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HVAC surge protector

Cyclops56 | Posted in General Questions on

After power spikes or when the power very briefly goes off then on, my A/C will continue to run but not cool. I have to flip the main panel breakers off and wait for a while before turning the A/C back on. This happens mostly during storms. I installed a whole house surge protector on the main panel but the problem persists. Should the protection be exterior breaker for the compressor? Or an additional one? The power company knows that there is a problem but can’t find out what.

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  1. Expert Member
    BILL WICHERS | | #1

    Anytime inductive things (motors, transformers), there are voltage transients produced that travel down the line. Those are small in comparison to a lightning strike, but they can still cause problems. "Whole home" surge protectors (known as TVSSes -- Transient Voltage Surge Suppressors -- in the trade) are great for protecting against these smaller transients, and will help with lightning although a strike will usually kill the protector since it will sacrifice itself to help protect your equipment.

    It's probably not the voltage transient that locks out your A/C, it's probably a power loss detection, or possibly undervoltage detection, in the equipment. This is a protection feature. Compressors don't like to rapidly stop and start, so it's possible there is a delay after a power interruption to make sure the system settles down prior to restart (pressures equalize in the refrigerant lines, etc.). When you say your A/C continues to run, but won't cool, that implies the blower in the indoor air handler is running, but the outdoor compressor is not. The logic in the furnace/air handler may be what is locking things out, or possibly the thermostat, and if that's the case, it will probably restart on it's own after a period of time. Look in the manuals to see what it says. It would be easy enough to track down what is locking things out if you have a volt meter and can probe around in the control wiring, but I wouldn't recommend doing that if you aren't familiar with working on this kind of equipment.

    The next time this happens, I recommend you wait up to an hour or so to see if the system automatically restarts, and keep track of how long it takes. If it consistently restarts after a nice number (10 minutes, 15 minutes, etc.), then it's probably under control, and working as designed. If it restarts after random amounts of time (7 minutes, 33.2 minutes, etc.), then it's probably something goofy. We engineers like round numbers, so delays are likely to be in multiples of 5 minutes, probably 10-15 minutes or maybe a little more.

    Regarding placement of the TVSS device, that should go on your main panel. The only reason to add one at the compressor (the outdoor part of the A/C system) is if there is a very long (100+ feet or so) run of power cable between the main panel at that compressor. I doubt you really need to add a protector there, although I would check to make sure your existing protector is working since they are consumable devices that don't last forever. I like the little units made by Ditek, which are around $50-60 or so for the residential configurations.

    BTW, there is one other thing you should check. It is VERY VERY important that you have what is known as a "single point ground". That means all your services -- power, telephone, cable TV -- anything with a wire, be grounded at the same place your electrical service comes into your home. This makes sure that if there is a lightning strike nearby, you don't have voltage differentials between different cables inside your home. This is actually even more important than the protector itself. Check this, and fix it if it's not done right. It is current code to bring things in this way, although it's not always done correctly, especially on older homes.


    1. Cyclops56 | | #4

      Thanks. The compressor also keeps running. Maybe at a slightly lower speed. I use a temp reading gun to make it easier to tell if the system is not cooling. An electrician friend has also mentioned that a power loss or low power is triggering the system to partially shut down. The power company does not seem motivated to find the issue so I hope that there is no permanent damage being caused. I appreciate your diagnosis.

      1. Expert Member
        BILL WICHERS | | #7

        There are really only two parts to a typical residential A/C system: the outdoor compressor unit, which has the compressor inside and the cooling fan on top, and the indoor air handler, which is pretty much just a blower motor when in cooling mode. If the indoor and outdoor units are both running, you should be getting cooling. It can sometimes take a bit for the system to get the refrigerant up to pressure though if it's been shut down for a while.

        Power interruptions during storms are sometimes a normal part of the operation of the power company's system. Lightning strikes may trigger surge arrestors on the lines, which will cause short disruptions to service. It's entirely possible there is nothing "wrong" with their system, so nothing for them to fix. If you have times of low voltage (usually defined as under about 110v or so, 120v is normal), that's something you should be able to get them to fix if the duration of the issue is long enough.


        1. Cyclops56 | | #8

          The interruptions are a millisecond and blower continues to run for at least 10 minutes without cooling. My neighbor is have the same issue and his A/C is brand new. Here in Florida the inside temp rises quickly. Thanks again!

          1. Expert Member
            BILL WICHERS | | #9

            That sounds like the compressor has locked out.

            Quick power hits as you described are usually caused by one of two things:
            1- lightning arrestor firing, which would only happen during a thunderstorm
            2- tap changer operation, which is a problem usually at a substation, but sometimes in a pole-mounted voltage regulator on a very long distribution line.

            Tap changer problems are often tied in with consistent over or under voltage situations which the utility company should be able to fix, but you'd need a volt meter to be able to monitor things.


  2. walta100 | | #2

    When it is acting up what does the thermostat display say?

    On most systems the only logic is in the thermostat you may want to try a different make and model thermostat.

    If the problem was some type of power interruption timer resetting the breaker would only restart the timer.


    1. Expert Member
      BILL WICHERS | | #3

      >"If the problem was some type of power interruption timer resetting the breaker would only restart the timer."

      That's often not the case with these simple systems. Many HVAC controls have no backup power for the control logic, so the timer only runs when it has power. Shut off the power and restart, and you wipe out the timer. Many controls can detect fast glitches, but a longer shutdown of 5-10+ seconds will clear many timers.


      1. Cyclops56 | | #6

        Bill, the shutdown is an instant. I guess I need to be more patient and wait for the system to recover. I would guess that it taking at least 15 minutes. Thanks.

      2. walta100 | | #10

        Yes Bill, very simple systems that see any interruption in power as a reason to inhibit the compressor for one time cycle on power up because the simple system has no idea what the compressor was doing when the power went down or for how long the power was down.

    2. Cyclops56 | | #5

      The thermostat reads normally. The only indication is warm air. Thanks!

      1. walta100 | | #11

        Generally, when the thermostat is set to call for cooling but is locked out on time the cooling indicator will be blinking and when the timer expires the cooling indicator is on steady and the compressor starts.

        Kind of subtle difference that one might not notice unless they were expiating to see it.


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