UNDERSTANDING SYSTEM OPERATION (READ BEFORE STARTING SYSTEM)
Mode and Damper Positions
The thermostats determine the system heating or cooling mode. The first call in any zone sets the mode to satisfy that call. It will remain in that mode until all calls in that mode are satisfied and the equipment has been off for the time set by the auto changeover time dipswitches. (See Timers section.)
In normal heating or cooling, the damper of any zone with a call in the current mode will be open and all other zones will be closed. When there is no call, any zone with its fan set to ON will be open, and any zone with its fan set to AUTO will be closed. If any zone fan is set to ON, the blower will be energized.
If all fan settings are AUTO, the dampers will remain in their last position before the equipment turned off and the blower will be off. (This normally means one damper open and all others closed.)
When dampers are to move, all opening is done first, followed by all closing.
Stages
In multi-stage systems, the equipment stage is set by the greatest thermostat call, but may be delayed by the control’s cycle and staging timers. (See timers explanation below.) The AC Control supports only single stage heat and cool. The HP control supports two stage cooling, two stage furnace heating, and three stage HP heating (lo HP, hi HP, hi HP + aux heat.)
Emergency Heat
Emergency heat (aux heat without compressor heat) can be selected for a HP system by either of two ways: First, by selecting Eheat using the Eheat override switch on the HP Control, or second, selection of Eheat on each of the thermostats, provided they have the Eheat function. When either of these Eheat selections is made, a heating demand provides a W signal without a Y signal to the equipment.
NOTE: The second Eheat method requires HP thermostats and that they all must be set to Eheat.
Indicator LEDs
There are 7 indicator LEDs on the AC Control and an additional 3 on the HP/2S Control. Their locations are shown on Fig. 8 and 9.
Each damper has its own green LED which is ON when the damper opens due to a calling condition or partially open due to an LAT or HPT limit condition.
Each equipment output has its own LED which is on when that output is energized. Y and O outputs are yellow, W outputs are red and the G output is green.
In addition, there is a status LED whose operation is described under the section Error Codes.
Timers
To control excessive equipment cycling or rapid staging up, the control has two timers. The cycle timer prevents the same stage from turning on within 10 minutes of the last time it turned on. This allows a stage to turn on for as short or as long as the thermostats request, but will not allow more than six cycles per hour.
The staging timer prevents a higher stage from turning on until the next stage below it has been on for 15 minutes. This minimizes use of electric heat with heat HP systems.
There is also a timegaurd timer which will not allow the compres- sor to be turned on until it has been off for five minutes.
A changeover timer, which can be set from 0 to 30 minutes, limits
the control’s ability to switch between heating and cooling. The opposite mode is prevented from coming on until the first mode has been satisfied for the selected time.
Timer Override
A momentary switch is located near the bottom of the control circuit board. Pressing it momentarily overrides all of the system timers, allowing the control to immediately jump to the highest calling stage.
Temperature Limits and Sensors
Both the AC and HP controls have a LAT (leaving air temperature) sensor which is to be placed in the downstream air path of the heating /cooling equipment. It is used in both heating and cooling to limit LAT to a safe value. It must be connected. The system will not operate without it. Its setting is fixed for cooling and is adjustable in four settings for heating. Selection of best setting is discussed under LAT Limit Selection.
The HP control also has an optional HPT Sensor (heat pump temperature) which is to be placed downstream of the coil but ahead of the electric heater. This sensor measures the temperature of the air leaving the coil during HP heating. It is not included with the control, but may be ordered separately as part number TSATXXSEN01-B. A 10K ohm resistor is factory installed in its place when the actual sensor is not used. In the HP control only, dipswitch 11 allows the installer to temporarily disable both the LAT and the HPT sensors. Disabling of these sensors is only to be done on a temporary basis.
UNIT DAMAGE HAZARD
Failure to follow this caution may result in unit damage. Operating equipment with sensors disabled can cause perma- nent damage to HVAC equipment.
Bypass
The purpose of a bypass is to limit noise in the duct system when the dampers are excessively restricting it. When a direct bypass (outlet air fed back directly into the return) is used, bypassing decreases entering air temperature in cooling and increases it in heating. Excessive bypassing will lead to limit trips, either through the LAT /HPT sensors or the equipment internal limits.
Setting the Bypass
Setting the bypass is a balance between too much noise (bypass trip pressure set too high) and excessive bypassing which will cause limit trips, diminishing performance. As a general rule, the bypass should remain closed as much as possible. It should never open when all the dampers are open and only open as much as needed to bring noise to an acceptable level when only one damper is open. (See System Setup for details.)
LAT Limit Selection
Cycling on internal equipment limits is to be avoided because it overstresses and can shorten the life of the equipment. Therefore, the LAT limit setting should be selected to trip below the equipment limit. See System Setup for details on how to choose one of four available LAT limits.
Limit Levels and Actions
The response of the system to the LAT/HPT sensors are shown in Table 2. Cooling and HP limits are not adjustable. Looking at Table 2, there are eight limit level index numbers, 0 through 7. These represent the closeness of the actual LAT/HPT temperatures to the final shutdown limit. 0 represents no limit challenge while 7 indicates a final shutdown of the equipment. Note that progres- sive actions are taken by the control as the LAT/HPT limit is approached. Each action progressively reduces the limit challenge by increasing airflow. Normally, the system will stabilize at limit level 1 or 2 because opening all closed dampers 2 or 4 positions (out of 15) will reduce LAT to a level below its limit.
