Hydroflame 8500-III series, ignition resets TDR

[DSKUP7]

Hi there. I'm wondering if anyone has run into this issue before. The primary problem I'm running into with my furnace is that the blower shuts down momentarily the instant the control board fires ignition. I pulled the unit, and tested it thoroughly. Sail/Limit circuit is good. Solenoid/valve works reliably. The board does create spark at the igniter. The TDR works as expected IF the igniter cable is disconnected, even if the solenoid is connected.

It appears that the TDR is resetting as a result of the ignition.

1) Board fires igniter
2) Motor goes off (no power to motor even though the TDR has 13V and ground on the coil
3) Sail switch kills power to the board
4) Short delay, TDR turns motor back on
5) Sail switch closes, board is powered up again.
6) Cycle repeats.

I bench tested the TDR and it seems to be working fine, even at greatly reduced voltage. Is control board faulty, maybe creating current surge or voltage spike when ignitor fires that causes the TDR to hiccup? Would appreciate any advice you can lend.

The only thing I cannot thoroughly test is the control board, since I don't have a tester.

[DSKUP7]

I didn't get any replies to this post, which is a little disappointing. But I did manage to solve the problem through trial and error, so I'm posting again with the solution so that I might save the sanity of others who have run into the same issue. Again, the problem was that the furnace ignition was causing the Dinosaur TDR module to reset. This occurred even if the valve solenoid was disconnected. I swapped in the OLD igniter (which I intended to replace) and the issue went away. Comparing the old and new igniter, I saw that the gap on the new was just a wee bit smaller (slightly less than the nominal 1/8"). I bent the electrode slightly to increase the gap and then swapped it back in and.. what do you know? Problem gone!! The is strange, because even when the gap was too small it was sparking. But the moment after it sparked the TDR reset.

Not sure it will solve your problem, but it's one more thing to try. I racked my brain over this for days on end, an even wasted some money on replacing the control board. Grr. Good luck!

[Tinstaafl]

Excellent detective work; glad you got it sorted out AND reported back. Not too surprising you had no responses since it was a fairly bizarre problem most folks would never encounter.

To be fair, one part of the puzzle you didn't mention is that you had replaced the igniter. In hindsight, your solution does make sense. That spark is loaded with "dirty" RF energy, and by changing the gap, you changed which part of the spectrum was getting enough energy to upset the TDR electronics.

Kind of a moot point now, but it's possible a new TDR could have fixed the problem. Your fix was cheaper, assuming it holds up long term.

[DSKUP7]

Thanks for the reply! Yes, I understand this is probably a rare issue. But I just want to add a few more points for the record, (again) in case it helps someone down the road. I did actually try another TDR, but saw the same behavior. I also upgraded to a FAN 50 board, but again saw the same result. This lead me to think about the igniter, because it was pretty much the only component in the circuit that I hadn't modified. But here are a few more learnings I thought I would pass on:

1) The instructions of the FAN 50 instruct the consumer to wire the fan control relay on the control board in series with the TDR relay (when using on "older furnace models). In talking with Chris at Dinosaur I learned that this is done to make things easier for less technical folks, but it's really not necessary. By wiring the thermostat, protected power (from the circuit breaker), and motor positive to the matching terminals on the FAN 50, the TDR can be eliminated completely. He confirmed the issue is caused by spark EMF and advised that the FAN 50 is less susceptible than older boards using an external TDR. With fewer parts and fewer electrical connections to worry about corroding, that was a no brainer for me to upgrade.

2) You will see references online to the Fan 50 Plus needing a "local sense" igniter. This is not true. The board can perform flame sensing using the electrode side of the "remote sense" igniter just fine. The flame sense probe of the remote sense electrode is not used, so the wire can be disconnected (or not, since the connection terminal goes nowhere on the control board). Seems to me the local sense igniter is just a remote sense igniter without the extra flame sense probe.

3) Another nice thing about upgrading to the Fan 50 Plus is that it is capable of turning off the blower when if the furnace fails to light after 3 ignition attempts, and it will try again in an hour. That means the furnace won't blow cold air all night and drain my batteries if it fails to ignite. Now that my furnace is back to working like new, this is not as likely to occur. But it used to happen regularly... which is really no fun when you wake up in the middle of the night chilled to the bone.

Sincerely hope this helps someone out there who is restoring an older Hydro Flame!!

[Tinstaafl]

The nerd is strong in you, young paduan. I sense that you'll be a valuable addition to this forum's resources. Thank you so much for the added details!

[JohnB]

Hi DSKUP7 and welcome!

First, we want to thank you for your extra efforts in reporting back in a perfect technical way about your problem and what you found out to correct the problem.

Next up, you mentioned you were disappointed you did not get any responses. Sorry about this; we have several of us who are "into" helping out folks with tech issues, and your post is one of them. And we also camp. In my case, I'm usually all "in" on any tech problem; however, this time, my wife and I were camping off-grid (boondocking) for 30 days straight, and in these remote areas, trying to get a reliable internet signal is futile. We like the wilderness type of camping in the woods, but it often comes with very little cell signal. By the time I saw your post, I saw that you had sorted out the problem. We are now back home, and I have a good signal and keyboard to add to your excellent post.

Here are some tech things to add for you and others.

The original controls of the 8500-III furnace used a more mechanical type time delay relay (TDR) for fan control. It had a heater element that heated a metal disk that flipped to trip the contacts, creating a timing device for the fan to purge the combustion chamber at startup as an on-delay before ignition and as an off-delay to cool down the combustion chamber when the T stat was satisfied.

Dometic discontinued this older mechanical TDR a while ago, creating a problem for anyone with the Rev III Hydroflame furnace. An aftermarket company made a heater-type TDR that could replace the Atwood TDR, but it was a lot bigger in size and made it hard to mount in the space available. This older tech TDR was very tolerant of electrical noise, and the sparking noise of the ignition never bothered it. By design, it was, well, noise-proof. This may also be why others have not seen your specific problem if they are still on the older TDR. We usually have folks report they cannot get the older TDR to turn on the fan as the TDR has failed.

You did not mention in your first post that you have the Dinosaur TDR they created as a replacement for the old heater type. I suspect the Dinosaur TDR uses a more modern electronic timing circuit or chip to accomplish the TDR function. In this day and age, why would they not do it that way? There is good learning here for all of us that the ignition sparking electrical noise can sometimes create a malfunction within the electronic TDR. They most likely did not put in enough noise suppression in their device or knew to at the time. While I know of the Dinosaur TDR, I have not yet had to install one. I wonder if it can be mounted in the same space as the older heater-type TDR, which is very close to the gas burner/electrodes. Where did you mount yours out of curiosity? From my work background, distance from the noise-creating item is sometimes a factor. Would mounting the TDR further away help, or could it be practically far enough to solve the issue within the confines you have to deal with? I'm really surprised you tweaked the electrode gap, and the problem stopped. GREAT! The noise suppression of the TDR must have been right on the edge.

Next up is the Dinosaur FAN plus 50 board you mentioned. This one https://www.dinosaurelectronics.com/Fan_50_PIN.html

Atwood has evolved its design since your furnace was made using the flame sense feedback and the TDR setup. I have helped others on the forum where the flame sense on the Rev III furnace was the first "local sense" with three probes at the area of the electrode. This evolved down to only two probes using the "remote sense". The Rev III could have been either 2 or 3 prodes pending when it was made. Was your furnace set up on the three probes' local sense? And what year camper do you have? Putting all these changes and dates together for future troubleshooting on other campers is good thing.

The next change leads to your great explanation of the FAN plus 50 board. Sometime I "think", not sure yet, in the early 2,000's (I know by 2003 for sure) Atwood created the 8500- Rev IV. This rev IV upgrade simplified the gas valve and manifold; they had already created a remote sense, which is cheaper to build and works well. They also eliminated the older heater-type TDR. The PC board now has a fan control, blinking LEDs on the PC board for faults, and the ignition system all in one. The FAN plus 50 board is the newer replacement board for the Rev IV and jumpers for the older Rev III's. My hands-on furnace work (I have worked on many) is all on the Rev IVs to date, but we can go back into what the older systems did.

Your second great tech reply confirmed my thinking. Do some rewiring and convert to using the FAN plus 50 board to control the whole furnace system and eliminate the standalone TDR; that is the best outcome. Trying to make sure the electrodes do not bounce closer together over time so as to avoid making enough noise not to affect the TDR, is just too close to the edge when it works and when it does not. These things only seem to happen on very cold nights.

We can tell from your troubleshooting efforts and tech understanding that you have an electronic/electrical background, which is great. . That same background can allow you to troubleshoot the water heater, the AC system, and the fridge. They are different brands and applications, yet they all use the same older style of half-mechanical/half-electrical/electronic controls theory. If you understand the control systems of the 60's, 70's, and 80's, you will be right at home working on your camper.

If you have not found them yet, we have the service manuals for all the appliances on the forum in our FILES area. See here for the Atwood furnace system. Get the ones for your furnace. It takes a "learning" on which version of the appliance they are talking about, so make sure you are reading for the correct model and rev number, but they may have the wiring diagrams of the systems. If you can read the wiring diagrams, they help with troubleshooting.

https://www.sunlineclub.com/forums/d...hp?do=cat&id=6

The FILES area is at the top of the forum. When logged in, look for the words FILES, click it, and you will be in.

Thanks again for reporting back.

John

[DSKUP7]

Hi John,

Thanks for the great info. I've read numerous threads in which you've helped folks, and I greatly admire your skills and willingness to assist those in need. And OF COURSE I totally understand that you might be off having fun.

I am an electrical engineer by trade. My dad was too- he designed cathode ray tubes for living. Growing up, he had a shop in the house where repaired TVs, radios, VCRs, and just about anything else as a side job. I've been around this kind of stuff all my life. Today one of my hobbies is micro-controlled automation. RV mechanicals? Well, I'm also frugal.. so I just can't see throwing away this old furnace when I can bring it back to life, good as new!

Thanks for all the great history. I enjoyed reading it. And the "Files" section, oh my!
I wish I had seen that earlier! It will be useful in the future for sure. My camper is a mid-90s model I believe. I suspect all that you've written is spot on. I installed the Dinosaur TDR a while back, honestly just throwing parts at a temperamental furnace at that point. It was mounted in the exact same location as the original bi-metal disc/relay module. Same mounting holes in fact. But I agree, it must have been right on edge in terms of glitching with the spark noise. At some point I realized I wasn't going to figure this thing out unless I yanked it and dug into it on the bench.

I'll leave one last helpful hint, as it just came to mind (and maybe you've already posted this elsewhere)...

You'll read about folks bench testing an RV furnace using a propane grill regulator, which I did myself initially. But you really need to use an RV propane regulator for this. My grill regulator is rated 0-30psi. My furnace is rated for 11 column inches of water, which is just 0.5 psi! Initially i had the dial set up high, and of course the unit would not light. That's waaaay too much pressure. I connected a manometer to the test port on the propane manifold and found that even at the lowest setting, I could not get a stable reading. Grill regulators are generally single stage, and simply are not designed for the same level of regulation as a 2-stage RV regulator. I bought proper regulator and found it was rock solid at 11 inches.

Well, thanks again JohnB. I your post to me was #12,739. HOLY COW. You da man!!!

[JohnB]

Hi DSKUP7,

Thanks for the good words; they are much appreciated.

We have similar backgrounds and maybe even passions. In my case, I grew up in a farming background, and working on a farm in the 1970s, farmers rarely hired out repair work. So you learned how to do it by doing and being mentored, in my case, by my Uncle Art. You were exposed to and learned a lot, from woodworking, metalworking, engine, tractor, farm equipment rebuilding, and repair, pouring concrete barn floors, and putting new shingles on the house, and the list goes on. When it was time to go to engineering college, I learned the science behind what Uncle Art had been teaching me all those years.

I'm a machine designer by trade, and I started out in the machine tool industry in 1979. I designed mechanical power presses and feed systems in the metal stamping world—primarily the mechanical work. However, being in a small company, you also do the control systems. In the 80s, controls started changing from heavy-duty relay controls to solid state and then to the PLC (programmable logic controller). Once the PLC came out, the industry was exploding in technology; every year, a new, faster PLC would come out. Then, I moved into the food industry, doing food sterilizers, filling, and packaging machines. Process control also started changing from stand-alone loop controllers to the P&ID control loops inside the PLC.

I learned from the rapid growth in control systems from 1979 up to about 2000. By then, process control by disturbed control systems started coming on the screen, and there were still many PLCs. I had so much mechanical work to do, and the company I worked for had an entire department of EEs, I had to stay in the mechanical world constantly, with failure analysis of mechanical systems being a significant part of my workload.

My wife and I started camping "in a camper" in 2003 when the kids were all off in college. My son and I tent camped in the Boy Scouts until he was 18, and I stayed as a leader for 4 1/2 more years. The wife wanted nothing to do with sleeping on the ground but loved the camper. In 2004, we traded in our pop-up and bought a new Sunline 2004 T-2499, which started my travel trailer obsession. We sold the T2499, and in 2007, we purchased the 2004 T310SR, which we are still camping in a 20-year-old camper. Owning and using a camper a lot, you learn how to draw on your work experience to fix the camper. I have had a lot of things to repair over the years on my campers.

I retired in 2016 and started a somewhat extreme retirement hobby. They seemed to have found me, but I restored and repaired older campers. I have 4 of my own project campers to restore, and I help my family and friends with repair and water damage restorations. All my work experience comes in handy, and doing enough campers and helping others, the learning continues.

On your furnace gas bench test, getting the manometer and working through the issues is good. FYI, this link will drop you in the middle of one of our restorations. I am doing a bench test on a 2003 vintage 8500-IV furnace before I install it back in the camper. You can see the evolution of some of the things Atwood upgraded since your Rev III. Scroll down a little to get to the furnace. https://www.sunlineclub.com/forums/f...tml#post154909

Have fun camping this season,

John