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Old 05-09-2013, 07:47 PM   #1
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Gas furnace wiring


I have a older gas furnace with a Robertshaw 720 series gas valve that is new. I am wondering if anyone can tell me how to wire this. There is a thermostat, transformer, high limit, blower motor and the valve. The valve has three wire terminals, 1 (to transformer), 2 (to thermostat) and the third is only labeled #3. Attached is a photo.


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Old 05-09-2013, 08:13 PM   #2
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Gas furnace wiring


I'd say first
show us another photo after you've removed that old redundant reg and the manual pilot shut off fitting before the new gas valve and have moved the pilot tube over to the new valve.
There are a few different ways of wiring this valve using either two or three connections.


Last edited by how; 05-09-2013 at 08:24 PM.
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Old 05-10-2013, 09:09 PM   #3
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Gas furnace wiring


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I'd say first
show us another photo after you've removed that old redundant reg and the manual pilot shut off fitting before the new gas valve and have moved the pilot tube over to the new valve.
There are a few different ways of wiring this valve using either two or three connections.

OK, got new photos today.







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Old 05-10-2013, 10:26 PM   #4
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Gas furnace wiring


In your first new picture, please tell me that there is a drip leg or sediment trap in that gas line. If there isn't one, there should be. I know this is a wiring question, but the red flags went up when I saw that picture.
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Old 05-10-2013, 10:47 PM   #5
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In your first new picture, please tell me that there is a drip leg or sediment trap in that gas line. If there isn't one, there should be. I know this is a wiring question, but the red flags went up when I saw that picture.
Not sure they are required here. I almost never see them and I am deal with hundreds of homes like this one every year. Adding one is not a big deal, just not sure it is required. I will look into it. Even if it is not required can you give me a good reason why I should put one in?
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Old 05-11-2013, 03:20 AM   #6
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Gas furnace wiring


Drip legs catch the rust, scale, moisture and old pipe cutting oil that can come through the piping that can trash a new valve.

Your new photos show that....
Your upper limit & fan control are used parts from a different furnace.
The wiring is mostly adhoc non stock.
You have drip lines down the face of the furnace.
Your gas valve is miss wired.

I'd confirm that the heat exchanger is intact before going further.
But...
I for one do not feel comfortable advising you how to get this unit running as I see it now.

Last edited by how; 05-11-2013 at 03:30 AM.
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Old 05-11-2013, 05:29 AM   #7
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Gas furnace wiring


Quote:
Originally Posted by how View Post
Drip legs catch the rust, scale, moisture and old pipe cutting oil that can come through the piping that can trash a new valve.

Your new photos show that....
Your upper limit & fan control are used parts from a different furnace.
The wiring is mostly adhoc non stock.
You have drip lines down the face of the furnace.
Your gas valve is miss wired.

I'd confirm that the heat exchanger is intact before going further.
But...
I for one do not feel comfortable advising you how to get this unit running as I see it now.
Good advice. Inspect that heat exchanger well. Units that old can have cracks and that is never a good thing.

As far as the drip leg is concerned, it is good safety practice to have one whether they are required or not. If sediment travels through the pipe and gets lodged in the gas valve it can cause an explosion.
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Old 05-11-2013, 11:02 AM   #8
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Good advice. Inspect that heat exchanger well. Units that old can have cracks and that is never a good thing.

As far as the drip leg is concerned, it is good safety practice to have one whether they are required or not. If sediment travels through the pipe and gets lodged in the gas valve it can cause an explosion.

OK, I will install a drip line. The furnace just needs to run for an appraiser. We are going to be replacing it before winter. We just can't get a loan without it running. We inspected the heat exchanger and it is in good condition with no cracks.
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Old 05-13-2013, 05:32 PM   #9
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Gas furnace wiring


Anyone willing to help?
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Old 05-13-2013, 06:43 PM   #10
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Simple series circuit one wire from xmfr to limit from limt to valve out of valve to tstat then out of tstat to other side of xmfr. The other camstat is line voltage with a heater not in the control circuit
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Old 05-13-2013, 07:18 PM   #11
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OK, I will install a drip line. The furnace just needs to run for an appraiser. We are going to be replacing it before winter. We just can't get a loan without it running. We inspected the heat exchanger and it is in good condition with no cracks.
Do you mean an appraiser to appraise your house??? They have to see if your furnace works to appraise your house??? I have never heard of such a thing. Where do they do this?
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Old 05-13-2013, 07:54 PM   #12
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Do you mean an appraiser to appraise your house??? They have to see if your furnace works to appraise your house??? I have never heard of such a thing. Where do they do this?

We are purchasing the home. Lenders require the home to be habitable so heating and cooling needs to be working. The seller is out of state and is not able to make repairs so we are stuck with it. The furnace just needs to fire once for the appraiser. We are planning on replacing it and the evap cooler with a new heat pump after we own the home.
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Old 05-23-2013, 05:13 PM   #13
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This worked. Thanks for the help.

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