Diagnosing & fixing propane RV furnace

I don't know which subforum would be best, so hopefully this isn't too out of place.  I'm migrating this to a new topic from my old topic about fixing an RV water heater.

Furnace:
Suburban Propane SF-35F furnace
Auto-ignite (doesn't have a constant pilot)

Furnace symptoms:
- usually won't light
- sometimes lights but won't stay lit
- all other functions (fan. blower, etc.) seem to work

Troubleshooting:
- all other propane systems function perfectly
- unit appears to be clean and not dusty or grimy

Other:
- Furnace was working great for almost 18 months, then stopped working right one morning.  RV had been parked, totally stationary over winter, at least 6 months.  So it wasn't a bump/jostle issue.  
- Likely culprits are physical obstruction or electronic control failure.  I had similar symptoms in my water heater prior to this, turned out to be the "gas valve" electronic propane delivery control unit.  A
- To physically remove the furnace unit I think I have to take apart the propane lines. I  haven't done that yet but can if need be.  No one is actively living in or using the RV so it won't be an issue to crack open.  If I need to do this, any pointers would be appreciated!


So here are some photos of the unit.  Looks great!  I'm 'really' hoping that my culprit might be the "Limit Switch" (this would be my replacement part here: https://www.amazon.com/Suburban-231630-Limit-Switch/dp/B003G9IWZW/ref=sr_1_2?keywords=SF-35F&qid=1563662047&s=automotive&sr=1-2).  I'm curious on other input though as to any kind of diagnostic, troubleshooting, and/or repairs I could do.

Is there a flame sensor that could need cleaning?

I have heard minor obstructions in the propane lines/orifices can be 'resolved' with a bumpy ride upside down in a pickup... this was advice for 3-way fridges but it seems like the principle should be generally applicable...

Looks like it's in pretty nice shape.  Here's a link to download the service manual - has all sorts of good stuff to help you out, including wiring diagrams and troubleshooting guides! :)

http://www.bdub.net/manuals/Suburban_Service_Manual.pdf

Hey Jen!

I tend to doubt it's the limit switch, those usually are a safety feature that turns the unit off if it gets too hot. Your furnace symptoms you provided (and I want to compliment you on the excellent post providing such detailed information) provides a clue, "sometimes lights but won't stay lit". Sounds to me like 1) fuel air mix is off or 2) something is obstructing the fuel vapor flow which could be something like dirt plugging the orifice or the solenoid that actuates the valve and it won't stay open. If I were there next to you, those are the first two things I'd be checking.

I glanced at the manual that Phil provided in the link, and the whole sequence of events for power up and starting is listed starting on page 23 which I think will help you narrow down the problem if it's not something simple. Good luck!

I appreciate the fast and helpful responses, thank you.  

I will flip through the manual and see what I can see.  I guess I'll probably have to remove the unit which will mean disconnecting the propane lines.  Not quite sure how I'm gonna do that yet.  The line turns 180º via 2 brass elbows at the front of the unit, which traps it in place.  I photographed this.  There is a brass coupler on the outer elbow tying it into the rubber propane line.  I can get a wrench around it but I only have 3-4" to work with as far as twisting room.  

Any ideas on the best way to disassemble the propane line?

Sounds like that might be enough room to turn the connecting nut one flat at a time.  Plumbers have all sorts of interesting tools to get to nuts in difficult places.  Check the geometry carefully to confirm the direction to turn to loosen, and remember to turn off the gas supply first (sometimes I get into trouble with the simplest things . .  . . :) )

However, if there is play in that feed line, I am guessing you might be able to remove the anchor screws (for example, there is one angled-in holding the frame of the heater to the underlayment just under that brass connector) and slide the unit out a few inches to get better access.  You will have to get those out anyway to remove the heater so no lost effort.

Good luck!

Phil Gardener wrote:Sounds like that might be enough room to turn the connecting nut one flat at a time.  Plumbers have all sorts of interesting tools to get to nuts in difficult places.  Check the geometry carefully to confirm the direction to turn to loosen, and remember to turn off the gas supply first (sometimes I get into trouble with the simplest things . .  . . :) )

However, if there is play in that feed line, I am guessing you might be able to remove the anchor screws (for example, there is one angled-in holding the frame of the heater to the underlayment just under that brass connector) and slide the unit out a few inches to get better access.  You will have to get those out anyway to remove the heater so no lost effort.

Good luck!

There is play in the outside feed line, yes, but the inner propane line is metal and solid.  I took those mounting crews out and you can't wiggle the unit but 1/4" side to side because of the propane line.  It's gotta come out!  I wrenched on the lines a little bit but was unable to get either end of the union to move.  I know that our propane connection for the cookstove at the house is reverse-threaded for some reason.  I have no idea how to tell if a given thread is going to be normal or reverse, so I've been trying both ways.  Like I said, haven't gotten it to budge yet.  Will try with a different, less cumbersome set of tools.

This is the sequence guide from the manual.  Based on how the furnace runs, it probably narrows it down to 2 culprits:

The thermostat controls the operating circuit to the furnace by reacting to room temperature to open and close a set of contact points which allows current to flow to the ON and OFF switch then to the relay.
Operational
The relay receives the power and allows power to pass through to the switch within the relay. This is done by a heater coil within the relay which actuates a bi-metal disc closing the relay circuit.
Operational
The power then flows to the motor and allows the blower to operate. One end of the motor shaft drives the room air wheel. The other end of the motor shaft drives the combustion air wheel that delivers the required air to the burner for combustion.
Operational
The limit switch is an in-line device which protects the furnace from over heating conditions. The contacts in the limit switch open at a given temperature setting, shutting off power to the ignition system which controls the gas valve.
Operational
As the room air wheel comes up to speed, air flow closes the sail switch completing the circuit. The sail switch is placed into the system as a safety to prove there is adequate air for combustion.
Operational
The next operation is controlled by the Direct Spark Ignition, (DSI) system as power is applied to the DSI board. The system will do the following:
1. The board has a timing circuit which allows the blower to purge the chamber of any products of combustion or gas.
Operational
2. The board will then apply power to the gas valve. At the same time it produces a high voltage power supply to the electrode producing spark at the burner.
Operational but not igniting
3. The board will also confirm the presence of a flame. If the flame is not sensed after 7 seconds, the module will try two (2) more times and then go into lock-out. The flame is sensed through the spark wire and electrode.
Operational
When the thermostat has reached the set point with the room air temperature, the contacts will open removing power from the controls. The blower will remain on until the relay opens and stops the motor.
Operational
NOTE: On some models, sail switch is before limit switch.

So... probably the gas valve?  The board is supplying the needed power simultaneously to produce the spark, but the gas is not being let through.  The board is also confirming that there is no flame present and functioning accordingly.  

Assuming the lines aren't obstructed.  Although I don't think that's the case.  The lines were clean when my hot water did nearly this same thing, it ended up being the gas valve in that instance.

(edited so many times because every time I re-read it it made a little more sense :P)

Ooooh, so perhaps not the gas valve.  According to the manual's troubleshooting map, it might also be an issue with the 'module'.  So the unit is sparking, you can hear it quite well, but it's not establishing a flame.  I'm going to continue reading, but for the moment I'm trying to understand how it can successfully be creating spark, but not sparking 'at the electrode'.

Edit; I would answer "no spark at electrode" with "no" since, yes, it's sparking.  But as I'm reading this map, some of the yes/no answers don't quite make sense to me.  Would the correct answer to "no spark" be "no" or "yes"?  "Yes, there is no spark" or "No, there is a spark" could also be one interpretation.  Does "no" simply mean "incorrect" and yes mean "correct"?   The more I read into the yes/no answers the more confused I feel.  :x  I am more hesitant to think the module board is bad, since the module controls A LOT of the processes from start to finish and ALL other processes are functioning.  Guess I don't know til I take the thing apart and put a meter on it and watch it work!

I've been wrenching on these connections like mad and can't get them to budge.  Trying to think of things that are safe to use that might help crack the threads on these couplers...  Any ideas?

Jen Fan wrote:I've been wrenching on these connections like mad and can't get them to budge.  Trying to think of things that are safe to use that might help crack the threads on these couplers...  Any ideas?

In the 6th picture you provided, the backside of the 90 degree elbow, right next to the wires, looks to me like a compression flare fitting. The image looks like there is a skinny hex for a wrench hold closer to the elbow and then behind that towards the wires another fatter hex nut for another wrench, and the fatter one will turn.

Jen, you're sure ambitious! <g>

If you _must_ remove the heater (and thus the gas connection) it may be worth looking "up the line" a ways for easier connections to reach. W/out more pics of the installation (and maybe not then because it's hard to see important buried stuff in pics) it's hard to say. But you mentioned a "hose" and hoses are both skinny and flexible. So if you can reach a connection on that hose upstream, open that and then feed the hose through it's access hole while  you pull the unit, that may do the job for you. IOW, do an end-run around that union. And, incidentally, all unions I have ever seen in 30 years plumbing were right hand threaded. But that's something you could ask a local RV service guy, whether there's such an animal as a left thread _union_. There are left thread connections, but they usually occur at points where they will be used at least occasionally, like hose connections at gas bottles.

Also. Most copper tube is bendable and that is actually required during many installations - and thus when doing R&R.

BUT: The thing was/is installed properly (it worked ok a while back). That condition is not something to be tossed aside until you determine s l o w l y and carefully, repeating yourself often, that you absolutely must pull the unit. I have done a whole lot of work I didn't really need to because of not taking time, (seemingly a LOT too much time, but...) up front to painfully go through each step of testing a problem. When doing R&R with stuff like this, you will quite likely break things and thus need to recreate piping and connections. Don't let it bother you, but don't bet the farm on getting it done w/out the right tools and materials when it comes down to it. Also, if you do take it out, there is a chance that you can figure out a better way to put it back; but check any innovative ideas with somebody who does propane work first. If you crack fittings, have a way (ready ahead of time) to test the system as you put it back. That actually includes testing the existing propane system (for leaks) after you have removed the unit - because when testing it after you get the heater back in you will also be testing the _whole_ system all together and if there is leaks or seepage _other_ places you want to know it _before_ you freak out and start pulling hair when you've just finished the heater.

If the vehicle moves it would be worth the trouble to take it to a place claiming to service stuff like that and see what story of woe they tell you and what they say is the reason they have to charge you so much. It's a fine line between acting smartty and eliciting important information, but considering the scope of your proposed project, all info will help. James could be right about the fitting behind the elbow. But from long experience, I can tell you that plumbers don't use a lot of fancy tools - they fall back on one, the Sawzall to get them where they can use more or less regular wrenches. Which may be something you should consider - cutting up the cabinetry (in ways that you can put it back together nicely) in order to reach what you need to reach and do what you need to do. I just had to do that with a little travel trailer that had split its seam at the wheel wells inside the cabinets both sides. Went back together quite acceptably and was stronger than original; just a little less space inside the cabinets.

> clicking
If you know _where_ the spark is supposed to occur, make it perform after it's dark out and try to ensure that the spark is where it's supposed to be. Not somewhere else in the high tension lead that's supposed to be super insulated.

Some of those manuals are very good. I think you can figure this out pretty surely, but those tests all assume the unit is installed properly. You well might not be able to go back and do the tests after you've messed with the unit, so get it done now. There's probably a little plug on the side of the gas control valve which you remove to attach a low pressure gas meter called a manometer. The manual should show these connections and what you want to see there; it helps a lot to know what you're dealing with. The manometers come in various shapes etc and also for various pressures. I think simple ones can be had off amazon for $20; just make sure you get the one that covers the range of pressure your control needs.

It's been 10 years or more since I did surgery on an RV water heater so somebody will have to verify my memory here, but: I believe that the "main line" on the RV, directly after the pressure regulator over the tanks, is 11" water column. The stove can use it's own regulator to reduce that to 3-5 water column and ditto the refrigerator and the heater. However, some appliance _may_ use the full system pressure (supplied by the regulator at the tanks). Hopefully somebody will comment and/or any RV tech you talk with should know right off. IAC, there may be _two_ relevant pressures in your propane lines: the main supply and then the pressure for a particular appliance that has it's own regulator. If either of those is significantly off, an appliance doesn't really have a chance. And the pressure needs to be there _while_ all the gas appliances are running and drawing their fuel.


Ok, "Nuff and more than...
Regards,
Rufus

Again thanks for the replies!

James:  Yes, skinny nut in front, thicker nut in back.  They're pretty big too, 1"+.  I will try focusing on the fat one.

About the piping:  I was mistaken about it being flexible.  It has a plastic sheath on it which made it appear like a black propane hose.  The line is metal all the way around, and I've tried bending it to no avail.  I wasn't being terribly forceful though

About alternative connection points:  I already checked on that one.  The metal piping behind the coupler hooks up like a big horseshoe, coming back down where it goes through the floor.  On the outside of the unit, that pipe comes through the floor over 12" with several bends before its first union with the main RV line.  Unless I cut the pipe that's not an option.  I also checked for a back-entry to the unit and popped off the exhaust panel, but I only get a 4x6" opening to the back of the unit and it's clogged by 2 pipe ends.  So no hope there.

Good to know the nut is likely right-hand threaded!  Yes, the only reverse threading I've seen is on our propane tank hook up here at the cabin.

As far as whether or not I must pull the unit; if I don't, it doesn't get fixed.  There is literally no access to the rear of the unit where the electronic parts are.  It must come out or remain broken.  The reason I do these things myself is because I cannot afford to pay someone to do it for me.  That, and I value learning and knowing how to tend things myself.  A professional's knowledge is not out of reach to the average person.   Their degree or certification does not mean they know something others can't learn without that same piece of paper.  I'm not saying the average person can do it as well or better, but we CAN do it.  Part of the beauty of learning to do these things yourself is that you gain uncommon knowledge and have the opportunity to pass that freely on to others, with, naturally, a healthy dose of disclaimers and practical cautions, as you have done
So, round-about point being, it's not a matter of ambition.  It's a matter of necessity and principle.  Being self sufficient isn't simply material.  It's intellectual and skill-oriented as well.  

Cutting the line: It seems to be the most simple way to pop this thing out is to loosen this hex nut.  That's literally the only thing holding it in.  As per the manual, the removal instructions are "disconnect propane line, unscrew 3 screws, pull unit out".  Seems pretty straight forward.  However, with the space constraint, if I can't get this nut off, the next option may just be cutting the line behind it.  There's a lot of pipe to work with back there.  It would have to be coupled back together upon reinstallation, which will make it more prone to leaks and problems, and I really don't want that.  
Cutting the cabinetry: I will not be cutting any cabinetry.  This RV is REALLY nice and in excellent condition.  I'm actually trying to get it sold but the non-working heater has been the fail point of every near-sale, even though I'm knocking of $3,000 to compensate for it.  So getting it done myself will get it sold and get me a little more on the sale price

Taking it to a shop:  There is only 1 shop in the area and they charge like $180 an hour, told me it might take 1-3 hours to diagnose, and 1-3 hours to repair, assuming there's no complications.  She said to reasonably guess on at least $500.  They're also booked out 3-5 months at a time at all times of the year.  There are also complications with moving it off the property due to our remote location; the trip this RV takes off the property will be the last time it's ever here.  I do NOT want to drive it back up here and have to park it again!  So, ultimately, it's not going to a shop for fixing.

New idea: Just had this idea whilst typing... I wonder if I can pop the propane stove out and get top-access to the unit.  I don't recall if there's a wooden divider between the stove and the furnace, or if the bottom of the stove makes the ceiling of this compartment.  I will investigate this today.

Gas valve check and spark check: unfortunately I can't test any of these until the unit is out, or if I can remove the stove, until I can see it from above.  Which raises a good point; if the unit is totally removed, I may need to splice and temporarily hook up/extend electrical and propane lines to test these things.  

Line pressure:  All of the appliances are working great and as they're  supposed to, as did this furnace before it stopped lighting.  I have a hard time understanding how there could suddenly become a pressure shift without a major influencing event to the propane lines or appliances, especially if the furnace is not trying to regulate the pressure itself.  Granted I don't know the ins and out of propane systems that in-depth at this time, but it seems unlikely to me

Spark location:  This is one thing that I find confusing.  How  can the spark be happening in a different/new place than where it's supposed to?  Though, the fact that it sometimes lights and stays lit for under 60 seconds tells me that's probably not the culprit.  I'm really inclined to think gas valve here based on my experience with my water heater.  It was doing the same thing.  Usually not lighting, but when it did light, it stayed lit for a short time then went out again.  I did all kinds of testing, cleaning, and puzzling before replacing the gas valve ($42) and it's good as new now!  Anyway, that combined with the indications of the manual.  Still puzzles about the 'yes/no' map.  My partner seems to think that the applicable answer in this case is "yes", but the double negative of "no's" really has me thrown for a loop.  While I'm no mechanic, logic tells me that if the symptoms are virtually identical to the last gas valve that went out, and if, according to the manual, the other option is the module being bad, despite the module seeming to preform every other function it's in charge of perfectly, then it's very likely the gas valve.

Jen Fan wrote:

James:  Yes, skinny nut in front, thicker nut in back.  They're pretty big too, 1"+.  I will try focusing on the fat one.

I'm 99% sure that will be the disconnect and allow you to remove the unit. The skinny "nut" actually doesn't turn but is to put a wrench on to apply resistance while turning the fat nut. Just wrenching on the fat nut could possibly tweak fittings in a not good way.

 Though, the fact that it sometimes lights and stays lit for under 60 seconds tells me that's probably not the culprit.

Gaaah!!! The thermocouple!! This is a little electrode thing that sits in amidst the flame when it's burning. When working properly, it tells the furnace that there is a fire and all is functioning. When they don't work right or go bad, it thinks there is no fire (even though there is) and the unit will turn off the gas supply to prevent the rv from filling up with unburned gas and causing an explosion. They can be tested with an ohm meter.

> pipe... horseshoe

Copper tube, more like, maybe? How are the other appliances installed? Ie. is it with copper tube? The horseshoe configuration often is used to allow making the connection with the tube accessible, then pushing the appliance into place which bends the tube gently over the range of the "horseshoe". IOW, extra tube forms a long loop which bends relatively easily back onto itself. Again, can't say w/out seeing it up pretty close. Sorry, cant' recall if you posted, but: What happens when you remove the mounting fasteners (you said "three"?) and try to move the unit? Does it shift around some, maybe bending the "horseshoe" out a little? Beware, there may be some kind of flue connection which also has to be released.

> spark location
I think you're right, it's probably fine. However, there is several THOUSAND volts making that spark and there is usually a short piece of special electrical cable (a few inches at most) that is designed to contain that voltage. That cable is actually a highly stressed (electrically) bit of equipment. There are various ways it could fail, electrically, and when it does, the voltage gets out and the spark can occur where you don't want it.

> under stove
Good thought. And it might work, sorta - hold that thought. But I'd bet the heater is supposed to come out all by itself.

> thermocouple +
James hit it - common fail point for all gas appliances. Usually tested by replacing it because it's almost the same cost to replace it (when billing $) as to test it. And you then know that you have a new part where there was an old (very old?) one.

Another common fail point is electrical connections. Both plugs/sockets and the screwed down kind. Usual test is to disconnect, look for obvious problems, reconnect and see what happens.

I have read many complaints of "sludge" causing problems in RV propane systems. There are various claims and explanations, but it's a recurring theme. I don't recall for sure where the choke point usually is - probably regulators. Something to look at maybe, although you say the other appliances are all fine.

And yes, it's very handy to be able to operate the thing fully while it's sitting on the table, sorta speak. Usually a bit of a pain, but it can really help when you're new to the territory and not sure just what you may have done right or wrong.

In the end, if it's not the thermocouple, it sounds like a gas control issue like you suspect. The question is whether it's from "dirt" which _sometimes_ can be cleared. But I'm afraid the surest route is slow and steady, walk through the testing logic if the manual provides it.

Regards,
Rufus

I appreciate the input, Rufus.

I took my stove all apart and encountered the same exact style of coupler/connection.  I thought, cool, I'll figure out how to take this one apart since it's open and totally accessible, then I'll be able to take the other one apart.  Nope.  No manner of tapping or wrenching seems to budge it in the slightest!
Also I was able to move the stove enough to see that it has solid cabinetry under it.  So even if I took it out I'd be ripping paneling off and I want to avoid that.

I'm actually now thinking of cutting the propane line and coupling it back together...  

The 'horseshoe' propane line is super stiff and unyielding.  Basically the whole unit is resting totally freely in there without those 3 screws, but the propane line is worked into a notch in the metal framework of the furnace at a 180º angle.  So the propane connection and the 3 screws are all that's holding it in.  If I could unbend that propane line?  I might be able to pull the unit out far enough to work freely around the coupler that needs to be taken apart.  But I still can't budge the darn thing.  I was thinking of hitting it with some penetrating oil, not sure if that would have any adverse affects, being a propane line.  So far the input I've gotten is "no, it won't affect the gas lines"

What more can you tell me about the thermocouple?  Does it go by another name?  The manual for this unit doesn't contain the word "thermocouple" a single time.  The only "thermo" word is thermostat.  

If I can get it open I can definitely observe and test electrical connections.  With how clean all the appliances and wiring are, I'm really not expecting to find bad connections.  But I won't know til I can look at it!  My RV has no rodents or rodent damages, no water damages, no issues.  It's very clean and well cared for  Doesn't mean appliances don't go bad though!  Obviously!

Jen

> cutting propane line...
Good: Thinking out of the box.  ...   Or...   Desperation! <G>      Seriously, that's sometimes how one must proceed. HOWEVER: Don't do it yet. <g> Figure out with some degree of confidence _exactly_ how you're going to splice it back together or re-run the whole line. Before you go cutting it.

You say you got an identical connection you can reach to work with? Gonzo! Take a picture of it so we all can get in on the fun. <g> Assuming you really can reach it ok, and that it's some kind of flare connection (or union, if you prefer), this is how I'd approach it:

1) Threaded connections connections _almost_ always will yield to brute force provided you get a solid hold on each side of the connection with the lever arms (the length of the wrench) long enough to apply overwhelming force. Here is a short dissertation on using force to get pipes apart. Fear and awe doesn't cut it. There is some real thought and technique needed. You must have long enough lever arms;  for a 1/2" pipe that would be 24" of lever arm (both of them) in my case.

edit: You get 24" lever arms onto small pipes by using _steel_ 8" to 12" wrenches with cheater bars slipped over the handles. Try the with aluminum wrenches, you'll likely break the wrench handle.

Both wrenches must HOLD. That would seem obvious but in fact setting those wrenches properly to NOT SLIP actually takes thought and care. Then the problem often comes down to how to keep the "backup" wrench immobile while applying very excessive force on the side you hope to move. When I attack an old galvanized joint that I have removed from a wall (sometimes I've  been on a job, have the problem solved, except... I NEED that one fitting on the old pipe I removed) I have set the two wrenches and placed the "fixed" wrench on a concrete floor and stood on it while a wrenched with an equally long handle on the other side of the joint. What I'm trying to say here is that even when the joint is wide open and you know it can come apart and will... It's not necessarily duck soup to get it yield. The tools by themselves won't do it for you, nor will, often, huge muscle - you have to apply some real thought and care and imagination, even. This stuff can require some 3-dimensional ingenuity. Ideally when fighting with a joint you want the fixed wrench to be backed up by something totally solid. For example, I've been in situations where I failed to move the joint even though I had it immobilized on the floor. But when I moved the operation to the CONCRETE garage floor, I got it apart. Now, move this hair raising recital to the insides of some delicate piece of equipment and you start to see why some plumbers can and some... can't.

OK, there's more. When wrenching (or any seriously extreme application of force) _don't_ bend your elbows. There is no strength there and you might hurt yourself. Pull or push with from your shoulder keeping your arms straight, elbows locked. You can actually apply several hundred pounds of force pushing with one straight arm and pulling with the other using your chest muscles. Don't pull with your weight unless you _really_ know what you're doing - something lets go and bad things might happen. Your hands (and  the handles) need to be close together because your shoulders only have a few inches of movement with full strength. Finally, in a tight place, if you cannot position yourself to use your shoulders, set the opposing wrenches so they are almost aligned, then _squeeze_ them together with your hands. If you can get your hands out near the ends of the handles, his is one of strongest forces you can apply and is often my preferred way to open joints.

Not done yet. No matter what approach you are executing there is a reasonable chance of something slipping, letting go. Take a few seconds to visualize what will happen to your tender flesh _when_ that happens and don't put any body parts where they might get maimed - caught between wrenches, jammed into the concrete forced into the sharp sheet metal edge of an appliance. It happens, I promise, so do allow for it.

All part of the plumbers day. The second approach:
2) The "hot wrench". Although some people blanche at the thought of applying a torch to gas line, I and all the plumbers I know have done it repeatedly. Just make sure the gas is out of the line _and_ that the line is open, not closed. Lighting the gas in an (almost) closed line can (but probably won't - not much oxygen in there) cause an explosion while lighting the gas in an open line will cause a brief (.2-.5 second) PUFFF of flame and residual gas burns and your hand might have a really bad burnt hair smell for an hour. OK. I don't believe in soft pedaling stuff. If you don't feel like applying heat to a gas joint, don't.  I just gave you legitimate reasons to be concerned and not to do it with a clear conscience. However. For many plumbers it's SOP - you have to decide whether you're on top of things enough to do _all_ the necessary checks before sparking up your flame. In your situation I'd buy a small hand torch like found at home centers for about $20. They're about the size of an old fashioned C-cell flashlight with a right angle for the burner at the top. They usually run on butane and you want one that locks on. Click start is worth the extra $10. A trigger safety is just a PITA, but if you can't avoid it, whatever. Get the largest version of this little toy torch - the size is right for this job, whereas the standard propane torch is probably too large and its flame is almost too large. Make a plan: Lay out your weapons. You're going to make the "nut" quite hot, but you still need to get wrenches onto the joint and apply significant force and you have to do it quickly and hopefully w/out burning yourself too much. You want to heat the female part of the joint, the part that goes around, encloses, the male threads; make this part expand with the heat and all a sudden, the joint is a LOT easier to open. Apply the flame to the "nut" and try _not_ to apply it anywhere else. You want the nut to grow - which is does when it gets hot - and you'd rather not have the rest of the joint grow as fast. If it starts to glow cherry red you're definitely "there",  could'a stopped sooner. But you probably won't get it red with a little toy torch; that's part of the reason it's the right tool for this job (but you _will_ get it hot enough to burn yourself good, so take care). Drop the torch (click it off first) and wrench it. Finer points include trying to heat evenly around the nut, not burning stuff behind the joint (thin sheet metal or even tin foil does quite well as a flame shield but remember to check behind it occasionally...).

There's a lot of other detail that you'll come across if you proceed here. Be sensible, go slow, etc. As I said earlier, it's quite possible to break things. But it's also possible to get the part or equivalent and put them back together right. This isn't brain surgery.

The gorilla option:
3) This isn't always appropriate, especially if you haven't yet broken stuff. It's an acquired taste. But it can get a lot of force onto an awkward joint. Set the "backup", fixed, wrench _solidly_ (against something so that side of the joint can't turn). Then use a small (3-5 lbs) sledge hammer (the tool is usually shorter than a normal hammer) and hit the handle of the opening wrench which you've set onto the joint. Or, set the opening wrench and then place then end of a short piece of 1/2 or 3/4" pipe against that wrench and hit the other end of the pipe. That's another one that almost always get the joint open, but... It might also do other things, especially if the backup wrench slips off. Oh, and this also will break aluminum wrenches. Gotta use steel wrenches.

OK. I've gone through the long, long detailed post because from what you have said you have a threaded joint available and threaded joints _will_ open. Going through this and opening the one you can reach will show you what is involved and just how hard it is or is not, for you. The "hot wrench" is probably the most likely bet, but because you have to practice setting those wrenches, trying it cold to start is the way to go. Failing at that will also tell you just how hard you might have to wrench and how well your wrenches, and you, will work. Because you have to do almost the same thing to open the hot joint but you need to do it quickly and w/out burning yourself (too much).

I guess to be fair, I should mention that when you open a joint you usually must get it back together so it doesn't leak. And _that_ doesn't always "just happen". There might be another adventure there. So are you _sure_ you've done _all_ the trouble shooting and testing of that heater in place, hooked up and all that? <g>

Oh, thermocouples. I think you said you have the manual. If you do and you're sure it's the right manual, just do exactly what it says. A good manual is really the first place to spend lots of time; best bang for the buck, fewest mistakes. If it's not clear, google the product name/model including the words "problem" "flame" "not light" or various other likely hints.  Good chance somebody else has been there. If nothing pops, drop the model number and try again. Might save you lots of hassle, even though that will certainly improve your character. But plumbers cheat, take no prisoners and try to get home by 5pm. Good character and $5 will only get you a latte at Starbucks - after the job is done.

Cheers,
Rufus