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Central AC not even turning on....

K

krptwo

Junior Member
My central ac unit as well as my furnace will not turn on when I set the thermostat to do so. I checked the fusebox and all appears well. Both worked last week but today, nothing. Any clues or ideas?
 
If you're absolutely sure the breaker hasn't been tripped and is on, check to see if there is a switch on or around your furnace that may have been accidentally turned off.
 
Absolutely sure the breaker is set and there are no switches around unit. The ac unit won't even kick on to blow warm air so I checked the heater and that doesn't turn on either. Never had a problem with either unit.
 
bad connection between the thermostat and the system? or something isn't receiving power? These seem like the most logical causes. (not an HVAC tech).
 
if you have a nest thermostat make sure it doesn't need rebooted. a friend of mine learned that lesson the hard way after calling out a repairman.
 
Since you seem to be getting zero power to your unit you need to focus on that.

How old is your house? Mines old and I have not only breakers but fuses along the power line in places. I.e., checking the breaker is insufficient.

Do you have some sort of circuit or current tester? I have a very inexpensive one that is two lead wires with a LED. IF the LED lights up, you've got current. I got it at Home Depot or Lowes or a couple of $'s.

Anyway, you can use something like that to test that you've got power to your unit(s). If so, then maybe it's your thermo. If not, I think you've another problem.

Good luck with it.

Fern
 
FWIW thermostats do just die. I had one that looked okay, but wasn't sending a signal to hvac. Had a tech come out and he told me that. Just went and got one from home depot. Before you buy, replace the batteries. That was another cause of it not kicking it on once.
 
Close the breaker and turn it back on. If it tripped, it will often appear to be on. Though usually they should go half way or have some kind of indication.

Once you confirm that, double check for a switch near the furnace, or maybe near one of the house entrances. In case of fire you turn it off so it does not feed air to the fire. Though installers tend to forget the purpose of these and I've seen them right next to the furnace, kinda defeats the purpose. :/

Once you confirm that, open the front panel of the furnace, and look for a set of terminals, something like this:

img18411.jpg


Take a piece of wire and put it between red and white. That should kick on the furnace. Red and green should star the blower IIRC. If that does not work, then try to determine at what point you have power. Check the 24v transformer. Could be it's blown. Hopefully it's not the board.

This is the extent of my furnace knowledge though, but given even the blower wont turn on I'm leaning towards an issue on the control board and not any of the furnace parts like the valve etc.


In fact, I'm even getting ahead of things, first look for where the main AC line goes into the furnace and find the first spot where you can access the bare wires (probably somewhere on the control board). Take a volt meter to that and check if you are getting 120v. Maybe for some reason the circuit is dead. If you are not getting 120v then you need to troubleshoot upstream of that.

Layer 1... 😛
 
You say it is an old thermostat. Examine the glass mercury capsule that is attached to the coil spring. See if you can discern if the contents have leaked leaving the capsule empty.
 
Red Squirrel said:
Close the breaker and turn it back on. If it tripped, it will often appear to be on. Though usually they should go half way or have some kind of indication.

Once you confirm that, double check for a switch near the furnace, or maybe near one of the house entrances. In case of fire you turn it off so it does not feed air to the fire. Though installers tend to forget the purpose of these and I've seen them right next to the furnace, kinda defeats the purpose. :/

Once you confirm that, open the front panel of the furnace, and look for a set of terminals, something like this:

img18411.jpg


Take a piece of wire and put it between red and white. That should kick on the furnace. Red and green should star the blower IIRC. If that does not work, then try to determine at what point you have power. Check the 24v transformer. Could be it's blown. Hopefully it's not the board.

This is the extent of my furnace knowledge though, but given even the blower wont turn on I'm leaning towards an issue on the control board and not any of the furnace parts like the valve etc.


In fact, I'm even getting ahead of things, first look for where the main AC line goes into the furnace and find the first spot where you can access the bare wires (probably somewhere on the control board). Take a volt meter to that and check if you are getting 120v. Maybe for some reason the circuit is dead. If you are not getting 120v then you need to troubleshoot upstream of that.

Layer 1... 😛
Click to expand...
Central AC is going to 220volt, the older thermostat's usually run off a 24volt tap from the main line. OP, as suggested above get an amp meter or a cheap DMM and start looking to see what's getting power and what's not..
 
Y and C are the air conditioning terminals. R and W are for heat, R and G are for fan only. There's usually an LED on the board somewhere to indicate power if you don't have a meter. Also, if there's no ambiant noise you can usually hear a buzz. As suggested above, short out the R and G terminals with a pair of needle nose pliars (make sure the failsafe switch is pressed since the lid is off). If nothing happens, your 24v transformer is no good, or you simply have no power at all to the unit. Pics will take us to the next level if this doesn't work.
 
Actually before opening the furnace/air handler cover, just take the thermostat off and short the proper wires together. That will at least rule out the thermostat before you start getting into the furnace/air handler. Considering the blower or furnace does not start either, the AC unit itself is probably fine, the issue is in the furnace/air handler.
 
Have you got a volt-ohm meter?
If so, check for voltage (24) across the "Y" and "C" points.
If no volt meter. How can you be sure the unit is getting power?



.
 
For some reason your pic doesn't show up for me. What ever wires are connected to the R and G terminal, short them together. That will make a call to the blower motor only, so you won't have to worry about other problems that can affect a call for heat (dead exhaust fan on high-efficiency units, bad igniter, bad thermocouple, etc). If the fan doesn't kick in immediately, take the door off and short the R and G terminals on the board, remembering the safety switch will have to be manually pressed with the cover off.
 
I had similar issue, turn out to be a clogged water drain switch on the drain pan. The safety switch cuts power to the furnace when it detect water not fully drained out and may flood the drain pan. Worth checking if you have one.
 
PVC drainage pipe was indeed clogged. I blew out the entire the line, how do I reset the furnace? The pilot light is still burning but neither the heat, AC or fan will turn on.
 
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