By Kevin O’Neill

Humidity plays as much a part in our comfort as air temperature.

It’s easy enough to cure a case of low humidity during heating season in a typical residence by simply installing a humidifier.

However, during the rest of the year, too much humidity is the problem. This isn’t only a comfort problem, it’s also a potential health problem. Mold is more likely to grow when the relative humidity level is above 50%. Most molds really like relative humidity above 70%.

High indoor humidity is often hard to control. Air conditioners remove moisture from the air as a by-product of cooling, and it’s a normal practice to use an air conditioner to dehumidify the space as well as cool it. But moisture removal isn’t an air conditioner’s primary function. This is because the air conditioner is controlled by a thermostat, which senses temperature only.

Using an air conditioner for dehumidification works fine when the outdoor air is both hot and humid, in which case a properly sized air conditioner will run nearly all the time. Just remember, when sizing the equipment, it’s important to pay as much attention to latent load as to the sensible load.

Unfortunately, even when using a proper load calculation, we create air conditioning systems that are oversized about 90% of the time, such as in the spring and fall. During these non-design conditions, air conditioners don’t run enough to do a good a job of dehumidifying the air. For example, when it’s raining and cool, the air conditioner has no reason to run, since it’s not getting hot in the house. However, the moisture in the air outside will get into the house just by normal infiltration and ventilation. Then the house air will be too humid, with no way to get rid of the moisture.

Enhancing Air ConditionerPerformance

When the air conditioner is the only way you have to control high indoor humidity, there are several ways you can enhance its performance. One of the simplest ways is to adjust airflow. Most brands of equipment can be run with 350 cfm of airflow per ton of cooling with little loss of overall performance. This makes the indoor coil colder than normal, resulting in better moisture removal.

An accurate measurement of overall airflow is required. In fact, if you’ve done an accurate load calculation, and know your sensible/latent split, you can optimize airflow even more. Set your airflow at 350 cfm per sensible ton of cooling. The latent load is then taken care of at the indoor coil, because it’s so cold. If you use this method, include a freeze-stat on the coil because under light loads, the sensible percentage of the load may be higher than at full load conditions.

The next method is very similar. Install a single-pole, double-throw relay in the indoor unit, connected to a dehumidistat. This requires a multi-speed blower. Connect the common terminal of the relay to the cooling speed tap of the blower relay. Connect the normally open and normally closed terminals of the relay to two different blower motor speed taps. Set the dehumidistat to the desired relative humidity.

When the humidity in the house rises above this setting, the blower motor is switched to a lower speed tap and runs slower. This makes the evaporator coil colder and removes more moisture. When the humidity falls, the blower speed is increased, resulting in a warmer coil, slightly increasing overall capacity. It also reduces the chance of freezing the indoor coil.

If you install a system with a variable speed indoor blower, the blower can be set for the exact airflow you want, as long as the duct system is capable of supporting that setting. Most variable speed blowers have enhanced dehumidification modes. These blowers run at about 200 cfm/ton for the first few minutes. Then they ramp up to 80% required airflow for five to eight minutes more. If the thermostat is still not satisfied, they ramp up to full airflow. This produces very cold indoor coils when the system has short run times due to mild weather, but still supplies full rated airflow during periods of extremely hot weather.

Variable speed blowers are also good in houses with slightly restricted duct systems. They can generally provide full-required airflow at 0.8 to 1.0 in. w.g. external static pressure, as opposed to 0.5 in. or less for a standard blower.

Some manufacturers make multi-speed condensers, which vary their capacity as the load changes. These
systems can be expensive and complicated, but they provide superior humidity and temperature control. We once put elapsed time meters on the compressor controls of such a unit, and found that it ran five times as much in low speed as it did in high speed. Since these units run so much of the time in low speed, they also offer very high
efficiency operation.

Another option is a unit with two compressors in a single cabinet but only one refrigerant circuit. This has the same advantages as a two-speed compressor. Most multi-stage equipment is designed to provide optimum performance when used with variable speed indoor units.

Some manufacturers offer dual compressor units that offer similar benefits. I have also seen at least one indoor unit with a split coil having two refrigerant circuits. This is made to run with two separate outdoor units. An added advantage of this system is that if the compressor in one outdoor unit fails, the other one gives partial back up.

Keep in mind that when using multi-stage residential equipment, superior dehumidification occurs only when the system is properly sized. An oversized two-speed system will have the same type of comfort problems as a single-speed system.

Another method of humidity control, which we use for people who take extended vacations, is to put in a three pole, double throw relay. The normally open contacts are connected to the R terminal in a heat pump. The common terminals are connected to Y, O and G terminals in a heat pump. The normally closed terminals are connected to the Y, O and G terminals on the cable from the thermostat. When the customer goes on an extended vacation, the thermostat is set up to about 85F and the dehumidistat is set at 50%. If the relative humidity rises above the dehumidistat’s setpoint, the system comes on in cooling, and runs until the relative humidity is reduced. This can be done with air conditioners as well, but you don’t need to energize “O” since there is no reversing valve.

This method can also be used when the house is occupied. However, the temperature in the house may become uncomfortably cool. If you have an automatic changeover thermostat, your heat may come on after the dehumidistat is satisfied, depending on the settings. This can lead to expensive utility bills.

Another way to dehumidify a house is to use a dehumidifier. This requires another piece of equipment and another controller, but it’s more energy efficient than using reheat. Since the controller senses relative humidity, it works well in mild weather, unlike a heat pump or air conditioner.

Dehumidifiers are often used in basements. As they’re usually not very elegant, most people don’t want one sitting in their living room. They also tend to be rather noisy for an occupied area.

Most dehumidifiers use the hot gas as condenser reheat, so they’re self-contained and require no refrigerant piping. Therefore, they’re easy to install. The only installation problem is installing a drain line to get rid of the condensate. They also add some load to your air conditioning system, because the electricity they use shows up as heat in the house. They’re usually available in 20 to 50 pint per day models. Some manufacturers have models as large as 80 pints per day.

Another option is a whole-house dehumidifier. These units can be installed in a basement, crawlspace, or attic and tied into the duct system to dehumidify the entire house. Whole house dehumidfiers cost significantly more than stand-alone units, but have a higher dehumidifying capacity and don’t cost any more to run. They’re also out of the living area, so they operate quietly.

In cooling-dominated climates, a large part of the problem with high humidity levels is caused by the coupling of uncontrolled areas external to the living area with the living area itself. Thus, if the crawlspace has high humidity levels, that moisture will be prone to infiltrate into the living space through cracks and holes in the building envelope. The floor over a crawlspace in my area often has no vapor retarder to slow moisture movement into the occupied spaces.

In dry climates, excess humidity problems often show up as window sweating on the interior surface of the glass during the heating season. This is often caused by too high a setting on the humidistat and can be eliminated by reducing the run time of the humidifier.

In heating climates, the vapor retarder is normally installed on the interior surface of the wall. This ensures that any moisture that comes in contact with the vapor retarder is unlikely to condense because this is the warmest point of the wall for most of the year. Any moisture in the wall cavity will migrate toward the outside, where it’s absorbed by the outside air. The same is true for ceiling and floor assemblies.

In mixed climates, the use of a vapor retarder is not as important as in heating or cooling climates. If a vapor retarder is used, it should be installed on the interior side of the wall cavity if the mixed climate is heating dominant. If the climate is cooling dominant, the vapor retarder should be installed on the exterior side of the wall cavity.

In humid climates the vapor retarder should be installed in the wall cavity facing the exterior surface. If the vapor retarder is on the interior surface, condensation may occur in the wall cavity, degrading the wood and plaster or dry-wall due to mold growth.

The new International Building Code recommends not using a vapor retarder at all in very humid areas.

Vinyl wallpaper and high gloss paints shouldn’t be used in cooling climates on exterior walls or ceilings on the interior surfaces. This is because they act as a vapor retarder and can cause condensation on the backside of the wall covering, wetting the wall material itself.

As you can see, high humidity in a residence is a multi-faceted problem. However, knowledge is power and a knowledgeable HVAC contractor, armed with the tools we have available to us today, can solve virtually any indoor humidity problem. Believe me, doing so will make many of your customers very happy.

Kevin O’Neill, CM, is the owner of O’Neill Cooling & Heating, Myrtle Beach, SC. He has 27 years experience in the HVAC service business and is a 20-year member of RSES. Kevin can be reached at 843/385-2220.