The first method directly measures both the indoor relative humidity and outdoor temperature. By using an outdoor temperature sensor to measure the actual outdoor temperature and adjusting the control for every 0.5°F fluctuation, it can increase the setpoint during warmer weather to achieve greater comfort and decrease the setpoint during colder weather to avoid condensation.

The second method calculates dewpoint by measuring both the indoor relative humidity and temperature and estimating the outdoor temperature.

No outdoor temperature sensor is required; instead, an algorithm built into the humidistat infers outdoor temperature by monitoring furnace runtime.

Because both dewpoint and relative humidity methods are able to control indoor humidity levels, the use or non-use of an outdoor temperature sensor becomes the critical component for determining which method can achieve optimum humidity.

Impact of measuring outdoor temperature

The technology that directly measures outdoor temperature, used in the relative humidity-based method, can automatically adjust indoor humidity levels close to the condensation point without exceeding it.

Even during winter months, with temperature fluctuations in excess of 40°F, the relative humidity-based method can achieve optimum humidity because it uses an outdoor temperature sensor to directly measure the outside temperature.

The dewpoint-based method estimates outdoor temperature based on furnace runtime. Because several rather common conditions can affect furnace runtime, it’s only natural that indoor humidification would be affected as well.

These conditions include:

• Use of a multistage furnace;
• Furnace over-sizing;
• Sunlight/solar heat gain;
• Use of a fireplace;
• A zoned home;
• Natural air infiltration; and
• Window type.

For example, in a home with a fireplace and thermostat in the same room, when the fireplace is in use, room temperature will increase. Therefore, the thermostat needs less assistance from the furnace to maintain its setpoint, which causes the furnace to run less.

Because the furnace is running less, the humidification method based on furnace runtime will also run less, thus increasing the humidity setpoint even if the outside temperature is dropping.

Although each system has a dial to adjust to individual structural and comfort needs, a simple knob adjustment cannot compensate for every outside condition.

The use of an outdoor temperature sensor eliminates the need for manual adjustments. Instead, it automatically makes adjustments based on changing outside conditions.

Wet windows

Optimum humidity is a balancing act between what the home can handle and what the occupant prefers.

It does little good to achieve a comfortable level of indoor humidity if the homeowner experiences condensation on cold windows — or worse, inside of insulated walls.

The relative humidity-based method has the ability to achieve optimum humidity because the humidistat setpoint adjusts 1/2% for every 1°F fluctuation in the outdoor temperature. In a 24-hr period, the unit has the ability to monitor the indoor and outdoor environment 86,000 times.

Home system study

In order to compare the abilities of each method in real life applications, a 16-week study was conducted in 10 homes in a Midwestern city during the 1998-99 winter season.

In an effort to obtain accurate, unbiased data, each home was wired with both humidification methods. Each week, five homes used the relative humidity method while the remaining five used the dewpoint-controlled method. The following week, the methods were reversed.

During the first week of the study, occupant comfort levels were measured and dials were set based on comfort needs. This study was conducted using 4.5 as the average setting for the relative humidity-controlled method, and 7 as the average setting for the dewpoint-based method.

In the month of January, the relative humidity-based method operated 44% of furnace runtime, whereas the dewpoint-controlled method operated 22% of furnace runtime.

Over the course of the 16-week study, the relative humidity-based method was able to generate an average of 126 gal more moisture per house than the dewpoint-based method.

The end result

Looking at the information from this study, it is clear that the relative humidity-based method is able to fulfill the delicate balance of optimum humidity, satisfying the needs of the homeowner while avoiding condensation.

As stated earlier, the biggest difference between the two methods is the measurement of outdoor temperature.

Estimating outdoor temperatures is not enough. An outdoor temperature sensor is needed to accurately measure outdoor temperature.

Automatic humidity controls, when properly installed, create greater homeowner comfort and help to eliminate callbacks for lack of humidity, evidence of condensation, or questions on humidistat adjustments.