User Guide

Ap plication Bul l etin
Aprilaire Application Bulletin
Model 4851 Blower Activation Relay
Aprilaire humidifiers are designed to operate in conjunction with the home’s heating
system to deliver the comfort and protection of whole-house humidification. In typical
installations, the humidifier operates when there is a call for heat from the thermostat.
However, there are installations where humidifier operation beyond a call for heat may be
required; e.g. when installed on a heat pump or during times of the year where there is a
need for humidity but not a need for heat.
For these applications, we recommend the use of the Aprilaire 4851 Blower Activation
Relay. When wired as shown in the diagram below, the installation of this relay allows
the humidifier to energize the HVAC blower on a call for humidity; the humidifier is no
longer dependent on a call for heat. This means the Aprilaire humidifier will deliver the
comfort and protection of controlled humidification whenever it is needed.
Important Note: Plumb humidifier to hot water when using this wiring configuration.
Form #246