Air conditioner vs power failure

[Tom Bender]

Our power went off and on a few times and the A/C lost it's capacitor. The serviceman said he changes several every week. This would be prevented if there was a start delay. Any easy way to add this? I'm thinking it should be done in the 24 volt control circuit.

[Brian Elfert]

Air conditioning capacitors fail for all sorts of reasons besides power failures. Some of them get all puffed up. Many air conditioners already have a time delay somewhere in the circuit so you might be able to take advantage of that. The delay is because they don't want A/C units short cycling.

[Mike Henderson]

All the air conditioners that I've worked with have a delay between startups. You can test this with your thermostat. Set the temperature low to cause the AC to come on. Then switch the unit off with the thermostat switch (move from cool to off).

Then switch the thermostat back to cool. You'll notice that the AC does not come on right away. The fan may come on but the compressor does not start.

The purpose of the delay is to allow the pressures in the system to equalize so the compressor is not trying to start against a high pressure.

Even if the compressor stalled, I doubt if that would cause the starting capacitor to fail. Most AC units also have stall detection (something that detects excess current) and will disconnect the compressor. Once the detector cools down, the compressor will try to start again.

I can't say that every AC has these safety features but I'd think most would.

Mike

[Tom Bender]

Ok that's right.

Maybe the power failure caused a short brown out which caused high amps. Preventing that would require a whopping big UPS. Better to just replace the capacitor as needed.

[Stan Calow]

Tom, what about a whole house surge protector? Our utility is really pushing them now. They'll install free, with a $6-9 monthly fee, including $5-10K of insurance. I'm thinking about signing up, as we've had at least a dozen outages this year.

[Bill Dufour]

Square D QO breakers sell a special whole house breaker. It just plugs into a 240 volt breaker slot with a neutral wire to the neutral bus. Cost around $50 on the bay to $150 in stores. Locate it in the slot closest to the main input.
Other makes may offer similar. Or install a generic whole house hard wire surge protector inside or closely attached to the main breaker.
Bill D

[Kev Williams]

Dumb question: How does an AC unit's "delay function" know exactly when to delay? High pressure relay, some sort timer, or ?

[roger wiegand]

Buy an extra cap and you'll be running again in 10 minutes the next time one fails (which it will!).

[Tom M King]

Any time we have had an HVAC capacitor fail, which they will always do at some point, I replace it with whichever model of these Rectorseal Kick Start things is the right one. I used to just keep regular spares, but decided I'd try these. I don't even remember when I started using them, but I've never had to replace one of them.

35 bucks, and fifteen minutes work (including troubleshooting), versus call, and wait, and pay $250 for the convenience of getting a "Pro" to do it.

https://www.amazon.com/dp/B008A3UJ7I...v_ov_lig_dp_it

[Rod Sheridan]

Dumb question: How does an AC unit's "delay function" know exactly when to delay? High pressure relay, some sort timer, or ?

Hi, they use a timer….Rod

[Mike Soaper]

Any time we have had an HVAC capacitor fail, which they will always do at some point, I replace it with whichever model of these Rectorseal Kick Start things is the right one. I used to just keep regular spares, but decided I'd try these. I don't even remember when I started using them, but I've never had to replace one of them.

35 bucks, and fifteen minutes work (including troubleshooting), versus call, and wait, and pay $250 for the convenience of getting a "Pro" to do it.

https://www.amazon.com/dp/B008A3UJ7I...v_ov_lig_dp_it

Tom, thanks for the link.

After a same value but physically smaller replacement cap failed the following year I then used BMI and Amrad caps from Airstar Supply | Solutions for Today's HVAC Problems (no association besides being a customer).

From what I've read some inexpensive caps use inexpensive vegetable oil instead of a better oil, that and the film between the conductors can't handle voltage spikes as well as better films. Voltage spikes can burn through the film and short things out.

Part of me understands techs using inexpensive caps because the customer doesn't want to pay for a better cap, but another part of me wonders if having the ac fail again a couple of years later due to cheap caps helps to nudge homeowners into buying new complete systems.

[Bill Dufour]

Power company here, and many others, have a program to add a control box to your ac unit. They can turn the compressor off remotely when demand is high. Thing is it has a delay and the compressor will not restart instantly. It has a built in start delay time circuit. They pay about $5.00 a month during cooling season if you have them install one. the air handle continue to run and they promise not to turn the compressor off more then 15 minutes every few hours.
Bill D
Bill D

[Tom Stenzel]

The on-delay for my air conditioner is provided by the thermostat. If you turn the air off and then on again too soon the little snowflake in the display will blink indicating that the air is set to run but is in the time-out period.

If there's a power failure there's no timeout. The thermostat has batteries and is unaware if the main power is on or off. I've seen commercial air conditioners with a loss of power delay but haven't seen a household one with it. This is just the ones I've worked on, mine, my Mother's, my Brother's, my neighbors, a few friends. The sample selection isn't that large although larger than I'd prefer.

-Tom

[Kev Williams]

The on-delay for my air conditioner is provided by the thermostat... If there's a power failure there's no timeout.

-Tom

That's why I asked if a timer is used. If the delay is based on an electric-operated timer, then it's useless if the power goes out.

What comes to mind here is pressure washers; the pump mechanism disengages the pump mechanically once the water pressure passes a certain limit, re-engages once the pressure falls... I've always thought that's how AC compressors were set up... guess not?

[Mike Henderson]

That's why I asked if a timer is used. If the delay is based on an electric-operated timer, then it's useless if the power goes out.

What comes to mind here is pressure washers; the pump mechanism disengages the pump mechanically once the water pressure passes a certain limit, re-engages once the pressure falls... I've always thought that's how AC compressors were set up... guess not?

Unless the power just blinks, it will likely be off for the amount of time the thermostat would keep the compressor from starting.

But if you had a very short power failure, you could have the compressor attempt a hard start against high pressure.

Mike