Micro Lite 21 FRBS Furnace Fan Runs Continuously

[NeilandHeather]

Here's a tough one I think. The furnace fan has started running continuously. RV is 2011 21' Micro Lite FRBS. Furnace is a Suburban NT20S. The thermostat is off, not an aftermarket. With main power connected and furnace circuit breaker tripped, the fan still runs but much slower as it seems to be drawing power from the battery. If I disconnect the battery, the fan stops. There are no blown fuses. Really appreciate some input as this is a strange one. Thanks.

[dward51]

Check the wires behind the thermostat and where those wires connect to the furnace. It sounds like one wire may be a little long and with the vibration of the trailer moving, may be touching the other so the furnace thinks the thermostat is calling for heat. It should work on 12v like you described as that is how they are made to work if that connection is complete, which normally is what the thermostat does (with no 110v shore power it should still work on 12v).

If you don't see anything obvious there, I would complete disconnect the wires from the thermostat at the heater screw terminals. If it still wants to run, you have a problem inside the furnace control unit in the furnace. Take a photo with your cell phone of the wires hooked up before you take them off. That way you have a reference to which color wire goes to which terminal when hooking them back up. Also I would make double sure the 110v power is disconnected and disconnect the battery before touching any wires on the furnace side. Some of those are 110v and can be dangerous.

[NeilandHeather]

Thanks for the advice. I disconnected the two main wires at the thermostat as suggested. The fan still runs. Looks as though the fan module board inside the furnace itself is bad. In the manual, the part number in the parts list appears to be 520820.

I can see a few of these available on Amazon. Just not sure if that's a trustworthy choice. Any feeling about that?

Also, it is a big deal removing the furnace to get at the board? The wires look as though they need to be cut and reconnected/crimped. The gas disconnected too. I'll check the manual too. Or could I get at it from the side without removing the furnace by removing the wood panel between the furnace and lower cabinet beside it. Thoughts?

[TitanMike]

Originally Posted by NeilandHeather

Thanks for the advice. I disconnected the two main wires at the thermostat as suggested. The fan still runs. Looks as though the fan module board inside the furnace itself is bad. In the manual, the part number in the parts list appears to be 520820.

I can see a few of these available on Amazon. Just not sure if that's a trustworthy choice. Any feeling about that?

Also, it is a big deal removing the furnace to get at the board? The wires look as though they need to be cut and reconnected/crimped. The gas disconnected too. I'll check the manual too. Or could I get at it from the side without removing the furnace by removing the wood panel between the furnace and lower cabinet beside it. Thoughts?

If your furnace is anything like the one in my 25BDS you may not be able to access electric connections and circuit board without actually removing unit.

I think mine is an NT-20 ? And wiring goes to vent/exhaust end which is against the outer wall.

Gas is the easiest to disconnect as it's right in front of you when you open return air grille (again, on mine).

I looked mine over for the likelihood of future service needs. It will be difficult to remove.

[Fergynb]

This happened to me a couple weeks ago following a few days of heavy rain. I checked this forum, and found a reply suggesting that moisture in and around the furnace box may short out the wires and cause the fan to run continuously.
I pulled the fuse to stop the fan and left it for a few more days. After some drying time, I replaced the fuse and resumed normal operation.
I suppose a blow dryer may speed the process if necessary...

[Less is More]

Mine did that while the trailer was under warranty. The dealer replaced the circuit board. First time out we had the same problem. Dragged it back and they replaced the sail switch. Works fine now.

[Larry-NC]

Originally Posted by TitanMike

If your furnace is anything like the one in my 25BDS you may not be able to access electric connections and circuit board without actually removing unit.

I think mine is an NT-20 ? And wiring goes to vent/exhaust end which is against the outer wall.

Gas is the easiest to disconnect as it's right in front of you when you open return air grille (again, on mine).

I looked mine over for the likelihood of future service needs. It will be difficult to remove.

If the NT-20 is like the Suburban SF series (lots of illustrated installation and repair manuasl readily found online), it's much easier to access than it looks.

For an SF-series,
--Turn off the propane
--Take off the grille
--Turn off the propane (check again)
--Disconnect propane at the furnace flare nut. (May need to use a crowfoot socket)
--Remove the plenum front cover (one or two sheet metal screws at the top)
--Remove the firebox anchor screw at the bottom center
--There are four wires going to the furnace. Either they will be long enough to allow you to pull it out, or there's a plug-jack to unplug
--Pull the firebox out of the plenum
--All of the circuit boards, motor, sail switch, etc. are accessible
--You can even run the furnace (without propane) when it's out.

[5picker]

Originally Posted by NeilandHeather

Here's a tough one I think. The furnace fan has started running continuously. RV is 2011 21' Micro Lite FRBS. Furnace is a Suburban NT20S. The thermostat is off, not an aftermarket. With main power connected and furnace circuit breaker tripped, the fan still runs but much slower as it seems to be drawing power from the battery. If I disconnect the battery, the fan stops. There are no blown fuses. Really appreciate some input as this is a strange one. Thanks.

First... what is this 'circuit breaker' you are turning off for the furnace?

The furnace is 12v. It has no circuit breaker but it does have a fuse.

Often times pulling the fuse and cycling the power to the furnace can reset the control board.

And yes, a bad control board will make the fan run constant.