tekmar 371 House Control - Manual (Page 4)

4
DHW Priority Override
There is always the possibility of an excessively long DHW call for heat due to a broken
pipe, faulty aquastat or other problems. At cold outdoor conditions the building may
freeze when there is a long DHW draw with DHW Priority selected. In order to prevent
this, the control must override the DHW Priority and simultaneously operate the DHW
and heating systems. The maximum time allotted for DHW Priority should decrease as
the outdoor temperature drops.
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 tempera-
ture, 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 Zone Temperature Control

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.
DHW priority demand time limit (hours)
(minimum 20 minutes)
Outdoor air temperature
100 ° F
(38) ° C
-20
(-29)
1
-40 ° F
(-40) ° C
20
(-7)
60
(-16)
2
3
4
5
6
7
P
Mixing
Heat
Source
90%
on time
85%
on time
100%
on time
M M M
70 ° F
(22 ° C)
15 minutes 15 minutes
5 minutes 10 minutes
on
72 ° F
(22 ° C)
68 ° F
(20 ° C)
P
70 ° F
(21 ° C)
15 minutes 15 minutes
13 minutes
72 ° F
(22 ° C)
68 ° F
(20 ° C)
droop