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Old 03-23-2020, 06:52 PM   #1
jregalia11
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Suburban SF-30

No heat. Now I have watched you tube videos.. repaired the furnace once before and this time I am stumped.
I have a furnace that will bring the blower on,purge,then NO spark and then open the gas valve.
I have a new board and new spark igniter installed to eliminate a failed old part. Not a parts replacer but there can only be 3 things really wrong.
Maybe someone can help me.
I have made sure spacing is correct on the spark ignitor. 1/8 inch and also shined the light into the burner to line it up. Checked 13vdc. Now I am not getting any bolts of of the board where the spark ignitor is.
My last guess is the sail switch but... before I order one as a guess... correct me if I’m wrong and that the gas valve would not even open if the sail switch or high temp limit was OPEN. I do know the sail switches are the most common failure tho.
Btw have gas.. and gas pressure is proper.
At least it appears that way with other appliances.
No tick tick tick before the gas valve opens.

Searched for this issue before so sorry if it’s a repost but no one seems to ultimately ask the question... will the gas valve open even with a failed sail switch ? I don’t think so from what I have read but I could be an idiot. Ha.

Thanks in advanced.
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Old 03-23-2020, 07:26 PM   #2
Roscommon48
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Like I suggested to someone else, it could be a number of things. You need a professional to get into it. Or, just start changing things out.
might be the lp regulator or low pressure in tanks as well as the other things.
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Old 03-23-2020, 07:40 PM   #3
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I have the same symptoms. I removed the heater and "bench" tested the unit and it worked. So my conclusion is it is the regulator. I have not had a chance to replace it yet to confirm, but that it's my next step.
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Old 03-24-2020, 04:33 AM   #4
jregalia11
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A regulator would prevent the spark ignitor from sparking? That’s the part of this furnace that I am trying to figure out.
What is my sequence of operation in the board/system that just “close” or “make” for power to be allowed to the spark ignitor.
In my opinion.and experience . and I am an HVAC tech in the residential industry a gas problem does not cause the spark ignitor to not perform properly.
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Old 03-24-2020, 05:32 AM   #5
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I have read that there is not enough gas pressure to get it to process.
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Old 03-24-2020, 05:40 AM   #6
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The sail switch is a very common problem. If you don't make up the sail switch it will not try to ignite. There's a LED on the control board that will flash fault codes.
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Old 03-24-2020, 05:58 AM   #7
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Here's the "sequence of operation" from the Suburban Furnace Service Manual:

Start
Thermostat Calls for Heat
The wall thermostat controls the operation of the furnace by reacting to room temperature, this allows current to flow through the On/Off switch to the module board. The module board constantly checks for a minimum 9.5 volts. If there is not 9.5 volts, the module board will go into a stand by mode until adequate power is supplied. It will then resume normal operation. Upon a call from the thermostat, the module board thermostat circuit will go active. The sail switch circuit is verified as being open. The blower output is energized.

Blower motor starts.
15 Seconds Purge Cycle
The module board will then verify that the sail switch circuit is closed and motor is up to speed. If this circuit remains open for 30 seconds after the blower motor starts, the module board will go into lock out and shut down the blower motor. The module board checks that the gas valve relay contacts (which are located on the module board) are open before the ignition sequence starts. The board has a pre-purge timing circuit of (approximately 15 seconds). This allows the chamber to purge.

7 Seconds Ignition Cycle and Flame Sense
The module board will energize the gas valve and enable the high voltage spark output to the electrode for 7 seconds of ignition time. The module board will then check for flame sense to verify successful lighting of the main burner flame. Sparking will then be terminated and the gas valve and blower outputs will remain energized. If ignition is successful the module board will monitor the flame sense, sail switch and limit switch circuits, and the thermostat inputs during the heating period. The flame is sensed through the spark wire and electrode. Therefore, it is essential that the electrode is properly positioned in the burner flame.

2 and 3 Ignition and Flame Sense Cycles if Required
3 Try Ignition Board If the flame is not sensed after seven (7) seconds, a second 15 second purge cycle will begin followed by a second Trial-For-Ignition sequence. After three (3) Trial-For-Ignition attempts with no ignition of the main burner, the module board will de-energize the gas valve immediately and blower will run for 3 minutes and then shutdown in lockout. Heating Cycle If during the heating cycle, the limit switch circuit opens and remains open for 5 minutes, the module board will go into lock out and shut down the blower motor. If this occurs, the thermostat will need to be reset for the furnace to operate.

90 Second Shut Down
When the thermostat has reached its set point and the demand for heat ends, the gas valve will be de-energized and the flame will go out. The post purge period of 90 seconds begins. When it times out, the blower motor output is removed, and the blower stops.

The above "sequence" calls for several "safety interlocks" to function properly or the sequence will not move forward.

First, the voltage monitor circuit must sense 9.3VDC. If not above that level, the system goes into "lockout".

Next, the sail switch must be OPEN at the start, before the sequence can begin. Then, during the sequence, when the fan switch closes, the sail switch must also close. If that "two step sequence fails" the system goes into "lockout".

Third, the gas valve control circuit opens the gas valve and starts the ignition sequence for 7 seconds. If a flame is sensed, the system advances. If not, then the next step allows for two additional "ignition attempts". At the end of the third attempt, the system goes into "lockout".

Any of those "lockout sequences" will shut down the furnace and prevent operation.

You should be able to download the Suburban Furnace Service Manual here: http://bryantrv.com/docs2/docs/subntnew.pdf the sequence of operation is on page 24. The pages that follow are extremely important in troubleshooting that sequence. Any lockout will cause ignition failure, so it could be something as simple as a "sticking sail switch" or as "elusive" as an intermittent capacitor on the control board or even a crack in the printed circuit on the board.
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Old 03-24-2020, 06:42 AM   #8
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Only thing I think you missed is the limit switch, that is verified before the sail switch.
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Old 03-24-2020, 06:53 AM   #9
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Originally Posted by chuckster57 View Post
Only thing I think you missed is the limit switch, that is verified before the sail switch.
Chuck,

If I remember the "yes/no logic tree" correctly, the limit switch is verified closed during the ">9.5VDC check". If the limit switch is open, there will be no "minimum voltage and the system shuts down in step #1. So, even though it's not "stated as being in the start-up sequence" the limit switch is verified closed and functional in "step #1"....
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Old 03-24-2020, 07:00 AM   #10
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Only thing I think you missed is the limit switch, that is verified before the sail switch.
True I didn't mention the high temperature safety switch that's in series with the sail switch. I don't think they fail that often though. Have you seen many problems with them Chuck?
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Old 03-24-2020, 08:16 AM   #11
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Thank you for responses. I did actually speak with tech support and he verified that if the gas valve is opening the. That proves the sail switch and limit switch are both doing as they should.
In my case the gas valve is opening and no spark. In my case he mentioned that if you take the orange wire off of the connector on the board and hold it 1/8 inch from the connector you should see spark. If not then it’s a failed board. Everything else is proving in my case and maybe just a DOA board. Will update it after I replace the board.
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Old 03-24-2020, 08:42 AM   #12
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Thank you for responses. I did actually speak with tech support and he verified that if the gas valve is opening the. That proves the sail switch and limit switch are both doing as they should.
In my case the gas valve is opening and no spark. In my case he mentioned that if you take the orange wire off of the connector on the board and hold it 1/8 inch from the connector you should see spark. If not then it’s a failed board. Everything else is proving in my case and maybe just a DOA board. Will update it after I replace the board.
DON'T DO THAT HOLDING THE CONNECTOR IN YOUR BARE FINGERS while wearing damp socks...…
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Old 03-24-2020, 09:10 AM   #13
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In my years, the control board has failed more often that both the switches put together.
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Old 03-24-2020, 03:19 PM   #14
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DON'T DO THAT HOLDING THE CONNECTOR IN YOUR BARE FINGERS while wearing damp socks...…
That would be a "hair raising" experience!
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Old 03-24-2020, 05:54 PM   #15
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Years ago, a RV furnace went out. I double and triple checked everything I could and finally took the furnace out and to a shop. It was the board. Hours after install and testing still nothing. The new board was also dead.
That time the shop put the next board in it and tested. It worked for years after that. Sure is frustrating buying/installing a new part and having it bad from the start.
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Old 03-24-2020, 08:43 PM   #16
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DON'T DO THAT HOLDING THE CONNECTOR IN YOUR BARE FINGERS while wearing damp socks...…
John, reading this again brought up an old memory that made me laugh.
40 years ago when DW and got married we moved into our first abode. Our neighbor was retired from a nearby military base after 20 years of pumping gas. His lack of basic knowledge was astounding. He cut grass in the neighborhood for money. Now this guy was constantly bugging me to help him fix everything from his lawnmower and trimmer to his cars. Wanting to help and be a good neighbor I obliged. So a year of this goes by and I ask him if he could cut the grass 1 time while we went on vacation and thought gee, all the times I did a favor no problem right?

So we return and I go over and thank him and out of politeness asked how much did I owe him. He said his "normal" price so I paid him and walked away without comment. Two days later he knocks on the door asking for help as he can't get his mower to start. I told him I'd be happy to help.

I walked outside, pulled the spark plug wire off the plug and told him wrap his hand around it. Then I told him to place his other hand on the metal cage on top of the motor. And you guessed it, I pulled that string with all my might. When he finally stopped screaming and jumping I said "looks like you got good spark, must be a problem with gas".

He never asked me to fix another thing for the 10 years we lived there.
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Old 03-24-2020, 10:30 PM   #17
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John, reading this again brought up an old memory that made me laugh.
40 years ago when DW and got married we moved into our first abode. Our neighbor was retired from a nearby military base after 20 years of pumping gas. His lack of basic knowledge was astounding. He cut grass in the neighborhood for money. Now this guy was constantly bugging me to help him fix everything from his lawnmower and trimmer to his cars. Wanting to help and be a good neighbor I obliged. So a year of this goes by and I ask him if he could cut the grass 1 time while we went on vacation and thought gee, all the times I did a favor no problem right?

So we return and I go over and thank him and out of politeness asked how much did I owe him. He said his "normal" price so I paid him and walked away without comment. Two days later he knocks on the door asking for help as he can't get his mower to start. I told him I'd be happy to help.

I walked outside, pulled the spark plug wire off the plug and told him wrap his hand around it. Then I told him to place his other hand on the metal cage on top of the motor. And you guessed it, I pulled that string with all my might. When he finally stopped screaming and jumping I said "looks like you got good spark, must be a problem with gas".

He never asked me to fix another thing for the 10 years we lived there.


And there I was, thinking I was "that" guy!
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Old 03-25-2020, 01:10 AM   #18
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And there I was, thinking I was "that" guy!
I'm betting there's several more of "us" hanging around in here!
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Old 03-25-2020, 05:02 AM   #19
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Old 03-25-2020, 06:53 AM   #20
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I can't count the number of times I've bit my tongue and wanted to do just that.... It's reassuring to know that, at least with that one neighbor, you "made a difference".... Good on ya !!!
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