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Old 10-10-2009, 05:27 PM   #1
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


I am working on a really old wall heater with a B-60 gas valve. I know the generator is bad. The problem is the generator is not made anymore. The B-60 gas valve has instructions on which generator to use. It says...

USE WITH G.C. 250 MILLIVOLT (YELLOW BAND) PG-1 PILOT GENERATOR

it also says....

WHEN USED WITH T-70 HEAT ANTICIPATOR USE ONLY G.C. 500 M.V. BLUE BAND PG-9 GENERATOR.



So my question is, which replacement generator do I buy this is reasonable price? I know the PG-1 is not made anymore. I found a website that told me to replace it with a 750 millivolt generator. However, I don't want the extra millivolts to damage the gas valve. The replacement gas valve is really expensive.


Last edited by stanlam; 10-10-2009 at 07:01 PM.
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Old 10-10-2009, 06:31 PM   #2
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


Is there a newer replacement wall heater available? At that age the valve may fail soon and is the heat exchanger safe? I always protect my behind first and won't retrofit anything even if there is the smallest chance of it not meeting some code or rating or design. Had a bad experience with a customer with lots of $$, and no brains try and sue me and my company. Do someone a favor and they stab you in the back.


Last edited by yuri; 10-10-2009 at 07:00 PM.
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Old 10-10-2009, 08:20 PM   #3
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


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Originally Posted by yuri View Post
Is there a newer replacement wall heater available? At that age the valve may fail soon and is the heat exchanger safe? I always protect my behind first and won't retrofit anything even if there is the smallest chance of it not meeting some code or rating or design. Had a bad experience with a customer with lots of $$, and no brains try and sue me and my company. Do someone a favor and they stab you in the back.

The Gas Company I use actually have technician that come out and do free safety inspections and diagnosis. The technician said the heater is in perfect shape with the exception of the generator that needs to be replaced.

Therefore, thanks for your concern. Now I just need advise to know on which replacement generator to buy.
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Old 10-11-2009, 10:30 AM   #4
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


To drop 750 mV to ~400 mV at some unknown, small, current level you need a series dropping resistor. It costs less than a dollar.

Since the elec. specs. on the internal resistance of generators and valves are a State Secret, you'd have to try different resistor values unless you measure the resistance of the gas valve coil and post back with the value.

E.g., if the coil reads 10 ohms and you want 500 mV across it the current in the coil must be 500/10 = 50 mA.

(750-500)mV/50 mA = 5 ohms
If the gas valve has an internal resistance of 3 ohms then you only need a 2 ohm resistor but let's start with 5.

The resistor will be dissipating 5 (0.05^2) = 13 mW. Choose a 1/8w or 1/4 w, 5.1 or 4.7 ohm resistor from (yuck) Radio Shack.

If you use no resistor the coil power will be twice normal and that will shorten the life of the coil, somewhat.

How old is that heater?

Last edited by Yoyizit; 10-11-2009 at 11:19 AM.
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Old 10-11-2009, 11:03 AM   #5
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


I think he needs a variable resistor as when the new generator starts to get weak the resistor needs to be variable or he will have no heat?
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Old 10-11-2009, 11:22 AM   #6
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


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I think he needs a variable resistor as when the new generator starts to get weak the resistor needs to be variable or he will have no heat?
A few fixed resistors, jumpered in as needed, are probably cheaper.
You have any measurements on what is considered "weak?" Open circuit output voltage goes down, internal resistance goes up, or both? How do these type of generators normally fail?
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Old 10-11-2009, 12:07 PM   #7
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


They get old and lose voltage. I don't dissect em just replace em. They are a series of thermocouples in a daisy chain. Read Fundamentals of Gas Utilization for more info.
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Old 10-11-2009, 12:11 PM   #8
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


I would just put it in. Not a real big deal and I don't see it doing any damage.
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Old 10-11-2009, 01:29 PM   #9
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


If the control valve is designed for 250mv, then damage will occur if you hook up a 750mv TP. That would be like hooking 300 volts AC to an appliance designed for 110 volts AC.

Does the TP make any voltage? I have seen people replace the TP only to find that the problem was only a clogged pilot assembly.
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Old 10-11-2009, 01:35 PM   #10
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


Those ITT General B60s had a small steel vent line from the side of the gas valve to the pilot burner. Very common that the end of it would get burnt off/plugged and not allow the valve to open. Now you got the old bean thinkin "old school".
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Old 10-11-2009, 02:11 PM   #11
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


Quote:
Originally Posted by log_doc_rob View Post
If the control valve is designed for 250mv, then damage will occur if you hook up a 750mv TP. That would be like hooking 300 volts AC to an appliance designed for 110 volts AC.

Does the TP make any voltage? I have seen people replace the TP only to find that the problem was only a clogged pilot assembly.

If you measure the wires directly from the generator, then it produces 150 milivolts. However, if you measure it through the thermostate, which is on the other side of the room, then it only produces 75 millivolts. I guess it loses millivots since the wire to the thermostate is either to long or to old.
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Old 10-11-2009, 02:11 PM   #12
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


Quote:
Originally Posted by yuri View Post
They get old and lose voltage. I don't dissect em just replace em. They are a series of thermocouples in a daisy chain. Read Fundamentals of Gas Utilization for more info.
OK, book by Dutton.
OK, open circuit or closed circuit voltage decreases.

Quote:
Originally Posted by stanlam View Post
If you measure the wires directly from the generator, then it produces 150 milivolts. However, if you measure it through the thermostate, which is on the other side of the room, then it only produces 75 millivolts. I guess it loses millivots since the wire to the thermostate is either to long or to old.
#26 AWG is ~4 ohms for 50' [100' loop distance] so I = 75mV/4 = 19 mA, assuming the wire and connections are good.

What's the voltage across the valve with this current? If it's 75 mV then the resistance of the valve should be 75/19 = 4 ohms. You may want to confirm this.

Last edited by Yoyizit; 10-11-2009 at 02:27 PM.
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Old 10-12-2009, 05:33 AM   #13
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


I would try to disconnect the pilot line at the valve and blow the pilot assembly out with compressed air. Also, I deal mainly with gas fireplaces that use a 750mv TP and the manufacturers say that the stat should not have a wire run of more than 25 feet....that means the stat cannot be more that 12 1/2 feet away. Check the connections of your stat at the control valve and at the thermostat. I've seen a slightly loose connection cause voltage drops.
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Old 10-12-2009, 06:11 AM   #14
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


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Originally Posted by log_doc_rob View Post
I would try to disconnect the pilot line at the valve and blow the pilot assembly out with compressed air. Also, I deal mainly with gas fireplaces that use a 750mv TP and the manufacturers say that the stat should not have a wire run of more than 25 feet....that means the stat cannot be more that 12 1/2 feet away. Check the connections of your stat at the control valve and at the thermostat. I've seen a slightly loose connection cause voltage drops.

Aman to that brother!

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Do twenty calls in a 12 hour shift just polishing the copper withe a SOS pad (steel wool fine grade). You might still be able to knock out 20 calls in this day and age if all you have are dirty flame sensor cleanings.
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Last edited by hvaclover; 10-12-2009 at 07:03 AM.
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Old 10-12-2009, 10:44 AM   #15
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Can I use a 750 millivolt generator on a 250 - 500 millvolt Gas Valve


Cool dude. Was that you in your "Glory days". Bruce wrote a song about them, you can sing along to as well.

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