tekmar 512 Programmable Thermostat Installation - Manual (Page 4)

© 2009 D 512 - 09/09 4 of 20
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
When operating with only a slab sensor, the on times for the Heat 1 and Heat 2 relays
are 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 underheating
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 1 relay to satisfy the slab sensor’s requirements and on times for the Heat 1
and Heat 2 relays to satisfy the air sensor’s requirements. The thermostat operates the
Heat 1 relay for the longer of these two on times.
While the minimum slab temperature is satisfied, the on times of the Heat 1 and Heat 2
relays are calculated to satisfy the air temperature requirements
.
During heavy loads, the maximum slab temperature setting limits the on time of the Heat
1 relay. In this situation, the Heat 2 relay may be on while the Heat 1 relay is off.
NOTE: During light heating loads, overheating can occur due to the minimum slab
temperature requirements
.
Mode
Heat In the heat mode, the Heat 1 and Heat 2 relays are operated to satisfy the
temperature requirement of the zone .
Off In the Off mode, the Heat 1 and Heat 2 relays are not operate d.
NOTE: If an air or slab sensor is active in the Off mode, a freeze protection is enabled
that allows the relays to be operated to keep the zone above 35°F (2°C).