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Old 10-06-2012, 11:02 PM   #1
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Newbie w/ my first question - old gas furnace issues


Hey guys and gals, first post on this forum, just signed up and figured this will be a helpful place as I work on a newly purchased older home. I am by NO means a real home repairman, but I am fairly mechanical and own and run my own automotive shop. I am learning all aspects of home repair as I go at this point.

I'll try and make this quick and cover what I have checked into with my furnace and avoid the, "my furnace won't work, what's wrong?" question.


Furnace is a Ruud, supposedly around 40 years old. Plan is to replace furnace and air soon, just bought the house and the furnace worked at the inspection and when I tested it, but now that I need it, of course it isnt working.

Thermostat is working correctly, turn on heat to auto, set temp and furnace lights the burners. Burners run for a while (seems a little long to me) and then finally blower kicks on, decent heat comes out, fan runs about 30 secs to a minute, then turns off. Burners never turn off and whole process repeats, over....and over. Eventually after so many failed cycles (maybe 30) the gas valve seems to decide its over and shuts off all the gas, including the pilot.

It seems to be that the issue is with a limit sensor not letting the fan stay on, but when I turn the fan to "on" or bypass so it stays on, I get fan running, and the burners stay going, but only lukewarm air comes out of the vents and the house wont heat up. Instead of a switch problem, it seems to me like I am actually below a "low limit" and the burners/ heat exchanger isnt actually getting warm enough to keep the blower turned on.

The ONLY thing that happened from the time the furnace worked in my tests originally and now was that I had a contractor install a new gas valve before the heater because the valve was found to be leaking when the house was inspected. I am wondering if crap got loosened up and has clogged the gas valve so it isnt feeding the burners enough to get to full heat? The burners seem to burn well and the flame is blue but I am not sure how much is enough, ya know?


Thoughts? Am I way off base?

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Old 10-06-2012, 11:33 PM   #2
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Newbie w/ my first question - old gas furnace issues


"Fan on" the blower motor is running in high speed Black wire )....in heat mode the blower mode use med speed ( Blue or Red wires)

Check the capacitor.

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Old 10-07-2012, 12:54 AM   #3
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Newbie w/ my first question - old gas furnace issues


A fan/limit control with the lowest temp nib adjusted too high will cause the fan to short cycle.
Post a photo of a close up of the uncovered fan/limit control.

If the summer fan or manual fan switch runs the fan at a higher speed than the regular heating speed, then this would just make the problem worse.

If you have a 1972 furnace then I assume you have a standing pilot system. The pilot should only go off if you have a faulty gas valve, a weak thermocouple, a weak pilot flame, an oversized main flame or pilot assy gas leak that overheats the cold junction of the thermocouple or something is causing a draft on the pilot flame like a cracked heat exchanger.
Firstly watch the pilot flame to see if it is being blown away from the thermocouple. This will probably be most apparent just before all the gas shuts off. Report back what you see.
A partially debris plugged gas valve might cause both the fan and gas outage problems but it is unusual. (I've only seen it twice in 30+ years and I look at 6-8 furnaces a day.) Clocking the meter is the best test to tell you if it's matching the furnace BTU rating plate.

Last edited by how; 10-07-2012 at 12:58 AM.
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