Original message
| zennoh
 |  "lennox gh19 furnas"
, posted Tue 8 Nov 15:31  
I started checking the gas furnas made by Lennox for a recently purchased building and it is a GH19P3/4-100. I noticed that is is cycling the flame on and off every few minutes. I have good air flow and I seem to think it is wired to the wrong burner temperature limit switch. There are two plunum temperature limit switchs and the funace is mounted horizontally. The safety circuit wires go to the temperature limit that has a tag that states: "If furnace is field reversed, this alternate limit swith must be wired into the safety circuit in place of the factory wired switch. Failure to follow proper field reversal proceedure will result in nuisance limit cyling and/or safety hazard." The temperature switch without a tag has no wires on it. My question is, should the wires for the safety circuit go to the temperature limit that is not hooked up and that someone accidentally or without knowing, hooked up the alternate limit switch.
alpine
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Replies:
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| pilight
| "Re(1):lennox gh19 furnas" , posted Sat 27 Jan 15:57
Something else came to mind after posting the last message.
I have come across some furnaces that have a device attached that act like a limit switch and cycle the burner in an attempt to con-
serve on fuel used by the furnace. I can't say whether these actually work but I rarely see them any more. I believe that they would cycle the gas valve so much that the valve may go out prematurely. You don't state whether maintaining temperature is really a problem.
Pilight
pilight
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| pilight
| "Re(1):lennox gh19 furnas" , posted Sat 27 Jan 15:48
Trace the open circuit back from the gas valve. Is it a limit switch or could it be a pressure switch. Did you measure the air temperature at the limit switches. Could they be opening prematurely because of fatigue due to age. If so. replacing should resolve it, given that this is not caused by some other issue like airflow restriction i.e blocked filter, insulation fallen into duct etc. If it is a pressure switch the furnace can come on for a few minutes ok and then start to short cycle the burner. This is due to bad connections either internally or externally at the switch, plugging of pressure switch tubing and ports or a breach in the heat exchanger that shows up after a few minutes of operation or the inducer draft motor housing or inducer motor. Finding out where the break in power is will show you where to look. If replacing the limit switch look at the nameplate rating for max. outlet temperature and keep that in mind.
Pilight
pilight
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