Furnance /Bench testing

[nazoomer]

I have NT-30K suburban furnace in 89 dirfitwood. Fan will run when thermostat is set to call for heat but no spark. Tried many times on different days. Finally got nerve up to pull furnace today. Tested sail switch with ohm meter, it closes circuit when pushed shut. Put 12 vdc to red wire (+) and neg to yellow. Fan would not run, jumpered the blue thermostat wires still no fan. Can anyone help with how to test system on the bench without thermostat? I am preparing to order the dinosaur board just because but thought one of you experts may have a solution! BTW I did read the Bryant PDF, and wonder how anyone tests current or voltage when it is installed? I could never get to any of the terminations in the cabinet it is in??? Happy New Year

[johnsonjm209]

Had a similar problem w/my furnace last year. Called a mobil tech. I had used him before. I was quite a way from his location. He recommended that I take the board out and bring it to an RV Place about 10 miles away. He told me that they can put it on a test board and determine if it needed replacement. I did, they did and it ran a poor test. I got a new one, put it in and everything worked fine.
Maybe there is an rv place near you that sells parts and has a similar tester. It saved me a lot of time and frustration.

[FreshAir]

No expert here. Are you sure your battery was charged enough to spin the fan fast enough to transfer the sail switch with air? A common problem worth considering.
Back to you on that Happy New Year.

[bob109]

Quote:

Originally Posted by nazoomer I have NT-30K suburban furnace in 89 dirfitwood. Fan will run when thermostat is set to call for heat but no spark. Tried many times on different days. Finally got nerve up to pull furnace today. Tested sail switch with ohm meter, it closes circuit when pushed shut. Put 12 vdc to red wire (+) and neg to yellow. Fan would not run, jumpered the blue thermostat wires still no fan. Can anyone help with how to test system on the bench without thermostat?

Check out the following files which contain service manuals for Suburban Heaters
Service Documents

[George Schweikle]

I had furnace problems, and finally took the circuit board to our local dealer for testing. The board tested bad, and a new board solved the problem.

Quote:

Originally Posted by johnsonjm209 Had a similar problem w/my furnace last year. Called a mobil tech. I had used him before. I was quite a way from his location. He recommended that I take the board out and bring it to an RV Place about 10 miles away. He told me that they can put it on a test board and determine if it needed replacement. I did, they did and it ran a poor test. I got a new one, put it in and everything worked fine. Maybe there is an rv place near you that sells parts and has a similar tester. It saved me a lot of time and frustration.

[nazoomer]

I think the batteries are fine, I just charged them but good thought. I have called 3 local dealers to have the board tested. All are off for the week. Thanks for the responses.

[Dave78Chief]

Your Theromstat is nothing more than a temperature controlled On/Off switch. Referring to the wiring diagram on pdf page 16 of http://bryantrv.com/docs2/docs/ntseries.pdf , +12VDC from the battery is applied to the red (R) wire and ground (-12VDC) is applied to the yellow (Y). +12VDC is routed (disconnect block connection to red wire) to the thermostat on the Blue (BL) wire. When the thermostat is turned ON, the +12VDC blue wire signal is routed back to the furnace over the other blue wire to the Limit switch. That signal is also applied to the Time delay relay "coil" via the brown (BR) wire. Return path back to ground for the time delay relay "coil" circuit is the green wire to the grounding terminal then the yellow (Y) wire back to -12VDC (ground).
Energizing the Time delay relay coil allows +12VDC voltage to be applied to the blower motor via the red wire (R) across the time delay relay contacts to the blower motor via the other red wire (R). When the blower motor gets up to speed, the force of the air moves the flapper on the sail switch allowing it to close (turn on). This allows +12VDC to flow from the Limit Switch through the sail switch to the control module Printed Circuit Board (PCB) over the brown (BR) and red (R) wires. This is also the power source for the Control board. The control board waits about 30 seconds before sending +12VDC to the gas valve (Brown wire BR) in order to allow the combustion chamber to be purged of any residual gas (prevents an explosion). When it does send the signal to the gas valve it also ends the high voltage signal to the electrodes (Black wire *BK). The control board monitors the flame sense wire, the other Black wire (¤BK). If the control board does not see flame sense within 15 seconds in shuts the gas valve off then retries the lighting sequence 2 more times before going into lockout mode.
You monitor signals (except HV) along the way using a voltmeter to ensure they happen at their applicable time during the process. For HV, you just verify you are getting a spark. Flame sense is a very small millivolt reading. Proper adjustment of the electrodes is critical to both getting a spark as well as flame sense (see manual).

Dave

[nazoomer]

Thanks for such a detailed explanation! I had the board tested and it is bad. I ordering new tomorrow.

[retiredtech]

Quote:

Originally Posted by Dave78Chief Your Theromstat is nothing more than a temperature controlled On/Off switch. Referring to the wiring diagram on pdf page 16 of http://bryantrv.com/docs2/docs/ntseries.pdf , +12VDC from the battery is applied to the red (R) wire and ground (-12VDC) is applied to the yellow (Y). +12VDC is routed (disconnect block connection to red wire) to the thermostat on the Blue (BL) wire. When the thermostat is turned ON, the +12VDC blue wire signal is routed back to the furnace over the other blue wire to the Limit switch. That signal is also applied to the Time delay relay "coil" via the brown (BR) wire. Return path back to ground for the time delay relay "coil" circuit is the green wire to the grounding terminal then the yellow (Y) wire back to -12VDC (ground). Energizing the Time delay relay coil allows +12VDC voltage to be applied to the blower motor via the red wire (R) across the time delay relay contacts to the blower motor via the other red wire (R). When the blower motor gets up to speed, the force of the air moves the flapper on the sail switch allowing it to close (turn on). This allows +12VDC to flow from the Limit Switch through the sail switch to the control module Printed Circuit Board (PCB) over the brown (BR) and red (R) wires. This is also the power source for the Control board. The control board waits about 30 seconds before sending +12VDC to the gas valve (Brown wire BR) in order to allow the combustion chamber to be purged of any residual gas (prevents an explosion). When it does send the signal to the gas valve it also ends the high voltage signal to the electrodes (Black wire *BK). The control board monitors the flame sense wire, the other Black wire (¤BK). If the control board does not see flame sense within 15 seconds in shuts the gas valve off then retries the lighting sequence 2 more times before going into lockout mode. You monitor signals (except HV) along the way using a voltmeter to ensure they happen at their applicable time during the process. For HV, you just verify you are getting a spark. Flame sense is a very small millivolt reading. Proper adjustment of the electrodes is critical to both getting a spark as well as flame sense (see manual). Dave

What is going on with this Suburban NT30SP' Turn T-Stat up Furnace come on and heats,but will no initiate relight when temp falls.must move T-stat to off then back to temp and unit works but won't restart when temp falls,have replaced t-stat twice?

[wa8yxm]

Someone says I missed a step or two but ... I just spent quite a bit of time toubleshooting my furnace, wound up with a new Dino board and now it works 100%.

When the T-stat calls for heat the control board fires up the fan (or the T-Stat does) once the fan is up to speed and air is moving in the proper (Well minimum) amount a sail switch should close.... There is a test point on the control board where you can monitor the voltage and see when that switch closes. IF IT CLOSES.

Then the control board sends power to the gas valve and ignightor. And, hopefully, ignition happens. (You did say it was not happening) .

Since you said it's not ignighting, I suggest you stop reading here.


Once ignition happens, we have options depending on the model. ON MINE, the ignightor continues sparkign for a short period of time (Seconds) and then the board switches mode and the very same wire that had a thousand volts or more AC on it now has a few millavolts DC on it (Perhaps 1/2 to less than a volt) This is in my opinion a very poor design but at least DInosaur boards has included some protection for the low voltage detector that Attwood left off.

On the other kind of board there are 2 wires, one for high, one for low, voltage and the detector works independent of the sparker.

IF. the flame sensor is hot, and the board is working, operation then continues till the T-stat says "ENOUGH ALREADY" and then it enters it's shutdown sequence which I won't go into.

In my case everythign worked save the board's low voltage sensor chip was fried. So though the flame sensor was hot enough, and sending the proper voltage to the board, the board was ignoring it and shutting the system down after 7 seconds.

By the way.. I am a certified electroincs technician (or so says some wallpaper I used to know the location of) ... And based on the "Look and feel" that Dinosaur board is much better than the OEM. And not as expensive either.