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Billy
Billy, HVAC Technician
Category: HVAC
Satisfied Customers: 19777
Experience:  Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
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I have a Reznor gas heater Mod. # UDAP30. How can I check

Customer Question

I have a Reznor gas heater Mod. # XXXXX How can I check the flame sensor to see if it is good?
Submitted: 3 years ago.
Category: HVAC
Expert:  M Oglesby replied 3 years ago.

M Oglesby :

Do know where the flame sensor rod is?

Customer :

Of course I do! It is mounted on the side of the furnace fire box with the rod going through a hole in the fire box. The question I have for you is how do I test the sensor to see if it is good or bad.

M Oglesby :

Do a resistance check with a ohmmeter.

Customer :

OK? Resistance from where to where? And what range of resistance should I expect for a good or bad flame sensor?

M Oglesby :

from the wire lead to the tip of the flame sensor rod. should be 5 ohms of resistance.

M Oglesby :

Why do you suspect the flame sensor rod?

M Oglesby :

Are the burners igniting?

Customer :

Because I put in a new control board after the old one failed to generate any spark. The old board failed after the xformer supplying power to the building shorted out. With the new board the sequence is; boost motor starts, ignitor sparks, the burner fires up, then shuts down. The XXXXX XXXXXght (flame indictor on the board) never lites up.

Customer :

I measured the resistance from the rod tip to the terminal. The rsistance is 0.2 ohms. I also have a new sensor, still in the factory package, and it also measures 0.2 ohms. Futher information; this is the second control board I have put in. I got the distributor to cover it under warranty because they and I both agreed the flame sensor seldom ever go bad. The only other things in the curcuit are the wire from the sensor to the plug on the board and the wire and plug have continuity from the sensor teminal to the pc board.

M Oglesby :

The only other thing that I can recommend is clean the Flame sensor rod with a fine piece of sandpaper.

Customer :

That was the first thing I did.

M Oglesby :

I will have to recommend that you call a technician out to troubleshoot.

M Oglesby :

Have you checked the batteries in the thermostat if it has any? Bad batteries can cause intermittent behavior from the furnace.

M Oglesby :

If that doesn't work, I would recommend you also try to bypass the thermostat to see if it is the problem. This is how you bypass it: Remove the thermostat cover. You should see a R terminal W terminal G terminal Y terminal. Use a piece of wire to jumper the R terminal to the W terminal or Remove the R wire and the W wire and twist them together. See if the heater runs with the Tstat bypassed.

Customer :

Are you nuts? How would the heater ever go through it's start up to get up to where it dosn't see the flame even though it is in fact on? I think this help(?) session is over.

M Oglesby :

Yes it is going through start-up but it is cutting out. Something is causing it to cut out. I assure you what I recommend is not nuts at all.

Customer :

Look, it goes through the start up sequence that I gave you earlier. It tries THREE times then locks out until I lower th thermostat to OFF then raise it to 80 degrees again and it again goes through three cycles before locking out again. How can that be the thermostat?

M Oglesby :

Are there any lights flashing on the control board?

Customer :

There are two lights on the board. The frist (green) is the ready light. It blinks slowly when the thermostat is not calling for heat. When I raise the thermostat it starts blinking fast. The second light (amber) is the flame indicator. It never does anything not even after the burner cycles the three times and the heater goes into lockout. Also, I just installed the new flame sensor I got day before yesterday and the above symtoms remained the same. Also, remember that this is the second new board I have tried and I am absolutely 100 percent sure all the wires are on the right terminals and that the two plugs are plugged in properly and securely.

Expert:  Tom replied 3 years ago.

Hi...is this a new installation in this location? Meaning ... has it worked in it's present location?

 

Tom

 

Expert:  Tom replied 3 years ago.

It sounds to me like the 120 leads are reversed. On the UDAP the board is polarity sensitive and if the 'hot' and neutral are reversed it will do everything except recognize flame..

 

I would verify with a meter which is which or just reverse the hot and neutral on the L1 and L2 terminals and try it.

 

Let me know how it works....do not ACCEPT until we resolve your problem...

 

Tom

Customer: replied 3 years ago.

No this is not a new install. Please read my third response of this session.

Expert:  Tom replied 3 years ago.

Ok..I read them all....possible the leads got switched when you changed the board or something changed when they changed the building transformer?

 

Tom

Customer: replied 3 years ago.

The board has only L1 and neutral terminals and L1 is connected to the "hot" line and neutral is connected to "neutral" (the white wire).

 

Expert:  Tom replied 3 years ago.

Ok...well I would try switching them anyway....but I'll opt out maybe someone else has an idea...

 

Best Regards...Tom

Customer: replied 3 years ago.
OK
Expert:  Billy replied 3 years ago.

Resistance will have nothing to do with the sensor operation. It is based on the microamps delivered in flame rectification. The meter must be attached in series with a sensor in flame.....then the micro amps can be determined.


 


Each control can be a bit different ranging from from .05 microamps to 5.0. Here is a handy test chart with different manufacturers ratings http://hvac.amickracing.com/Furnace%20Info/Flame%20rectificatoin.pdf


 


The ground to the unit as well as the ground to the ignition control is critical when reading microamps.

Customer: replied 3 years ago.
Am I down to where a dog is answering my questions? Ha ha. I think finally installing a new flame sensor kind of proves that it is not the sensor that is the problem. Any other ideas?
Expert:  Billy replied 3 years ago.

Hello,

 

the dog is the one that answered your initial question correctly..How to test the flame sensor? I found it strange to test the flame sensor when you said no flame appears. Sensor is irrelevent without flame.

 

I think you would be better suited to get info here -

https://www.rezspec.com/catalog-udas.html

 

The dog also is supplying factory direct info here on your unit...go to udap installation and then troubleshooting.

https://www.rezspec.com/index.php?pageid=000000000014&mod_files[currpos]=2

 

 

I also hope you got that Reliance Water heater fixed back in May

 

 

 

 

 

Customer: replied 3 years ago.
I think you have mis-understood what has transpired with earlier techs - no flame has never been an issue. Please read over the session.
Expert:  Billy replied 3 years ago.

 

There is no need.

 

Your issue is grounding.

 

New board

new sensor

no XXXXX XXXXXght as you say...

 

Pressure switch would not allow progression, as well as , any other precondition safety.

 

This is a perfect case of eliminate what it isnt....I also will be opting out and wish you luck in your project. Once you get 120v, neutral and proper ground in place the components will react accordingly.

 

Also test that sensor the way I mentioned to affirm.

 

 

 

 

Billy, HVAC Technician
Category: HVAC
Satisfied Customers: 19777
Experience: Endorsed for unlimited heating, cooling, oil burners, boilers, refrigeration, hydronics
Billy and other HVAC Specialists are ready to help you
Customer: replied 3 years ago.
Well, I am happy to say you were right about the voltage being turned around. But apparently it was cross wired all along and when the new xformer was installed they did it right because come to find out the curcuit the heater is on had been crossed so the heater would work. Thanks
Expert:  M Oglesby replied 3 years ago.
I have opted out.

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