Any HVAC specialists here?

I have a newer Carrier AC/furnace unit, and yesterday it conked out.

Does not come on at all, either the inside blower (nothing coming out of the vents), or the outside unit.

Things I have checked:

  1. The house circuit breaker. Nope.
  2. The thermostat. Everything seems fine there. It uses a battery and they are charged up.
  3. The outside “plug” or whatever that’s called (it’s the thing in the box that you turn upside down if you want to temporarily shut off the outside unit).
  4. The inside circuit board. Looks fine, no scorch marks. Fuse on it was intact. There are, however, no lit LEDs or anything that would indicate it is getting any power.
  5. The electrical wiring into the unit appears intact, as far as I can see it.

Any more steps I can take to diagnose/fix it? I have a strong feeling it is something on the inside of the house. I’d like to do a last ditch effort to fix it myself because the company that installed it are huge dicks and I’d prefer not to have to deal with them again if I don’t have to.

#4 it would appear that either the board has failed or there is no power.

tripped breaker faulty wiring.

If the furnace fan (inside fan) is not running hopefully the outside unit will not be running.

I am thinking as I type.

Where the inside unit is should be switch to turn the furnace on or off to work on the unit. Look at the inside unit power should be connected to the unit, either by romex or metal flexable wire connector. See where it come from. There should be a switch near there check it.

Normal operation on cooling demand should be inside fan comes on, after air flow is established outside unit should start.

I’ll take a couple pictures of the board and post it tonight. There is a tiny on/off switch in the inside unit that I flipped on and off (originally set to on). Didn’t seem to do anything. There is a big red lightswitch-like switch outside the furnace that I switched on and off, didn’t seem to do anything either (was in the on position).

I’m not an electrician by any means but it seems like it’s not getting any juice at all. I suppose I could get a voltmeter, but I wouldn’t know where to test it.

If you get a voltmeter, you need to test two important points.

  1. DANGER - High Voltage! Put the meter on AC Volts, and >250v range, and test the voltage across the thick wires leading from the outside of the unit to the big relay (contactor). Those should have ~240vac on them at all times. If they don’t, you have a bad breaker or bad wiring somewhere. You can also try measuring the voltage from each lead to chassis ground (a screw or other clean connection on the metal case). Each lead should have ~120vac with respect to ground. If only one lead does, the other is bad, and indicates a bad wire, bad breaker, or bad buss bar in the breaker panel.

  2. If you have 240v, test the low voltage coming from the thermostat transformer. These are always in different locations for every unit, but you should be able to tell the transformer because it will be connected to high voltage on one side, and the thermostat wires on the other. There should be 24vac on the secondary side.

Check your condensate drain. If it feeds to a pump you could have a problem in the pump and the safety switch. Pump

I wire the safety to kill all power to the stat, so even though the batteries keep the program, time of day, etc. it can’t activate anything.

If it’s a gravity drain you could have a safety on the secondary drain. Secondary. Check the floats and switches to be sure nothing is hanging them up
You say nothing works. That narrows it down to loss of 24v control voltage. Check any other safety switches in the furnace section. If there’s a tripped safety, find out why it tripped.

(Checking the Dope while waiting for my pump to reclaim 150 lbs of refrigerant. 2 hrs and counting)

The inside unit is probably only 120 VAC.

What happens when you set thermostat to “fan only”?

Okay, here’s the pics. All of the pictures but the first one are on the inside of the house.

This intrigues me. We have a white pipe that empties into a french drain. We have had heavy rains over the last few days, and perhaps that french drain filled up and maybe that caused this? So you’re saying if that gets clogged, EVERYTHING goes kaput? No light even on the control board?

Turn the Burner switch off. Wait 10 seconds then turn back on. If the little yellow light does not blink at all then I doubt that you have power going to the unit.

There is probably a power kill switch that operates when the cover is removed from the furnace. Try manually pressing it in. If you are searching for voltages with a meter you will likely need to keep that switch engaged manually.

I recommend meter leads that have a good clamping lead end on the ground. So you do not need to hold both leads at the same time. Rubber gloves are good too.

The circuit board may have a ground that is isolated from the incoming AC ground or neutral. The board will have an AC to DC converter that may isolate things.

The AC may not be making it to the furnace even if the breaker and switch is on. Poor contact at the switch or inside the furnace. Turn off the breaker and ensure the furnace power switch connections are tight. Not scorched due to arcing at a loose connection.

A mostly good board that is getting power should show led’s. The AC/DC power supply may be cooked.

Well, if it just dumps into a French drain then you don’t have a Pump or Condensate safety switch. Also I see that you have nothing connected to the secondary drain, that’s where that red plug is.
You say you check the three amp fuse on the board at checks good. I would double checked it three interviews and make sure it is good. If that’s not the problem then you’ve lost 120 V power to the furnace all together. Could be a bad breaker or your disconnect switch. That will burn or safety switches left over from the oil burning furnace that used to be there. It may be old and deteriorated or you might not be getting voltage to it. Be safe and check for 120 V. If you don’t know what you’re doing it will be time to call pro. Good luck

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What that should have read his check the three amp fuse. Further down that should be oil burner switch.
I’m on a rooftop using my phone and tap talk.
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Thanks. Damn, I was really hoping it was going to be fixed by clearing that pipe. I don’t really know what I’m doing with live wires (I’ve done minor electrical work but only with the whole house shut down). Plus I don’t have the skill to find a break once it goes into a wall (the wire seems totally intact up to the ceiling where there is a hole).

Do you think it would be better to call HVAC or an electrician? The fuse is definitely fine.

Yeah; it’ll only hurt a little. :slight_smile:
It is probably something simple as there’s not a whole lot that can kill all the power to the furnace.
Good luck.


If you call an electrician and the problem turns out to be in the HVAC system you will probably have to call an HVAC guy.

If you call a HVAC guy he will be able to tell if the unit is getting power. If it is not he should be able to chase down where the problem is. And unless you have a real electrical mess on your hands he should be able to handle it.

Update: [SOLVED]

I bought some electrical testing equipment and confirmed there was no electricity getting to the indoor unit. This caused me to investigate further and I discovered there was a second red switch on another other floor I was unaware of that had been switch off. Flipped it back on and everything fired up.

Eureka! Thanks all.

Saved yourself a service call, and now you have a new tool.

Hey! I think you’d better call some service. Because Last summer my AC has broken too and I was struggiling with its reparing till I called fonko service and their repaired everything.

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