Air Conditioning Modes of Failure

Question

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?

Answer 1

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.