24V goes to zero for 30 seconds on heat start?


Old 12-08-13, 09:35 AM
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Join Date: Dec 2013
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24V goes to zero for 30 seconds on heat start?


I have a gas/blower furnace. I am observing that, on starting of heater (jump terminal R and W), as expected, the exhaust fan starts, furnace fires up, and blower starts.

However, a minute or so after the blower starts, the voltage at terminal R would go to zero for about thirty seconds. I can hear a relay click when this occurs. After about 30 seconds, the voltage goes back to 24. After that, the heater continues to run normally and no more voltage drop out until the next start cycle.

The condition occurs whether or not the furnace is warm.

If I were using a conventional thermostat, I would not have noticed the problem (since the blower continues to run during the zero-volt period). I am using a Z-Wave thermostat that requires 24 volts to run. During the zero volt, the thermostat resets causing the heat to shutdown (causing short cycling).

Question is, what can cause the problem? What can cause the voltage to drop to zero every time the heater initially starts? Again, this occurs when after the furnace fires up and blower starts to run, and only happens once each time.

I'd really appreciate any clues.

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Old 12-08-13, 11:43 AM
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Welcome to the forums.

There is a duplicate/similar thread running on this same problem.


See if this helps. If not, we can go on from here.
Old 12-08-13, 12:03 PM
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Join Date: Dec 2013
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Thank you. I read through the thread. The problems are similar, but with a few key differences:

--My 24V goes to zero on startup, only once, and the heat runs fine for the rest of the cycle.

--I'm testing and monitoring with the air filter/blower cover open, so there is no obstruction.

--If the limit switch is tripped, why does it get tripped so early on and not later?

--My 24V does not go away after the heat is off.

Thanks for any help.
Old 12-08-13, 12:47 PM
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Join Date: Dec 2013
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I tested the limit switch, that is not the cause. I tested it by jumping the leads (bypassing the switch) and measuring the switch (normally closed).

The exhaust fan is okay too.

Incidentally, when the limit switch opens, it does not cut the voltage to zero volts (The gas does shut off as it should).

What switch/device can cause the voltage to go to zero?
Old 12-08-13, 02:47 PM
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Join Date: Dec 2013
Posts: 4
*** UPDATE ***

I did more testing and traced the wiring. I found that the 24 volt source at the transformer is solid. The wiring goes from the 24V transformer -> fan limit switch -> R terminal. It is the fan limit switch that is interrrupting the 24V at the R terminal.

Now then, I removed the fan limit switch, cleaned it, reinstalled, and watched it work. It starts to rotate as the flame comes on, when it reached the proper temp, the blower fan turns on, (furance stays lit), and the fan limit switch keeps rotating until it goes to about 200 degrees F and shuts off the flame and 24V. In about 30 seconds, as the switch cools, it rotates back and turns on the 24V. After that, it maintains about 150-160 degrees without any more issues. The flame goes on and cuts off normally and no more 24v cutoff for the duration of the cycle.


1) There is no air obstruction. The filter is new. The airflow is strong from the registers and they are all open.

2) On the initial start (flame comes on, then blower, flam stays on for a while), it seems that the flame runs a lot longer than later in the cycle. That is, I think the flame stays lit longer than it should, causing the fan limit to reach 200 degrees. Later in the cycle, the flame goes off and on more frequently, therefore no over temp.

What controls the flame on/off other than the fan limit switch? Mid-cycle, I see the fan limit switch steady at about 150-160 degrees while the flame comes on and goes off. Something is maintaining the flame activity, what?


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