Steffes HPB22 Owners Manual - Manual (Page 33)

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H. THEORY OF OPERATION
(For units after serial/ARL number 00110)
•OUTDOOR TEMPERATURE SENSING
Core charging of the Steffes ETS Heat Pump Booster (HPB) is regulated by a Three-Stage Outdoor Air Tem-
perature Sensor. The mercury bulbs of this sensor are normally calibrated at 45
o
F, 35
o
F, and 25
o
F. These temperatures
are set points for charge levels 1, 2, and 3 respectively. (See Table 1). Optional temperature set points are available.
The sensors will initiate core charging a drop in outdoor temperature. By design, the on-board charge control
system will respond to the coldest temperature signal. As a result, should the charge level 1 sensor fail to respond to
the outdoor temperature, the charge level 2 or 3 sensors will initiate core charging. This feature provides control
redundancy, reducing the chance of having an uncharged core during cold weather.
•CORE TEMPERATURE REGULATION
Above 45
o
F (outdoor air), the HPB will not maintain a core charge; but, 1/3 of the core charging elements will
be energized only during a home heating call.
During moderate weather (35
o
F to 45
o
F) the charge level 1 sensor opens. This signals the HPB charge control
circuit board to energize charge level 1 heating elements through the charge control sequencers. In this mode, the
charge control circuit board will also apply 24 VAC to both the lower and upper segments of the charge control
resistor’s center tap. This resistor acts as a setback device by applying heat to the charge control thermostat sensing
bulb. When both segments are energized, a 1/3 core charge will be allowed.
During cool weather (25
o
F to 35
o
F degrees), charge level 1 and 2 sensors open signaling the HPB charge control
circuit board to energize the charge level 1 and 2 elements. 24 VAC will be applied to only the lower segment of charge
control resistor allowing a 2/3 core charge.
In cold weather (below 25
o
F), all sensors open signaling the charge control circuit board to energize all heating
elements. In this mode, no voltage is applied to the charge control resistor, and unit will be allowed to charge to its
maximum charge level.
•AIR DELIVERY
On a call for heat, the wall thermostat will energize the Y terminal on the HPB's SFIII relay interface board.
Depending upon the heat pump control strategy, the O terminal may also be energized which in turn energizes the HPB's
internal blower.
A remote mounted duct temperature sensor modulates the HPB’s internal damper. This allows the HPB to mix
its warm discharge air with the cool output air of the heat pump to ensure comfortable supply temperature.
On a stage 2 heat call, the wall thermostat energizes the Y terminal on the HPB’s SFIII relay interface board,
and the W terminal will be energized through the HPB’s internal controls. The duct sensor is bypassed, and control of
the HPB’s output temperature shifts to the internal air discharge controller. In this mode, the HPB will provide dis-
charge air at the maximum allowable temperature depending upon the core charge.