South Carolina Dumb Short cycling question

Discussion in 'Maintenance and Troubleshooting' started by RonL, Apr 29, 2021.

  1. RonL

    RonL Member

    I read a post below about a compressor short cycling . So my question is, how long should your compressor run ? We had one of our first semi warm days yesterday (low 80s) and checking how my system was working. It seems pretty consistent at 7 min utes on, 20 minutes off. I would think the cycle will change as it gets hotter. But how long should it run before it would be considered overpowered for the house ? I know there is a happy median somewhere, just not sure what that is. In other words how long is to short of a cycle where it is bad for the system..
  2. gsmith22

    gsmith22 Active Member Forum Leader

    the short answer is no one knows for sure. But just like your well pump, turning the system off and on repeatedly isn't good for it as lots of wear accumulate during the startup phase. Probably more important, short cycling during AC mode will not reduce humidity because the air coil never gets cold enough for long enough to pull water out of the air moving across it. So the system drops interior temps but doesn't drop humidity making it not all that comfortable inside even if the thermostat temp is satisfied. Its probably not a problem right now as summer hasn't really started, but midsummer anywhere east of the Mississippi and short cycling means no dehumidification. On a single stage unit, there is pretty much no way to avoid short cycling because it is sized for the worst day (say mid July) so way oversized for late April. Dual stage units are better, variable speed the hold grail matching load to output all throughout the season
  3. RonL

    RonL Member

    I was always concerned that my unit was running to much and now there is a possibility of running to little. This is a very confusing industry. I finally installed an almost whole house monitor for my electricity so I can see what runs and when and for how long. So I will be monitoring this. My main concern is running to much water out of my well. So i never liked the 2 stage approach where the hp runs longer but on less amps. Good for the hp. Bad for the well.
    And even all the new single stage units seem to have variable speed fans.. that's a bit confusing does it know when to go fast or slow... maybe monitors the air temp of what its generating...?
  4. SShaw

    SShaw Active Member Forum Leader

    Variable speed fan doesn't mean what it sounds like on a single or dual-stage system. A variable speed fan would be better called a "constant CFM" fan. The motor will adjust speed to maintain the target airflow in cubic feet per minute as duct conditions change, e.g. closing a supply vent, dirty air filter, etc. It won't change airflow based on air temp. Most will be programmed to ramp up slowly when the equipment starts to reduce noise. Some systems may be programmed to reduce airflow by a set amount (say 15%) during cooling in response to high humidity sensed by the thermostat. This lowers the coil temperature and removes more water from the air.
  5. gsmith22

    gsmith22 Active Member Forum Leader

    running isn't an issue for a motor, starting is the issue for motors. obviously running full speed all the time becomes an issue for your wallet which is why variable speed/variable frequency/whatever you want to call it systems were invented to slow down motors when full speed isn't needed (it typically is never needed). look up "affinity laws" and you will see that power (what you pay for) is the cubic of speed so even small amounts of slowing down speed can save big on power consumption. now put this in a variable speed heat pump -runs all the time mostly at very low speeds so not only does it match output to load better by varying compressor speed, it does so at substantially reduced power/cost. win-win

    Yes, if you run a single speed AC compressor too little, the system won't dehumidify properly. This is true of every AC system/heat pump that has ever existed and isn't unique to anything ground source heat pumps.

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