9
Boiler Differential
The Boiler Differential adjustment sets how much the actual
supply water temperature may deviate from the desired
temperature before stages are turned on or off, and is
determined by the flow rate past the supply sensor relative
to the amount of heat produced by each stage. To prevent
short operating cycles of the boiler(s) the control has a delay
of at least 1 minute between firing cycles. On an installation
where flow rates are known, the Boiler Differential can be
calculated as follows:
Outdoor air temperature
Supply water temperature
50
(10)
30
(-1)
10
(-12)
-10
°
F
(-23)
°
C
110
(43)
70
(21)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
3.6 3.0 2.4 2.0
0.8
0.4
1.0
0.6
90
(32)
50
°
F
(10)
°
C
Heating
Curve
1.2
Minimum Supply
Setting 130
°
F
WWSD
Point
70
°
F
1.6
120
°
F
Off
170
Minimum
Supply
80
Minimum Supply Temperature
This dial should be set according to the requirements specified by the
boiler manufacturer. Many boilers require a minimum operating
temperature to prevent corrosion from flue gas condensation. The
control raises the supply temperature to the Minimum setting when the
outdoor temperature drops below the WWSD point, and holds it there
until the outdoor temperature becomes cold enough to require opera-
tion on the heating curve. When an Indoor Sensor 074 or 2K RTU is
installed in a room and the selector switch set for "Indoor Sensor", the
control will cycle the boiler on and off at the Minimum Supply tempera-
ture to prevent overheating of the room.
Typical Minimum Boiler Operating Temperatures: • Steel Tube
Boilers …140
°
to 180
°
F (60
°
to 82
°
C) • Cast Iron Boilers …130
°
to
150
°
F (54
°
to 66
°
C) • Copper Tube Boilers …105
°
to 150
°
F (41
°
to
66
°
C) • Condensing or Electric Boilers …Off
100,000 Btu/hr
20 US GPM x 500
Btu/hr
US GPM x 500
Boiler Differential =
For example:
=
10
°
F (6
°
C)
All boilers will eventually turn on when the temperature falls 5
°
F (3
°
C) below the desired
temperature. Delays of 1 to 5 minutes for staging on depend on the degree of control error.
All boilers eventually turn off when the temperature rises 5
°
F (3
°
C) above the desired
temperature. Delays of 8 seconds to 3 minutes for staging off depend on the degree of
control error. Within the differential range, no boilers are staged on or off.
Trial setting = 22
°
F if
flow rates are unknown
22
°
F
2
42
Boiler
Differential
Test button
The control can be made to cycle through a test routine whenever the Test button is pushed. The test can
be halted at certain times by pushing the button a second time. For details of the test routine, refer to the
description starting on page 10.
Test
Indicator lights
There are eleven LEDs on the front of the control that will aid in testing and troubleshooting. During normal operation, these lights
indicate the following functions:
Power light on
• the 24Vac power supply has been connected and the control is energized.
Heat Demand light on
• the control is receiving a 24Vac external Heat Demand signal at terminals
Heat Dem — Heat
Dem (1 and 2) or the Heat Demand selector switch is in the "Permanent" position or a 10K Zone
Control causes a Heat Demand.
DHW Demand light on
• the control is receiving a 24Vac DHW Demand signal at terminals
DHW Dem — DHW Dem
(3and 4) or a DHW sensor is connected and has determined a need for DHW generation.
WWSD light on
• the outdoor temperature is above the WWSD point and the control has shut the heating system off.
Unoccupied light on
• the terminals
Com Sen — Uno Sw (17 and 18) are shorted together, switching the control into
Unoccupied (setback) mode.
Minimum light on
• the control has calculated that it must operate the boiler(s) to maintain the Minimum Supply
temperature since the outdoor temperature is not cold enough for Heating Curve operation.
System pump light on
• the system pump relay is on, closing the contacts between terminals 7 & 8.
DHW pump light on
• the DHW pump/valve relay is on, closing the contacts between terminals 9 & 10.
Stage 1 or 2 light on
• a boiler relay is on, closing the contacts between terminals 11 & 12 or 13 & 14.
Test light on / flashing
• the control is going through the programmed test routine / is halted.
Desired temperature
this example
160
°
F(71
°
C)
T
emperature
W
armer
Cooler
Time
Differential
this example
10
°
F(5
°
C)
165
°
F(74
°
C)
155
°
F(68
°
C)
Te
m
p
e
ra
tu
re
fa
ll
Staging
OFF ON
Te
m
p
e
ra
tu
re
r
is
e
