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Humidification is important when it comes to indoor air quality, skincare, minimizing static electricity, and protecting string instruments, just to name a few concerns. When the seasons change from hot, muggy summers into cool, dry winters, a common question homeowners tend to have related to humidity is why their furnace-mounted humidifier is not keeping up. Why does the Weather Channel say it is 80% outside, but it is only 20% in the house? To understand why this happens, everyone should know about the bypass humidifier, a very common accessory found in residential settings.

Why does the Air ‘Dry Out’ in Winter?

Summer air is consistently hot and thus has a greater ability to hold moisture from lakes, rivers, and streams, so the air feels moist. Winter air is consistently cold and thus has a lower ability to hold moisture. Interestingly enough, meteorologists and HVAC technicians alike say the cold air actually has a higher “relative humidity,” which is a percentage of the actual amount of moisture air is holding compared to what it can hold. The low capacity of cold air causes moisture to “fall out” of the air until the air is holding, relatively, as much as it can – nearly 100%. When this air enters a warm house, the air’s capacity to hold water increases. However, if water is not added to the air, the air effectively “dries out” and in the process attempts to “soak up” as much water as possible – from the body’s skin, nasal passages, eye sockets, etc.

Because of this process, indoor air in winter tends to feel less comfortable. A solution is to maintain humidity between 30-and-60%, as recommended by health experts as well as ASHRAE. Keeping the humidity above 30% can be accomplished by installing a whole-home humidifier, such as a flow-through evaporative humidifier that mounts onto the furnace.

The Basics of an Evaporative Humdifier

Although there are many ways to control a furnace-mounted humidifier, they all work on the same principle of evaporation. A bypass humidifier is easy to spot because of its round, usually six-inch, duct connecting the humidifier to the opposite plenum, in effect bypassing the furnace. Having this bypass allows the furnace’s indoor blower motor to draw air across a water panel. Another style of a furnace-mounted evaporative humidifier is a fan-powered humidifier, and as the name implies, it has its own motorized fan and does not use a bypass duct. These furnace-mounted humidifiers are also referred to as “flow-through” humidifiers.

When a humidifier control (“humidistat”) senses that the humidity of the indoor air is low, and the furnace blower motor is running, it allows an electrical signal to be sent to a 24-volt solenoid valve. The solenoid valve opens and allows water to flow from the hot side of a hot water tank, connected by a saddle valve, and into the humidifier’s distribution tray. From the tray, water evenly flows by gravity down a water panel, also called a pad, which is made of an expanded-aluminum mesh coated in liquid clay which is highly absorbent to allow water to evenly coat the pad.

As mentioned previously, air is pushed across this pad, causing the water to evaporate into the plenum on which the humidifier is mounted. Finally, the excess water is drained out of the bottom and into a drain system. This type of humidification works with the same principle as a swamp cooler whereby warm, dry outdoor air is pulled across a wet pad to inject moisture into the space and absorb heat. This is why flow-through humidifiers work best with warm water which removes less heat from the air than cold water.

Generally, it does not matter if a humidifier is mounted on the supply or return. Though some technicians will have differing opinions on this, in the field, technicians will equally see both configurations, and the deciding factor is usually space requirements, where installation is easiest, or if the installation manual specifies which side of the furnace is preferred.

IDENTIFYING KEY COMPONENTS: According to Lianna Schwalenberg, an HVAC service technician at The K Company, located in Akron, Ohio, Generally, it does not matter if a humidifier is mounted on the supply or return. Though some technicians will have differing opinions on this, in the field, technicians will equally see both configurations, and the deciding factor is usually space requirements, where installation is easiest, or if the installation manual specifies which side of the furnace is preferred. (Lianna Schwalenberg)

Bypass Humidifiers and their Effect on CFM

A problem that bypasses humidifiers specifically can have is if too much hot supply air travels through the bypass, mixes with warm return air, and causes the furnace’s high limit safety switch to trip. This nuisance trip can be alleviated by increasing the cubic feet per minute (CFM) by increasing the blower motor speed. Some thermostats for modulating blower motors can be configured in such a way that if the thermostat knows the furnace has a bypass, it will compensate by increasing the CFM in heating mode.

Why does Outside Temperate Matter?

On some humidistats, homeowners are usually curious about the outdoor temperature chart which seems counterintuitive to what the humidifier is designed to accomplish. The chart on the humidistat will usually say something like, “WHEN OUTSIDE TEMP IS -20°F, RESET HUMIDITY TO 15%. WHEN OUTSIDE TEMP IS 0°F, RESET HUMIDITY TO 25%. WHEN OUTSIDE TEMP IS +20°F, RESET HUMIDITY TO 35%.” As outdoor temperature drops, it wants the humidity setting to drop, too. Likewise, some humidistats come with outdoor temperature sensors wired into their controls to automatically reduce the humidity setpoint when it gets very cold outside.

This is to prevent condensation on cold surfaces such as windows, which can lead to mold and bacteria growth. With updated, better-insulated windows, this might not be an issue for some homes, but it is not always windows where condensation can occur.

Maintenance Considerations

It is recommended to change the humidifier pad once a year as they do get very dirty and sometimes plugged up with water softener salt. During summer, it is also recommended to turn off the humidistat and close the bypass damper.

Things to Consider when Humidity is Still a Problem

When homeowners are concerned about humidity not keeping up in the wintertime, it is almost never the humidifier to blame because it is a very simple device with very few moving parts. In most cases, the lack of humidity is caused by air infiltration and exfiltration through the home’s walls, ceilings, windows, doors, etc. As warm air escapes to the outside and cold outdoor air infiltrates, the indoor air loses relative humidity, making the air feel uncomfortable if moisture is not added to it.

Because bypass humidifiers interfere with airflow, waste a significant amount of water that just goes down the drain, and they constantly fight with issues of infiltration, homeowners with strict humidity concerns should consider steam humidifiers. These come in the form of whole-home humidifiers, which create steam and put that steam directly into the ductwork without the involvement of the furnace, or “tabletop” humidifiers which can be bought at local drugstores and simply plug into an outlet.

A more high-end solution, especially if homeowners are concerned about stale air as they improve the home’s insulation, is an Energy Recovery Ventilator (ERV) which brings in fresh outside air and tempers it with the heat and humidity of the exhaust air from inside.