Honeywell Thermostat Acting Strange

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Old 01-19-12, 10:52 AM
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Honeywell Thermostat Acting Strange

I've had a Honeywell RTH7500D thermostat for 4 years and never had a problem. Lately, every once and a while I notice that the room temperature gets to as much as 5 degrees below that the programed setting before it calls for heat. There is no pattern as to when it happens. Any ideas? Thanks in advance.
 
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Old 01-19-12, 03:42 PM
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Sounds like you may have a defect, and not much you can do.. Have tried replacing the batteries?
 
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Old 01-19-12, 04:34 PM
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I believe that the batteries are only used to save the program in the event of a power failure.
 
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Old 01-19-12, 08:29 PM
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Ok, Take them out for kicks and see what happens. If nothing changes, then you may need to replace it.
 
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Old 01-20-12, 10:38 AM
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if the stat is digital and programmable you might just need to change the batteries if you have no C wire to the subbase.older stats use to work on a back feed to power the programmables but once the LED screens and SS workings started to kick in that C was required with the backfeed not doing the job.so with the stat off the subbase do you see a C connection terminal? clean the pin contact on both sub/stat might help..something is dragging your temp run
 
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Old 01-20-12, 11:19 AM
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Don't have my manual nearby..but I think I have the exact same stat...could be a 7600? anyway....

Mine has done this VERY rarely in the last few years. I'd wake up in the morning and the house would be 6-8 degrees below setpoint. New batteries didn't matter. It showed "HEAT" being called for but nothing was running. It always happened when we had apparently had a power outage during the night. I'm just thinking something in the furnace locked out and the call for heat didn't work.

Turn it to off..then back to heat and it worked fine.


Also...this came up an another post. Not sure if it applies...normally the symptoms were a frozen/blank screen. Some stats have a button battery in addition to the AAAs that are easily replaceable. You have to pry apart the body of the stat to access it.
 
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Old 01-20-12, 12:52 PM
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Vic, what you described is called a "soft" lockout of the furnace and turning off the power to the furnace or raising the temperature setting of the thermostat above the room temperature momentarily will reset the furnace.

Johnam, some furnaces have a timer function so that if it goes into a soft lockout it will reset an hour or so later and try again. My suggestion is that when the furnace fails to fire that you look in the window on the blower compartment door and see if there is a "code" blinking on the LEDs inside. There should be a "cheat sheet" pasted inside the blower compartment or on the back of the door to decipher the meaning of the code blinks.
 
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Old 01-23-12, 07:24 AM
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I tried Gunguy45 suggestion of turning off and on and it work both times when the problem occurred. What do I look for now? Thanks to all for you help.
 
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Old 01-24-12, 04:45 PM
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Please refer to Furd's post on what to do next.
Johnam, some furnaces have a timer function so that if it goes into a soft lockout it will reset an hour or so later and try again. My suggestion is that when the furnace fails to fire that you look in the window on the blower compartment door and see if there is a "code" blinking on the LEDs inside. There should be a "cheat sheet" pasted inside the blower compartment or on the back of the door to decipher the meaning of the code blinks.
 
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Old 01-29-12, 03:48 PM
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Checked the code, fast flashing red = flame sensor. Removed it, cleaned the carbon build up, working fine. Thanks for all the help.
 
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