Zone load staggering, Zone post purge, Unoccupied (night setback) – tekmar 370 House Control User Manual
Page 5: Optimum start / stop with water temperature boost, Optimum start / stop with only indoor sensors, Optimum start / stop with only an outdoor sensor, Control strategy
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Zone Load Staggering
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.
Optimum Start / Stop with only Indoor Sensors
When only indoor temperature feedback is available, the control must base all Optimum
Start / Stop calculations on
only indoor temperature. If there are large variations in outdoor
temperature, this method cannot provide the same level of accuracy as when both indoor
and outdoor sensors are used.
Optimum Start / Stop with only an Outdoor Sensor
Every building, and often each zone within the building, has a different response time.
When only an outdoor sensor is used, the control must assume a particular response time
for the entire building. Therefore this is generally the least accurate method of calculating
Optimum Start / Stop times.
Outdoor
Sensor
Room
Temperature
Units (RTU's)
Room
Temperature
Units (RTU's)
Outdoor
Sensor
Zone 1
Zone 2
Zone 3
Zone 4
Post Purge
(Boiler off, Pump on)
On
On
On
On
On
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)
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)
Control Strategy