Reznor 75 needs help

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  #1  
Old 01-09-09, 06:53 AM
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Reznor 75 needs help

Glad I found this forum! I have a ceiling-mounted Reznor forced air model 75 furnace in my shop. I used to just flip a wall switch and the burner would kick on, then after maybe 20 seconds the fan would kick in. There is also a pull chain switch that allows the fan to turn on at the same time as the burner (no pre-heat). Recently the burner stopped coming on when the wall switch is flipped, but it still came on with the fan when I used the pull chain. Gradually it started coming on less and less to the point of no burner at all.

Went to hardware store and guy said to replace thermocouple. Did that, no help (pilot light stays lit). I opened the access panel on the side to find two switches. I tapped one and viola, the burner came on! That didn't last long, and I'm back to square one, no heat.

Called heating repair company. He came out and said I needed a new limit switch and gas control valve. Replacement price on gas valve alone was $350 + $250 to install...OUCH!! By the time they charge me for the initial visit, I'm looking at around $750! I called another furnace guy who said valve + switch + labor should be more like $370 total.

How do I determine if it's just the switch, and not the valve as well? I looked around for a diagram to re-hook everything up but couldn't seem to find what I need. It's a 24 volt system I believe (there is a small transformer) I'm fairly handy and have a volt meter, and know I can physically replace the parts myself, but am unclear on making sure I get the right parts and hook up the wires correctly. I also notice there are a couple adjustments screws, one for pilot and can't remember the other. I have pictures of everything.

I live in San Francisco where it obviously doean't get that cold, but it's all relative! The thing only really gets used Nov-May, so I'm also wondering if it's cost effective to replace with a more efficient new furnace.
Thanks!
 

Last edited by robs10; 01-09-09 at 07:19 AM.
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Old 01-09-09, 09:44 AM
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A limit switch that is stuck open and therefore won't turn on the burner is a common enough defect.


Did the repairman say why the gas valve needed to be replaced? There should have been a specific reason, such as an unrepairable gas leak. Just replacing it "on spec" "because it's old" is a waste of time and money.

The limit switch is the obvious repair to make.
 
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Old 01-10-09, 06:07 AM
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He recommended both

Originally Posted by SeattlePioneer View Post
A limit switch that is stuck open and therefore won't turn on the burner is a common enough defect.


Did the repairman say why the gas valve needed to be replaced? There should have been a specific reason, such as an unrepairable gas leak. Just replacing it "on spec" "because it's old" is a waste of time and money.

The limit switch is the obvious repair to make.
He didn't say why he recommended the valve, but he said both were "bad". He returned the switch yesterday and I'd like to replace it and see if that doesn't fix the problem. I figure if that doesn't get it going, I can always replace the valve afterwards. Don't know why the repairman wouldn't do the same unless the valve tested bad (or he wanted to make more money). The repair company is supposidly reputable.

The switch has 4 or 4 wires to connect it, the other (that sits above it) has two, which I assume is the fan switch? I assume this is something a furnace repair place would have? Outside of it just not working (the burner not kicking on), is there an accurate test for the control valve so I don't waste time and money replacing it if it's not bad? The guy at the hardware store said they rarely go out.
Thanks!
 
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Old 01-10-09, 08:27 AM
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Guessing about which switch is which is not allowed.

If you look at the part numbers, limit switches usually have an L in them while fan switches usually have an F in the part number.


And I can't really suggest tests on the gas valve without knowing what to test for.

To describe a part as being "bad" is a poor practice because it is vague. A good repairman identifies particular defects in parts that suggest they should be replaced.

Whether the repairman thinks they are naughty or nice isn't relevent.
 
  #5  
Old 01-10-09, 09:05 AM
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Originally Posted by SeattlePioneer View Post
Guessing about which switch is which is not allowed.

If you look at the part numbers, limit switches usually have an L in them while fan switches usually have an F in the part number.


And I can't really suggest tests on the gas valve without knowing what to test for.

To describe a part as being "bad" is a poor practice because it is vague. A good repairman identifies particular defects in parts that suggest they should be replaced.

Whether the repairman thinks they are naughty or nice isn't relevent.
I'll check the switch, and see if the local shop can match it up, then retest the furnace (without replacing control valve). I don't suppose there is an easy test for the limit switch?
 
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Old 01-10-09, 09:37 AM
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If the limit switch opens, the burner wont turn on.

A correct way to test the switch is to turn the thermostat up and measure the 24VAC power on both switch contacts. If there is voltage on one contact only when the heater is cool, the switch is sticking open rather than staying closed when it should, and should be replaced. Tapping on it lightly will often close the switch and cause the heater to turn on, a useful double check.
 
  #7  
Old 01-10-09, 01:56 PM
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Originally Posted by SeattlePioneer View Post
If the limit switch opens, the burner wont turn on.

A correct way to test the switch is to turn the thermostat up and measure the 24VAC power on both switch contacts. If there is voltage on one contact only when the heater is cool, the switch is sticking open rather than staying closed when it should, and should be replaced. Tapping on it lightly will often close the switch and cause the heater to turn on, a useful double check.
When I first started messing with it, tapping on the switch worked, so I guess that's it. Now, however, even tapping won't get the burner to kick on.

The one switch that I believe is the limit switch (the one the repairman removed) has 4 terminals. Which ones do I test?
 
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Old 01-10-09, 02:24 PM
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This is not about belief, it's about knowing and testing.

Why do you believe the switch is the limit switch?


What does this switch look like, what is given for a part number.

What voltages are on the connections to the switch and when are they there?
 
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Old 01-10-09, 02:29 PM
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Originally Posted by SeattlePioneer View Post
This is not about belief, it's about knowing and testing.

Why do you believe the switch is the limit switch?


What does this switch look like, what is given for a part number.

What voltages are on the connections to the switch and when are they there?
I believe it says limit switch on the estimate. Both switches are disc type. The upper has two terminals, the bottom has four. I'm not at the shop to get a part number, but check the next time I'm there.
 
  #10  
Old 01-10-09, 05:45 PM
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the limit switches should be stamped with a ***F temperature 220 is hi limit burner,and the other is the fan 160F-180F
 
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Old 01-10-09, 06:56 PM
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Sminker, there are other possibilities, such as a fan switch with a 24vac heater coil to insure that the fan turn on. That would be fairly common in an old Reznor unit heater.
 
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Old 01-11-09, 04:17 AM
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yes,if he has a relay mounted on the 1900 box along with the TR,but usually they run the fan 115 HOT thru the fan limit disc
 
  #13  
Old 01-11-09, 07:13 PM
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OK, the part the repairman took out is a 4 terminal "Therm-O-Disc". Two terminals are brass in color and larger than the other two. There are a lot of thongs stamped on the flange...24V, 29T12, 32539, 2-73, F125
 
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Old 01-11-09, 08:12 PM
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As I suggested earlier, that part appears to be a fan switch with a 24 VAC heater coil to assure that it turns on whether the burners heat up the heat exchanger or not.

It's not clear to me why the repairman would remove the fan switch, though.
 
  #15  
Old 01-12-09, 06:01 AM
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Originally Posted by SeattlePioneer View Post
As I suggested earlier, that part appears to be a fan switch with a 24 VAC heater coil to assure that it turns on whether the burners heat up the heat exchanger or not.

It's not clear to me why the repairman would remove the fan switch, though.
I agree, since the fan was turning on, though when it was in Pre-heat mode (burner on first, then fan) the fan never came on since the burner never pre-heated. Is there a test for either switch, like using a heat gun on the sensor button and testing continuity across the pairs of terminals?
 
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Old 01-12-09, 12:01 PM
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Why did the repairman remove the fan switch?


Presumably the other disc is the limit switch, which typically has an "L" in the part number and the temperature at which it opens.

when the heater isn't turning on but the thermostat is calling for heat, measure the AC voltage between the chassis and each side of the limit switch. With the heat exchanger cold, both should be 24 VAC, indicating that the limit switch is closed and allowing the gas valve to turn on.

Other ways to double check that are to use a jumper wire to connect the two prong on the limit switch, which should cause the burners to turn on.

You can also tap lightly on or near the switch, which will often cause a poor electrical connection in the switch to make and turn on the burner.
 
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Old 01-14-09, 07:11 AM
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Originally Posted by SeattlePioneer View Post
Why did the repairman remove the fan switch?


Presumably the other disc is the limit switch, which typically has an "L" in the part number and the temperature at which it opens.

when the heater isn't turning on but the thermostat is calling for heat, measure the AC voltage between the chassis and each side of the limit switch. With the heat exchanger cold, both should be 24 VAC, indicating that the limit switch is closed and allowing the gas valve to turn on.

Other ways to double check that are to use a jumper wire to connect the two prong on the limit switch, which should cause the burners to turn on.

You can also tap lightly on or near the switch, which will often cause a poor electrical connection in the switch to make and turn on the burner.
The smaller, two terminal switch has a sticker on the back, and unfortunately most of the printing is faded. I did pick out what looks like an L125. Protruding to the inside (towards burner when installed) is a round, loosley crimped metal disc. When you shake it it rattles a little. Anyway, I reinstalled the larger sensor (you say is the fan switch) and jumped the terminals on the smaller switch. No Burner. I notice the red wire from the limit switch goes directly to the fan switch. Is there no simple way to test the gas control valve by jumping something, without replacing the switches? I'll try my heat gun on the L switch and see if anything in continuity changes.
 
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Old 01-14-09, 11:58 AM
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Well, a limit switch on that unit heater that opens at 125'F isn't going to get very hot.

You might do a test for continuity through the gas valve --- perhaps it's burned out and the repairman's recommendation to replace the limit switch and the gas valve was correct.

I wish people would describe in more detail WHY they are making their recommendations!
 
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Old 01-14-09, 12:06 PM
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You should also test for 24VAC across the gas valve when the thermostat is turned up.
 
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Old 01-14-09, 12:22 PM
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Reznor parts identification

Hi, Iwill be posting my reznor question soon. The following is a link to reznor parts catalogs. https://www.rezspec.com/index.php?pageid=000000000007 at the top of the page pick Manuals & Media folder tab. then pick "parts Manuals". The pdf for my F100 is 2/3's of the way down the page. Make sure you have the serial number with you because that info to get you to the right section.
I just replaced my gas valve and pilot assembly, and yes it is pricey. Retail $130-150, I was able to get it wholesale and it cost $62.78 for the valve, 26.65 pilot assembly, plus tax. All this to find out it was not the valve. mine still does not work so I'm going to check the limit switches before I post.
OH, by the way I can make you a hell of a deal on a like new valve!!
Terry
 
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Old 01-14-09, 12:44 PM
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24 vac across valve ?

SeattlePioneer, How do you test 24VAC across valve?
 
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Old 01-14-09, 12:44 PM
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Originally Posted by tinkerer Terry View Post
OH, by the way I can make you a hell of a deal on a like new valve!!:


Thanks, but I have plenty of gas valves left over as inventory from my furnace repair business since I retired!


Start a new thread and perhaps you can get help identifying the actual problem.
 
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Old 01-14-09, 01:13 PM
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post new thread

As I said in my original post SeattlePioneer, I will post soon, after I check out the limit switches, and I'm sorry, I was offering the valve to Rob10 with a "Big Grin"
Terry
 
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Old 01-15-09, 05:56 AM
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Originally Posted by SeattlePioneer View Post
Well, a limit switch on that unit heater that opens at 125'F isn't going to get very hot.

You might do a test for continuity through the gas valve --- perhaps it's burned out and the repairman's recommendation to replace the limit switch and the gas valve was correct.

I wish people would describe in more detail WHY they are making their recommendations!
I heated the limit switch's sensor disc until it popped, which I assume was an indication it was up to the set trigger temp, but it still showed as an open circuit. I also jumped the two terminals on the furnace that connect to it, which shopuld be the same as the switch functioning, and still no burner kick on. I think I measured across the two large terminals, and two small ones on the other switch (four terminal, fan switch, as you called it) and no voltage. I'll try again as maybe I did this wrong. I think I also measured across the transformer terminals, which I assume drops the 120VAC to the 24 volts that the furnace works off, and nothing. BTW, the little transformer outputs 24 volts AC, or DC?
 
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Old 01-15-09, 06:24 AM
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Originally Posted by SeattlePioneer View Post
You should also test for 24VAC across the gas valve when the thermostat is turned up.
I'll try jumping the limit sensor and test across the two terminals of the gas valve. Shouls I also jump the two smaller terminals on the fan sensor?
 
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Old 01-15-09, 06:27 AM
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Originally Posted by SeattlePioneer View Post
You should also test for 24VAC across the gas valve when the thermostat is turned up.
I'll try jumping the limit sensor and test across the two terminals of the gas valve. Should I also jump the two smaller terminals on the fan sensor, and would that be the same as bypassing both sensors, therfore "forcing" the gas valve open?
 
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Old 01-15-09, 06:32 AM
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Originally Posted by SeattlePioneer View Post
You should also test for 24VAC across the gas valve when the thermostat is turned up.
I'll try jumping the limit sensor and test across the two terminals of the gas valve. Should I also jump the two smaller terminals on the fan sensor, and would that be the same as bypassing both sensors, therfore "forcing" the gas valve open?
 
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Old 01-15-09, 06:44 AM
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Sorry for the multiple duplicate posts! When I hit post I got website cannot be displayed message, and I assumed the message didn't post, all those times! Sorry!
 
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Old 01-15-09, 08:05 AM
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I wouldn't bother jumpering out the fan switch. It's not supposed to turn on the fan until after the heat exchanger is hot, so there no reason to suppose there's a problem with the witch or the motor.
 
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Old 01-16-09, 07:57 AM
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No voltage across transforner

Maybe this is a dumb electronics 101 question, but if I measure either Volts AC or DC across the two drop-down transformer (that's connected to the gas valve, etc.), I get no power. Is that correct, or do I measure each terminal to the heater chassis (ground?)? Sorry about the lag between Q & A. My computer is home and heater is at work.
 
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Old 01-16-09, 09:28 AM
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You should have 120 VAC across the primary windings of the transformer and 24 VAC or thereabouts across the two secondary windings of the transformer.

If you get no voltage, I'd do a double check by disconnecting the wires to the secondary and measuring the ohms across the winding ---- infinite resistance verifying a burned out transformer as the problem. If the secondary winding are OK I'd do the same thing across the primary windings, disconnecting the wires to the primary first as well.
 
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Old 01-17-09, 07:01 AM
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Originally Posted by SeattlePioneer View Post
You should have 120 VAC across the primary windings of the transformer and 24 VAC or thereabouts across the two secondary windings of the transformer.

If you get no voltage, I'd do a double check by disconnecting the wires to the secondary and measuring the ohms across the winding ---- infinite resistance verifying a burned out transformer as the problem. If the secondary winding are OK I'd do the same thing across the primary windings, disconnecting the wires to the primary first as well.
Yeah, I figured there should be something across the secondary terminals. Didn't think to check resistance of windings
 
  #33  
Old 01-21-09, 07:16 AM
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Transformer resistance

High resistance across primary (110VAC) side, if I read meter right 200K. This was measured with wires disconnected from source. None across secondary, measured with Primary source wires (110VAC) connected. Short? Also, no voltage across secondaries when everything turned on.
 

Last edited by robs10; 01-21-09 at 07:59 AM.
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Old 01-21-09, 07:41 AM
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Zero ohms is a short. Replace the transformer with another of the same volt-amp rating as the one you have --- that's a measure of the power output rating for the transformer.

you need one with a 120VAC primary and a 24 VAC secondary.
 
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Old 01-21-09, 07:44 AM
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I take it that you got no voltage when you measured across the secondary windings of the transformer?
 
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Old 01-21-09, 05:31 PM
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Originally Posted by SeattlePioneer View Post
You should have 120 VAC across the primary windings of the transformer and 24 VAC or thereabouts across the two secondary windings of the transformer.

Rob,

No need for further confusement. That test above is all that needs doing. That test will say it all.
 
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Old 01-26-09, 05:10 AM
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Originally Posted by SeattlePioneer View Post
Zero ohms is a short. Replace the transformer with another of the same volt-amp rating as the one you have --- that's a measure of the power output rating for the transformer.

you need one with a 120VAC primary and a 24 VAC secondary.
I took the transformer out and measured the resistance of the connections completely unhooked from the furnace:

Primary (110 VAC side, white and black wires) - infinite.
Secondary (24 VAC side, terminal screws on housing) - 2 ohms.

I'll stop by the near by furnace place tomorrow and pick up a new transformer. I think the woman on the phone said they get about $24-30 for them.
 
  #38  
Old 01-26-09, 05:20 AM
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Originally Posted by SeattlePioneer View Post
I take it that you got no voltage when you measured across the secondary windings of the transformer?
With everything hooked up on Primary side and secondary side disconnected from gas valve, I got zero volts across terminals. Just for fun I'm going to connect the Primary side to 110VAC when I get to the shop, now that it's unhooked from the furnace, and measure again. I'm guessing it will be the same since I assume infinite ohms across the Primaries are an open circuit/burned out Primary windings?
 
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Old 01-26-09, 04:54 PM
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Let us know how you made out.
 
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Old 01-27-09, 06:30 AM
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Bench test

Originally Posted by robs10 View Post
With everything hooked up on Primary side and secondary side disconnected from gas valve, I got zero volts across terminals. Just for fun I'm going to connect the Primary side to 110VAC when I get to the shop, now that it's unhooked from the furnace, and measure again. I'm guessing it will be the same since I assume infinite ohms across the Primaries are an open circuit/burned out Primary windings?
Same resistance, and when I hooked up Primary wires, secondary gave no AC voltage. I'm going to pick up a new transformer and limit switch on the way to work (if the shop has them in stock) and reconnect.

If I'm getting 24VAC from the new transformer, can I just jump the limit switch (which they may have to back order) terminals to test the gas valve without damaging anything?
 
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