furnace issue

ptbietz

furnace is a NUGK100DH11, has a S8600M ignition.
I'm going thru troubleshooting guide for the ignition...
Have spark at igniter.
Next step voltage across PV-MV/PV terminals. Meter is bouncing from 0 to 14vac. Should this be bouncing or a continuous 24v?

Might be a coincidence, But i had gas company out a few weeks prior to fix an outside leak. All other appliances are fine


Thanks for any helps and replies

EBEBRATT-Ed

voltage must be steady 24 v. disconnect the wire and then check the voltage. The valve could be bad. If you could put a temporary 24 volt load on the wire that would tell you if something on the supply end is bad. Could be the S8600M as well

109A_5

Hello @ptbietz,
Even though it is producing a spark, I would verify the 24 VAC to the S8600M is relatively steady (Red annotations) and there are no loose connections all the way back to the transformer. If the power to the S8600M is compromised expecting the output to the Gas valves to be correct is silly. And as @EBEBRATT-Ed stated, if the Gas valve is defective and overloading the S8600M and/or the transformer the Voltage to the Gas valve may do abnormal things.

EdTheHeaterMan

The PV 24v power to MV/PV which is Common is what sends power to the gas valve. It should be 24 voltish constant. 14 volts is not enough to operate the pilot valve solenoid in the gas valve. Check the voltage with the from the PV terminal to the MV/PV terminal with the wires disconnected. If you get 24 Volts, then your gas valve may be defective. If you get 14 volts then the control may be bad. To check if the control is bad you will want to test the power into the control where the 24 volts is connected to the S8600M when there is a call for heat.

The reason you want to check the voltage going onto the S8600M control is to see if the limit control, fan control, transformer or other control is defective. You see, if you only put 14 volts into the S8600M you can't make 24 volts come out of the control. That control does not make 24 volts... it only switches it.

ptbietz

got it
thanks guys
i'll check tomorrow. got company a beer now. shoot

ptbietz

I took voltage at 24v / 24gnd on the m8600M and was getting nothing.
Follow the 24v line to the induce pressure switch since it can shutdown the 24v to S8600M from transformer.
Disconnected and plugged back in and got flame.
My next step would been check transformer.

Thinking about this today and my best strategy would be, follow the 24v line in.
Then think about hardware failure.

I'm thinking dirty terminals. I gave it smack with screwdriver and cleaned terminals.

Thank you for quick replies

109A_5

Hello @ptbietz,

Yes, bad connections, pressure switch, system relay, thermostat, fan control, a few limit switches. Many things in series with the R transformer terminal before the 24 VAC gets to the S8600M. Resist the urge to wiggle stuff, do strategic tests with the meter. The transformer output 24 VAC should be verified very early in the troubleshooting process.



https://f01.justanswer.com/ServiceTechRus/cc203482-688c-4c66-9bf3-60db28e527ee_NUGK+SERVICE+MANUAL+PART+2.pdf

EdTheHeaterMan

ptbietz said:

I took voltage at 24v / 24gnd on the m8600M and was getting nothing. was this during a call for heat?
Follow the 24v line to the induce pressure switch since it can shutdown the 24v to S8600M from transformer.
Disconnected and plugged back in and got flame. getting flame means that the ignition worked and the valve opened so the S8600M is fine
My next step would been check transformer. Getting flame also means the transformer is good

Thinking about this today and my best strategy would be, follow the 24v line in.
Then think about hardware failure.

I'm thinking dirty terminals. I gave it smack with screwdriver and cleaned terminals.

Thank you for quick replies

The sequence of operation will tell you where to look. There are things that happen with a call for heat in a certain order to prove that it is save to make a flame. if one of those things as not happening, then NO FLAME

If @109A_5 selected the correct wiring diagram for your system, then the sequence of operation should look something like this:
Before a call for heat can be successful, the limit control, the flue temperature switch and the spill switch all need to be closed as if the furnace is at room temperature. Only then can the R from the transformer get to the R on the thermostat. Then on a call for heat, R closes to W in the thermostat. This will send 24 volt power to the System Relay terminal # 8. The return path for that relay coil is #7 back to the C on the transformer. Step one completed
At the same time as the system relay coil is energized, W from the thermostat also sends power to Terminal # 6 on the system relay that has normal open contacts to terminal # 4 where that power is now at the pressure switch which is normally open. The system relay contacts connected to terminal #3 and #5 also close powering up the exhaust blower. So as far as you are concerned on a call for heat the system relay turns on the exhaust blower.

Next step, the exhaust blower will cause the pressure switch to close as long as the vent or the flues are not blocked. Only after the vent pressure switch proves that the venting system is clear, will the S8600M get 24 V power to the 24V terminal. The 24V (GRD) is the return path to the 24V transformer. Then the spark will start, and the pilot valve can open

So call for heat. Then the blower starts, then the ignitor will start to spark and the pilot valve will open.

When the S8600M gets 24 V power the PV terminal also gets power when the spark is happening. The gas valve pilot solenoid will open the pilot valve. You should get a pilot flame at this point. Once the pilot flame is sensed by the flame sensing circuit in the S8600M, the main valve MV terminal in the S8600M will energize and send power to the gas valve main valve solenoid. When this happens your call for heat is completed and the main valve will stay open until the call for heat is satisfied, or a limit switch opens or the flame sensor no longer senses the flame or the flue vent pressure switch opens as a result of a bad fan or a blocked vent.

Assuming none of these things happen, after a short time the fan switch will close as a result of the heat in the furnace or plenum, the Furnace Blower will start and circulate heat into the ductwork.

Once the home thermostat reaches the set point temperature the thermostat will satisfy and the R and W connection in the thermostat will open stopping 24 V power to the gas valve, S8600M control and the System relay, and the burner will stop. Once the plenum temperature drops to a set temperature, the furnace blower will stop.

Summarize: Call for heat from thermostat >>> System relay >>> Exhaust blower >>> S8600M powered >>> Spark >>> Pilot >>> Main burner >>> Furnace Blower >>> Thermostat satisfied.

If you follow the sequence of operation and see where the trouble happens, that will eliminate looking where the problem is not and you can focus on where the problem is.

109A_5

EdTheHeaterMan said:

If @109A_5 selected the correct wiring diagram for your system, then the sequence of operation should look something like this:

ptbietz said:

furnace is a NUGK100DH11

EdTheHeaterMan

109A_5 said:

EdTheHeaterMan said:

If @109A_5 selected the correct wiring diagram for your system, then the sequence of operation should look something like this:

ptbietz said:

furnace is a NUGK100DH11

Agree
Do you concur with the Seq. of Op.

109A_5

Hello @EdTheHeaterMan,

As much as I went back and reviewed what you wrote about the sequence of operation, it seems good. And yes, the furnace operation can verify that significant parts are working correctly, those parts and the power to them are probably fine. Although I probably would verify the transformer 24 VAC is good under load. So as loads are accumulated by the operational sequence a power supply issue is not overlooked. Relay and/or solenoid hysteresis behavior may confuse some folks if the power is marginal.

I assume this may be a typo, I knew what you meant but others may be confused. Maybe "can't" instead of "can" ? So I think you meant, You see, if you only put 14 volts into the S8600M you can't make 24 volts come out of the control.

EdTheHeaterMan said:

You see, if you only put 14 volts into the S8600M you can make 24 volts come out of the control. That control does not make 24 volts... it only switches it.

EdTheHeaterMan

@109A_5 was a typo. i corrected it. Good catch.