Increase Your Ability to Control Humidity

Increase Your Ability to Control Humidity

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 ( 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.

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The amount of moisture air can hold increases as it becomes warmer. Air in a hot attic that's pulled into an HVAC system has up to three times more moisture in it as the same volume of air at normal room temperature. When it's that hot, saturated air hits a cold coil and air conditioners are forced to operate far outside manufacturer's specifications. The coil can not possibly dehumidify this air in a single pass. The moisture passes through the coil and is dumped into the building, causing humidity to skyrocket.

Fix the return duct leak, and eliminate this moisture source.

Supply duct leaks can also create moisture problems. When supply ducts are outside the building envelope and they leak, the amount of supply air in a home may be less than the amount of return air into a building. This will cause a negative pressure inside the building.

When a building is operating under a negative pressure, whenever a door or window is opened or there are penetrations in the envelope, moisture will be pulled into the building whenever the fan is operating. This is especially true in the more humid regions of the country. Some building humidity levels are constantly high due to this effect.

Fix the supply duct leakage, and eliminate this moisture source.

Changing Equipment Latent Capacity
In the attempt to win the efficiency war, some manufacturers have significantly reduced the moisture removal capacity of their cooling equipment. Just read their rated latent removal capacity under normal operating conditions from their engineering data and compare it to older or less efficient equipment.

Many contractors are finding that in order to deliver adequate indoor air conditions in the more humid parts of the country additional dehumidification systems are becoming mandatory. Stand alone equipment or auxiliary dehumidification equipment is often required to maintain acceptable humidity levels, even when the system is operating at near perfect performance.

In winter months, many parts of the country require additional moisture to be added to the home to maintain comfortable humidity levels.

As air is heated, moisture is removed through the combustion process. Also, if it's cold and dry outside and the building envelope is leaky, physics take over. Warm air will move to colder air and take the humidity in the home with it. This leaves an uncomfortably dry home that begs for a moisture infusion.

A number of effective humidification systems can accomplish this task.

Reduce Airflow, Increase Dehumidification
Most of us know that as airflow is decreased through cooling equipment that the latent (moisture) removal capacity of the equipment will increase. This principle is correct, but is often misused in the field. The outcome is a significant decrease in system capacity.

What we've learned over the years is that many contractors simply lower fan speed to increase moisture removal assuming they’re starting with 400 CFM per ton. So while they thought they were reducing airflow down to 350 CFM per ton, in reality they were only at 300 CFM per ton to start with and actually lowered airflow to 225 CFM per ton. There’s little argument that there’s no way a normal DX cooling system can operate effectively at 225 CFM per ton.

Moisture and Humidity Actions You May Take
So, before decreasing airflow to increase dehumidification, first make sure your system is operating at full airflow and cooling capacity.

First, you can verify 400 CFM per ton; not just over the coil, but into the return grilles and out of the supply registers too. You can also measure the system delivered BTU into the building. Unless you fully commissioned the system previously, the odds are you’ll find system defects preventing good dehumidification as you verify airflow and delivered BTU.

Repair and adjust the system to eliminate the HVAC causes of humidity.

Then, operate the system under its conditions for several days at 400 CFM per ton. If humidity remains too high, search for evidence of moisture sources in the building performance issues that allow moisture to enter the home as described above. Cure those defects or connect your client with able tradesmen to solve their roofing, plumbing, landscaping, or water sealing problems.

Allow the system to run again for several days. If the humidity levels remain high, then consider reducing fan airflow. Measure system performance and verify the moisture removal of the system has increased.

Once you've verified that the HVAC system and the building are both operating as they should, you then may add auxiliary humidity removal equipment and systems as needed to remove excess moisture from the building.

In more humid regions of the country, adding moisture removal systems will become standard operating procedure as you learn to serve your customers above the level provided by competing contractors in your area.

Hopefully you can begin to see the opportunity to serve your clients and earn substantial profits as you learn to solve moisture and humidity problems.

Rob "Doc" Falke serves the industry as president of National Comfort Institute a HVAC based training company with technical and business level membership organizations. If you're an HVAC contractor or technician interested in a free procedure that will show you how to measure a system's total, latent, and sensible BTU, contact Doc at [email protected] or call him at 800-633-7058. Go to NCI's website at for free information, articles, and downloads.

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