Boiler shock protection, Dhw post purge, Dhw setback – tekmar 370 House Control User Manual
Page 4: Zoning operation, Integral (i), Derivative (d), P + i + d = pid, Control strategy
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Boiler Shock Protection
When DHW priority is used, the temperature within the heating system terminal unit
may be significantly lower than the boiler temperature once the DHW operation is
complete. If the DHW pump or valve is simply turned off and the heating system pump
turned on, a large
∆T can develop across the boiler. This may induce thermal shock of
the boiler. In order to provide a smooth transition between the DHW and heating system
loads, the control must simultaneously operate the DHW pump and heating system
pump for a short period of time. This mixes the water returning to the boiler and
minimizes the possibility of thermal shock.
DHW Post Purge
During the DHW operation, the boiler temperature is normally raised above 180
°F (82°C).
Once the DHW tank is satisfied, the residual heat within the boiler should be purged in
order to reduce stand-by losses. When the heating system does not require heat, the
boiler can be purged into the DHW tank. This is accomplished by turning the boiler off but
keeping the DHW pump or valve operating for a purging period. If the boiler supply
temperature drops below the DHW tank temperature, heat will be removed from the DHW
tank. Therefore, the post purge is terminated if the boiler supply is not hot enough. This
means that the DHW post purge will not always take the same length of time.
DHW Setback
During the night, or when people are not within the building, energy can be saved by lowering the DHW tank temperature. A lower tank
temperature is achieved when the system control ignores the call for heat from the DHW aquastat. In order to prevent a cold DHW
temperature at the start of the Occupied period, the system control must raise the tank temperature before the setback period ends.
ZONING OPERATION
In a multiple zone heating system, the zones may have different internal heat gains, heat losses or different temperature settings.
Each zone must therefore have individual temperature control. For maximum comfort, the heat should be continuously supplied
to the zone at the same rate the zone is losing heat. The most accurate method of accomplishing this is by outdoor reset; however,
it is not normally economical to modulate the supply water temperature to every zone.
Outdoor reset can be combined with zoning for a more cost effective solution. Through
indoor sensors, a zone control can provide indoor temperature feedback to the outdoor
reset control. The outdoor reset control will then adjust the supply water temperature to
satisfy the zone with the highest water temperature requirement. Heat to the remaining
zones will be cycled on and off by the zone control using zone valves or pumps. Since the
heat is cycled on and off, accurate PID control logic should be provided to maintain a stable
indoor temperature.
PID Zoning Logic
Proportional (P)
In order to prevent indoor temperature swings, the heat supplied to each zone must be
proportional to the heat required by the zone. Proportional control logic can be
accomplished by pulse width modulation (PWM). A typical PWM system has a fixed
operating cycle. During this operating cycle, the on time of the zone relay is varied
based on the difference between the desired zone temperature and the actual zone
temperature. As the zone temperature drops, the relay on time increases and as the
zone temperature rises, the relay on time decreases.
Integral (I)
Controls that are strictly proportional suffer from a problem of offset. The amount of heat
supplied to the zone depends on how far the space temperature is below the desired
setpoint. This implies that as the heating load increases, the average room temperature
droops. On the coldest day of the year, the most heat is required and therefore the room
temperature must be coldest.
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.
DHW
Tank
P
Mixing
P
M
Heat
Source
90%
time on
85%
time on
100%
time on
M
M
M
70
°F
(21
°C)
15 minutes
15 minutes
13 minutes
no heat
72
°F
(22
°C)
68
°F
(20
°C)
droop
70
°F
(22
°C)
15 minutes
15 minutes
5 minutes
10 minutes
on
no heat
72
°F
(22
°C)
68
°F
(20
°C)
Control Strategy