In many areas of the country humidity control is the most essential function of the HVAC system. The HVAC system pulls the largest amount of moisture into a building from outside the envelope and is the primary cause of excessive humidity in some homes. There are also a number of building defects that bring moisture into a home. Let's take a look at how to address these problems.

Use the Right Test Instruments
Humidity measurement has come a long way in the last decade. A reasonably accurate hygrometer cost over $2,000 ten years ago, and now, very accurate hygrometers are available for less than $150. To find a hygrometer that'll do the job, look for one that measures from 5% to 95% RH and has a readout to the tenth of a degree. There's no need to apply water to these new instruments; the high quality sensor does the job dry.

Use caution when making a purchase, however; most hygrometers on the market are poor performers that will not provide the consistent reliable readings you'll need. Our old friend the sling psychrometer has slipped into the past. It just can’t read accurately enough to provide the critical readings we need today.

Ideal Temperature and Humidity
Ideal indoor humidity levels vary in different climate zones around the country and also change, depending on the indoor humidity or with outdoor conditions. Take a look at the NCI Indoor Comfort Equivalent Temperature and Humidity Tables (http://bit.ly/NCI_Table) and find the indoor temperature and humidity that will suit your projects in either heating or cooling mode, under varying humidity and temperature levels.

ASHRAE Standards specify ideal comfort ranges of 68F to 74F at 40% to 60% relative humidity in the winter months and 73F to 79F at 40% to 60% relative humidity in the summer months.

How to Measure Humidity
To effectively evaluate current indoor conditions we'll need to measure room relative humidity and the entering and exiting wet bulb air temperatures of the HVAC system.

Room Humidity
To measure humidity in a room, allow the room to stabilize if the system has been running. Set your hygrometer to the humidity mode, then simply take the humidity reading. Don't take the measurement near a wall, or near a heat-generating appliance. Do not measure in the system air stream or directly in the sun. Measure humidity near the center of the room.

Take and record the humidity reading in several rooms served by the same system and average the humidity reading.

System Humidity
The NCI recommended way to measure moisture in an HVAC system is to set your hygrometer to wet bulb mode. To get the whole picture of the latent heat (moisture) moving through a system, measure wet bulb temperatures at the return grilles, where the air enters and exits the equipment and at the supply registers.

Wet bulb readings are then converted to enthalpy. Enthalpy is used to calculate the total, sensible and latent BTU removed by or added to the building by the system. The only way to identify the moisture movement through the system and identify its moisture removal capacity is to measure it. If you need the procedure, I'll send it to you if you email me. See the information at the end of the article.

Building Performance and Moisture
Moisture will enter a building from a number of sources unrelated to the HVAC system. However as the building performance and HVAC industries unite to deliver better comfort and efficiency to consumers, moisture control is becoming a piece of HVAC projects.

In a building, moisture may enter through roof and wall leaks from rain. Moisture may seep in from surface water from rain or irrigation that may find its way into the building. Moisture may also enter a building from water vapor in soil or from underground water sources that may wick or migrate into a building. (See Illustration A.)

Internal sources of moisture include leaks from plumbing systems, or water vapor discharged by showers, cooking, laundry, or plants. Today's HVAC contractor should become familiar with these sources of moisture and be able to advise building owners of the sources of excessive moisture and encourage them to solve their moisture issues.

The best solution to excessive moisture is to not allow it to enter a building in the first place. Once moisture enters a building, it requires significant cost and energy to remove it.

HVAC Causes of Excessive Humidity
When it comes to discovering the sources of excessive humidity in a building, most of us are surprised to find large amounts of moisture brought into the building by the HVAC system.

Return air leakage is the largest source of moisture you’ll often find. Return air leakage from unconditioned attics is often the worst source of moisture. A 200 CFM return leak from a 120F attic can easily raise the humidity in a home by 30% in several hours.