Furnace - ignition sequence help

[A W Warn]

Yes, I do know it's July, and it's to hot to be working on the furnace!

Last time we went camping, we went to the mountains where it got down into the 40's overnight. The furnace kicked on, but did not fire up. Never a problem before, so what's up?

In preparation for next fall, I'm trying to get that 15 year old furnace going. The trailer is a 2000 Safari 25C, built Oct 1999. The furnace is an Atwood Hydraflame 8500-iv.




This is what I've done so far:

1. checked voltage - got 13.1 vdc (output from the converter)
2. check all blade connections at the furnace - tightened a loose one
3. jumped thermostat wires - thermostat ok
4. removed and cleaned the exhaust tube
5. visually check the burn chamber by looking inside where the exhaust tube was removed. No visible rust or burn on the metal. (it's really clean)
6. no corrosion on circuit board or plug, removed and cleaned dust off of circuit board and plug
7. removed fan cover
   1. cleaned dust off sail switch confirmed it functions correctly
   2. cleaned dust off fan
8. Confirmed gas valve is opening - I hear it clunk and smell gas after.

Still will not ignite!

Because I have to loosen the propane line and remove the gas valve to get at some items, I have not. I know how to do it safely, but do not want to if I can avoid it.

This is what I have not checked and a few questions:

1. high temperature limit switch function. is this normally closed or open when cold ???
2. ignition electrode function (I cannot hear spark clicking but the air noise is loud enough that it could mask it. normally I do not hear it)
3. corroded or dirty ignition electrode
4. proper placement of the ignition electrode
5. mud dauber or spider nest on the electrode or deep inside the burn chamber (no problems before)
6. I do not know if the circuit board is working properly. Can I check with a multi-meter??? (I do not have a circuit board tester)
7. I do not know the sequence of events in the starting cycle. Can any of the not checked items be ruled out because it occurs in the sequence prior to the gas valve opening???

How can I check/confirm function on any of the above without removing the gas valve?
Knowledgeable advice is welcomed!

Thanks!!

[HowieE]

I have found that if the gas tanks were removed from the system often air gets into the line and the pilot valve is so small that it can take forever to purge the line.

Crack the gas line at the gas valve and wait till you smell gas. Close the valve and then try it.

The heater fan should come on and about 15 seconds and then you should hear the gas valve click open. There is no proble with cracking the gas line for that period of time.

[jdalrymple]

I have had that problem in the mountains, too.

You didn't mention the altitude, but I had trouble around 9,000 feet.


Best I could tell, the main blower was not moving enough of the thinner air to make the sail switch happy. We were COLD and ended up adding just a bit more surface area to the sail with some duct tape. All was well after that.

It seems worse when the humidity is very low, so the air is very thin.

I have also had a board failure exhibit the no light/no spark issue.


Regards,

JD

[A W Warn]

Originally Posted by HowieE

I have found that if the gas tanks were removed from the system often air gets into the line and the pilot valve is so small that it can take forever to purge the line.

Crack the gas line at the gas valve and wait till you smell gas. Close the valve and then try it.

The heater fan should come on and about 15 seconds and then you should hear the gas valve click open. There is no proble with cracking the gas line for that period of time.

I turned the first tank off, then opened the second full tank and switch the valve between the tanks. Then I lit the cook top burner to purged the line, which also feeds the furnace. I am getting fuel through the furnace gas valve. I can smell it as soon as the valve opens.

[Buck944]

Everytime the furnace did not want to start, it was doing the same thing as you and that was related to a bad ground... I think that your ground connection are on the left side of the picture, on top...




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[A W Warn]

Originally Posted by jdalrymple

I have had that problem in the mountains, too.

You didn't mention the altitude, but I had trouble around 9,000 feet.


Best I could tell, the main blower was not moving enough of the thinner air to make the sail switch happy. We were COLD and ended up adding just a bit more surface area to the sail with some duct tape. All was well after that.

It seems worse when the humidity is very low, so the air is very thin.

I have also had a board failure exhibit the no light/no spark issue.


Regards,

JD

I'm guessing I was around 5,500 ft. I'm back home at 900 ft. It was raining while I was up on the mountain, but dry where I am now.

I think it's either the board or igniter, but not sure.

[Buck944]

Oups... I just cleaned them!

Good luck!


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[HowieE]

Originally Posted by A W Warn

I turned the first tank off, then opened the second full tank and switch the valve between the tanks. Then I lit the cook burner top to purged the line, which also feeds the furnace. I am getting fuel through the furnace gas valve. I can smell it as soon as the valve opens.

Where are you smelling gas? You should not be able to smell gas in the trailer when the gas valve opens.

Just having flame at the stove does not insure there is gas at the heater. If there was an air slug in the gas line from having removed a tank in the past. That air likely moved towards the heater because you have been running the heater for several attempts

[A W Warn]

Originally Posted by Buck944

Everytime the furnace did not want to start, it was doing the same thing as you and that was related to a bad ground... I think that your ground connection are on the left side of the picture, on top...

Envoyé de mon iPhone à l'aide de Airstream Forums

Ground is good.

[Al and Missy]

I presume since you are talking about electronic ignition there is no pilot, and you smell gas, so it must be an ignition problem.

The limit switch, along with the sail switch is in series with the 12volts to the ignition module, so it should normally be closed. The sail switch is normally open, but should close with air flow. Yours looks mostly like mine, but the wiring on the control board connector is slightly different. The black cylindrical part on the circuit board is the transformer that generates the spark. You could unplug the red wire from its top and place the end of a grounded wire near the terminal to see if you get a spark.

My furnace has the manual in it. Do you have one? I could take a picture of the wiring diagram and send it to you if you don't.

Al

[A W Warn]

Originally Posted by HowieE

Where are you smelling gas? You should not be able to smell gas in the trailer when the gas valve opens.

Just having flame at the stove does not insure there is gas at the heater. If there was an air slug in the gas line from having removed a tank in the past. That air likely moved towards the heater because you have been running the heater for several attempts

Definitely there is gas to the furnace. I smell strong gas outside at the furnace exhaust tube as the fan purges the burn chamber each time the gas valve opens. Maybe a minute or so between each time the valve opens, but does not ignite. No smell inside. I only let it go 3-4 cycles then I turn the furnace off.

[A W Warn]

Originally Posted by Al and Missy

You could unplug the red wire from its top and place the end of a grounded wire near the terminal to see if you get a spark.

My furnace has the manual in it. Do you have one? I could take a picture of the wiring diagram and send it to you if you don't.

Al

Good idea, I'll check the spark in the AM. Though, until I know the sequence of ignition events, there is no way for me to tell if it should or should not spark at the time I am checking it.

Thank you for the offer. I emailed Atwood, now I have the owners manual and also a service manual. Since most of the wires originate from the control circuit board, Looking at the wiring diagram I cannot tell which order things should happen.

I think the order is (1) thermostat calls for heat (2) fan activates (3) sail switch activates (4) current flows through limit switch (5) igniter sparks (6) gas valve opens (7) flame begins (8) sensor heated to keep gas valve open (9) flame continues until thermostat is satisfied.

If I have the order correct, then either the circuit board is bad or the igniter is not sparking for another reason.

I'm not sure of the order, if (6) comes before (5)?
Also, if (4) is after either (5) or (6) the limit switch could be bad.

[Al and Missy]

There is no power to the igniter board until the sail switch activates. So I think you have the order right. In fact, this is a very simple control system. Deducing from the circuit, the thermostat calls for heat which applies power to a thermal relay and the sail switch at the same time. I have to presume that the thermal relay immediately turns on the fan, but delays turning it off until some time passes to allow the burner to cool down. When the fan turns on, the sail switch activates and applies power to the limit switch which should be closed because it is cold. The power passes through the limit switch to the ignitor circuit which controls both the spark and the gas valve. My schematic shows red, white, and black on the ignitor board. Yellow is where the black should be and yellow is the new black for rv and marine circuits so maybe Atwood was ahead of the times. Red goes to the gas valve and we know that is working. So the sail and limit switches must be ok.

My best guess is you either have a failed board or a bad igniter somehow. The spark jump test will tell you which. Spider webs or other crud on the igniter could be shorting out the spark.

Edit: I just read the label on the black module in your picture. It says it is a 12 volt automatic ignition system and the ignition time is 7 seconds. Don't know whether that means 7 seconds of delay or it will spark for 7 seconds after it starts. But it appears that the igniter board is pretty stupid, just applies 12 volts to the gas valve and 12 volts to the ignition module. There may be some sequencing logic on there, but it appears the spark generation is all in the black cylinder.

Good luck.

Al

[A W Warn]

Originally Posted by Al and Missy

There is no power to the igniter board until the sail switch activates. So I think you have the order right. In fact, this is a very simple control system. Deducing from the circuit, the thermostat calls for heat which applies power to a thermal relay and the sail switch at the same time. I have to presume that the thermal relay immediately turns on the fan, but delays turning it off until some time passes to allow the burner to cool down. When the fan turns on, the sail switch activates and applies power to the limit switch which should be closed because it is cold. The power passes through the limit switch to the ignitor circuit which controls both the spark and the gas valve. My schematic shows red, white, and black on the ignitor board. Yellow is where the black should be and yellow is the new black for rv and marine circuits so maybe Atwood was ahead of the times. Red goes to the gas valve and we know that is working. So the sail and limit switches must be ok.

My best guess is you either have a failed board or a bad igniter somehow. The spark jump test will tell you which. Spider webs or other crud on the igniter could be shorting out the spark.

Edit: I just read the label on the black module in your picture. It says it is a 12 volt automatic ignition system and the ignition time is 7 seconds. Don't know whether that means 7 seconds of delay or it will spark for 7 seconds after it starts. But it appears that the igniter board is pretty stupid, just applies 12 volts to the gas valve and 12 volts to the ignition module. There may be some sequencing logic on there, but it appears the spark generation is all in the black cylinder.

Good luck.

Al

I made pretty much the same assumptions as you did. Though, I believe the board is somewhat smarter in sequencing/control.
Guess I'll dig into the combustion chamber tomorrow to get at the igniter to see if it is ok.
If there is no spark from the circuit board to the igniter, I'll take it somewhere to get it checked out. I'm about 40 miles from ODM, so I can run it over there.

Thanks for your help!

[Siegmann]

The furnace in my '98 has needed both a circuit board and an igniter. Regardless of what you find out, both are worthwhile parts to have on hand. Particularly if you camp someplace remote, and find you really need the furnace one day...

[A W Warn]

I check the igniter wire for spark as Al recommended, but none was present. So, I ordered a new Dinosaur board. It arrived today. I installed it in about 5 minutes. The furnace fired up immediately. I'm ready for cold weather!

[Siegmann]

Good to hear you got it fixed. I had gotten the Dinosaur board also. My OEM board lasted 12 years. I hear that the Dinosaur board is even better.