tekmar 371 House Control - Manual (Page 2)

2
Control Strategy
OUTDOOR RESET
In order to properly control a hot water heating system, the heat supplied
to the building must equal the heat lost by the building.
• The heat supplied to a building is proportional to the temperature of the
water and the surface area of the heating element. A small surface area
such as baseboard radiators requires a higher water temperature than
a larger surface area such as radiant floors.
• The heat lost from a building is dependent on the outdoor temperature.
As the outdoor temperature drops, the building heat loss increases.
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 temperature is raised for every 1
°
drop in outdoor air
temperature. The heating curve is sometimes called an outdoor
reset ratio.
Heating Curve 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 temperature is
too cold, the control shifts the starting point and the heating curve up.
Warm Weather Shut Down (WWSD)

When the outdoor air 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.
Boiler Minimum 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 tempera-
ture. Some boilers are designed to condense and should be operated at
low water temperatures as much as possible for maximum efficiency.
Constant
Room Temperature
Outdoor
Temperature
Drop
Heat
Loss
Increase
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
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
Boiler Minimum
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
DOWN
Parallel Shift of Heating Curve
0.2
UP