Furnace Issue - Suburban NT-30 won't light

[tmsnyder]

There's a 'sail' switch too, which senses that the burn air is moving through the unit.   Those are prone to sticking and it won't release gas and try to ignite if that switch doesn't work.

[DaveVA78Chieftain]

Based on what I have read here, there is a gas delivery issue.

• When turning the Propane system on at the tank, turn the gas on very slowly.  There is a safety valve in the propane tank that is used to drastically reduce the flow from the tank to the regulator if it senses a large volume of propane is discharged from the tank.  This is safety feature where if the RV is in an accident and the propane lines are ruptured, the gas flow is greatly reduced.  If the safety valve is triggered, to reset it, you have to turn the tank supply OFF for 60 seconds and then very slowly open the tank supply valve.
• If the propane system in the RV has been opened up (e.g. furnace removed for service), then it is best to purge most of the air out of the system using the burners on the stove prior to attempting to light the furnace.  Even so, it my still take multiple attempts to start the furnace in order to purge the air from the furnace gas lines
• Gas nozzle is clogged up not allowing gas into the combustion chamber.  Use compressed air to clear out.  Best not to use any sort tool to clear it as it could increase hole size resulting in to much gas into combustion chamber. Replace nozzle if not able to clear.
• Oil from propane can build up in supply lines resulting in clogged lines and valves.  Maybe able to clear with compressed air, otherwise, replace lines.

Please note:
You would NOT get ignitor spark if:

• Limit switch was open
• Sail switch was not operating properly

[moonlitcoyote]

Thank you Dave, we know about the safety on the propane bottle. We have run the burners on the stove to purge air. You may be on to something with the gas lines maybe being clogged. The lines in the furnace are ok, we did blow through those. BUT maybe there is a problem between the bottle and the furnace. Now to go blow air through that one. Will update if it worked.

[moonlitcoyote]

Well, I thought you could be on to something until I got in there and realized my supply line also supplies my wave 6 heater and it runs fine. And to be clear.. the wave 6 heater is connected to the supply line 1 inch from the furnace connection.


So to summarize: I have spark... Sail switch was checked and working, ignitor/ thermocoupler (whatever its called on this) was replaced, gas valve was replaced, limit switch was ordered( dont believe it's a problem though) No nests or clogs in furnace... Sparks 3 times before going into lock out due to non-start.

Everything seems to work PERFECTLY except that it wont light..

[DaveVA78Chieftain]

NT-30 Parts Drawing

Then my only other thought is that the Main Burner Orifice (item 49), Manifold Pipe (item 52), Gas Inlet Pipe (items 54, 57 - 60), or Gas Burner (item 48) is/are clogged up preventing the gas to get through.  Spiders love to get in those areas and nest over the summer.

[moonlitcoyote]

Dang it! Those have all been cleaned and blown out. We have literally gone through everything. We have taken it out about 10 times now. The only other thing we can think of is "gas pressure"

Is there any possible way that the furnace needs more pressure than the wave 6 heater? We did blow through the supply line just to be sure it wasn't partially clogged, (guess it could still be partially clogged) But our only other thought is maybe the pressure regulator on the tank? What do you think? It's going to be 27 degrees here tomorrow night and I dont think the wave 6 heater is going to keep us toasty.

[DaveVA78Chieftain]

To test the gas pressure you will have to make a DIY Manometer.  Manual says your supposed to have 11" Water Column.

The only other thing I can think of is the control board is not sensing the initial lighting however, you indicated that you do not hear/see any evidence of a flame (just hear/see spark) which is why I did not mention it before.

[moonlitcoyote]

You are correct,  I watch the sparking and it never lights.

[moonlitcoyote]

Here we are a year later and still have no heat.

Recap: Furnace runs but wont light. Sail switch is good, new gas valve, new limit switch.

Now, in all my testing today I found the problem (sort of) If I apply 12 volts to the gas valve (both sides) when the HV wire is creating it's spark then the furnace lights right away.

There is 12v getting to the plug on the side of the furnace (checked and verified).

I spoke with someone at Dinosaur boards and I was told that 12v must get to the gas valve, so I attached my voltmeter to the leads that attach to the gas valve. When the valve "clicks" open the volts jump to about 5-6 volts (happens too fast to see an accurate voltage)  then drain down to .09 Volts where it stays until the furnace gives up trying.

I have looked at a price for a new board so I can replace it IF that is the only way. Now I have 2 questions..

#1 Where can I check for my missing voltage? Bad or loose wire or something?

#2 My board doesnt seem to have a connector that you pull off the pins. So I was wondering if anyone has changed one of these and could tell me how you disconnect it.

[Oz]

Do you have the manual for this?  It may help a lot. Check in the member area. We have NT furnace manuals.

[moonlitcoyote]

I have read through the manual, thank you. The manual does state that if there is "no" voltage at the gas valve to replace the board. I do have some voltage but not enough. I just want to be sure that there isn't something I can check before paying over $100 for a new board just to end up with the same problem.

[Oz]

Trace the wire back to the source. Check it before where it connects. If it's good there, you know it's the board.

[AOBrodie]

Lower than anticipated voltages can be caused by the ground side as well. Trace back the ground that is supplied to the board if you haven't done it already.
You said you can open the gas valve with 12v. Rig it up so you can light the burner manually at the appropriate time, and see if it will sense the flame and keep running after you remove your 12v. I bet it shuts off because the board can't keep the valve open.

The board thinks it is turning the valve on, but not detecting a flame, so gives up. The board is satisfied with the order of operations all the way up to flame detection.

Sure seems like it's the board.

[moonlitcoyote]

You are correct, it does shut down as soon as I remove the 12v to the gas valve. But that still only shows the valve isn't receiving enough power to stay open. I am going to take it back out and trace the ground wire and check for any loose or corroded connections before I order a new board. Mainly because I need to find a part number for the board. And I still don't know how to disconnect the board.

[TerryH]

Hello MJ. Sorry to hear you are still fighting this issue. At this point I would agree you have it down to a board problem. Should you decide to replace it I would highly recommend you go with a Dinosaur Electronics Board. Their products are considered to be far better than OEM.

http://www.dinosaurelectronics.com/
They do not sell direct, but provide you with a list of dealers, and prices are comparable or better than OEM. Their Customer Service was excellent when I dealt with them some 3 or so years ago. If you provide them with proper furnace and board info they will tell you exactly what replacement is required.
The board number is usually on the back, board generally fastened with 4 screws into stand – offs on the board.
Wires are generally pin connectors to the board.
Best of luck.

[Oz]

Spot on, thanks, Terry!

[DaveVA78Chieftain]

Assuming you have a NT-30SP model furnace (Fan Control Module Board), as described below, there should be 12VDC +/- 1VDC applied to the gas valve throughout the entire 7 second ignition cycle period (ignitor running).  7 seconds is plenty of time for a volt meter to read the value.  If flame is detected, the board will leave the gas valve ON until thermostat shuts OFF.

Gas valve IS NOT on during Purge cycle.

If the Control Board is bad, Dinosaur is the way to go.  Just make sure to get the correct model.



Figure 28: NT30-SP Furnace Wiring Diagram

[moonlitcoyote]

Thank you Dave, I guess I just need to purchase a new board and try it. The only reason I hesitate is because the gentlemen at Dinosaur Boards said there could be a wiring issue that would cause the voltage drop. Oh well, time to just throw more money at it.

[AOBrodie]

Put your meter across the + and -12v at the (supply) connector and try it. If the voltage stays up, it's not the wiring to the board.

[DaveVA78Chieftain]

1.  Connect the NEG lead of the voltmeter to a ground point.

2.  When the Thermostat is ON, you should have 12VDC +/- 1 VDC at all points labeled as 1 (fan comes ON also)

3. When the fan gets up to speed, the sail switch closes and you should 12VDC +/- 1 VDC at all points labeled as 2 (start purge cycle to evacuate any excess propane in combustion chamber to prevent an explosion)

4. 15 Seconds after you get voltage at 2 , you should get 12VDC +/- 1 VDC at the gas valve (turn ON) and high ignitor voltage (do not measure ignitor voltage) at electrode (7 second ignition cycle).
Warning: do not measure ignitor voltage

Please note that the single letters on the drawing represent wire color (Y=yellow, BL=Blue, BR=Brown, R=Red, G=Green, O=Orange, BK=Black)

The drawing reflects a common grounding point that all components connect to (eg: the yellow ground wire from the gas valve).  It also is connected to RV -12VDC.

The gas valve has 2 solenoids in it so the Brown wire from the control board connects in 2 places.  Likewise, the yellow ground wire connects in 2 places also.

Dave