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I replaced the transformer and the thermostat and the gas valve still doesn't open.

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  • Mike Butler
    Mike Butler Apr 02, 2011

    Lets start by getting a make and model number off furnace to better determine your problem.



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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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The heating element on an electric dryer can
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Thermostat doesn't activate unit

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Pilot light on thermostat click main burner doesn;t come on

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The water heater red light keeps flashing rapidly on my Powervent 2 model 41VRP40N. It is not producing hot water and when I read the manual it says it needs more heat. What does this mean. How do I fix...

There are many things to check. The single most important test is for 24V at the gas valve after the ignitor is supposed tp be on. 19 V or less and there is either a bad ground or the ingition control board has failed. (Wire ground problems are a high probablilty)

Here is what the book says:


(Remember to wait for purge time, 17/34 seconds, on models so equipped).

Possible causes

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


Step #1

Perform normal system checks of main power, transformer, thermostat, limit

switch, blower interlock switch, and replace faulty component.

Step #2

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



Possible causes

A. Improper ignitor alignment

B. Faulty ignition control

C. Faulty gas valve

D. High inlet gas pressure

E. Polarity reversed

F. No earth ground


Step #1

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.

Step #2

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.

Step #3

Check for proper ignitor position.

Step #4

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.

Step #5

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
ignition module.

That is pretty much all of the troubleshooting required.

Thanks for choosing FixYa,

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2 Answers

I replaced the thermostat because it wouldn't turn on, it still doesn't turn on. What else can I check?

Jump the two thermostat wires together at the thermostat, if it fires up; it is a bad thermostat. Jump the terminals where the thermostat connects to at the gas valve, if the unit fires it is a bad thermostat wire or connection. If the unit does not fire, check for 24 volts AC out of the transformer. If yes, bad gas valve, if no bad transformer or no power to transformer.

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Did you bypass with the 24v.right to the gas valve and it opened? Sounds like that's what you are saying. This is a good way to test the valve but can be dangerous. Well, we do it all the time.If the pilot is on and the transformer is good, even though you replaced the stat, that is what controls you're gas valve to open on a call for heat? Double check the stat wiring, and make sure that you have 24volts from the stat to the valve when stat is on. You may have a bad spot in one of the wires.Check all of your control wiring,24 volts. You will find the problem in your wiring. Let me know. Good Luck,

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