New member. Furnace in 5th Wheel Heartland Bighorn 3670rl stopped working.

[MikeT16]

Hello, the furnace in my 2010 Heartland Bighorn 3670rl stopped working. Fan doesn't come on. Air conditioning and its separate fan work fine. Thermostat had been acting a little weird, so replaced it. But furnace fan still not working.

Got a voltmeter and checked voltage to furnace fan. With thermostat furnace control off, it was zero; with thermostat furnace control on, it was over 12 volts. Wondering if this means that the fan motor is definitely shot or if something else might still be wrong.

Also, if it is busted fan motor, wondering how hard it is to replace that. Supposed to be 28 degrees here Friday night so trying to get it fixed before then so hoses don't freeze. Any help greatly appreciated.

[lynnmor]

If there is voltage supplied to the motor and it doesn't operate, then it must be bad.  You didn't say what brand or model of furnace, but the fan motors can be a pain to replace mainly because of access.  Search YouTube for some help.

[Gary RV_Wizard]

The thermostat merely signals the fan to start, and the fan uses the furnace's own 12v connection for the actual power.   You should be able to jump +12v to the fan terminals and the motor should turn. If it doesn't , it's either a bad ground or a bad fan motor.

Where did you check for the 12v?  The thermostat "heat needed" signal wire is +12v when heat is needed and 0V otherwise.  The fan power wire should be receiving +12v from the furnace circuit board as soon as the thermostat signals for heat.  Once the fan starts & runs, the rest of the furnace ignition cycle gets underway.

I've written an article for the RVforum Library that explains how RV furnaces operate - it may be helpful.

http://www.rvforum.net/miscfiles/Furnace_Trouble-2.pdf

[MikeT16]

Thanks. I came across your article when I started researching this and have it bookmarked. VERY helpful. Don't really know much about maintenance/repair and it was the only thing I found that gave me any idea of how it worked. But I still may not be totally getting it.

When I took access-panel to furnace off, there were two wires plugged in to it that were easily accessible. I just unplugged them and put the red "pen" tip from the voltmeter to red and the black to black. That's where I was getting the readings I mentioned: over 12 V when thermostat is on and set above room temp, 0 when thermostat was off or set below room temp. But, after reading your post, I now realize that I don't know whether those wires I unplugged lead directly to fan or whether they go to circuit board first. Should have mentioned that furnace is Suburban.

[MikeT16]

Sorry, should have said it's a Suburban SF-42 furnace.

Here's a pic of wires to which I attached voltmeter. When thermostat is set to "furnace" and above room temp, voltmeter reads a little over 12 V. When thermostat is set to "off" or set to "furnace" but below room temp, voltmeter reads 0. That, plus the wires' location makes me think they must go directly to fan. But totally new to all this so could be wrong and would appreciate input from folks who have more experience with this stuff.

Other thing I just noticed is that, when I turn the on/off switch located to left (from viewers perspective) of fan off, I hear a click that sounds like it's coming from somewhere behind and above the switch. When I turn it back on I hear another click coming from same place. Think click occurring when switch is turned on is louder than the one occurring when it's turned off, but need to check again. Initially thought that the clicking when I turned switch on didn't happen when the wires in the pic were unplugged. But was wrong and seems like clicking occurs regardless of whether both, one, or neither of the wires is plugged in.

Wondering if that clicking sound is normal. Could it be the sound of some "lock out" mechanism that stops fan from running? Or is it some mechanism that supposed to kick in and has nothing to do with stopping fan?

EDIT: I guess click could be blower motor trying to run and failing. Not sure. Again, any input from people with experience greatly appreciated.

[MikeT16]

Found a post on a Heartland owners forum from 2010. Guy said his fan wasn't working and, like me, he thought it couldn't be the sail switch since that is supposed to stop furnace from igniting after fan has already started. But turned out that in his furnace, control board checks to see if sail switch is stuck before letting fan start. Guy had a 2009 but didn't say make or model; said furnace was Suburban but didn't say model. I have a 2010 Bighorn with a Suburban SF-42 furnace, so seems very possible that my control board also won't let fan start when sail switch is stuck. Trying to find where sail switch is, but not having any luck. Found video of guy showing parts of SF-42 furnace, but they must have tweaked the model a bit as the one he's working on is a little different than mine. Will keep looking.

[TonyL]

I can't say much about RV furnaces, but in a lot of domestic heating units, the modern ones look to see if the fan proving switch has reset before any ignition sequence begins, this ensures that it seems a change in position when the fan starts.

[MikeT16]

After a couple of days of research, I think I've sorted out issue with furnace. But I don't know how to access the problem area.

According to the Suburban Service and Training Manual for my model (Suburban SF-42), the control board first checks to see if sail switch is open (i.e., not getting air) before blower starts. If sail switch is stuck closed (i.e., thinks it's getting air when it's not), the blower won't start. I had connected a voltmeter to the only two wires I could access enough to disconnect and result was they were getting 12 Volts when thermostat was set to "furnace" and above room temp and zero Volts otherwise. Initially thought those wires went to motor. But after studying wiring diagrams and viewing pics of motor, I'm now pretty sure they go to the sail switch.

All that plus two clicks I hear 30 seconds apart when the fan ought to start blowing but doesn't and what the S&T Manual says under "Sequence of Operation" for my model make me think a stuck or completely malfunctioning sail switch is the likely culprit. Unfortunately, I can't access or even see the sail switch. Nor can I access the terminals of either of the wires that I now suspect go to the motor to test it directly unless I cut them. And after A LOT of googling, I STILL can't find anything that explains what I have to do to remove some or all of the furnace so I can get to the sail switch or motor wires.

If anyone knows or can point me to something that explains how to access the sail switch OR the motor wires in a Heartland Bighorn 3670rl with a Suburban SF-42 furnace, would be VERY grateful. Temps drop to 24 tomorrow night for two nights and then a few days after we're supposed to have a four-day-straight spell of nights in the mid-20s. So really need to get this fixed pronto or else figure out a way to keep guts of trailer warm enough so no water lines are damaged from below freezing nights till I sort it out. Also, as interesting as it all is, need to get back to my paying job really soon, lol.

EDIT: Pretty sure sail switch is behind part of casing circled in green in pic below.

[Gary RV_Wizard]

I'll have to research the Suburban operation more thoroughly (I don't have time right now), but that doesn't sound right to me. I think you have misunderstood how the sail switch works and what it does.   The sail switch sits in the air path in front of the fan and literally gets blown closed by the air the fan pushes out.  It is driven by the fan rather than causing it to start. In other words, the sail switch closes to verify that the fan is running and at a sufficient speed to circulate air properly.  The fan runs first, then the sail switch verifies it and that tells the circuit board it is safe to ignite the burner.

[MikeT16]

Fan engaging without sail switch being checked is the normal sequence, Gary, but it doesn't appear to be the way my model's control board works. See attached screenshot from Suburban RV Furnace Service & Training Manual (http://techsupport.pdxrvwholesale.com/wp-content/uploads/2018/12/Suburban-Furnace-Master-Service-and-Training-Manual.pdf).

This is verified by a post I found at another RV forum. (https://heartlandowners.org/archive/index.php/t-13849.html) Though he doesn't mention the specific model, so it could be about another Suburban model that functions like mine. Fan wasn't working in his Suburban furnace, and he eventually discovered:

"It was the sail switch, a mechanical switch inside the blower that senses airflow and allows the furnace to fire ONLY if airflow has closed it. The tech (and conventional wisdom on the internet) concluded that if the fan never started the sail switch was not involved - and maybe for some furnaces this is true. However, on the Suburban furnace in my 2009 the controlled startup is smarter. Before spinning the fan the controller checks to see if the sail switch is OPEN, eg no airflow detected. My sail switch was stuck CLOSED and the controller would not start the fan with the switch closed. Conversely it will not continue the startup sequence if the switch does not transition from open to closed once the blower has run a little."

If you discover any of this is wrong, definitely let me know. But currently not seeing how statement in S&T Manual can be read any other way. Also, if you can direct me to anything that would tell me how to get access to sail switch and wires going into motor, that would be greatly appreciated.

[MikeT16]

Ha! Got the furnace out.
I need to cap the propane line to it so I can turn tank back on for hot water while I'm figuring out what's wrong. I have the right size plug and from what I understand I put it in and then test the seal with soapy water.
But wondering how much I tighten the plug? Seems like I could break line if I try to make it too tight but obviously want it tight enough for seal.

[MikeT16]

Capped furnace propane line and tested it with soapy water. No bubbles, also no smell I can detect. Stove/oven is working fine but for some reason hot water heater won't light and fridge indicates it's not getting propane. Thought maybe there was air in line but so far only thing I'm seeing in terms of how to bleed it is turning stove on. Which I already did. Not sure what issue is. Will Google more.

[Wi1dBill]

Mike...just a thought...did you guys buy throw rugs for your rig?  Are the floor vent covered?  Had a neighbor in Az do that...shut the furnace off.  Cost him a over a hundred bucks...service call and labor to find that out.

Wi1dBill

[MikeT16]

Thanks for suggestion Wi1dBill. Didn't put rugs over vents but had read that covering vents might cause problem and made sure all were unobstructed. Figured out how to remove furnace from trailer and problem persists when I bench test it. So definitely not obstructed vents.

[MikeT16]

Removed furnace from trailer, capped furnace propane line, and turned propane tank back on. After running stove for a while to purge propane lines, fridge is running on LP and hot water heater is working.

Connected battery directly to furnace motor and it ran fine.

Made sure sail switch wasn't stuck and wired furnace so it was getting power just like it does when thermostat signals it to start. Found video on how to bench test furnace and I think I did it correctly. Result was exactly the same as when furnace was connected to trailer: clicking sound...fan doesn't start...another clicking sound exactly 30 seconds later...fan still doesn't start. However now that furnace is out, can tell that clicking sound is coming from something on control board.

Even though making sure sail switch is unstuck doesn't help, problem might still be that sail switch is defective and not sending proper signal to control board indicating it's unstuck. Did some googling but couldn't find anything helpful on how to test sail switch.

Other possibility is defective control board, but couldn't find anything helpful on how to test that either. One guy said to see if power is running to blue wire in white connector on control board. But not sure how to do that. Blue wire doesn't seem to detach from white connector and there's no exposed wire surface for me to touch with voltmeter pen.

If anyone can explain how to test sail switch and/or control board, that would be very helpful. Can't believe I've gotten this far given that I knew nothing about RV furnaces, disconnecting propane lines, or electrical wiring until a few days ago. Am very close to narrowing down what's wrong. Necessity really is mother of invention. Temp drops to mid-20s tomorrow (Friday) night.

If I can figure out what's wrong by tomorrow, can order necessary part by overnight mail and will only have to deal with one night of below freezing temperatures without furnace to heat trailer guts. Ordered mini electric oil heater which I plan on putting in basement storage area and heating coil to wrap water line. Thinking that can get me through one night without damage from freezing. Hoping it will work but would REALLY like to avoid pushing my luck and making it two or more nights. So any guidance on how to test sail switch and/or control board would be greatly appreciated.