I have a question about air conditioning and the probable modes of failure.

I recently installed a Nest thermostat in my home. I've noticed that it seems to prefer to have the air conditioner's compressor on for multiple short durations instead of one long period. This got me thinking...

1) This is going to kill the starter capacitor of the air conditioner more quickly than normal.

2) I wonder why they made the control system like this... It MAY extend the life of the compressor.

Here's my thought process:

The AC compressor is effectively a non-linear constant-Q source (i.e. you pump energy into it (by compressing the freon) at a rate (which is technically a function of the temperature differences involved) and it heats up until the energy coming in (from the compression) equals the energy going out (by the blower)).

If you were to run this continuously, the compressor itself would get really, really hot, always running at its maximum temperature (i.e. the crossover point between the efficiency-drop of the compressor at the high temperature vs the convection-cooling of the compressor). If you work it in smaller increments, you can keep the temperature down of the compressor.

The starter capacitor is an annoying $100 capacitor... but the compressor dying can effectively total the AC.

So: how much of the wear on the compressor is due to over-heating? Is this actually going to make an appreciable difference, or is Nest just unnecessarily eating my starter capacitor?

  • 2
    $\begingroup$ The goal of the Nest is energy efficiency, not lifetime of any particular part. As such, my bet is that the more rapid cycling is to keep the temperature more uniform so that the owner doesn't turn the temp down during a hot swing. $\endgroup$ Apr 8 '15 at 16:56
  • $\begingroup$ That's true at steady-state, but even when it's doing a massive-cool (i.e. "owner just got home!") it tends to not keep it on continuously. Besides, my question was not "is this was Nest is doing?" as much as it was "is this going to accidentally help me out?" $\endgroup$
    – iAdjunct
    Apr 9 '15 at 3:30

I don't know for sure without more details of your setup, but I have two possible explanations (both of which may be partially or wholly responsible for the behavior you see):

  1. Your thermostat doesn't directly control the compressor in your cooler. It tells your heating/cooling system when to come on and go off.

    In this case what could be happening is the thermostat is telling your cooler to come on, and the cooler control system is deciding how much to run the compressor.

    The physical heat exchange interfaces (on both the hot and cold sides) can only transfer so much heat in a given amount of time, but the compressor is capable of higher performance than the minimum required to maintain this level of heat exchange. This would be by design, so that it can quickly come up to maximum performance when switched on. If you notice that it comes on for a bit longer initially, then tops up the pressure with short bursts after a while, this is probably what is happening.

  2. The smart-thermostat has learned about a time lag in your property between enabling heating/cooling and registering that change. It is therefore cautiously applying heating/cooling to avoid large swings one way or the other.

    Based on the location of the thermostat and your heating/cooling system(s), and the layout of your building, it may take quite some time after switching on the heating/cooling before any effect is noticed by the thermostat. It may have learned this when first installed and applies heating/cooling for a short time, then waits to feel the results of this before deciding if it needs some more.

  • $\begingroup$ This makes me wonder something I hadn't thought of before: is the compressor on the entire time the blower in the compressor unit is on? Or can the system leave the blower on while the compressor is off? $\endgroup$
    – iAdjunct
    Apr 15 '15 at 22:29
  • $\begingroup$ I know that the thermostat can say "Air Conditioning" (meaning AC + Fan) or "Fan" (meaning Fan), and it appears to use that to minimize compressor use (by switching from AC to Fan to Off instead of AC to Off), which is what I meant, assuming the compressor is active the entire time the blower is on... $\endgroup$
    – iAdjunct
    Apr 15 '15 at 22:30
  • $\begingroup$ I understand #2, as I have a strong background in control systems and transients. I was just thinking about the effect on the compressor lifespan. $\endgroup$
    – iAdjunct
    Apr 15 '15 at 22:33
  • $\begingroup$ Note: I probably used the wrong term. I realize that "blower" usually refers to the circulation fan, but I am trying to refer to the fan that increases convection across the compressor in the outside unit. I apologize for any confusion there. $\endgroup$
    – iAdjunct
    Apr 15 '15 at 22:35
  • $\begingroup$ I would imagine the fan that blows air across the radiators on the hot/cold side would stay on the whole time the unit is actively cooling, but the compressor only comes on as needed to maintain the cold-side temperature. If the incoming air is relatively cool anyway, the compressor only needs to pump heat from one side to the other less often (since the compressor is either on or off, time-division allows in-between amounts of cooling). $\endgroup$
    – jhabbott
    Apr 16 '15 at 16:18

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