Monitor house A/C condenser for failure

The run capacitor has failed again this year on my A/C condenser. For those not aware, that means the fan and the compressor won’t start running on the outside A/C unit, but they will be humming and groaning away trying to start. If you don’t catch it in time and leave it in that state it can burn out the compressor and/or the fan motor.

It would be great if somehow you could monitor that (1) the thermostat called for the A/C to come on but (2) it did not start so (3) the thermostat will be told to give up and turn off the A/C to prevent it from burning out. … and then it would be nice if it sent you an alert.

Anything like this already on the market? I tried to search but only come up with anti-theft alarms for the condenser, I cannot find anything to monitor operation.


You could also put an energy meter on your ac circuit at the breaker to monitor current draw. Couple that with a temp sensor near or even in one of the registers.

Use webCoRE logic to cook up your own rule (Piston)

If therm is cooling
Temp sensor is less than xxx
Power is between xxx and yyy
With therm
Wait xxx minutes
Turn Off


How big (power rating) is your a/c?

Is it connected with a plug or fixed wiring?

Use a Z-Wave powerplug (Neo CoolCam), a clamp-on current transformer (Aeotec) or install a a switch module/relais with power meter built-in (Fibaro Switch) in the circuit.

Neo CoolCam powerplug

Aeotec clamp-on current transformer

Fibaro Switch module

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Yes it is a regular whole-house central air condenser. So it is 220VAC hardwired. If I had current monitoring device not sure that the current in normal operation and the current while trying to start with a bad capacitor would be different enough to tell that it is running or not. Can a specific current range be chosen and be a trigger?

Maybe this is what I need…

In the powerplug I use is the current measured and you can set an alarm limit to eg 12A and switch off (protect) at 13A.

A motor with a broken capacitor will not start/run and draw a higher current.

How much higher is hard to say, depends on the power rating of that motor (HP/kW).

Option is measure the current with a clamp-on Amp meter when running and with the condensator disconnected, as if it is broken, measure the current.

What is the electric rating of your A/C? Is it 1-phase or a 3-phase motor (higher power rating)? Sorry, 3-phase doesn’t use a capacitor to start.

Look for a suitable zwave or zigbee relay that measures current and can be adjusted to alarm you.

PRM2-ZBS Series Power Meter Switches:

The slim-sized PRM2-ZBS and PRM2-ZBSR power meters plus relay switches can be installed inside power sockets. Connecting to power cables to automatically control appliances and measure and report their energy consumption, these devices allow you to turn on/off appliances from a smartphone. The PRM2-ZBSR also functions as a repeater associated with up to 40 ZigBee devices to extend a security system’s wireless range.


Or via Z-Wave (check for the proper frequency)

Aeotec Nano Switch

Grtn Ben

I would advise that you put a reminder in your calendar to check the capacitor once a year at least with a multimeter. That is the best way to check if it’s going bad. There are YouTube videos and instructions like this on how to do it.

As the capacitor gets old, the readings gradually fall out of range.

Capacitor failure every year is a hint of something else wrong. You are just putting on a band-aid when surgery is needed. It should not fail every year.

In the washing machine it lasts for years

Maybe a wrong type, the one that failed, was replaced by the same wrong value.

The capacitor size could be calculated by this formula:
C= 50 x P x (220/V)2 x (50/F)

C= microFarads, P= power output in HP, V= Voltage, F= Frequency (Hz) .

Your MOTOR is P (HP) = 1, Volts: 240, and F = 50 then C = 42 microfarads.

And the max voltage of the condensator, at least 400v in a 220v system

PS in English it is capacitor!? :sunglasses:

Not accurate to say it fails every year, the A/C has been in service for 12 years. . The original failed early last year for the first time after being in service for 11 years. The replacement failed after approx 1.5 years, so maybe it was not a good quality one. Just 2 times total.
Cap is a dual cap, 55/5 370V proper specs replacement installed each time.
Preventive maintenance check the cap each year, probably not going to happen, that I why I am investigating smart technology/home automation to be constantly on guard.
Thanks everyone for your time and input, certainly an interesting project!

You need to clean your coils every Spring why not check the cap then? While automation is great sometimes manual elbow grease is required.

There’s bound to be some level of confusion here!

The “condenser” in an HVAC system is the outdoor part of your air conditioner. A “capacitor” was also called a “condenser” back in the day. That use has pretty much fallen out of use—at least in the USA—but, nevertheless, you still see it used from time to time.


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OK please assume the following are true conditions for purposes of my question and request for help, the proper specification capacitor is installed, and proper periodic preventative maintenance is performed.,

So, as a home automation enthusiast like many on this forum, I have an interest in being able to monitor my HVAC unit for proper operation 24/7 , and it would be great if it could be integrated into my SmartThings system.

A simple approach may be putting temperature probes in the ducts: one in the output duct, the other in the return air duct. Assuming you already have the ‘running’ state of your HVAC system, some clever programming could show you if the system is ‘cooling’ or ‘heating.’ If not, send the appropriate notification that something is wrong.

In the summer, if your condenser fan doesn’t spin up, the system won’t cool. It also “won’t cool” in a number of other failure cases: low coolant, tripped circuit breaker, faulty air circulation blower, etc.