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SELECTION
Humidifier Selection
TrueSTEAM Humidification System
With the multiple humidifier styles and sizes available to distributors and contractors, it can
be difficult to discern which model provides the best solution for the homeowner. A common
method of rating the humidification capacity is to state the output in terms of gallons per day
(GPD). However, that rating can be misleading since evaporative pad and steam-based
products can perform differently depending on the operating conditions.
Evaporative Pad
GPD rating is based on the amount of moisture the humidifier can deliver if it is running
continuously over a 24 hour period 120°F air passing through the wetted pad. This rating does
not account for periods when the furnace is off, and cooler plenum temperatures and duct air
flow may impact performance.
Steam-Based
GPD rating is based on the amount of water evaporated by heating the water. These units are
typically installed so that they turn on the HVAC fan when there is a need for humidity so they
are not dependant on furnace run time. Steam is delivered to the air duct regardless of plenum
temperatures.
It is important to understand the difference between these two as it directly affects which
product is best for each home. For example, a evaporative pad humidifier rated for 18 GPD that
is installed in a home where the furnace and fan only run 8 hours (33%) of any given day may
actually be delivering only 5.8 GPD of humidity. If that furnace air temperature is less than 120°F,
that delivery rate will be even less – perhaps closer to 4 GPD.
One of the distinct advantages of using steam-based humidifier designs is that there are no dependencies on air temperature or furnace
operation in order to deliver its rated GPD. For example, the TrueSTEAM HM506, which is rated at 6.5 GPD will deliver 6.5 GPD regardless of
furnace operation or air temperature. This makes steam based designs ideal for moderate climates where the furnace doesn’t run as often, as
well as for homes that have high-efficiency or multi-stage (heat pump) equipment that run less frequently and at lower air temperatures.
There are situations, however, where evaporative pad humidifiers can meet and in some cases exceed the delivery rates of smaller steam-
based units. This becomes relevant with gas/oil conventional furnaces that have plenum air temperatures over 120°F and in colder climates
where the furnace may run 50-75% of the time. As established earlier, the TrueSTEAM HM506 will deliver 6.5 GPD regardless of furnace run
time. The HE225 evaporative pad humidifier has a 12 GPD rating, which if applied to a conventional (gas/oil) furnace in a colder climate where
the furnace and fan run for 12 hours (50%) can deliver up to 6 GPD – comparable to the TrueSTEAM HM506.
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