Millivolt gas valve

tc-vt · December 19, 2008, 01:28:16 AM

Hi all,

Does anyone know anything about millivolt gas valves? I have an old Hearthstone Sterling direct vent propane stove I purchased used. The pilot was staying lit but the main burner would never come on. I tested the thermopile and the output was low. I replaced the thermopile and it works now.

My question is, is there any importance as to the polarity of the two wires wires from the thermopile, red and white, and how they are connected to the gas valve? The gas valve has three terminals TH, TP, and TH/TP. The thermopile goes to the TP and TH/TP and the thermostat goes to the TH and TH/TP. There is no thermocouple on the stove. The thermopile operates both the pilot valve and the main valve. Does it matter if the red and white are switched? Of course, I forgot to note how the old one was wired when I took it apart. Searching on the web, I come up with 50/50 as to which way the wires go. I have found nothing that says if polarity is important or not. I do have a call in to the service center but haven't heard back yet.

I'm asking here because I might get an answer and because I am surprised that it works with the wires in either polarity. Can anyone explain how these valves work such that the DC output from the thermopile operates the valve in both polarities?

Tom

glenn kangiser · December 19, 2008, 03:04:56 AM

I don't know that one, Tom, but will add that I just checked out a gas oven today and found that the ignitor can be partially burned out and will not light the oven as it requires a certain amount of amps current draw to open the safety valve.

As they start to burn out the draw goes down and the gas will not flow until the ignitor is replaced. The solenoid requires a certain amp pull to operate and this prevents explosions from a burned out ignotor. The ignitor is in series with the valve and can be checked for flow with a clamp type ammeter. The one I checked today for a customer pulled 1.95 or so but required 2.94 to 3.3 amps to open the valve.

December 19, 2008, 07:22:12 AM

It should not make any difference in that application. the thermopyle is the power source for the thermostat so it is just a common and a neutral that get sent through the gas valve to the thermostat to determine if heat is needed or not. On some furnaces and boilers it matters but they tend to have more controls on them that are polarity sensitive.

Brian

tc-vt · December 19, 2008, 09:07:18 AM

Thanks Brian and Glenn,

I'm still wondering how the valve works even with reversed polarity. A DC operated solenoid would be expected to go in an opposite direction if polarity is reversed. I think these gas valves like mine use an electromagnet to open the valve, but then I would expect opposite poling of the magnet with reversed polarity which I would think would cause improper operation of the valve???

Tom

John_C · December 19, 2008, 09:37:38 AM

You are forcing me to use brain cells that haven't had much exercise in 40 years.

If you reversed the polarity to the field of an electromagnet what would happen is the N - S poles of the magnet would be reversed. But the open/closed status would not be changed. ie. the valve would be closed when there is no field and when energized the magnet would have the same strength and attract/open the valve. .....
Open when idle ----- closed when energized would also stay the same, if that was the configuration.

December 19, 2008, 10:30:00 AM

John, I was thinking that the when the magnet poles are reversed because of the electrical polarity being reversed that the magnet would repel the valve instead of attracting it. Is this wrong?

Tom

glenn kangiser · December 19, 2008, 11:06:31 AM

I think I have it The valve actuator or armature is not a magnet therefore has no force to repel from either the north or the south pole of the magnet. It is always attracted to either. I think...

John_C · December 19, 2008, 11:32:36 AM

QuoteI was thinking that the when the magnet poles are reversed because of the electrical polarity being reversed that the magnet would repel the valve instead or attract it. Is this wrong?

Yep. That's wrong.

Science experiment 101

If you have a bar magnet one end is N, the other S. Either end will attract a ferrous metal. The N end would repel the N end of another magnet... as would a S - S orientation, but either will attract (most) non magnetized ferrous metals. There would be no advantage, functionally or cost wise, to have two opposed magnets in the switch. It's just one magnet and a piece of metal... either magnetic pole will work.

tc-vt · December 19, 2008, 01:25:30 PM

Thanks John,

I was thinking that might be the reason but I still imagine all magnetic things as having poles. Time for a physics review. So, then the moral of the story is that the polarity of a thermocouple or thermopile on a miilivolt gas valve doesn't matter?

Tom

MountainDon · December 19, 2008, 02:00:08 PM

FWIW

www.robertshawtstats.com/spaw2/SiteContent/Files/faq/ControlTips-MillivoltSystems.pdf

tc-vt · December 19, 2008, 02:08:27 PM

Don,

I saw that one and thought it was going to explain all. No mention of polarity, though. I googled and some manufacturers do and some don't stipulate which wire goes where. I have read a thread in hvac-talk.com about a particular valve setup where it is mentioned that polarity does matter. I think this valve they were discussing there used two thermopiles together somehow.

Tom

brian_nj · December 19, 2008, 04:46:25 PM

the reason some manufacturers stipulate position is to keep their technical support from being over run with calls on which wire to hook where. It is like some of the filters that are bought for a furnace there are ones that both sides are constructed exactly the same but they still put an airflow arrow to keep people from calling and asking which way to put it in.

tc-vt · December 19, 2008, 10:30:54 PM

Brian,

That sensible of them, and then it isn't. Wouldn't it make more sense to say that the 'wires can be installed in any orientation'?
I got a hold of the service guy, so now I am one of the statistics you mentioned, a person who calls to ask. He told me it doesn't matter.

Tom

lobster · December 21, 2008, 12:04:36 PM

QuoteScience experiment 101

If you have a bar magnet one end is N, the other S. Either end will attract a ferrous metal. The N end would repel the N end of another magnet... as would a S - S orientation, but either will attract (most) non magnetized ferrous metals. There would be no advantage, functionally or cost wise, to have two opposed magnets in the switch. It's just one magnet and a piece of metal... either magnetic pole will work.

yes - that is the how an AC-powered solenoid works.

the coil of the solenoid generates a magnetic field which attracts the movable iron plunger no matter what the polarity of applied voltage.

(but in addition on AC solenoids there is a small, usually copper "shading ring" which sets up eddy currents and continues the attraction while the AC supply voltage passes through zero volts.)

for example: http://answers.yahoo.com/question/index?qid=20070717200320AAOpRgt

Millivolt gas valve

John_C

John_C

John_C