Trane Tracer VV550 Installation, Operation And Maintenance Manual page 47

Modulating (three-wire) hydronic heat
The amount of modulating hot water reheat required is determined by a control
algorithm. The algorithm determines the percentage of available reheat capacity needed
to satisfy zone heating requests. The modulating valve then is driven to the calculated
position.
It is difficult for the modulating types of reheat to control to the heating setpoint. The
proportional integral algorithm would normally overshoot slightly and settle in at the
correct amount of heat. However, this overshoot is not allowed for reasons relating to
energy consumption. When the zone temperature reaches the heating setpoint plus 0.5°F
(0.28°C), the reheat is turned Off. This causes the operation with modulating heat to
oscillate over a long time period and not control at the setpoint.
The modulating hot water valve is over driven closed (by 20 seconds) whenever reheat
is no longer required. This maintains position calibration of the hot water valve.
Electric Heat
Thermostat control and pulse width modulation (PWM) control of electric heat are
supported. The controller supports up to three stages of electric heat without a fan and
two stages with a fan. For electric heat, the reheat enable setpoint is able to be
configured. The electric heat range is 70°F to 100°F [21.11°C to 37.78°C]).
Local electric heat is enabled when there is no valid primary air temperature and the
configured default primary air temperature is cooling. Local electric heat is disabled:
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Local electric heat is only allowed when a valid primary air temperature is available. If a
valid primary air temperature is not available, the controller uses a derived primary air
temperature. The derived primary air temperature is warm or hot when the system is
heating, or in morning warm-up, or in auto with a configured default primary air
temperature indicating heat.
Local electric reheat is allowed to run for 30 seconds prior to the airflow being at the local
reheat minimum airflow setpoint. If the airflow never reaches that point, electric reheat
is turned off until the airflow is at least at the local reheat minimum airflow setpoint. Once
the reheat is on, it is allowed to run as long as the airflow is greater than the local reheat
minimum -3% of the normal airflow. If the reheat is turned off because the airflow falls
below the local reheat minimum -3%, the airflow has to rise above the local reheat
minimum before the reheat is turned back on.
CNT-SVX17G-EN
When there is no valid primary air temperature and the configured default primary air
temperature is heating;
If the primary air temperature is greater than the configured reheat enable setpoint.
It remains disabled until the primary air temperature drops 5°F (2.78°C) below the
configured reheat enable setpoint;
When the derived primary air temperature is warm or hot. If the configured reheat
enable setpoint is invalid, the local electric heat is disabled.
Sequence of Operations
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