If you live in a dry climate, a humidistat is pretty straightforward; it just turns a humidifier on and off. That's about as simple as it gets.

If you are in a humid climate, some people have taken to wiring in humidistats as dehumidistats to help control humidity when they are away.

The first thing that must be said is:

If you want to truly control humidity AND have a system that cools for comfort, get a system and thermostat that has that function. This setup won't help with that.

The reason for this is simple…

These humidistats just open and close a switch and aren't generally very accurate to boot. So, the only reason to have one is so that the system will run based on humidity and NOT temperature during periods the customer is away.

This is why it is common to see these in second homes of snowbirds in Florida or people who travel a lot. The thinking is that when you are away, you care about the humidity in the house and not the temperature, which has a measure of truth to it. However, these controls often cause more heartache than anything else due to common misunderstandings.

The Honeywell H46C1166 (link to manual)

The most significant cause of misunderstanding is that they can be wired in two different ways:

1. Open to prevent the system from running (most common)
2. Close to make the system run

Let's look at each method and how to wire them and set them.

Open to prevent the system from running (Series)

The H46C is pretty simple in that it opens and closes at around the set humidity and only has two connections. In this strategy, you break the Y wire, leaving the thermostat or the R wire entering the thermostat with the humidistat. This way, it shuts the system down when the humidity in the space is lower than the setting. If you break Y, the blower may still run continuously when the humidistat is open; if you break R, then the thermostat may go blank, or the batteries may eventually die.

If you wire it this way:

• Set the humidistat to ON when the home is occupied
• Set the humidistat to 55-60% when away
• Set the thermostat to 75-80° when away

This way, the system will only run when the humidity is above the humidity setpoint and the temperature is above the temperature setpoint. So, a “safer” setting would be 75° at 55%, and a riskier but more energy-efficient setting would be 80° at 60%.

Close to Make the System Run (Parallel)

This method works on the idea of setting the thermostat on a high setting when you are away and allowing the dehumidistat to turn the system on and off in cooling mode.

As shown above, this method connects between Y and R on the thermostat in parallel with the thermostat control.

First off, this strategy doesn't work well on heat pumps, though you could do it by adding a relay, and I've seen people wire it O to Y, which really won't work right in the heating mode because it will keep the system locked in cool by back-feeding O from Y.

So, to be clear, I don't like this way at all, but if you choose to wire it this way:

• Set the humidistat to OFF when the home is occupied
• Set the humidistat to 55-60% when away
• Set the thermostat to 80-85° when away

So, there you have it—if you want humidity control while occupied, get a thermostat and system that have the ability to control airflow and runtime based on humidity as a factor.

If you want the system to run primarily based on humidity during away periods, this can be done as shown above.

—Bryan