July 10, 2025

    That darned Rheem HVAC compressor has a mind of its own. Martin had seen it freeze up when the AC supply temp was warmer than 39°F, so he figured out how to do what we’ll call “Direct Freeze Detection”. If we see the supply temp go below 43°F we know that the compressor is running. If we then see the supply temp go above 45°F we know that the compressor has shut itself down, so we should set the evaporator frozen and let the supply temp go to 60°F to make sure all the ice is melted.

    Why not just set the Evaporator Freeze temp up higher? 

        1. We don’t know just how high a temperature the Rheem will shut itself off at.

        2. We really want to drive the HVAC as hard as possible. That’s why we upgraded to 5 tons, the house needs the extra cooling.

That blue line is where the compressor turned itself off. The supply temp was about 40°F at that point. Martin noticed the problem and manually set the evaporator to frozen and started putting the new conditions into the controller. He had them loaded before the Evaporator Frozen had cleared, hence the short cycle at about 15:30.

A new register, “Compressor On” has been added to remember that we saw the AC supply temp go below 43°F. Then, if we see that the Compressor is On, but the temperature has risen to 47°F, we declare an Evaporator Freeze condition, but set that register to 25 so that when we chart the log file, we can see the two conditions clearly.

Here’s a couple of very nice examples of the new direct freeze detection at work. At about 10:30 the compressor turned itself off, so the direct detection spotted it. Note that the Evaporator Freeze register was set straight to 25 in both the cases at the right side of the above chart.