This tech tip was submitted by Steve Rogers, the current president of The Energy Conservatory (TEC). Steve has been on the podcast several times in the past and is a valued contributor to HVAC School. Thanks, Steve!

As summer approaches, HVAC technicians all over the US are preparing for those difficult conversations with occupants about houses that are too hot and uncomfortable. In high-humidity climates, they will also be having conversations about how muggy it feels and possibly even biological growth. Occupants of a home may even feel like the system is “not keeping up” because the high humidity makes them feel uncomfortable, even when the system is maintaining the temperature set point.

First, let’s understand the problem of high indoor humidity. At its most basic level, the problem is very simple. If a space is too humid, it is always because there is too much humidity coming in or being generated inside, not enough humidity is being removed, or both. So, we could compare humidity control to a leaky boat with a bilge pump. The amount of water in the bottom of the boat depends not just on the bilge pump's capacity to remove water but also on how fast the water is leaking in through the hull of the boat, plus any water from the activity of people on the boat.

By now, those of us who follow HVAC School know that often the best, most permanent solutions are some combination of a new air conditioning system that is properly sized, adding a whole house dehumidifier, and/or doing air sealing work to improve the airtightness of the building and the ducts. (You can learn more about testing the tightness of a building HERE.) Those solutions will probably remain the gold standard of humidity control for the foreseeable future. But all those solutions are also expensive and may not be economically realistic for many occupants of the homes we serve. So, you might be asking—are there any solutions that can improve humidity control that are less expensive? I believe there are. These may not completely resolve the problem, but they can often improve it.

Short Cycles, aka “Humidity Catch and Release”

Before discussing cost-effective options to improve humidity control, let’s consider how we remove humidity from the living space and where it goes. That way, we can better understand how we can affect it.

When a system first comes on, it takes several minutes before the coil gets cold enough to start condensing water. It takes several more before it gets close to its coldest, steady temperature. Just like a cold beverage outside on a humid day, once water begins condensing out of the air, it doesn’t drip off the surface immediately. The droplets accumulate until they are large enough to drip off. But even after water drips off the coil, it doesn’t immediately go down the drain. It takes a while to accumulate in the pan until it’s deep enough to flow down the drain hole. But you probably already knew all that!

But have you ever considered at exactly what point we have won the battle against humidity? When water condenses on the coil? When it hits the drain pan? Or when it goes through that drain hole? It’s not until it goes down the drain! When the cycle stops, there is a pretty good chance some of the water still on the coil and the drain pan will evaporate and go right back into the house. How much will depend on how long it is between cycles, whether the circulation fan keeps running, whether the indoor unit is in a hot attic or cool basement, and even duct leakage?

In a worst-case scenario, the system could be so oversized (and the cycles so short) that on typical warm days, the system runs just long enough to fill the drain pan with condensation and begin dripping down the drain, and then it satisfies the thermostat. Then because the system is in a hot attic, or because the circulation fan is set to run continuously, all the water sitting on the coil and in the drain pan evaporates back into the air and is carried right back into the house where it came from. The result can be near-zero dehumidification! Back to our bilge pump example, this would be like a bilge pump that pumps water up through the discharge line but shuts off before much water actually reaches the end of the discharge tube. Then, all the water just drains back into the bottom of the boat.

With that example in mind, here’s a list of things that are relatively inexpensive to change. These changes certainly won’t solve humidity problems in all cases, but they may improve indoor humidity. So, they’re worth a try, especially if the homeowner really can’t afford the “gold standard” solutions we already mentioned.

1. Set the circulation fan to “auto” so that it stops running when the compressor stops running. If the circulation fan is in the “on” position, all the water that remains on the evaporator coil and in the drain pan when the system stops running will evaporate very quickly back into the space—often within minutes! If the fan shuts off at the end of the cycle, some water may still evaporate away, but it will take longer. Whatever water is still on the coil or in the drain pan when the system starts again will go down the drain sooner, resulting in a little more dehumidification for each cycle.

2. Set the Cool dissipation time to zero seconds. Check the thermostat settings to see if there is a setting that will run the circulation fan for a certain number of seconds after the cycle ends. It may have different names for different manufacturers. This feature can save a little energy and may be great for dry climates. However, in humid climates, it can make humidity worse for exactly the same reason described in #1.

3. Set the system airflow to 350 CFM/ton. Measure the system airflow and adjust the blower speed until the airflow is 350 CFM for every ton of cooling. A 2-ton system should be set to 700 CFM, a 3-ton system should be set to 1050 CFM, etc. The best way to measure airflow is with a TEC TrueFlow (learn more HERE), but using the manufacturer’s fan tables and static pressure can also give reliable results on newer systems. Airflow is important because all systems will remove more humidity at the lowest of the manufacturer’s recommended airflow settings. There are usually three airflow rates published in the extended performance table. They’re usually about 350, 400, and 450 CFM per ton of cooling. You want to use the lowest one for maximum dehumidification.

4. Increase temperature “swing” setting. This setting's name may differ by manufacturer. Some manufacturers may call it the temperature differential, the temperature swing, or the temperature deadband. In any case, it refers to the number of degrees between when the system cycles on and when it cycles off. For example, if the thermostat has a swing of 1 degree, and it is set to 75°F in cooling mode, it means that the system will call for cooling when the temperature gets up to 76.0°F and end the call for cooling when the temperature gets down to 75.0°F. Some manufacturers default this value to 0.5°F degrees or even 0.25°F, which can result in very short cycles and poor dehumidification. A better setting is 1°F or even 1.5°F. In general, if you double the swing temperature, the cycle times will be twice as long under the same conditions. 
   
   But with cycles that are twice as long, the system can do even more than twice the dehumidification! Here’s an example. Let’s say an AC system is running four-minute cycles, but it takes three minutes before water begins flowing down the drain, and then it’s 10 minutes before the system comes on again because (surprise!) it’s significantly oversized. During those 10 minutes, the hot conditions in the attic warm up and re-evaporate all the water on the coil and in the pan, even with the circulation fan off. So, water from three out of four minutes of run time re-evaporates back into the house. This system is really only doing 1 minute of dehumidification every 14 minutes! 
   
   Now, what happens if we double the temperature swing on the thermostat? The system will be on for about eight minutes and off for about 20. The water still begins flowing down the drain after three minutes, so it will be doing five minutes of dehumidification every 28 minutes instead of one every 14. That’s 2.5x more dehumidification just by changing a thermostat setting!

5. Increase compressor minimum on time. Some thermostats have a minimum time that they will allow the outdoor unit to run to prevent damage from very short cycles. This setting can also be set higher, resulting in longer run times and better dehumidification, just like #4. But keep in mind that this setting will take priority over the thermostat’s usual on/off cycles, so it could result in the indoor temperature getting colder than desired.

6. Run setbacks or make them larger. Setbacks are settings in a programmable thermostat that change the desired temperature at different times of the day. When the desired temperature gets cooler, the system usually has to run for an extended period of time to reduce the temperature of the dwelling. The larger the setback, the longer the system will need to run. This long runtime gets more humidity out of the air and down the drain, just like other settings that increase runtimes. So, if the occupants don’t mind sleeping with it a little cooler, you might have the setback drop the temperature from 74 to 70 when they go to bed. If nobody is home during the work day, you could also let the temperature rise up to 78 during the day and then get lots of dehumidification when the set point drops back down to 74 before they come home.

7. Check for large, obvious leaks in ducts that are outside of the conditioned space. Ducts that leak air outside of the conditioned space always have a negative impact on humidity control, regardless of whether the leaks are on the supply or return ducts. As already discussed, a duct replacement or renovation is likely to be expensive, especially if you need to do a duct leakage test to confirm whether that’s really the problem before making repairs. But sometimes, there are large, obvious leaks in accessible locations right near the equipment cabinet.  Leaks that are closest to the cabinet see the highest pressure, so they will leak more air than leaks farther away. Often, these leaks can be repaired in just a few minutes and will result in a large improvement in humidity control. Carry some foil tape and duct mastic in your service vehicle. If you can see a hole large enough to stick your fingers through, repair it immediately!

Some of you might be wondering how much to charge for these services. I can’t answer that. But I think there’s plenty of room to make money on the few minutes of additional time that most of these will take. Of course, the price and the discussion with the occupants should recognize that these changes might help, but they’re less expensive because there is no guarantee they’ll help much at all.

Ultimately, how much these measures will help depends on what conditions you find to begin with and what the real root cause of the humidity is. If your boat hull leaks 1000 gallons per hour, and your pump can only remove 500 gallons per hour, you’re going to have a lot of water in your boat! Similarly, a very leaky house may let in so much humidity that even a perfectly operating air conditioner with an auxiliary dehumidifier might not be capable of keeping it dry. Yes, you could get a better bilge pump, but the better solution is to find the leaks and fix them!

We are entering the hot and humid season. Hopefully, some of these quick and easy approaches will help resolve or reduce complaints and keep you moving when a customer is not ready to pay for the best solution.

—Steve Rogers

P.S. — If you work on many Ecobee thermostats, HERE is a page that describes many of the settings mentioned in this tech tip. Other manufacturers may have similar guides.