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stu7777 10-29-2012 04:09 PM

Furnace Ignitor question - voltage
 
I have an Armstrong Ultra Tech 80 furnace that won't light. The inducer blower comes on for about 30 seconds then shuts off and the ignitor never starts to glow. The LED code is 3 flashes. Things I've tried:

I checked the hose to the pressure switch and the nipples - all are clear. I can get the pressure switch to click by sucking on the opposite end of the hose. I also jumped the contacts on the pressure switch and cycled the heat with the same results. I'm getting 24V on both sides of the pressure switch to ground. The flue is clear.

I cleaned the residue off of the heat sensor with steel wool.

Nothing works to get the ignitor to heat up and glow. The element in it doesn't look cracked or broken. I put a voltmeter on the connections to the ignitor with it unplugged, cycled the heat and didn't get a change in voltage. It justs stays a zero. Shouldn't I get a change in voltage from the leads to the ignitor at some point when the inducer blower comes on?

Not sure what else to try. Any ideas? Thanks!

Wuzzat? 10-29-2012 06:08 PM

I've heard the ignitor should measure 40 to 90 ohms.

"Ignitor Doesnít Glow: If the combustion fan is working and the HSI ignitor doesnít glow then we
have to go back a step or two to determine the cause. Letís do an easy check before we go any
further to see if the ignitor is good. Using a non contact voltage sensor check the wires feeding
directly to the ignitor to see if there is voltage present. Be patient. The voltage will only be present
for about 7-15 sec so donít jump to conclusions. If you donít have a voltage sensor but you have a
voltmeter then disconnect the wires from the ignitor and check for the 120 volts. If your meter or
sensor indicates 120 volts then the ignitor is bad. You can confirm that with an ohmmeter if you have
one by checking continuity. If it reads Ďinfiniteí then it is open and needs to be changed. There also
might be burn marks or missing sections. In any case replace the ignitor. Be careful not to touch the
surface of the ignitor when replacing as your skin oils will cause it fail early.

If there is no 120 v present at the ignitor then it is either the circuit board or the air proving switch
which is not working. Letís check the air proving switch first. The air proving switch or air pressure
switch has to close in order to send a signal to the board to continue ignition process. There are a
couple of quick checks you can do at this point to confirm the circuit board is working. You could
place a jumper across the switch terminals or suck on the small plastic or rubber tubing that is
connected to the switch. The switch may not close for two reasons.

If there is blockage in either the small rubber or plastic tube that connects to the switch then it
cannot sense the pressure the fan is creating. Sometimes there is water present that has
accumulated from either water vapor condensing inside the tube. Water could also be present
because drain back up inside a condensing type furnace. If you remove the tube and blow or suck
thru it and water comes out then that might clear up that problem. Now you have to address the
furnace condensate drain problem. The tube also can have a leak in it so that the correct pressure
never gets to the switch. If that is the case then replace or repair the tubing.

If the tubing is not the issue then there might be a clog or obstruction in the flue. Obstruction can be
caused by animals enter the flue discharge opening and either dying or building nests there. The
opening can also be clogged with ice or snow. If the furnace brings itsí combustion air from outside
there may be a clog or obstruction in that intake opening as well. It is also possible there is
insufficient combustion air available from inside especially if the furnace is installed in a closet and
grilles openings have been blocked. Inspect the furnace intakes and exhaust and clear any
obstructions.

Now if the both the tubing and flue and combustion air paths are clear and the switch still doesnít
operate then place a jumper on the switch terminals and then if the ignitor glows then the switch
needs replacing. "

From
http://www.starheatingcooling.biz/gasfurntroubleshooting1.html


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