Amana gas furnace won't light with hot ignitor


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Old 10-11-16, 11:18 AM
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Amana gas furnace won't light with hot ignitor

I have a gas furnace, tag shows Amana model DMS90704CXA, believe it's original (house is 11 yrs old). Temps are dropping here in Pgh, PA at night and I noticed this morning that the furnace wasn't heating: thermostat set at 73, showed actual in house temp as 63. The furnace worked properly about 5 days ago when I turned it from a/c to heat, blew warm air from the registers.

I checked to ensure external gas valve was open (and gas is confirmed going to the water heater next to the furnace) and filter was ok, so I turned off external power at the switch, pulled the front cover, waited a few minutes, then turned it on after checking that the gas valve assembly gas control switch was "on". Draft inducer runs, ignitor glows, but I get no flame, so I'm guessing no gas getting to burners. I am not trained in furnaces (former electronics tech), have only changed filters, ignitors, and draft inducer when they needed it. I do have 2 spare pressure switches left by the previous owner/service tech, but I'm not sure it's worth me trying to go any farther before calling a pro (I won't break into gas lines/pipes myself). Any thoughts?
 
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Old 10-11-16, 12:10 PM
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Welcome to the forums.

It is not the pressure switch. The igniter will not glow until the pressure switch has proven.

The igniter should warm up for approx 30 seconds. The gas valve should open only seconds before the igniter shuts off. You can check for the 24vac from the control board to the gas valve.

There should be a trouble LED on the control board in the blower area. There may be a sight glass in the blower door to see the LED. When the blower door is open.... power is cut to the furnace.

Down around page 76 is the system description.
Your manual... Goodman_GMVC8_Service_Instructions.pdf
 
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Old 10-11-16, 02:01 PM
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PJmax- Thank you for the reply and the link to the manual. There is 1 LED, and it has a single flash which repeats continuously. I will check the flue next I guess, then it might be time to call someone.

Thanks.
 
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Old 10-11-16, 02:16 PM
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After the HSI heats up, what SHOULD happen next is that the main burner gas should switch on, and the gas light off the HSI.

You are correct to be SURE that the gas is on! Check to be sure that the electric gas valve is turned to the on position and that the furnace gas shutoff valve is turned on.

After that, use a multimeter to see if 24 VAC is being switched on to the electric gas valve. If no such voltage is being turned on, you likely have a bad circuit board.

If the voltage is turned on, you likely have a burned out gas valve ---- use your multimeter to see is the circuit scross the electric gas valve is an open circuit.
 
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Old 10-11-16, 03:31 PM
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Thanks Seattle Pioneer- Before checking the flue I pulled the pressure switch hoses, planning to gently blow into the switches to see if they seemed to "click" ok. The switch connected to the draft inducer seemed ok, but when I pulled the low hose (condensate box?) I found water in the end of it. After clearing that the furnace fired right up. Again, not knowing much about these, I have a couple more questions:

Is that a condensate drain box (lower right corner of the upper section, with the draft inducer just to the left and slightly above it), and isn't any water supposed to come out of the box/furnace through the small pipes to the outside drain?

Is water here any indication of a bigger problem, i.e. water coming in from outside? It looks like my setup has 2 PVC pipes, 1 coming in the right top of the furnace and connected to the draft inducer, 1 coming into the left top of the furnace (vent?). Both exit the home horizontally out near the a/c compressor.
 
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Old 10-11-16, 07:30 PM
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Your furnace follows a sequence of events, step by step which produces heat if everything is working properly. If something isn't working properly, that sequence will often stop at some point, and identifying that point helps identify the problem.

The thermostat turns on the inducer motor. When the inducer motor comes up to speed, the pressure switch closes.

When the pressure switch closes, the next step is that the circuit board turns on the hot surface ignitor.

When the HSI heats up, the main burner gas should turn on.


You original post described the HSI heating up but the main burner gas not turning on. That produced my post suggesting how to troubleshoot that situation.

But the HSI CAN'T turn on if the pressure switch doesn't close, and it wont close if the drainage system is plugged up.

So---- what's the problem? Is the HSI heating up or not?
 
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Old 10-11-16, 08:59 PM
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I left you a copy of the service manual. This IS a very involved furnace. It really is beyond the knowledge of most DIY'ers. I would recommend you identify your furnace and read all the requirements for venting and condensate lines positioning. Some of the instructions are very explicit.

It appears that your furnace uses two pressure switches. One is connected directly to the inducer. If you have a heating igniter.... that one is ok. Based on the troubleshooting chart the second one is called a front cover pressure switch and is in the condensate lines. You may need a manometer to troubleshoot this problem.

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Old 10-12-16, 05:17 AM
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Seattle Pioneer & PJmax- Thanks again for info. This furnace (again, Amana model DMS90704CXA; P/N P1256703F) has 2 pressure switches. I followed PJmax lead and went to the troubleshooting chart using the service manual PJmax provided in his original post. Not being an HVAC person, it took me some time to get a little more info about my unit from the manual (i.e., 90% efficiency furnace), skim the installation details (VERY detailed re venting, etc), and find the right chart. I already knew from looking at the unit that it had 2 pressure switches, but I was going to discount those based on the posts until I saw the entry in the troubleshooting chart (same section which PJmax excerpted) about the 'front cover pressure switch'. I couldn't figure out which that was, but after doing a quick Google/YouTube search on troubleshooting pressure switches, I found at least 1 result which said to pull the hoses and try to exercise the switches (my words). It was then that I remembered, in my last job as a Facilities Manager, watching an HVAC tech on our school roof doing that same thing to an inoperative rooftop package unit in the middle of winter (can't remember the result). That's why I pulled the hose on each one just to try it, and that's how I found the water in the one attached to what PJmax says is the front cover pressure switch. As I said, the furnace fired right up after that and now has run all night with no problem.

I think I'll spend some time today reading some more about the furnace to familiarize myself with it a little better. I especially want to check the vent/flue setup. I recall my next door neighbor telling me their friend, an HVAC guy, checked their unit and found their outside terminations configured wrong (Ryan Home also).

Thank again for your help guys!
 
 

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