Zebra Instruments Masthead

z•tech•tips #1

Using a two-stage thermostat to control humidity

You probably know that in the summertime, the perception of feeling cool has a lot to do with the relative humidity in an indoor conditioned space.

Guy Sweating  

Many of the newest variable-speed-drive air handlers and furnaces have an optional connection point on their control board that allows a humidistat to be connected to the system. When the humidistat indicates that the conditioned space has more humidity than the customer desires, the system blower will run in a somewhat slower speed in cooling mode. This allows the evaporator to operate at a colder temperature, and the greater temperature difference (TD) between the evaporator and the air flowing over it causes more moisture to condense out of the air and flow into the drain pan.

Most of these new systems also allow for a high-spread override: if the system is having trouble keeping the home cool while operating in this reduced fan speed mode, it will revert to the normal, high fan speed to use the full cooling power of the system.

Great engineering, you say, but what about the rest of the systems out there? Is there an easy way to make them able to help control humidity, too? Larry Brewer, CM, from Georgetown Texas offers a less expensive alternative than replacing the system with a variable-speed-drive unit:

how to do it:

By adding a relay, a humidistat, and a two-stage cooling thermostat to a multi-speed blower system, you can easily and automatically control humidity without sacrificing system capacity when needed. Larry obtained a Honeywell H890B 1002 TradeLine Humidistat, a #90-370Q (or MARS #92370) switching relay, and replaced the system thermostat with one that had a two stage output for cooling. Larry didn't need a programmable thermostat, so he chose Honeywell's Pro 5000 series TH5220D1029, which has a large display.


There are many other thermostats which will work well for this circuit; just remember it must have a two stage output for cooling. (Click any of the photos to bring up an enlarged version.) Also there is a schematic (and an enlargement) available near the bottom of this page.

The Thermostat

Two stage output uses two cooling terminals, Y (or Y1) and Y2. The Y terminal acts exactly like a normal thermostat's Y terminal; receiving power from the R terminal when the room is warmer than the thermostat's set point. The difference is that a two-stage thermostat will apply power from the R terminal to the Y2 terminal (also) when the room temperature goes a few degrees above the set point. To illustrate: If the room is at 75° and the thermostat is currently set to 80°, the system will not be running. If you gradually lower the desired temperature on the the thermostat, at around 75° the Y terminal will be activated, calling for cooling. If you continue to lower the desired temperature on the thermostat another 2 or 3 degrees, the Y2 terminal will also be activated.

Step One

Power off the system and remove the old thermostat, recording the wire colors/connections. You will need one additional wire running from the thermostat area to the furnace/air handler. Many times there is an unused wire in the cable already; if not, you'll need to run a single 18 gauge wire.

While the thermostat is removed, choose a location next to it (usually at the thermostat's side or below it) for the humidistat, and mount the humidistat, running it's two wires into the wall and then back in through the same hole that the thermostat's wires come through.

Thermostat & Humidistat

Step Two

Connect the new 2-stage thermostat to the existing wiring, using the diagram that came with it. Use the same terminal designations that the old thermostat used. Now, take either one of the two wires from the humidistat and connect it to the thermostat's R terminal (which is probably also connected to a red wire going to the air handler.) Next, take the other humidistat wire and connect it to the Y2 terminal of the thermostat, but before tightening the connection, also attach the 'spare' wire we discussed above. You can now mount the new thermostat, but don't apply system power yet.

Step Three

Take your tools and the new relay to the air handler/furnace, as well as some push-on insulated terminals and a few feet of wire of the proper size to carry the power the blower requires.

Mount the relay in a secure and appropriate place within the air handler. Choose a place where you can safely run wires from the blower motor and also connect to the low voltage wiring. Locate the other end of the 'spare' wire referred to earlier, and install a female insulated push-on terminal to the wire. Push that terminal onto the relay's terminal marked '1' (Coil). Now, take a length of 18 gauge or larger thermostat wire and connect one end to the COMMON side of the 24V transformer's LOW VOLTAGE side. (That wire is often Blue or Brown.) Crimp another female insulated push-on terminal to the other end of your wire and push it onto terminal '3' (Coil) of the relay.

The control circuit wiring is finished; you now just have to connect the blower motor wires to the relay.

Step Four

Nearly all systems that you are likely to encounter have the following configuration: cooling runs the blower on high speed; heating runs on low speed; and there is at least one medium speed wire. If your system uses different speeds, you may have to improvise. You might be able to use the low speed wire from the blower motor if you only have a 2-speed motor available; but the cooling system might run too slowly. If you only have a 2-speed motor, it would be a good idea to test for proper cooling operation at the low speed, before making any permanent changes.

Locate the wire that is currently in use for HIGH speed operation; it's often the black wire from the blower. (The other end of the wire is usually connected to the existing fan relay.) If possible, cut the wire so that the cut ends can be 'terminaled' and connected to the new relay you installed; if not, you will have to safely 'extend' the ends with an appropriate size and length of wire. Connect the end of this wire that comes from the system fan relay to terminal 4/6 (COM) on your new relay, using a suitable push-on connector. Connect the other end (the end that goes to the blower motor HIGH speed) to terminal 2 (N.O.) on the new relay. The final connection is to connect the Medium speed blower motor wire to terminal 5 (N.C.) of the new relay.

   (click here for larger window)      (click here for a printable .pdf window)

How It Works

When the thermostat initially calls for cooling, the outdoor unit is activated and the indoor blower is energized by the system's fan relay. BUT, we broke the wire that used to run directly from the system fan relay to the blower's HIGH speed and instead ran it to our new switching relay. Notice that if this new relay is NOT activated, the power flows from the COMMON terminal out to the N.C. terminal, which we connected to the MEDUIM blower motor speed. Therefore, the cooling speed will be MEDUIM unless the relay is energized. (When the relay IS energized, the fan speed reverts back to the normal HIGH speed.)

So, you can see that unless something energizes the new relay, the blower will operate in a somewhat slower, but higher-humidity-removing mode. This slower mode will not utilize the full BTU capacity of the system, because the blower is running slower. So, what will make the blower run faster? Either one or both of the following conditions:

  1. If the humidistat is 'satisfied'. For example, if the actual room humidity is lower than the set-point of the humidistat. This causes the humidistat's contacts to CLOSE, which energizes the new switching relay, causing the blower speed to return to HIGH.
  2. If the system cannot cool the home sufficiently using the reduced blower speed, the temperature difference between the room and the thermostat's set-point goes greater than 2 or 3 degrees, and the thermostat turns on the second cooling stage (terminal Y2) which activates your new switching relay, causing the blower to return to HIGH speed, bringing full output to the system.

Remenber to briefly explain to the customer how the system works, especially noting that at times the fan will run more quietly; otherwise they might think something's wrong and call you back. It also would be nice to leave a note for any future Tech to understand the modifications. Leaving a printout of the schematic may be useful to them (or you!) too.  

Note: if using this circuit for heat pump systems, remind the customer to turn the humidistat control dial to full "ON" when in the heating season - otherwise the first stage heat could sometimes run at medium speed!      ••z••

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