Armstrong Ultra SX90 gas furnace with ignition problem


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Old 10-10-10, 07:53 AM
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Armstrong Ultra SX90 gas furnace with ignition problem

Good Morning

I have an Armstrong Ultra SX 90 gas furnace with a Honeywell S8600C ignition module. Upon cycling the furnace for startup, the combustion blower comes on but ignition is not made and the furnace will not light up. To give a small background history for this furnace/air handler, this summer I had to replace both the air handler blower and motor on top of the unit. I don't know if doing this caused another problem but wanted to mention this in case it did make a difference. I did read about the S8600C being obsolete and that a replacement S8610U would be the replacement for it, but don't know if I am there as of yet. Is there a check or anything else I can do to see if the module is bad or if there is something else I can do to check why the pilot will not come on and light the furnace? Thank you.
 
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Old 10-10-10, 11:43 AM
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Do you have an ignitor that is supposed to glow, after the combustion motor has been running? IF that does NOT glow,...

......either it is burned out, it is not getting 120 volt power to it(easy enough to test by disconnecting ignitor jack and test across the 2 leads that lead back to the control module, while the furnace is running. Disconnect jack first, before starting the furnace, as this would cut down on your risk of getting a shock, but remember that the prongs in the jack are live with 120 when furnace runs), or there is something wrong where the furnace is not verifying that it has substantial pressure to proceed with the igntion.

You can confirm the latter by using a voltmeter and testing between a pressure switch teminal, to ground, when the furnace is running with the combustion motor going. Do each terminal- to- ground test, as mentioned one at a time. You are not trying to test across the two terminals. If one terminal to ground shows you have 24 volts but the other one does not, on a 2-wire pressure switch, then you have a pressure or pressure sensing problem we can discuss causes. You should get 24 - 26 VAC on the tests.

You can also ohms test the wires to the HSI hot surface ignitor(if that is what you have) by disconnecting the HSI jack wire and ohms testing across the ignitor leads leading into the burner. It should not be much higher than 100.
 
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Old 10-10-10, 12:50 PM
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A Honeywell S8600 is an intermittent pilot ignition system.


The combustion air motor is turning on. When that comes up to speed, a pressure switch should close, which would turn on the pilot burner gas and a spark to light the pilot burner. Since that is not happening it suggests that the pressure switch isn;t closing.

95% of the time when pressure switches don;t close, the reason is a defect in the venting system or heat exchanger of some kind.

Check to verify that the pressure switch doesn't close. If it isn't, start checking out the venting system for a problem.

It's usually a waste of time to replace the pressure switch until other, far more likely problems have been checked out.
 
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Old 10-15-10, 10:25 AM
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Thanks for replying. I just now got the chance to do the checks you asked. From what I can tell, with the combustion blower running, I tested with a voltmeter the pressure switch terminals. There are 3 wires going to the pressure switch ( to be sure, the pressure switch is located on the gas valve?). The wires from it go to the ignition module. The red one looks as though it comes from a jumper connection on the module and goes to the circuit board on the upper side of the unit. In other words, it looks as though this is where power is distributed down to the lower unit (not counting the combustion blower which is supplied with power independently). Anyway, testing those terminals, separately, and to ground shows no voltage(24vdc). Those terminals on the module (S8600C) are marked PV (orange wire), MV/PV blue wire, and MV (red wire). Let's talk about the ignitor. I removed the burners, cleaned them, disconnected the ignitor from the burners and looked at it. It has 2 rods, each with its own hood and wired to the module, one marked sense and the other marked spark. Any test of those wires using the meter shows no power to the leads. From this can you tell anything that I can do to look further?
 
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Old 10-15-10, 10:53 AM
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Still messing around with this thing to see if I can find anything out. I don't believe there is power coming from the circuit board to the module. I disconnected the wire bundle where it connects to the board, and then tested the connector on the board to see if I was getting power to it. I was checking DC voltage there and to ground. I specifically checked the red lead connector, thinking that this was my source for power to the module on the bottom. I also checked the other connectors just in case. I was unable to show any power anywhere there. I know when I disconnect the connector to the board, both the combustion fan and the blower come on, and then turn off when I reconnect to the board. I don't know if this will give you any clues as to what is going on but wanted to give you this additional information. Thank you.
 
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Old 10-15-10, 07:21 PM
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You have a pilot system.

It(furnace) will not fire unless you have the inducer motor first run, then pressure switch pass 24 volts AC(not DC) through it's contact points.

Since you have 3 wires there, rather than 2, your pressure switch has an extra safety bonus that will not allow the furnace to start the next heating sequence until it "knows" that the pressure switch vacuum diaphram has moved the piston to the relaxed position, which will be the NC position as likely is marked at that terminal on the pressure switch.

When the inducer motor then runs, the vacuum pulls on the pressure switch piston and shifts it to the NO (normally open) position, where from there, the current takes the path through the 3rd terminal/wire marked C (for common).

From there, the 24 VAC current goes to the ignition control module as part of the red wire safety circuit, and energizes that ignition module with the needed 24 volts AC, to allow the ignition sequence to begin - which would consist of the pilot first coming on, via 24 VAC current path PV to C. Only when the PV (pilot valve) is activated will then the gas valve fully open to allow MV(main valve) to C(common).

But this can only happen when the pilot flame stays on long enough to be "recognized", through a ground circuit back to the ignition module (a safety feature). The ground circuit will only work when the necessary metal at the pilot assembly in by the burner is not oxidized, and conducts a millivolt current.

If you get no pilot flame, it is wise to pull out of the furnace the entire pilot assembly to brighten up all the metal in the area of where the pilot flame, that comes out of the pilot hood, can contact.

But if you have an identifiable skinny rod, separate from your spark rod - usually only a few inches apart from each other (also at the business end of the pilot assembly), that rod would be your pilot flame sensor, and it is THAT you have to brighten up, with a scotchbrite pad or steel wool, to allow that millivolt grounding current I mentioned.

Then you also need to clean the pilot oricfice (also located down at the business end of the pilot assembly.
 
 

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