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Old 12-18-2011, 06:45 AM   #1
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Furnace wiring


Old single speed fan gas furnace is not coming on this morning. Before I call in the pros I'd like to see if it is the thermostat or wiring. Is there a table of standard thermostat wirings that I could reference to force the unit into the heat state?

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Old 12-18-2011, 07:12 AM   #2
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Furnace wiring


post the brand, model and serial # and some pics of it first.

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Old 12-18-2011, 08:07 AM   #3
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Furnace wiring


put the fan to ON with the subbase fan setting auto/off if you have AC..then if no ac or you get the fan...shut the stat system there remove the stat and jump R to W..this is a direct call for heat...lets hear back on either...if not still with stat off base...jump R to W at the furnace.NOTE with doing jumps and into the furnace wiring if it is ice cold wher you are and you have AC condenser outside shut the CB off out to it or the disconnect at the condenser.jumping control wires it is easy to hit the cooling call and then bumping the compressor in ow temp weather.... not healthy...let hear back furnace unit info as asked.if you need something quick to view YOUTUBE your brand and situation like won't start ot start operation for a quick video the furnaces are basically all typical standing pilot glow coil pilot spark ignition with or without Induced draft motor, if you have one it should be mounted above the burner and supplies air into the chamber....and is the first thing to run on a stat call for heat.check that RED PLATED emergency switch is ON
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Old 12-18-2011, 10:09 AM   #4
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Furnace wiring


jump r and w to call for heat.
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Old 12-18-2011, 12:55 PM   #5
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if its a single speed blower there should be no ac unit hooked to it,hook red/white together and see if it lights off .
if it lights that way id say thermostat
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Old 12-18-2011, 01:03 PM   #6
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Furnace wiring


Old single speed fan gas furnace is a bit vague.
Does your thermostat have a connected fan "ON" switch that you can try? Check house breaker, electrical shut off ( somewhere between the furnace and the entry basement door), is the furnace door with a door switch in in place.
Is your pilot normally "on" all the time?

Tator's (W & R) jumper advice.

And Yuri's advise with the age of the unit.

Last edited by how; 12-18-2011 at 01:05 PM.
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Old 12-18-2011, 02:13 PM   #7
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Went into crawl space and checked out a few things.

Pilot light is on. It's a continuous on pilot.

There is power to the main unit as I manually engage the solenoid and the fan started up. Red and White is a bit hard to distinguish down there as wireds are a bit discolored to say the least.

So back I go now with a few tools and my phone to take a picture of the labels and wiring diagrams. I thought the furnace was original with the house but it may be a later addition but prior to 81 or so. Heat exchanger doesn't seem to have any cracks.

I may take this opportunity to un-wire the humidifier and electrostatic filter that have not been used in over 20 years.

Can I tie R and W together at the thermostat without anything else connected?

There is NO AC unit.
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Old 12-18-2011, 02:16 PM   #8
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yes you can twist the red/white together at the stat.
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Old 12-18-2011, 02:50 PM   #9
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Furnace wiring


You probably have a horizontal furnace.

If twisting the thermostat lines together at the thermostat gives you heat, then all you probably need is a new thermostat. Except maybe some serious servicing if its been running that long without a filter.

Last edited by how; 12-18-2011 at 02:52 PM.
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Old 12-18-2011, 03:32 PM   #10
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Quote:
Originally Posted by how View Post
You probably have a horizontal furnace.
Yep.

Quote:
Originally Posted by how View Post
If twisting the thermostat lines together at the thermostat gives you heat, then all you probably need is a new thermostat. Except maybe some serious servicing if its been running that long without a filter.
I've been using regular filters since we moved in. Plan was to replace it at some point and it kept working and other things were always more important.

R & W at the thermostat doesn't fire it up. I'll go down and try it at the unit itself. I'm beginning to think the gas valve for the heat exchanger may be fubar. Sigh.
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Old 12-18-2011, 04:37 PM   #11
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Furnace wiring


How did you manually engage the solinoid for the (blower?)fan to start? Are you talking about the summer fan switch (button) on the fan/limit control?

Your heating problem is likely to be thermostat lines or connections, transformer, limit contol or the gas valve. A picture of the gas valve electrical connections and limit control would help.
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Old 12-18-2011, 04:45 PM   #12
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Furnace wiring


$10 bucks says it's a limit switch.
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Old 12-18-2011, 04:53 PM   #13
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Furnace wiring


If that furnace is 30 years old or older, I'll take that bet.


PS If you are pulling out that humidifyer and electrostatic, you may have a spare transformer to use.

Last edited by how; 12-18-2011 at 04:56 PM.
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Old 12-18-2011, 06:19 PM   #14
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Quote:
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$10 bucks says it's a limit switch.
depending on problem with limit switch blower should be constantly running
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Old 12-18-2011, 06:31 PM   #15
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Furnace wiring


I'll just shut up now.

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