4: Furnace – Suburban Model SF-42

Suburban Model SF-42 Gas Furnace 40,000 BTU

Back in ’16 when I bought this RV, the gas floor furnace worked very well in concert with the heat pump. It would surprise me sometimes in the cold of the high desert in the early mornings when it would start up even though I’d have the thermostat set for ‘Electric Heat’ but careful reading of the operators guide showed that was normal. And then it started faulting. Would work sometimes when I’d set the thermostat for ‘Gas Heat’ but other times wouldn’t. And it was confusing how it was behaving. But…I have several electric heaters I’d bought at thrift stores during the summer to save on propane and just for the possible event where the gas furnace or heat pump wouldn’t work. So there wasn’t any rush to repair it. And I always (or most always) have the heat pump…which works very well after I did that extensive maintenance on it back in April of ’19.

I have it in my notes that the furnace stopped working completely in June of ’19. And here it is mid-September of ’20 so it was time to work on it. And this article will cover what I found and what was fixed to get it working again. I’m lucky that it seems to be an electrical problem as that’s right up my alley.

I first gathered the documents I have for it, and read though them. Checked Youtube to see if there’s any videos about this furnace but no luck. There’s a simple test I found applied up at the TrueAir thermostat, and that is to disconnect the 3-terminal connector that heads down below to the furnace and jumper the +12volt to the Thermostat terminal.

This 3-terminal connector can be disconnected, which also removes battery power from the thermostat. Than a jumper wire can be connected from between the yellow wire that would be connected to the red wire over to the white wire opposite where blue wire plugs in. Back in June ’19, that test didn’t do anything other then cause a ‘click’ down in the furnace after a delay. So my best guess was that I’d have to remove the furnace from its cubby hole from the outside of the RV. I only have one gas furnace and it’s installed above the water heater and in the same compartment that the water heater and battery trays are installed. Owners of longer Winnie’s of this era will have two gas furnaces so this connector (on the right, below with the medical clamp attached to its wires) would have an extra hole and 4 wires.

So since the furnace doesn’t start with that jumper test I knew I’d have to work on the main unit, but first I crawled under the RV with a strong flashlight and inspected the back, underneath, and around the gas furnace to see how that looked. It’s tight in the chassis space so I really couldn’t tell much but I was looking for a connector that may have been disconnected or a wire broken or something. The wiring was well protected, didn’t find anything damaged so it was time to remove the furnace.

What I had to do first for better access was disconnect the gas strut from this compartment door over the furnace (that also covers the water heater below the furnace and the two battery racks to the left) and bungee it up a bit further. That gave slightly better access to the screws at the top of the furnace. I have an impact drill and that made short work of those screws that were reluctant to come out. Many of them were covered with black caulking even on the threads. Used an extension Philips screw bit that allowed access to the upper screws as they were still slightly covered by the cabinet’s door. Had to angle the extension a bit. I mention this so readers will know that the door itself doesn’t need to be removed to remove the top screws holding the furnace. You can see that I did have to lift the wheel well cowling as the left edge covers 3 of the furnace’s screws…it’s on hinges so it was made for that. Removed 4 easily accessible screws on it’s bottom edge. Supported both doors with bungee cords once opened.

After all the screws from the cover plate over the furnace itself were removed, had to carefully pry off the cover panel. There was tons of butyl tape and white caulking around the periphery and on other areas so I had quite the job prying the panel from the surface of the compartment. This is not unusual as this is a gas appliance and they want to keep it as air tight as possible with only the air they designed to flow to the flame actually reaching the combustion chamber, without any drafts. This is important to make sure the flame is just right and doesn’t produce soot. As I said, had to do a bunch of prying so the panel got bent a bit. Should be able to straighten it fine though.

There are a bunch of screws holding the cover and chrome pieces. These in the picture are all for the cover plate and the chrome exhaust’s intake tubes, chromed cover, and metal spacer. The black screws hold the cover plate, the longer rusted screws hold the chrome venting assembly and it’s bracket.

These are all the parts that keep the heated exhaust air coming from the combustion chamber from melting the side of the RV. Really had to strongly pry on these parts to get them out of the furnace and off the wall of the RV. You can see the rust here and there, that’s what usually makes them hard to remove from older RV’s. Eventually with some prying with a carpenters wide pry bar, this assembly slid out without damage.

Here’s where the above assembly attaches. Note the white goo sealing it up.

Okay, so here’s what we’re working on. That’s the blower motor assembly with the sail switch located there where it’s easily accessed, and the wires going to the left are heading to or coming from the control board. There’s the brass gas connection on the lower left. On the far right are the intake and exhaust ports. There’s two screws that hold the entire assembly in place and they are easily accessed. But I don’t want to remove the furnace unless I have to. So I did some testing and found that yes, the motor does work if I bypassed the control board.

It’s handy that the batteries are just to the left of the furnace so I can jumper 12 volts over to the furnace easily enough for testing. You can see my two jumper wires there behind the meter. Not everyone carries those clip leads like I do and they can be replaced with simple jumper wires you make up yourself. The motor draws enough current that I eventually made up an 18 gauge jumper wire for testing so as not to melt my clip lead.

 

 

Anyway, testing proved that the time delay relay that operates the motor has gone bad. It’s on the control board I think…hard to see back there inside the furnace…and I’d have to remove the furnace to access that board. Then I’d have to unsolder, remove & replace the relay because this model furnace it’s on the PCB rather than separated like in other Suburban models. Just my luck. Checking the schematic for this model SF-42 doesn’t show a separate Time Delay relay, it shows it on the main control board. The time delay relay for those other furnace models, like the NT-12 series, is p/n 230625 and Dinosaur Electronics has a replacement, here. It’s not clear if their replacement can be shoe horned into this model of furnace though.

It’s possible that this furnace’s ‘Time Delay Relay’ (TDR) is not a separate part but rather part of the control board. This based on the fact that the drawings I have for the furnace and my own search don’t show a TDR part number. However, I have found a replacement control board for my model, and it’s based on Suburban p/n 520820 shown in the Suburban drawings for my model, which shows up on Amazon here for ~$95. With the added effort of having to totally remove the furnace by shutting off the propane, bleeding the lines, pulling it out, replacing the board, repairing or replacing the foam seals where the furnace interfaces with the RV ducting, than resealing everything. I suppose I could just focus on removing that board and than replace just the relay but there’s no guarantee that it’s the relay itself, it could be the circuitry that drives and times it, though after testing I sort of doubt that.

Still…I’m thinking it would be easier, much more fun, and more economical to put some of the 10’s of thousands of electronic parts I carry with me to good use and design my own Time Delay Relay. And that’s direction I’m heading at present. I have a box of those standard automotive relays the RV’s use and I’ve temporarily installed one in the furnace as a proof of concept. It’s shown on the lower right of the next picture. It proved out that it definitely works if I manually actuate the circuit board and fan in the right sequence so the control board is happy and ignites the gas flame. The controlled sequence is: when heat is called for by the thermostat in the RV’s hallway, and the control board is powered, there’s a Delay On of 4 seconds before the fan motor comes on, when the fan is up to speed the moving air lifts the sail switch, than there is another circuit delay (~5 secs) before the control board sparks the gas. And it goes through the spark sequence 3-7 times trying to get a flame to ignite the gas, if necessary. The motor continues running and holding the sail switch on and when the flame starts (usually at the first spark) it heats up the spark gap probes, which produces a tiny voltage, which in turn signals the control board that the flame is on so the spark voltage is suppressed.  If this all works as designed, the RV is soon warm. In my Winnebago Journey RV, the cold air is draw from a single vent in the bedroom, circulates around the chamber where it’s heated, then the heated air is fan forced through the furnace ducting and out of the 3 floor vents throughout the rest of the RV.

When the thermostat set point is reached, the thermostat drops the voltage to the furnace to zero which causes the circuit board in the furnace to turn off the gas, but since the combustion chamber is so hot, the fan keeps running for another 2 minutes to cool down the chamber. After that two minutes, it shuts off the fan motor. There are some safety devices involved in the sequencing that have to tell the control board everything is okay and in my furnace they are working fine so I’ll not go into their functions at this time.

I did a test using jumper wires just to prove the concept of operation and it works every time doing it the way it’s mentioned above but really needs to have that special time delay circuit to automate the job. Mine has stopped functioning as designed. Incidentally, that microswitch there attached to the motor housing is the sail switch that tested fine in my furnace. Easy to get to and although a bad sail switch is a common failure for these furnaces, I didn’t get lucky this time.

So that’s what I’ll be doing…designing an electronic time delay on/off circuit that will operate that new relay. It’s called a ‘Time Delay Relay’ circuit. No hurry with the build as it’s still early September and shouldn’t get cold for another month, plus I have all those electric heaters.

Near the end of September here in N-E corner of Oregon it’s gotten a bit chilly in the mornings and I could use the furnace now and again so it’s time to build my personal TDR circuit. As I said earlier, I didn’t have to do any of this, I just wanted to. Here are the results. Created the circuit using parts I have in my stock and followed my own experience with the design. It was really fun revisiting old memory cells in my brain.

Helped use up some of my many many electronic parts, exercised the brain, and kept me from going Covid-19 stir crazy. Note this drawing has copyright but permission is granted by the author for hobby use.

The design was proven to work fine at the furnace. Using a build of the circuit on a ‘breadboard’ before committing to the hard wired board shown above. Then a bunch of parts were added to protect the circuit from the vagaries of battery operation in a vehicle with an alternator. Alternators are capable of 60 volt spikes called ‘load dump’ when something switches off, so spiking that high occasionally can damage unprotected circuits. And there are all sorts of other electrical devices switching off and on while driving and when parked or with the generator running 120 Vac devices like the near by heat pump. From these various sources sharing wiring runs, transients can be induced and play havoc with operation of a battery operated device so the design ends up quite a bit more complex and robust than what’s shown on the breadboard.

So that’s where I am now. I’ve got it all hardwired and it’s ready to install in the cabinet.

And a week later, here’s that tested circuit committed to a soldered board and installed. This shot shows the new fan relay and associated wiring at the lower right, the two added wire nuts where I needed to splice into the existing wiring, and the board attached to the frame with the green LED power light illuminated on the left side. I left the main metal cover off over night and tested it in the cool of the next morning and it performed as expected.

And now it was time to reinstalled the cover. It’s simple enough to remove so for now, I’ve not added new caulking. I’ll do that some other time, right now, I prefer let that go until I’m certain there’s no *gotcha* lurking in this design. Next morning I used it again to quickly and quietly get the house temperature from 68F to 74F. Works nice. Since I might need to spend the winter here in the Walla Walla valley, having an extra heat source that runs on 12 volt may turn out to be important.

So that’s the successful testing and modification of the Suburban SF-42 gas furnace resulting in a fix of the ‘no operation’ issue.

A couple things I’ve noticed while doing this work…the wheel well cover could be easily removed entirely with the removal of 6 screws holding the cover to the two hinges. If there needs to be some work done in the wheel well, I’ll know how to access that area in future. For a brake job perhaps. For this job, it only needed to be opened and bungeed up out of the way.

The other thing is that the water heater is mounted just below this furnace, and it would be much simpler to remove this furnace first if work is needed on that. There’s a large hole under the furnace between it and the water heater that would greatly aid in giving enough access for removing plumbing, wiring, and the propane pipe from the water heater. So keep that in mind if you have an RV similar to my Journey.

Finally, I had a devil of a time getting that chrome air intake and exhaust set up right so that the screws actually met the holes in the mounting bracket. Had to rely on tell tales like a large rust spot that corresponded to both the RV frame and the backside of the chrome assembly, or how the white goo matched up. There are several parts to that setup so if you work on yours, try to mark them in a way that you know the order to put them in when remounting. I even had to remove the cover again so I could force it down just a hair so the chrome venting assembly would mate up with it’s screw holes instead of missing by 1/16th inch. That work wasn’t all that fun and it took a good solid 2 hours but it eventually went together and I’ve got it all buttoned up, the wheel well cover back in place, and ready to heat. None to soon as it’s supposed to get into the mid 30’s F at night here soon.

So that’s the story of how I did an electronic design and construction to do a job that a new control board would have done, but that would not have been near as much fun.

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Update: June ’22

I’ve needed to use the furnace several times when staying in the desert SW with it’s cold nights and mornings and so far, this mod has performed flawlessly. I was in Cuba, Nevada one night back in late November and it got down to 19 F during the night. Happy to have this working so I was nice and toasty just 10 minutes after turning it on that morning.