Question about Honeywell Electronic Programmable Heat/Cool Thermostat - CT3300A100 Heater

4 Answers

Thermostat seems to be working, furnace does not always turn on

Hi, I have a 1997 Rheem Criterion II gas furnace with an electronic thermostat. On the thermostat, I have the temp set to 70 "hold", so it's supposed to keep that temp at all times. The problem I am having is that sometimes more often than others, the furnace should kick on, but doesn't. When this happens, the temp in the house might go down to 65, but the furnace never kicks on. If I flip the fan switch from "auto" to "on", still nothing happens. The only way to make the furnace start its cycle is to either turn the furnace power switch on & off again, or to adjust my temp on the thermostat to 65 (making it supposedly click "off"), then back up to 70, making it click "on". This seems to always work. I also have to add that sometimes the furnace will not turn off. The temp in the house might get up to 75, still running. I turn it off by cycling the thermostat up to 75 and back down to 70 again. I'm baffled that cycling the power switch on the furnace or cycling the temp up & down on the thermostat both work to kick the furnace on. Sorry for such a long post, but I tried to include as much info as possible. Thanks for any help! Joss

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  • jossde Jan 15, 2009

    I also forgot to add that I replace the batteries in the thermostat.

  • jossde Jan 16, 2009

    Hi, I did not find any flashing lights. I may have found a solution, though. Every time I noticed the furnace not kicking on was when it was already colder in the house. I read on other posts that the furnace will try up to 3 times to produce a flame, and then stop trying if gas comes out, but no flame is detected. I cleaned off with steel wool the flame sensor, and now the furnace seems to always come on when supposed to (the house stays at 70). Why it would eventually try again (at 65 degrees), though, is still a mystery since I am unable to duplicate the problem now. Also, since the heater not turning off when supposed to happened rarely, it's hard for me to pinpoint that one too. I will keep my eyes open, but so far so good. I guess it would basically quit trying to kick on if it thought there was no flame, until I manually told it to cycle on, or until some other factor made it cycle. Thanks for the prompt help! -- Joss

  • Anonymous Jan 18, 2009

    I got a Carrier furnace and having the same problem.

  • jossde Jan 19, 2009

    I already have a digital/electronic thermostat. Is there still a possibility that something inside that went bad? -- Joss

  • dt_trainer Jan 21, 2009

    I have had the same problem and cleaned the flame ingiter/ sensor. I actually installed a new one on 01-20-09. My tenant says the furnace is still not keeping the house warm enough. I have a digital thermostat that is about 4 years old. Should I replace the thermostat?

  • ltlshamrock Jan 21, 2009

    I am having the same problem. I had two hvac guys out and there baffled, gas starts up and shuts right off. It eventually will start working again after I shut it off and back on several times. The only smart answer I read online was possible the pressure/temp changes outside are affecting the flue sensor. I'll have to look into that one.

  • Craiger862 Feb 06, 2009

    I have a Janitrol with this problem. Flipping the switch to reset causes it to kick on and run, but it will get under the HOLD temp every time. I have replaced the thermostat with 2 different brand new digital Honeywells and still have the problem.

  • dwkcool May 11, 2010

    next time it will not start before resetting it look in the unit for the light flashing and count the code flashes


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Modern furnaces have many safety features that keep them from coming on. One likely cause is that the vauum hose to the flow inducer( the small motor that kicks on before the furnace lights) is clogged or kinked or filled with drainage water which will prevent the motor from ever coming on. The furnace will shut down until it is reset. Remove hose and check that is clear.

Posted on Nov 27, 2012

  • Mike Disibio Nov 27, 2012

    Modern furnaces have many safety features that keep them from coming on. One likely cause is that the vauum hose to the flow inducer( the small motor that kicks on before the furnace lights) is clogged or kinked or filled with drainage water which will prevent the motor from ever coming on. The furnace will shut down until it is reset. Remove hose and check that is clear.

  • Sherlock Ohms
    Sherlock Ohms Nov 23, 2014

    The tubing for the induced draft proving switch attaches to a tiny port on the induced draft assembly to "sense" the induced draft motors operation, this tiny port must be cleared every year or this will happen.


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Thermostats says that heater is on but furnace will still not come on

Posted on Dec 06, 2013

  • kiddouv Feb 16, 2015

    This sounds like a problem with your thermostat


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Joss sounds like the anticipator setting on thermostat i would replace it with a digital it will be more actturat . The furnace has to go over temp setting to turn off

Posted on Jan 18, 2009

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I need help rplce thrmostat with a digital braueburn my furnance wont turn on no flashing lights cant reset the burner though has power what do i do shoul i reset the circut breaker that was never tripped though

Posted on Jan 26, 2009

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