tekmar 352 House Control - Manual (Page 2)

Copyright © D 352 - 06/00 2 of 8
Heating Curve

A hot water heating system can be accurately controlled by modulating the supply water
temperature as the outdoor temperature changes. Using this approach the heat lost from
the building is exactly matched by the heat input to the building. A tekmar reset control
utilizes a heating curve to set the relationship between outdoor temperature and supply
water temperature. The heating curve determines the amount the supply water tempera-
ture is raised for every 1
° drop in outdoor air temperature. The heating curve is sometimes
called an outdoor reset ratio.
Heating Curve Parallel Shift

All heating curves begin at the heating curve starting point. If the heating curve starting
point is adjusted, the heating curve will be parallel shifted. The heating curve starting point
is either set manually through a dial, or it is determined automatically by the control
through indoor temperature feedback.
Indoor Temperature Feedback

Most buildings have internal heat gains due to people, passive solar heating and
mechanical or electrical equipment. If only the outdoor temperature is measured, the
control cannot compensate for these internal heat gains and the building may overheat.
In order to prevent overheating, indoor temperature feedback should be combined with
the outdoor reset strategy. From this indoor temperature feedback, the control can
change the heating curve starting point in order to match the supply water temperature
to the heat loss of the building. If the indoor temperature is too warm, the control
automatically shifts the starting point and the heating curve down. If the indoor tempera-
ture is too cold, the control shifts the starting point and heating curve up.
Warm Weather Shut Down (WWSD) 
When the outdoor temperature is equal to the heating curve starting point, no additional
heat is required in the building and therefore the heating system can be shut down. The
WWSD point is normally the same as the heating curve starting point.
BOILER OPERATION
The supply water temperature from a boiler can be controlled by cycling the boiler on and
off. Modulation of the boiler’s operating temperature in hot water heating systems not only
provides more comfort but also offers significant energy savings. The cooler the boiler runs,
the more efficient it is due to less heat losses up the flue and reduced boiler jacket losses.
Minimum Boiler Supply

Most boilers require a minimum supply water temperature in order to prevent corrosion
from flue gas condensation. The control should therefore only modulate the boiler supply
water temperature down to the boiler manufacturer’s minimum recommended operating
temperature. Some boilers are designed to condense and should be operated at low
water temperatures as much as possible for maximum efficiency.
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 mixing device into the system. An On / Off Injection Valve or Pump
can be used to modulate the system supply water temperature to
improve comfort in the building and provide a maximum target supply
water temperature limit. For more detailed information on mixing meth-
ods consult Essay E 021.
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)
Heating
Curve
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
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
Mixing
Device
Boiler
Loop
System
Loop
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
3.6 3.0 2.0 1.6
1.2
0.8
0.4
0.6
90
(32)
°F
( °C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
2.4
1.0
WWSD
Point
70 °F
Minimum Boiler
Supply Setting
130 °F
0.2
Outdoor air temperature
50
(10)
30
(-1)
10
(-12)
-10
(-23)
70
(21)
3.6 3.0 2.0 1.6
1.2
0.8
0.4
0.6
90
(32)
°F
( °C)
Supply water temperature
110
(43)
70
(21)
90
(32)
210
(99)
170
(77)
150
(65)
130
(54)
190
(88)
2.4
1.0
UP
DOWN
Parallel Shift of Heating Curve
0.2