Gas and Oil Home Heating Furnaces - HELP!!! No A/C !!!

it is up around 88 degrees in this house and i need a little help. here's the situation. our a/c was working fine for a couple of days and all of a sudden it stopped working. at the thermostat, when you set the fan to 'auto' and switch the 'cool' on, the a/c outside fires up, but the exhaust fan doesn't work. this same exhaust fan works just fine if the heat/cool is set to off and the fan is set to 'on'. while the fan is running, if you set the thermostat to 'cool' the a/c will not fire up. i did change the thermostat in the winter and had some (wrong) wiring issues, but the a/c was working fine a week and a half ago. we would really appreciate any help, we're melting...

hydro187:
I don't completely understand the description of your problem.
When you say exhaust fan I assume you mean furnace fan.

If you changed the stat and had wiring problems with the heat then there is a good possibility the wiring for the a/c is messed up as well.

If you go to the furnace and mark down the terminal number and wire color connected to each terminal you can then check at the thermostat to make sure the correct wire is connected to the right terminal.
The ID on the furnace should match the stat terminals.

and you still have wiring issues

if it worked properly last week and does as you say tonite, you have thermostat wiring issues now
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power should be shut down, first thing
you getting close to transformer burn out or possible circuit board burn out--
remove power to furnace or air handler unit--the unit with your (exhaust fan)... we call it the blower assembly
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maybe someone will help you tonite if I don't make it back
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you need to supply some important info before anyone can help
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list your equipment
heat pump?
central air?
air handler?
gas furnace?
brands or models if possible
old thermostat type--new thermostat type(brand name and model#)
need wiring terminals identification for new stat(R-W-G-O-W-B-Y-Rc-Rh ??? list all) and thermostat wiring colors and where you have wires connected)
--
and hope you have the note you made for yourself when you removed the old stat--wiring color to terminal markings--not necessary but useful if you still have it

once i actually wake up i'll get some more info, but i have central air, gas furnace, the old tstat i think was an old honeywell? circular dial, and the new one is a digital/programmable duracraft. i have my doubts as to whether or not the wiring is messed up, as it was working properly for a couple of weeks. i just got back from vacation yesterday so i will have some time to troubleshoot today. thanks again for all of your suggestions, and if any new ideas are floating around i would be very grateful. have a good weekend!

jump red, green and yellow together at the tstat. if it works, replace the tstat.

i jumped the r, g, and y at the tstat and still no dice. when i remove the green and keep only r and y, the a/c unit outside runs... if i replace the g with the y and jumper it with the r, the furnace fan runs. for whatever reason the furnace fan and the outside a/c unit will not work at the same time. it must be a low voltage problem, but where?? i work for the hydro company, and 27,000 volts makes a lot more sense than 24 volts!!!

Ok 27k that big real big. Think R is your + here and Y goes to the high side outside and the G goes to the inside fan . Has anyone did anthing to the wires or the tstat. Try and tie R G Y off the tstat and you should get ac and indoor fan. This should tell you if its in the tstat ;) ED

it's official: unfortunately r,g,y jumpered together do not work. after i jumpered them together, the furnace and a/c were working together for about 30 seconds before the furnace fan quit. the tstat is relatively new (less than one year) and no wiring has been touched since. i don't have a digital voltmeter, so determining 24 volts is basically impossible, but common sense tells me that if each unit (furnace fan and a/c) works independently, it must be some sort of relay or switch??? i'm getting really frustrated (and hot), someone help me!!!

???Did you put the R -Y -G together off of the tstat??
And it ran then stoped??? You didnt blow the board here if you have one on this unit? What kind of furnace is this ac on?ED

the furnace fan will not work for more than 30 seconds once the a/c outside is turned on. i checked for voltage on the board where the furnace fan gets power when the a/c is on and there is no voltage. i believe, and keep in mind i learned everything i know about a/c in the last 24 hrs, that the cooling is a high speed fan setting, whereas the heat is a medium high. is there a relay before the 120 v reaches the furnace fan? if there is, it obviously isn't working, and what can i do about it? is there a way for me to by-pass it? i would say my understanding of electricity is above average, so please do not hesitate to give me some insider's information, i promise i won't electrocute myself- i know how to flip the breaker to the furnace! thanks again for all the help, hope i am not inconveniencing anyone.

ed...

yes i put the r,g,y together off the furnace.

i have a keeprite furnace that is about two years old... i have no clue what kind of furnace (i am a layman) it is a mid-efficiency gas furnace.

wouldn't you be able to smell something burning if you blew the board?

thanks for all your help.

you may have had isolated heating and cooling on the power connections. Do you remember if the old T87f subbase had an Rh and Rc terminal that was with separate wires or did the red wire get a long stripping and connect to both terminals? (The old subbase was most likely the Q539 1147 or the 1014).
Do you still have the old subbase and tstat?
Connect the r to g and see if it runs for 15 minutes or so without stopping. What happens with the mode in cooling and the fan in the on position? Is it the fan switch in the auto position that is the problem?

i just have the one r connection @ the tstat- before and now. r-g runs the fan continuously. if the fan is on, the a/c outside will run, but the furnace fan will shut down within 30 seconds. if the a/c is on, and the fan is switched to 'on', the fan will not work. i am totally lost. the furnace is a brand new keeprite, not even two years old. the board shouldn't have problems like this. i guess i should mention that the furnace filter was pretty dirty before all this... just a thought. thanks for all the help once again...

To do this put together the green red and yellow. This should run the fan in the furnace and the outdoor unit. If the fan quits after 30 seconds as stated before, something amis is at the board or the fan motor. You'll have to check the motor for voltage then. I assume the fan relay is in the board?

hydro187:

To totally eliminate thermostat problems I would perform the jumper test at the furnace.
Carefully mark the wires connected to the furnace and remove them.
Jumper R to G, the fan should start. Leave it connected for awhile to make sure the fan continues to operate.
With R to G jumpered add one between R and Y and see what happens.
Cover C with a small piece of tape to prevent a short circuit.

i think the cooling mode is a 'high speed' fan setting for the furnace fan, whereas heat is 'medium high'. is there a way to bypass the 'cool' lead (on the board) going to the fan? beside the 'cool' is a 'cont' prong, which i am assuming means continuous. i isolated the furnace, put the cool lead onto the 'cont' prong and re-energized the furnace, resulting in the fan running continuously. when i put on the a/c outside, the fan shuts down after 30 seconds. i think the problem is with the 'high speed' fan setting at the furnace. does this make sense? thanks again for all your help, everyone has been great!

hydro187:

If you have jumpered the terminals at the furnace and the fan still shuts down in cooling mode, cct board replacement is required.

i was afraid you would say that. sounds expensive. i'll have to get someone in this week. what would make a new furnace have this problem? i have the receipt for the furnace, hopefully that will help. take care everyone. thanks for trying to help me, if nothing else i learned a lot about the inner workings of the furnace and air conditioning!

you'll need a meter. When the motor shuts down see if the leads to the motor have line voltage. If they do, the motor is toast!. If the board relay terminal is open, replace the board...the only thing is if the motor pulls high amps, the board might have some protective device that opens, I doubt it though, they usually just burn up. The best bet is to amp the motor, this is the only true way to know. If the motors bad and you put in another board and it burns up because of a overamping motor, you'll be out a board and have to replace it again AND the motor. .. try this....switch the speed leg of heating to the cooling post and leave the high speed one tapped up. If it runs OK there without shutting off, replace the motor.