Sequence Of Operation - Rehau PD1 Instructions Manual

Sequence of Operation

PD1 (One Stage Heat)
Air Sensor(s) Only Operation
When operating with only an air sensor, the on time for the Heat relay is calculated to satisfy the
requirements of the air sensor.
Slab Sensor Only Operation
When operating with only a slab sensor, the on time for the Heat relay is calculated to satisfy the
requirements of the slab sensor. The thermostat operates to maintain the slab at the minimum slab
temperature setting.
Note: Operating with only a slab sensor can lead to either overheating or under heating of the space.
Air and Slab Sensor Operation
When operating with both air and slab sensors, the thermostat calculates an on time for the Heat relay to
satisfy the slab sensor's requirements and an on time to satisfy the air sensor's requirements. The Heat
relay operates for the longer of these two on times. During light heating loads, overheating can occur due to
the minimum slab temperature requirements. During heavy heating loads, the maximum slab temperature
setting limits the on time of the Heat relay. In this situation, underheating can occur.
Mode
Heat In the heat mode, the Heat relay is operated to satisfy the temperature requirement of the zone.
Off In the Off mode, the Heat relay is not operated.
Note: If an air or slab sensor is active in the Off mode, a freeze protection is enabled that allows the Heat
relay to be operated to keep the zone above 35° F (2 ° C).
PD2 (Two Stage Heat)
The two stage mode of operation is selected using the DIP switch
located on the circuit board inside the thermostat.
In cases where a one stage heating system can not provide
sufficient heat under all conditions, a second stage of heat can be
added to supplement the first stage. A two stage system therefore
has one thermostat controlling two output relays.
Two Stage Logic
The temperature in a two stage zone is controlled by varying the on time of the Heat 1 and Heat 2
relays during a cycle. Under light loads, the Heat 1 relay is cycled on and off. As the load
increases, the Heat 1 relay on time is increased until it reaches the maximum of the cycle length or,
if a slab sensor is used, the slab temperature reaches the slab maximum setting. The Heat 2 relay
is then cycled and its on time is increased as the load increases. When the heating load decreases,
the on time of the Heat 2 relay is reduced until the Heat 2 relay is turned off completely. The
thermostat then reduces the on time of the Heat 1 relay
Note: When using a slab sensor, the Heat 2 relay may be on while the Heat 1 relay is off if the slab
temperature has reached the slab maximum setting
Air Sensor(s) Only Operation
When operating with only an air sensor, the on times for the Heat 1 and Heat 2 relays are
calculated to satisfy the requirements of the air sensor.
Slab Sensor Only Operation
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