DriSteem HUMIDI-TECH DI Installation Guide download pdf (Page 7)

DRI-STEEM Vapor-logic

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Control input signals

DRI-STEEM offers three control options for all its humidification

systems controlled by Vapor-logic

: on-off control, demand signal

control, and transmitter control.

On-off control

On-off control—the simplest control scheme—does exactly what its

name implies: the output device turns fully on, then fully off. Most

residential furnaces use this type of control.

The humidistat that controls the humidifier has a differential

between the on and off switch points. The differential is established

at a range sufficient to prevent output short cycling. In other

words, the humidity level has to fall below set point before the

humidistat closes and energizes the humidifier. Once the humidifier

is energized, the humidistat stays closed until the humidity is

above set point. This creates an operating range that prevents the

humidifier from running for very short periods of time.

In applications where there are multiple contactor outputs, such as

a Vaporstream humidifier, contactors for each heat stage are pulled

in one at a time with a one-second delay between them to prevent

a brownout condition. In applications with a variable output stage,

such as a GTS humidifier, the outputs are ramped up until they

reach 100%.

Demand signal control

With demand signal control, the Vapor-logic

controller provides

the output level that a master control signal calls for. This signal

can be generated either by a humidistat or by a building automation

system. The signal sent to the Vapor-logic

board is a modulating

signal (typically 4 to 20 mA or 0 to 10 VDC). Vapor-logic

responds

to this signal by producing a direct proportional output. In other

words, with a 4 to 20 mA signal, a command of 4 mA produces no

output. A command of 12 mA runs the humidifier at 50% demand,

and a command of 20 mA runs the humidifier at 100% demand.

With a humidistat provided by DRI-STEEM producing this signal,

the humidity set point is set at the humidistat. The keypad/display

then is used for maintaining and troubleshooting the humidification

system, with humidifier control stemming from the humidistat

itself. With a building automation system (BAS) providing the

signal, the humidity set point is established by the BAS, and the

humidifier responds to the BAS commands.

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