A Fix for Too-Dry Indoor Air

Enhance comfort and remedy dry-air problems with a whole-house humidifier.

When it comes to the structural components of your home, moisture is often considered the enemy. But, if the air in your home is too dry, you may suffer respiratory illness, sore throats, itchy skin, chapped lips, nosebleeds and static shocks. In addition, dry air can result in separating floorboards, gaps in moldings and loose furniture joints. Fortunately, you can avoid these problems by increasing the relative humidity in your home.

Intentionally increasing humidity might sound crazy, especially considering all the recent attention directed at mold and other moisture related problems. But some regions experience very dry conditions during at least part of the year, especially areas where temperatures remain below 40 degrees for a sustained period. Even if outdoor humidity levels are high, forced-air heating systems dry out the air inside homes in these regions.

You can increase humidity in many ways. Everyday activities such as boiling water or running a hot shower increase the humidity level, but these methods are not energy efficient, are difficult to regulate and tend to leave rooms feeling muggy.

A better way to add humidity to your house is with an evaporative or ultrasonic humidifier. Portable humidifiers are popular because they increase humidity quickly and are easy to set up and operate. However, some models lack accurate control, require frequent refilling and cleaning, take up floor space and create additional noise.

The best option for adding humidity throughout your home is a central or whole-house humidifier. These units will create enough moisture to serve a 4,500-sq.-ft. house. They connect directly to forced-air heating systems, so they take up no living space and create no additional noise.

Many early central humidifier models were criticized because they used a water reservoir that could become stagnant and encourage mold and bacteria growth. Most central humidifiers today have "flow-through" systems that keep the water circulating and either use all of the water or divert excess water down a drain. The only maintenance required is the annual or biannual replacement of the filter.

Choosing the right type
When selecting a central humidifier, you need to consider three main options: the type of airflow system, the type of humidistat and the drain system.

The two main airflow options are bypass and fan power (see the illustration below). Bypass models can be installed on the cold-air return (called the return-air plenum). Air is supplied to the humidifier from the furnace's outgoing warm-air supply plenum. A damper located in the bypass vent pipe closes off airflow through the humidifier during the air conditioning season.

Bypass (shown at left): Heated air travels through the humidifier to pick up moisture. Power Fan (shown at right): A fan built into the humidifier forces moisture directly intto the air flow.

Power fan models are most often installed directly on the warm-air plenum, but they can be installed on the cold-air plenum when connected to a hot-water heat source. No bypass is necessary because a built-in fan forces the moisture into the outgoing warm-air flow. These models are usually capable of producing more moisture than bypass models, so they're a better choice for larger homes.

The moisture that a humidifier produces is regulated by a humidistat. It is similar to the thermostat used to control a furnace or central air conditioner. The humidistat sends a signal to a solenoid valve that controls water flow across the filter in the humidifier. Central humidifiers are available with manual or automatic humidistats.

Manual-control humidistats require that you keep track of temperature changes that will occur during the next 24 hours and adjust the humidifier accordingly. Manual humidistats should be located in an easy-to-access location, such as next to the thermostat. They may also be located on the return-air plenum using an adapter plate.

Computer-controlled automatic humidistats are the best choice for controlling a humidifier. They monitor both the current indoor humidity level and the outdoor temperature to regulate the humidifier's operation, providing superior accuracy, faster responses to changes in conditions and "set it and forget it" convenience. An automatic humidistat should be located on the cold-air return a minimum of 6-in. up-line from the humidifier, any bypass connection or any fresh-air connection to prevent inaccurate readings.

One final consideration is how the excess water will be handled. Most central humidifiers feature a drain that is directed to a floor drain or to a pump that will move the excess water to a floor drain location. If you cannot divert to a floor drain or you have a septic system, choose a model that has a wicking filter or modified reservoir so it can use all of the water it receives.

Installing the unit
Installing a central humidifier requires basic plumbing, wiring and sheet-metal skills. We worked with Club member Alan Babatz of Ditter Heating Inc. in Hamel, Minnesota, to install an Aprilaire model 600 humidifier (see the how-to photos below). Most humidifier manufacturers prefer that a licensed HVAC contractor perform the installation, so if you choose to do the job yourself, keep in mind that you'll likely void the product warranty. Also, check with your municipality's building inspections department for code requirements before you begin.

Templates are included for laying out the locations of the humidifier and humidistat. Use aviator snips to cut the openings in the ducts and 1-in. sheet-metal screws to fasten the assemblies in the openings.

Use templates to mark the location of the humidifier and humidistat on the return duct. Use the vent-pipe starter collar as a template to mark the location of the vent-pipe bypass opening on the warm-air duct. Cut along the traced template lines with tin snips.

Fasten the humidifier housing with 1-in. self-tapping sheet metal screws.

Cut the opening for the bypass. Wear gloves when installing the starter collar.

It is essential that you follow the humidifier and furnace manufacturers' instructions for wiring and that you disconnect power supplies before you make any connections. In general, the humidifier is connected to the furnace circuit board's accessory terminal. This connection powers the humidifier whenever the furnace fan is powered on.

Pay special attention to whether the humidifier requires low-voltage or standard current. Bypass humidifiers operate on low-voltage power, so either they must connect to a low-voltage terminal on the circuit board or the power from the circuit board must connect to a transformer before connecting to the humidifier. Power fan models operate on standard current and include a low-voltage circuit for the humidistat.

Locate the humidistat on the cold-air return duct at least 6 in. up from the humidifier location.

Build the vent-pipe bypass between the humidifier housing and the starter collar with straight and elbow sections of 6-in. vent pipe. Keep the path as straight as possible, avoiding sharp turns.

Water can be supplied from hot or cold water lines. Many installers connect to the hot supply because it increases evaporative performance, even though a small amount of energy is used to run the water heater.

Connect the wires from the humidistat and the 1/4-in. water supply to the solenoid valve.

Tap into the hot water supply line. We installed a full-slip valve tee compression fitting.

The water supply line is 1/4-in. copper tubing. In the past, the water connection was almost always made with a saddle valve, but many cities now require a soldered or compression tee-valve connection. (Check with your municipal authority.) The drain line is 1/2-in. flexible tubing that runs from the drain port of the humidifier to a floor drain or pump.

Connect 1/2-in.-dia. tubing to the humidifier drain. Then install the filter and connect the water supply.

Operating the humidifier
Once the system is installed, you need to establish the best humidity level in your home. Keeping the structure of your house dry and the air humid might seem mutually exclusive, but there is an optimal humidity level that will keep you comfortable and won't cause moisture problems. The level that is right for your house at a given temperature is not the same for every house. Tighter homes typically require lower levels because they keep more moisture trapped inside. The goal is to find and maintain the right humidity level for your house.

For the initial setup, turn the humidistat control to the middle or "normal" setting as specified by the manufacturer. Wait 24 hours and adjust the setting accordingly. Set the humidity to the highest level that does not create excess condensation. If you install an automatic humidistat, very little adjustment is required. A manual humidistat requires adjustment as outdoor temperatures change.

Most central humidifiers require only seasonal maintenance. When the weather warms up and the furnace is no longer needed, neither is the humidifier. Taking four end-of-season maintenance steps can help preserve your humidifier (though not all models require all four steps). First, if you have a manually controlled humidifier, shut it down for the season when the outdoor temperature reaches 50 degrees. Some models automatically shut down when the outdoor temperature warms up. Second, turn off the water supply. Third, on bypass models, close the damper to stop air-conditioned air from flowing through the humidifier. And fourth, change the water filter now so that you're ready for next winter.

When we refer to humidity in homes we are talking about relative humidity: the percentage of the maximum amount of water that the air can hold at a given temperature. When the air temperature changes, so does the amount of water that the air can hold. To avoid condensation and potential damage to your home, the relative humidity of the air in your home must change when the outdoor temperature changes. For example, when the outdoor temperature is 30 degrees, an indoor humidity of 35 percent will typically not cause condensation. But if the outdoor temperature drops to 0 degrees, an indoor humidity of 35 percent is too high and will result in condensation that can lead to damage. The following list, provided by Aprilaire, shows appropriate indoor relative humidity levels at various outdoor temperatures.

Outside Temp - Recommended RH
40 degrees Fahrenheit - 45 percent
30 degrees Fahrenheit - 40 percent
20 degrees Fahrenheit - 35 percent
10 degrees Fahrenheit - 30 percent
0 degrees Fahrenheit - 25 percent
-10 degrees Fahrenheit - 20 percent
-20 degrees Fahrenheit - 15 percent