5
Differential
The differential adjustment sets how far the actual boiler supply water temperature may deviate from the desired temperature before
the boiler is turned on or off, and it is determined by the flow rate through the system pump relative to the heat output by the boiler.
The Differential can be adjusted to Ad (Auto differential) or from 2 to 42
°
F (1 to 23
°
C). The following formula can be used to calculate
the desired differential:
When the adjustment is set to
Ad (Auto Differential), the differential is continuously calculated by the control, and varies as the heating
load changes. This setting is suggested and therefore it is a factory setting.
Minimum Boiler Supply
Most boilers require a minimum operating temperature to prevent corrosion from flue gas condensation. The
Minimum Boiler setting
should be programmed to the lowest supply water temperature at which the boiler can operate without causing the boiler flue gases
to condense. Consult the boiler manufacturer for recommended minimum boiler supply temperatures. Some typical settings are as
follows:
• Steel fire tube boiler ….......... 140 to 160
°
F (60 to 71
°
C)
• Condensing boiler ................. Off
• Cast iron boiler ….................. 135 to 160
°
F (57 to 71
°
C)
• Electric boiler ........................ Off
• Copper tube boiler …............. 125 to 150
°
F (52 to 66
°
C)
Curve (Heating Curve)
The Heating Curve setting determines the number of degrees the supply water tempera-
ture is raised for every one degree drop in outdoor temperature. The
Curve setting can
be calculated from the following formula.
design supply temperature – Starting water temperature
Heating Curve
=
WWSD – design outdoor temperature
Curve (Heating Curve) Setting Examples:
Example #1
Example #2
Example #3
Design Supply Water Temperature
180
°
F (82
°
C)
180
°
F (82
°
C)
180
°
F (82
°
C)
Starting Water Temperature
70
°
F (21
°
C)
150
°
F (66
°
C)
160
°
F (71
°
C)
Design Outdoor Temperature
-5
°
F (-21
°
C)
-5
°
F (-21
°
C)
-5
°
F (-21
°
C)
Warm Weather Shut Down (WWSD)
70
°
F (21
°
C)
65
°
F (18
°
C)
60
°
F (16
°
C)
Minimum Boiler
130
°
F (54
°
C)
130
°
F (54
°
C)
130
°
F (54
°
C)
Differential
=
Example:
=
10
°
F (6
°
C)
Btu/hr input
100,000 Btu/hr
System US GPM x 50
20 US GPM x 500
Outdoor air temperature
55
(13)
40
(4)
25
(-4)
10
(-12)
70
(21)
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
-5
(-21)
Example #3
Example #2
Example #1
WWSD
Design
Condition
Starting
Temp.
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
3.6 3.0 2.4 2.0
1.6
1.2
0.8
0.4
1.0
0.6
90
(32)
Starting
Point
°
F
(
°
C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
0.2
=
Example #1
Heating Curve =
180 — 70
°
F (82 — 21
°
C)
70 — (-5
°
F) (21 — (-21
°
C))
110
°
F (61
°
C)
75
°
F (42
°
C)
= 1.5
=
Example #2
Heating Curve =
180 — 150
°
F (82 — 66
°
C)
65 — (-5
°
F) (18 — (-21
°
C))
30
°
F (16
°
C)
70
°
F (39
°
C)
= 0.4
=
Example #3
Heating Curve =
180 — 160
°
F (82 — 71
°
C)
60 — (-5
°
F) (16 — (-21
°
C))
20
°
F (11
°
C)
65
°
F (37
°
C)
= 0.3
Control Adjustments
Starting Temp.: __________________________________________
WWSD: ________________________________________________
Curve: _________________________________________________
Differential: _____________________________________________
Minimum Boiler: _________________________________________
