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Ask Douglas Your Own Question

Douglas
Douglas, HVAC Technician
Category: HVAC
Satisfied Customers: 2927
Experience:  Manufacture Rep for Major Brand, Technical Trainer, NATE
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My central air conditioner wont shut off and the fan to my

Customer Question

My central air conditioner wont shut off and the fan to my furnace wont turn on. The fan works in heat mode but not AC. I first replaced the Thermostat than I replaced the AC contactor. Any Ideas?
Submitted: 4 years ago.
Category: HVAC
Expert:  Douglas replied 4 years ago.
Hi, I'm Douglas and I am here to help you resolve problems with your heating and cooling equipment.

HOw about the blower relay? (It uses a different one for the heat fan speed). If you can, check to see that you have voltage from G to C or you can test the relay by installing a jumper from R to G at the thermostat subbase with the thermostat off.
Customer: replied 4 years ago.
I unfortunately don't have that capability. would that prevent the outside ac unit from shutting off as well as the fan on the furnace from kicking on?
Expert:  Douglas replied 4 years ago.
The outdoor not shutting off would be either a stuck contactor (you said you replaced it so I doubt that's it) or a short between the Red and Yellow thermostat wire. Neither of course would cause the fan not to work because it's driven by the Green wire
but I would start looking to see if somehow the thermostat wire didnt get hurt somehow.
Customer: replied 4 years ago.
Which thermostat wire? none was ever touched until I changed the thermo at the wall. What about the relay? And if it is the relay, would the furnace give you an error light? It is a Carrier Weathermaker 8000
Expert:  Douglas replied 4 years ago.
The weathermaker 8000 uses two different relays to control two different blower speeds. Does your fan work if you put the fan switch to on at the thermostat?
Customer: replied 4 years ago.
no
Expert:  Douglas replied 4 years ago.
So the fan doesnt come on with either the cooling or fan switch (both energize the same wire. Can you remove the thermostat from the wall? Just the thermostat you can leave the subbase on the wall with the wiring
Customer: replied 4 years ago.
ok
Customer: replied 4 years ago.
ok there are 4 wires connected to the base. and the unit has pins that fit into the base

Expert:  Douglas replied 4 years ago.
yes, can you find a paper clip somewhere and bend it so you can touch the R terminal with the Red wire and the G terminal with the Green wire? See if the fan comes on
Customer: replied 4 years ago.
Forgive my ignorance but the wires are connected to the base all 4. what do you mean by using a paperclip? cant you just touch both the red and green to the base terminals?
Expert:  Douglas replied 4 years ago.
are the wires not connected to the subbase? If they are, you could use a paperclip bent in a C shape to "short" the two wires together. I am referring to the wires in the wall, not the thermostat itself.
Customer: replied 4 years ago.
OK, the wires are all connected to the thermo base. I used a paperclip and touched both the g and r terminals at once and nothing happened. I still have it touching both. I didn't unscrew the terms or anything just pressed the paperclip to the terms.
Expert:  Douglas replied 4 years ago.
Ok. The next step would be to go to the air handler furnace and make sure those wires are connected. What we just did is verify the thermostat is not the problem with the fan. If you can find the R and G terminals in the furnace you can jump those also and see if it works.
Customer: replied 4 years ago.
OK, I took the paperclip off the thermo went downstairs to the furnace and put the clip on the red and green and nothing happened.
Expert:  Douglas replied 4 years ago.
is there power???? to the furnace?
Customer: replied 4 years ago.
Jesus! Forgive me! I had the cover off the furnace thus turning off the power to it. I put the cover on, jumped the thermo again and the fan is working.
Expert:  Douglas replied 4 years ago.
OK... .... so jumping R-G makes the fan run. Can you put the thermostat back on and try the fan switch with the door and power on?
Customer: replied 4 years ago.
currently I have the fuse block pulled on the unit outside, does this matter?
Expert:  Douglas replied 4 years ago.
no, lets leave that out for now
Customer: replied 4 years ago.
thermo says 84 in the house i put the thermo to cool and 75 and the fan to on
Expert:  Douglas replied 4 years ago.
and did the fan come on?
Customer: replied 4 years ago.
Only in my dreams Doug, only in my dreams.
Expert:  Douglas replied 4 years ago.
SO with the fan switch to on, it didnt but when you jumped R to G it did? If so your thermostat is screwed up
Customer: replied 4 years ago.
Could it be that both the old thermo and the new one is messed up? Does that also explain why the unit outside wont shut off?
Expert:  Douglas replied 4 years ago.
IF you take the thermostat off the wall again, and put the power back to the outdoor unit does it run?
Customer: replied 4 years ago.
Yes
Expert:  Douglas replied 4 years ago.
That's another problem alltogether.

Crud.... do you have a volt meter?

These low voltage circuits need rung out
Customer: replied 4 years ago.
I am a homeowner with a degree in Applied Social sciences. No volt meter here. The 2 issues must be connected, yes?
Expert:  Douglas replied 4 years ago.
Well, not always. The thermostat closes power to the Yellow wire and Green wire on a call for cooling. WHen you flip the fan switch to On, the fan comes on seperate from the outdoor unit. With the thermostat off the wall, it cant be the problem with the outdoor unit. But clearly it doesnt turn the fan on as it did with the jumper. You have two problems
Customer: replied 4 years ago.
Great! If you were to give a educated guess, what would it be?
Expert:  Douglas replied 4 years ago.
Well, yes, it's what I do, unless you find out some dog chewed the line outside. Clearly I am not there, but I would wring out the wiring in a similar fashion if I were there and didnt have a meter. I can only go by the responses I get in a case like this.
Customer: replied 4 years ago.
Right, I understand. There is not much to an outside AC unit. It appears tyo be working ok since it was freezing in the house when this issue all started. I replaced the contactor and now we are inside. I question the wiring since it hasn't really been touched except for the change out of the stat. So that leads to the Furnace or the Thermo? or both?
Expert:  Douglas replied 4 years ago.
I suppose the contactor could have been miswired hot to the coil in the process. Your thermostat controls the blower in cooling. I am convinced that is the problem with the fan part. Now if you look close back at the wiring in the wall, the red wire may be touching the yellow wire (the copper) which would cause your contactor to be closed.

Why did you replace the contactor?
Customer: replied 4 years ago.
Because I google the issue and it first said to replace the thermo and if that didn't work, replace the contactor. we have been here 13 years and the contactor has never been changed and the AC is ancient.
Expert:  Douglas replied 4 years ago.
they do fail. usually the contacts burn and dont pass voltage or they weld shut. I am worried about a new thermostat that's not turning the fan on though, especially if the old one had the same problem. That's not a common thing but the tests we did suggest that's the case unless the wire is in the wrong terminals.
Customer: replied 4 years ago.
I labeled them with little white stickers that were in the box. Honeywell. I put them on R G Y W do the R1 or the others with a 1 mean anything? Would suggest I bring the thermo back to HD and get a new one?
Expert:  Douglas replied 4 years ago.
what is the thermostat model number?
Customer: replied 4 years ago.
either RTH6350 or RTH6450
Expert:  Douglas replied 4 years ago.
ok one moment
Customer: replied 4 years ago.
ok
Expert:  Douglas replied 4 years ago.
was there a jumper from R to RC?
Customer: replied 4 years ago.
Yes I took it out
Expert:  Douglas replied 4 years ago.
lol.... you're killin me.Tongue out... can you put the little red jumper back in between R and RC? This will likely fix the fan not coming on problem.
Customer: replied 4 years ago.
Ha HA. I threw away. Shall I keep the red wire in the term as well. I can just use a cut paper clip yes?
Expert:  Douglas replied 4 years ago.
You can make a new one out of a small piece of wire or use the paper clip if you can cut it. That wire will have 24volts on it so it;'s best to use an insulated wire. Ifyou can cut a short piece from a spare wire or if there is enough slack somewhere to swipe a peice.
Customer: replied 4 years ago.
the jumper that was there before was like a paper clip. i will leave the red in the term and jump the 2
Expert:  Douglas replied 4 years ago.
Yes from R to RC.
Customer: replied 4 years ago.
Hey Guess what? I put the jumper in, turned the fan to on and the fan works.
Expert:  Douglas replied 4 years ago.
Surprised

So did you look at the yellow wire to see if maybe it is somehow touching the red....

Let me ask you this... what was your original problem that prompted the repairs?
Customer: replied 4 years ago.
Last Sat morning my wife woke me up and told me that the ac wasn't shutting off and it was 65 in the house. We switched the thermo to off and it didn't turn off the compressor/fan on the unit. So I pulled the fuse pack and than did my research, replace the thermo and than the contactor.; I just put the thermo at off and put in the fuse pack and the compressor went on.
Expert:  Douglas replied 4 years ago.
ok.... go to the furnace and disconnect the yellow wire coming from the thermostat to whatever wire or terminal it connects to.
Customer: replied 4 years ago.
Sweet Jesus! I have 2 reds wires. I had a red and a blue in the term marked Y I disconected them both.
Expert:  Douglas replied 4 years ago.
they may have used a red wire to the outdoor unit if there was no yellow.
Customer: replied 4 years ago.
OK well the red and blue have been disconnected. I am getting excited now.
Expert:  Douglas replied 4 years ago.
was the blue wire from the thermostat Y terminal?
Customer: replied 4 years ago.
yes both the wires were in the furnace Y term
Expert:  Douglas replied 4 years ago.
no, which came from the thermostat Y terminal? Was a yellow wire in Y at the thermostat or was a blue wire under Y at the thermostat?
Customer: replied 4 years ago.
A yellow
Expert:  Douglas replied 4 years ago.
where did it connect at the furnace???????
Customer: replied 4 years ago.
there is no Y wire at the furnace there are 2 reds a blue 2 whites 1 with a stripe going to the com term the red and blue is connected to the Y term at the furnace
Expert:  Douglas replied 4 years ago.
Nice.... someone has bugged the wires somewhere.... ok.... so the blue and red are disconnected from the y terminal and not touching each other.... fair? Does the outdoor unit want to run whith this being the case?
Customer: replied 4 years ago.
the blue wire is being used as a Y there is no yellow wire.
Expert:  Douglas replied 4 years ago.
Got it.... see if the outdoor unit wants to come on if you power it... hopefully not
Customer: replied 4 years ago.
The unit outside came on with the 2 wires disco from the furnace.
Expert:  Douglas replied 4 years ago.
nice.... how many of the little wires go to the outdoor unit?
Customer: replied 4 years ago.
It is 10:18 here. Not sure. My question is, if this bitch worked for 13 years and than just messed up, what would that matter?

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