull down on them to pull them out of a spring clip that holds them in place. Other thermocouples will be held in with a thumb screw you loosen from below to pull the thermocouple out. Basically, if it doesn't have a thumb screw on the bottom, pull down on it.

Some gas appliances, like natural gas burning fireplaces will have a "dual" pilot light that has two pilot light flames; one for a thermocouple and the other that continuously heats a thermopile. The thermocouple will produce the voltage to hold the safety valve open in the gas valve, and the thermopile will produce the voltage that goes through the control circuit to the second electromagnetic valve. In this way, the fire place can be turned on, turned up, turned down and shut off with just the turn of a switch even without external electric power being supplied to it.

This is what a thermoCOUPLE looks like:

The copper "tube" is really a coaxial cable. The outside of the cable is copper and there's an insulated copper wire that runs inside it. When the end of the thermocouple is heated, a voltage is generated between the two copper conductors.

This is what a thermoPILE looks like:

The distinguishing characteristic of a thermopile is that it connects to the gas valve with two electric wires (each with a terminal crimped onto it) rather than with a single nut like the thermocouple does.

And, finally, since both a thermocouple and a thermopile simply create a voltage difference between two wires when their ends are heated, different lengths of thermocouples will all generate the same voltage, just as thermopiles of different length. So you can use a longer thermocouple to replace a shorter one, and use a longer thermopile to replace a shorter one. They're like extension cords or garden hoses or battery booster cables in that respect. That's important to know when you're water heater's pilot light goes out and the hardware store doesn't have a thermopile of the right length. (Just buy a longer one.)
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Old 01-31-2009, 12:53 AM   #4
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Resource for understanding gas valve?


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They are not repairable. You only replace them. Do not attempt to repair a gas valve!!!!!!!!
I don't believe for a minute that this guy wants to start repairing his own gas valves. Since when can you buy parts for gas valves?

I think it's blindingly obvious that the guy simply wants to know how a gas valve works so that he can better understand his water heater and diagnose problems with it when it doesn't work as it should.

And, most of the time the tank will die before the gas valve anyway, so there is limited need to repair the gas valve. The new heater will come with a new gas valve.

That is, his story about wanting to fix gas valves is merely his justification for wanting to know how they work.
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Old 01-31-2009, 01:12 AM   #5
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Resource for understanding gas valve?


Thanks Nestor. I plan to review your info very carefully.

What I mean by repair is, if I find a gas valve has a problem, maybe I know to tap it and shake it a certain way to get it to work, or know just to throw it away. The mechanisms I found in side are so complicated, with such fine tolerances I am amazed they work so reliably. I wouldn't dare try to take it apart and put it back together and expect it to work.

For example, I've traced the passage for the pilot light gas. An extremely tiny passage is allowed to pass gas through the main valve when the valve is turned to pilot position (and that electromagnet is working or the red button pushed). But before it reaches the pilot line it opens up into a chamber that has some kind of diaphragm thingy. Then goes back to being a tiny passage to the pilot light tube. Why this additional complication?

I've traced the passage to the main burner line from the main valve. Before it reaches the main burner line it has to pass through a chamber that has a diaphragm with SEVERAL different layers, some rubber, some metal, some that look like paper. Why does it have to be so complicated? Or is this super technical stuff that I would never understand?
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Old 01-31-2009, 09:47 AM   #6
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Resource for understanding gas valve?


Kinda like me taking my computer apart and then going online for some DIY advice (with my spare trusty old HP windows dinosaur). That valve is way to complicated to describe. I understand where you are coming from. Someday I plan to take apart a car motor to trace all the ports etc and see how it works. Don't know if the garbage guys will haul away the parts. The diaphram thingys are the pressure regulators. One regulates the pilot gas pressure and the other the main valve pressure. Buy yourself "The Fundamentals of Gas Utilization" book by Dutton if you want to learn about regulators etc.
http://www.amazon.com/Fundamentals-G.../dp/0919852246

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Old 01-31-2009, 10:12 AM   #7
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Resource for understanding gas valve?


This post reminds me of a near panic attack I had when standing pilot furnaces switched over to IID DI systems (spark ignition). The thought of having to learn electronics was daunting. Well as it turned out I did not need to know what all those little transistors, micro chips ect were for.

What was important was knowing what functions it was supposed to perform. Knowing a system's sequence of operation is also key,

So if an inducer would not activate I would not try to find what resistor or other electronic piece on the board failed, I would just change out the board.

Same goes for a gas valve. If it does not keep the pilot lit and the gas flowing to the burners I know it is bad. All the stuff going on inside valve and how it's made are immaterial.

I hate when I read some electronics whiz thinks he is going to fix his malfunctioning furnace control module.

I saw a furnace blow the covers off because some computer guy tried to fix the board. Since he did not have oem parts he bought generics from Radio Shack. Some how it messed with the logic circuit and didn't allow the safety to work and ...KABOOM!
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Old 01-31-2009, 10:35 AM   #8
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Quote:
If the appliance is a boiler or furnace, it'll use 24 VAC to operate the "main" valve. Check to see if you're getting 24 VAC at the terminals on the gas valve. If so, and the boiler or furnace isn't firing up, then you need a new gas valve. If you don't have 24 VAC at the gas valve terminals, check for continuity across all the safety devices in the control circuit of the boiler or furnace as it's likely one of the safety devices is interrupting the circuit to the main valve.
I posted a similar thread the other day, but I will leave the question here. I am not sure where to check on the terminals on my particular gas valve. There are two identical black wires coming out from a connector (somewhat like a two-wire plug on a car light). One goes to a terminal marked TH, the other to a terminal marked TR. When I checked, neither had any current to it, so this may be a moot point as far as which one to check. I'm just not sure I'm checking the correct terminals. This is an older (1986) furnace, so I know things have changed since then. A visit to my post may help clarify my question and I apologize ahead to any who may offer assistance there and feel that I am "double dipping."

As for checking the continuity, without schematics my knowledge of what goes where is a little lacking.

Q Is the top left where the two black wires come out the actual main solenoid?

My problem is on a furnace in a greenhouse at work and I plan to probably either get another valve- if one can be found- or put in another later model heater that we have available. Personally, I would rather repair one than run new lines and start trying to make stuff fit to another heater. Long political story there.

PS-
NK, I think you were a little harch on John. He just wanted to err on the side of safety. I agree that a DIY should not be repairing gas valves, but I also can see where an understanding of what, where, how can help. Which is why I read this forum!
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Old 01-31-2009, 10:43 AM   #9
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Check where the two black wires(coming out of the valve body) connect to the blue terminal block. On a call for heat you should have 24v.
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Old 01-31-2009, 11:32 AM   #10
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Quote:
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PS-
Nestor: I think you were a little harsh on John. He just wanted to err on the side of safety. I agree that a DIY should not be repairing gas valves, but I also can see where an understanding of what, where, how can help. Which is why I read this forum!
I agree.

I make my own beer, and last night I dipped into it a little too deeply.

So, my first job this morning was to edit that post into a kinder gentler one. (I figure I better do that cuz the guy prolly knows way more than me so I better not get on his bad side cuz I might need his advice some day.)

I agree that he was only trying to steer the poster away from danger.

My own personal feeling is that there is NO DANGER in knowing how a gas valve works. My understanding is that on the very expensive gas ranges like Wolfe, you can buy replacement electromagnets to repair gas valves. However, these are not available on the gas valves used for home heating and domestic water applications. So, no repairs are possible on those.
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Old 01-31-2009, 11:53 AM   #11
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Darsunt:

Quote:
I've traced the passage to the main burner line from the main valve. Before it reaches the main burner line it has to pass through a chamber that has a diaphragm with SEVERAL different layers, some rubber, some metal, some that look like paper. Why does it have to be so complicated? Or is this super technical stuff that I would never understand?
I expect the reason it's not as simple as you'd like it to be is that the gas valve has additional features built into it, such as the override button to relight the gas valve. Also, and I don't know that this is the case, there is more than one way to shut off the flow of a fluid. Your toilet's fill valve and your washing machine's water mixing valve DON'T work the same way as your kitchen or bathroom faucet. The toilet fill valve and clothes washer's water mixing valve actually use the pressure of the water itself to shut off the flow.
Basically, you have a diaphragm that covers a "seat" through which fluid flows. The fluid pressure acts on both sides of that rubber diaphragm, but the area over which it acts is larger on one side than the other, so there is a net force holding the diaphragm tightly closed over that seat to shut off the fluid flow. If you then have a small spring loaded plug on the large area side that you can remove to release the fluid pressure on that side of the diaphragm, the diaphragm will pop open allowing flow out of the seat.

When you put that plug back in, the pressures equalize on both sides of the rubber diaphragm and it closes tightly over the seat again, thereby shutting off the fluid flow.

This is one method to use a very small amount of force to control flow of a fluid at relatively high pressure, and I have no doubt that gas valves use this closure system because it's very reliable. And, when you try to incorporate this method of opening and closing the two valves, it's gonna get pretty hairy inside that valve.

It's only a guess, but it's my best guess.
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Old 02-01-2009, 06:22 PM   #12
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Resource for understanding gas valve?


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I agree.

I make my own beer, and last night I dipped into it a little too deeply.

So, my first job this morning was to edit that post into a kinder gentler one. (I figure I better do that cuz the guy prolly knows way more than me so I better not get on his bad side cuz I might need his advice some day.)

I agree that he was only trying to steer the poster away from danger.

My own personal feeling is that there is NO DANGER in knowing how a gas valve works. My understanding is that on the very expensive gas ranges like Wolfe, you can buy replacement electromagnets to repair gas valves. However, these are not available on the gas valves used for home heating and domestic water applications. So, no repairs are possible on those.
No problem here. I saw "when things don't go smooth while repairing them." I understand you should know the basic operation inside and out as a service technition. It just makes me nervise when what might be a home owner starts tinkering where the shouldent. I know know now he is only looking for knowlege.
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Old 02-02-2009, 01:51 AM   #13
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There are two identical black wires coming out from a connector (somewhat like a two-wire plug on a car light). One goes to a terminal marked TH, the other to a terminal marked TR.
The terminal marked "TH" is where you connect the wire from your THermostat. The terminal marked TR is where you connect the wire from your TRansformer. Your gas valve requires that you connect the gas valve, transformer and thermostat all in series so that when your thermostat calls for heat it completes the circuit between the transformer and the main solenoid of your gas valve, thereby energizing the main solenoid in the gas valve with 24 VAC from the transformer. So, yes, those two black wires do go to the main solenoid in your gas valve.

Quote:
This is an older (1986) furnace, so I know things have changed since then.
1986 isn't that old. I know lots of things that are older than 24 years. Rocks, for example. Your best bet would be to find the model number on your gas valve and go to the manufacturer's web site and download the original documentation that came with that gas valve. That will tell you the correct wiring of the valve.

Ditto for the furnace. You should be able to download the documentation that came with that furnace, and it will include a "ladder diagram" telling you how the start up sequence is supposed to work on that furnace. If you find you can't understand it, then buy a $5 box of chocolate covered donuts and pop down to any of the major Plumbing & Heating wholesalers in your area. EAch of them will employ "heating specialists" who can explain that ladder diagram to you, and will devote their entire attention to your problem for chocolate covered donuts.

PS: The heating specialists aren't heating contractors. They're not even plumbers. They're just ordinary guys who have worked their way up in the company to have been sent on courses put on by heating equipment manufacturers to teach them how to size heating equipment for new construction, size radiators and ducting for different rooms in a house, size air conditioning systems, etc. So, they're just ordinary people, and as long as you're polite and respectful, they'll help you as much as they can, especially if you come bearing chocolate covered donuts.
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Old 02-02-2009, 01:57 AM   #14
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There are two identical black wires coming out from a connector (somewhat like a two-wire plug on a car light). One goes to a terminal marked TH, the other to a terminal marked TR.
The terminal marked "TH" is where you connect the wire from your THermostat. The terminal marked TR is where you connect the wire from your TRansformer. But, you say, "my thermostat is like a switch and has TWO wires coming out of it" and "my transformer has TWO 24 VAC terminals on it". That can cause some confusion until you realize that your gas valve requires that you connect the gas valve, transformer and thermostat all in series. So one wire from your thermostat and one wire from your transformer will connnect to the gas valve. The other wires from the thermostat and transformer will be connected together so that the thermostat, transformer and main solenoid on the gas valve are all connected in a series loop.

So, when your thermostat calls for heat it completes the 24 VAC circuit between the transformer and the main solenoid of your gas valve, thereby opening that main valve and firing up the burner trays.

Now, as explained in my prior post, that would mean that the burner trays will fire up as long as the thermostat calls for heat, even if one or more of the safety controls isn't happy with that idea. So, either you're also supposed to include all the safety controls in that 24 VAC series loop, OR your safety controls operate on the same 120 VAC circuit that the transformer is on. So that if one of the safety controls detects an unsafe operating condition, it interrupts the 120 VAC circuit to the high side of the transformer, thereby shutting off the supply of 24 VAC power to the main solenoid of the gas valve, thereby stopping the flow of gas to the burner trays. There are different ways to skin a cat, and wire a boiler or furnace.

But, yes, definitely, those two black wires do go to the main solenoid in your gas valve. The ONLY question is whether or not the safety controls are supposed to be in that same series loop connecting the thermostat, 24 VAC transformer and gas valve main solenoid terminals, or whether the safety controls are in the 120 VAC circuit to the transformer so they'll shut down that transformer in the event of an unsafe condition. Cuz doing that will shut the gas flow to the burner trays off too.

Quote:
This is an older (1986) furnace, so I know things have changed since then.
1986 isn't that old. I know lots of things that are older than 24 years. Rocks, for example. The Pyramids. The Eiffel Tower. And, me. Your best bet would be to find the model number on your gas valve and go to the manufacturer's web site and download the original documentation that came with that gas valve. That will tell you the correct way to wire that valve (with or without safety controls on the 24 VAC side).

Ditto for the furnace. You should be able to download the documentation that came with that furnace, and it will include a "ladder diagram" telling you how the start up sequence is supposed to work on that furnace. (That will include all the 120 and 24 VAC loops and what relays turn on which loops in order to get 24 VAC to the main solenoid of the gas valve.) If you find you can't understand it, then buy a $5 box of chocolate covered donuts and pop down to the Plumbing & Heating wholesaler in your area that sells that brand of furnace. That wholesaler will employ "heating specialists" who can explain both the wiring and ladder diagrams to you, and will devote their entire attention to your problem if you're offering chocolate covered donuts in exchange for their time.

PS: The heating specialists aren't heating contractors. They're not even plumbers. (They didn't even graduate from high school. ) They're just ordinary guys who have worked their way up in the company to have been sent on courses put on by heating equipment manufacturers to teach them how to size heating equipment for new construction, additions to existing buildings, size radiators and ducting for different rooms in a building, size air conditioning systems, etc. So, they're just ordinary people, and as long as you're polite and respectful, they'll help you as much as they can, especially if you come bearing gifts, like chocolate covered donuts. That is, the only reason they wouldn't take the time to help you is if they're in a real rush to get a project done, and simply can't take the time to help you. But, that will also mean that they'll feel obligated to do everything they can to help you understand your furnace's wiring when they do have some spare time.

If you have access to a digital camera, take pictures of everything that has wires connected to it, make a note of where those things are on the furnace (whether's they're near the bottom so that they can detect flue gas spilling out the bottom of the furnace, or high up on the furnace plenum chamber where they can detect the air temperature or on the flue duct to detect any back pressure in that duct or under the front cover of the furnace where the relays are located), print off those photos (B&W is OK) and take them with you. Lots of times they can tell what something does simply by it's location and shape, or because it's shape hasn't changed since 1986.

I'm not sure where your previous post is. If you copy and paste it in here, I'll take a look at it and see if I can help solve your problems.
Really, the solution to understanding your furnace lay in showing the ladder diagram to a "heating specialist" and having him explain it to you. The manufacturer of your furnace should have that information on his web site. If you can point me to a web page that has a ladder diagram, I'll see if I can explain it to you.

But, the people most competent to do that are the "heating specialists" at the wholesaler in your area that sells that brand of furnace. I'm much more familiar with hot water heating system boilers, but I'm willing to take a shot at it.
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Old 02-02-2009, 12:10 PM   #15
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Resource for understanding gas valve?


If you know the coil resistance for a 28mV gas valve, I'd like to know it.
Putting an AA cell across one of these valves may result in 700x the normal power dissipation, but I'd like a corroborating value.
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