Furnace dead. NT-34 suburban

[CapnDirk]

Replaced the thermostat when I first got it a few months ago.  Furnace has been working fine as I worked on the inside issues until two days ago.  I've read Dave's nice post about furnaces (thanks again Dave) and read the manual which outlines the series of events on startup, as well as some UTube videos on furnaces.


If I'm reading everything correctly when the thermostat is turned on the fan will engage in about 10 seconds then more stuff to ignition point.  My furnace will do absolutely nothing.


A note of something that now makes me suspicious of the delay relay.  These few months that it has been running, it would take a good 3 plus minutes from thermostat on, to fan motion then normal operation.


Thanks

[DaveVA78Chieftain]

1) Have you downloaded the Surburban Dyna-Trail NT Series manual from our members area?  Has a full description of circuit operation.
2) The Time delay relay is primarily used to keep the fan motor running for 3 - 5 minutes to allow the furnace to cool down once the Thermostat turns off (requested temp reached).  The module board performs the 12 - 18 second delay on start up to allow the fan to come up to speed AND purge any residual gas in the burner area to prevent an explosion.  However, the time delay relay must turn the blower motor on so that air speed gets up to 75% flow in order for the sail switch (air flow sense) to close so the module board gets the order to start the 12 - 18 second timer. Hmm, did I say that right?  Anyway, thermo ON -> Time Delay relay on -> Blower ON -> over temp switch closed -> sail switch closes -> module powered on (12 -18 sec delay) -> gas on -> ignitor pulse -> burner light -> ignitor sense (shut down if no sense) -> generate heat -> thermo off -> time delay relay count down (blower keeps running) -> countdown end - > blower shut down cycle end

[CapnDirk]

Thanks Dave.  I did already read that manual and much of your posts on furnaces.  What I was trying to confirm was that with the thermostat turned up the fan would immediately kick in.  I believe from there it then runs for a number of seconds to purge and set the sail switch THEN the board kicks in the gas and ignites.


So since there is no fan coming on, can I assume that it would either be a bad relay, or lack of 12V supply to it?


Dirk

[DaveVA78Chieftain]

Based on the schematic in the manual (pdf page 16), 12VDC is routed to thermostat and relay.   When thermostat turns ON, the relay is energized supplying power to fan motor.  So real simple circuit.  Either bad thermostat, relay, fan motor, or wiring.  Drawing is color coded for wires. 

Dave

[tmsnyder]

or blown fuse

[CapnDirk]

Dave:  You inadvertently gave me a reality check.  While I did have the furnace manual, there are two in our library that say NT models.  I had the wrong one.  Correct one gives a bit more detain and the schematic that the first lacked.  At this point everything indicates that with a good 12V supply the fan should kick with thermostat up.


Fuse is good, and thermostat new on a furnace that has worked for 3 months.  Thermostat up results in not a single click grunt, pop or poop.  I was going to replace the Formica on the galley counter, and the oven is over the furnace.  I'm going out now to pull the range which starts that project and exposes the furnace.

[DaveVA78Chieftain]

Dave:  You inadvertently gave me a reality check.  While I did have the furnace manual, there are two in our library that say NT models.  I had the wrong one.

In the world of RV's, one must remember there as many possibilities as there are dreamers.  tis why I have a huge reference library of info.

[TerryH]

  tis why

[M & J]

Tis why we oftentimes call him Super Dave.

[TerryH]

And very well deserved!

[CapnDirk]

I'm getting Tizzy 


Well, a fly in the ointment.  Replacement time delay relay part number from multiple manuals indicates a 231202 part number including info from suburban.  Multiple purchase sites on the web say that's a 24V relay.


Anyone replaced one?  Model NT34-SP


Thanks

[EldoradoBill]

The modern replacement board 521099 replaces the (obsolete) separate relay and eliminates the separate flame sense wire. It also retries lighting the burner without having to manually cycle the thermostat. I replaced my board and there is a patch harness 520839 you will need as well. Easy install and works well, so far.

[CapnDirk]

Thanks Bill.  Are you suggesting replacing the board as opposed to the relay?  I understand the logic (eliminate the relay), but that's $100 for the two items versus $35 for the relay.

[DaveVA78Chieftain]

I think I would wait until you get the furnace out and opened up before ordering anything.

Time Delay relay



or conversion approach that eliminates the time delay relay
Service Manual
Your original diagram on PDF page 31.
Diagram of new design on PDF page 33.

520839 wiring kit
521099 Control Board

You may find better prices so search by P/N

[CapnDirk]

Thanks Dave.  I already have it out apart and confirmed it is the relay.  If you blow up the picture of the time delay relay you posted you'll see it says 24 volt.  There are ones on eBay where the ad says they are 12 volt and one ad that shows multiple pictures shows it in the box that also says 24 volt.  Not sure what to make of it when the Suburban part number keeps bringing up a 24 volt part.


Your link to the Parts at PDX RV I had already investigated and found this (Same place, new web site).


http://pdxrvwholesale.com/products/suburban-furnace-time-delay-relay-48000


If you read the description it references the 231202 part number in bold letters as being for something else.  Not sure which way to go.


Both these links also reference the 231201 part number for my model.


http://www.sahararv.com/fckimages/pdf/586-590_Parts_Suburban.pdf
http://www.ourvanrv.com.au/files/Suburban_RV_Appliances_Parts_Manual.pdf


Thanks for your continued help everyone.

[EldoradoBill]

Thanks Bill.  Are you suggesting replacing the board as opposed to the relay?  I understand the logic (eliminate the relay), but that's $100 for the two items versus $35 for the relay.

A replacement board eliminates the separate relay and adds retry feature. If and when your present board fails you will eliminate the stand-alone relay anyway as it is now incorporated in the board. So, I guess you need to ask yourself if it's worth $65 to not have to pull the furnace anytime soon?

[CapnDirk]

Point taken,  I wonder if I could find wiring info on that board.  The harness looks like a few male/female terminals, something easily done with materials I have.

[EldoradoBill]

The harness is a $5 add-on though Amazon. But you can just reuse the old wiring, move wires in the board connector. I'd help but my diagram is tucked in with the furnace so I can find it when I need it. It's posted on line, however, probably on the Suburban site, it comes with the board.