tekmar 371 House Control - Manual (Page 5)

5
In order to overcome this offset, integral control logic is used. Only digital controls can provide integral control logic due to the lengthy
response time of buildings. Integral control logic is based on time. The longer the room temperature is below the desired setpoint,
the more heat is supplied to the room. With integral control logic, full heat can be supplied to the room on the coldest day of the
year without requiring that the room be cold.
Derivative (D)
In order to speed up the control’s response to quick changes in the heating load, derivative control logic is required. However,
sudden room temperature changes, for example from an open door or window, should be ignored by an intelligent control.
P + I + D = PID
If proportional, integral and derivative (PID) control logic are combined, the control is more able to prevent excessive temperature
swings and provide a stable room temperature under all conditions. It not only takes into account how much the room temperature
has drooped, but also how long there has been a droop and how fast the temperature is changing.
Zone Load Coordination

In a multiple zone system, there can be sudden load changes on the boiler and system
due to multiple zones turning on or off. These sudden load changes often lead to boiler
short cycling and unnecessary mechanical stresses. The operation of the system can be
improved by staggering the starting points of each zone relay within the operating cycle.
Staggering of the zones maintains a relatively constant system flow rate which improves
boiler operation. Controlled staggering can also minimize boiler running time and improve
system efficiency when only a few zones are needed for short periods.
Zone Post Purge

Before the last zone is turned off in a heating cycle, the boiler is turned off but the zone continues to draw heat from the boiler. This
post purge of the boiler reduces stand-by losses and reduces overall energy consumption.
UNOCCUPIED (NIGHT SETBACK)
During the night, or at times when people are not within the building, energy can be saved by lowering the building temperature for an
UnOccupied (Night Setback) period.
Due to the large thermal mass of buildings, it takes a long time for the indoor space temperature to significantly change whenever the
heating system is turned on or off. The building heat up or cool down time is further increased when high mass heating systems are
used (e.g. radiant floors). In most cases night setback cannot be used with these systems due to the long recovery time required in the
morning. A typical system is demonstrated in the adjacent diagram.
At the start of the night setback period the heat is turned off, but the heat
contained within the slab or radiator continues to heat the building and
there is a delay before the space temperature begins to drop. At the end
of this delay the temperature within the building gradually decreases, and
may eventually reach the required UnOccupied temperature after suffi-
cient time has elapsed. Once the setback period is complete, the heat is
turned on again but there is a long recovery time required to raise the
space temperature to the desired setpoint. The length of the delay and
recovery periods changes with outdoor temperature and is different for
each zone within the building.
A comfortable setback can be provided if the control “learns” the
response time for each zone within the building. Based on the zone’s
response time, the control can then calculate an Optimum Stop time and
an Optimum Start time. At the Optimum Stop time the control turns off the
zone valve or pump before the selected UnOcc time in order to overcome
the delay period. At the Optimum Start time, the control starts to raise the
zone temperature before the selected Occ time in order to overcome the
recovery period. This allows night setback to be used with most
heating systems.
Optimum Start / Stop with Water Temperature Boost

When Optimum Start / Stop is combined with Outdoor Reset, the
control can boost the water temperature during the recovery period.
This provides a faster recovery and allows a longer setback for greater
energy savings.
The accuracy of the Optimum Start / Stop routine depends on the
feedback available to the control.
Optimum Start / Stop with both Outdoor and Indoor Sensors

The response time of the building varies with outdoor temperature and is also different
for each zone. The most accurate Optimum Start / Stop routine is therefore achieved
when both the indoor and outdoor temperatures are monitored during transitions between
UnOccupied and Occupied modes.
Zone 1
Zone 2
Zone 3
Zone 4
Post Purge
(Boiler off, Pump on)
On
On
On
On
On
Delay
Period
8 A.M. 10 P.M. 11 P.M. 11 A.M.
Recovery
Period
Setback Period
Occ
70 ° F (21 ° C)
UnOcc
65 ° F
(18 ° C)
Delay
Period
5 A.M. 9 P.M. 10 P.M. 8 A.M.
Room Temperature
Setback Period
Optimum
Stop
Recovery
Period
Optimum
Start
Occ
70 ° F
(21 ° C)
UnOcc
65 ° F (18 ° C)
Delay
Period
6 A.M. 9 P.M. 10 P.M. 8 A.M.
Room Temperature
Setback Period
Optimum
Stop
Shorter
Recovery
Period
Optimum
Start
UnOcc
65 ° F (18 ° C)
Occ
70 ° F
(21 ° C)
Room
Temperature
Units (RTU's)
Outdoor
Sensor