Question about Heating & Cooling
Not knowing what make or model of equipment, I can only offer some basics. We are going to have to start the troubleshooting at the thermostat. Take the two wires at the thermostat and jump them together, If nothing; take off the thermostat wires at the transformer and jump the terminals R and W together. If nothing, you will need a test light or multimeter to see if you have voltage out of the transformer. You can test if you have a bad gas valve by applying 24 volts to the gas valve directly. Make sure you have a pilot going so it will burn off the gas if the valve opens. If the gas valve opens and you get ignition, check the wiring for any bad wires or wires on wrong terminals. If there is a pressure switch, jump that out once the inducer motor starts running (if you have one). Re-post with make and model and we can narrow down some other diagnostics for you. I am going to assume you have no blockage in the gas line or are out of gas. OK?
Posted on Apr 02, 2011
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Posted on Jan 02, 2017
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PROBLEM #1 - HOT SURFACE IGNITOR DOES NOT GLOW RED
(Remember to wait for purge time, 17/34 seconds, on models so equipped).
A. No main power
B. Faulty transformer
C. Faulty thermostat
D. Faulty limit switch
E. Faulty blower interlock switch
F. Faulty hot surface ignitor
G. Faulty ignition control
Perform normal system checks of main power, transformer, thermostat, limit
switch, blower interlock switch, and replace faulty component.
With power on and thermostat set at its highest position, check voltage
between "TH" and "TR" on HS780 ignition control. If 24 volts is not present
check output at transformer. If no, replace transformer. If transformer
checks out OK look at other controls in the 24 volt circuit I.E.: thermostat,
limit switch, blower interlock switch. Check for 120 volts between "neut"
(L2) and "120" (L1). Check for 120 volts at the ignitor across "IGN" and
"IGN" terminals on the HS780 ignition module. If 120 volts is present,
replace ignitor. If no, replace the 780 series module
PROBLEM #2 - IGNITOR GLOWS RED BUT MAIN BURNER
A. Improper ignitor alignment
B. Faulty ignition control
C. Faulty gas valve
D. High inlet gas pressure
E. Polarity reversed
F. No earth ground
Check for availability of gas at gas valve. Make sure the manual valve
upstream of the gas valve and the gas valve are in the full on position. Also
make sure gas is being supplied to the system at the proper pressure, too
high will lock-up the valve.
Check proper polarity of "NEUT" and "120" on the ignition control by reversing
the wires, if reversed gas valve will not open. If this does not solve the
problem, replace wires as they were.
Check for proper ignitor position.
Check for a good earth ground by using a meter connected between L1
and furnace chassis. Should read 120 volts, if no, check and/or repair ignition
ground wire or ignition control mounting screws.
CAUTION POLARITY MUST BE OBSERVED
Check for 24 volts at the gas valve terminals; "1" and "2" on a 7200 series,
"M" and "C" on a 7100 series, "TH" and "TR" on a 7000 series. If yes,
check wire integrity or change the gas valve. If no, change the 780 series
That is pretty much all of the troubleshooting required.
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