tekmar 370 House Control User Manual
Page 3
Copyright © D 370 -06/99
3 of 20
Boiler Differential
An on / off boiler must be operated with a differential in order to prevent
short cycling. When the supply water temperature drops below the
bottom rail of the differential, the boiler is turned on. The boiler is then
kept on until the supply water temperature rises above the top rail of
the differential. If the differential is too wide, there can be large supply
water temperature swings; however, if the differential is too narrow, the
boiler short cycles and operates inefficiently. Some controls automati-
cally calculate the boiler differential in order to achieve an appropriate
balance between temperature swings and boiler efficiency. This also
permits the control to adapt to changing loads and conditions.
MIXING OPERATION
The full range of water temperatures required through a heating season
can be provided with a standard (non-condensing) boiler by incorporat-
ing a modulating mixing device into the system. Mixing valves or variable
speed injection pumps are commonly used to modulate both the system
supply water temperature and the boiler return water temperature. The
modulation of water temperatures improves comfort in the building and
also protects the boiler from cool return water. For more detailed
information on mixing methods consult Essay E 021.
Boiler Protection
Cool water is often returned to the boiler from low temperature radiant floor heating systems or when the
heating system is recovering from night setback. This cool boiler return water may cause the boiler to operate
at such a low temperature that the flue gases condense. Alternatively, when the boiler surfaces are hot due
to previous loads such as domestic hot water generation, the large temperature difference (
∆T) between the
boiler and its return water can cause the boiler to become thermally shocked. Proper protection of the boiler
under these circumstances requires a modulating mixing device that can temporarily reduce the heating
load. This is normally accomplished by closing a valve or reducing the speed of an injection pump.
Maximum System Supply
Some systems, such as hydronic radiant floor heating, usually operate at water temperatures that are below
the minimum boiler supply temperature. This is due to the large surface area of the floors which radiate a
significant amount of heat at low water temperatures. Floor heating systems and flat panel convectors also
have a maximum surface temperature limit for occupant health reasons. In such systems a modulating
mixing device is normally required to limit the supply water temperature.
DOMESTIC HOT WATER (DHW)
An indirect fired DHW tank can be integrated into the hydronic heating system for greater
system efficiency and faster DHW recovery.
DHW Supply
Indirect DHW tanks are typically heated with a boiler water temperature of 180 to 200
°F
(82 to 93
°C). When outdoor reset is used, the boiler supply water may be well below this
temperature and therefore an outdoor reset override is required. When the DHW tank
calls for heat, the control must turn on the appropriate pumps or valve and ensure there
is hot boiler supply water.
DHW Priority
It is often desirable to temporarily suspend the flow of heat to the heating system when
the DHW tank calls for heat. This allows the DHW tank to recover faster than when both
the DHW and heating system operate simultaneously. If the heating system has a large
thermal mass, a relatively short interruption of the heat supply is not noticed by the
occupants. Heating systems with a smaller thermal mass may cool down while the
heating system is turned off. DHW Priority is therefore an optional feature.
Preventing Building Freeze Up when DHW Priority is used
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.
Mix
Mixing
Device
Boiler
Loop
System
Loop
P
Mix
Supply W
ater
T
emperature
Time
Differential = 10
°F (5°C)
165
°F(74°C)
B
o
ile
r
o
ff
B
o
ile
r
o
n
155
°F (68°C)
160
°F (71°C)
B
o
ile
r
o
ff
B
o
ile
r
o
n
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
Control Strategy