Experience Files

While reviewing client documentation recently, I stumbled upon some readings that highlight common misconceptions about HVAC systems and their role in drying.

First, let's clarify a couple of things about air conditioners:

1. ACs as Dehumidifiers: An air conditioner is essentially a large-capacity conventional refrigerant dehumidifier. As such, it's generally limited to dropping the GPP (Grains Per Pound) down to around 55 GPP. While you might occasionally see readings lower than 55 GPP, this often indicates a dirty filter or another restriction within the system, rather than superior drying capability.

2. Temperature Drop: Air conditioners typically achieve an 18-22 degree Fahrenheit temperature drop between the return and supply registers (i.e., between the affected/unaffected area and the HVAC output). You'll see variations from this range in extremely hot outdoor conditions or with systems nearing the end of their life.

Now, let's look at the peculiar readings I observed: Day 1 affected area readings: 66°F and 62% rH, which translates to 59 GPP. HVAC output readings: 44°F and 100% rH, which translates to 42 GPP.

This immediately raises a red flag: How is the AC pulling water (dropping from 59 GPP to 42 GPP) when I just mentioned it's generally incapable of dropping GPP lower than 55? And why is the HVAC output at 44°F and 100% rH, instead of closer to 55 GPP if it were truly dehumidifying effectively?

The straightforward answer: The AC isn't effectively pulling water, and the meter's reading is misleading. Since the relative humidity (rH) scale caps out at 100%, the meter can't register anything beyond that saturated point.

What's actually happening is that because water vapor isn't being efficiently converted into condensate and collecting on the HVAC coils, the system is cooling the air much more deeply. Dry air is significantly easier to cool than humid air. This additional cooling would actually push the relative humidity well beyond 100% if the meter were capable of showing that value.

This situation is unusual and was only occurring because the air in the room was already quite cool to begin with—temperatures that are generally too low for effective drying for several reasons. However, after a few hours of run time, the room temperature did rise into an acceptable range.

As for why the AC was running at such a low temperature initially, that's a good question. My best guess is that the thermostat was located near a heat source, perhaps a lamp, tricking the system into overcooling.