tekmar 279 Steam Control - Manual (Page 6)

© 2010 D 279 - 05/10 6 of 24
Heating Cycle - Heating Curve Section C
Outdoor reset is a method of operating a heating system
based on the fact that the rate at which a building loses
heat to the outdoor environment is mostly dependent on
the surrounding outdoor air temperature. As the outdoor
temperature gets colder, the heat loss of the building
increases at a proportional rate. This relationship between
heat loss and colder outdoor temperatures is called a
heating curve.
Heat Load = 20%
Outdoor = 50°F
Boiler On Time = 12 min
60 min
Heat Load = 60%
Outdoor = 30°F
Boiler On Time = 36 min
60 min
Heat Load = 100%
Outdoor = 10°F
Boiler On Time = 60 min
60 min
In order to calibrate the control to the heat loss rate of a
particular building, the installer is required to enter the
following heating curve settings:
Indoor Design
The Indoor Design is the starting point of the heating curve.
When the measured outdoor air temperature matches the
Indoor Design setting, the control calculates a 0% on time
is required for heating the building. The factory default is
to start the heating curve at 70°F (21°C).
Outdoor Design
The Outdoor Design is the end point of the heating curve.
When the measured outdoor air temperature matches
the Outdoor Design setting, the control calculates the
boiler on time to be at the Boiler Design % setting. The
Outdoor Design should be set to the average coldest annual
temperature recorded in the building area.
Boiler Design %
The Boiler Design % is the percent output capacity of the
boiler or steam system required to heat the building when
the measured outdoor air temperature matches the Outdoor
Design setting. The factory default is 100%. If the building
envelope has been upgraded to improve the building
insulation, the existing steam boiler or steam system may be
oversized for the building. In these cases, the Boiler Design
% can be reduced to fine tune the heating curve.
- 100%
- 10%
- 0%
0 (°F) 10 20 30 40 50 60
Boiler Cycled
On/Off
Boiler Cycled
On/Off
B
o
il
e
r
O
n
1
0
0
%
o
f
H
e
a
ti
n
g
C
y
c
le
Boiler On 1
00% of Heating C
y
cle
P
ercent On Time P
er Heating C
y
cle
Outdoor Air Temperature
70
A = Indoor Design
B = Outdoor Design
C = Boiler Design %
80
- 20%
- 30%
- 40%
- 50%
- 60%
- 70%
- 80%
- 90%
C C
B B A
Room (Occupied and Unoccupied)
The Room setting is the desired temperature of the building.
When a setback schedule is selected, there is a Room
Occupied temperature setting and a Room Unoccupied
temperature setting. When the setback schedule is turned
off, only the Room setting is available.
When an indoor sensor(s) is not connected to the control,
the Room setting operates as a parallel shift of the heating
curve. This allows fine tuning of the heating system. If the
Room setting is set above the Indoor Design setting, the
entire heating curve is shifted higher, resulting in longer
percent on times being calculated. Similarly, if the Room
setting is set below the Indoor Design setting, the entire
heating curve is shifted lower, resulting in shorter percent
on times being calculated.
- 100%
- 10%
- 0%
0 (°F) 10
75% 75%
65%
65%
42% 42%
20 30 40 50 60
P
ercent On Time P
er Heating C
y
cle
Outdoor Air Temperature
70 80
- 20%
- 30%
- 40%
- 50%
- 60%
- 70%
- 80%
- 90%
Ro
om
@
7
0°
F
Room @ 70°F
Ro
om
@
5
5°
F
Room @ 55°F
Ro
om
@
7
5°
F
Room @ 7
5°F
When an indoor sensor(s) is connected to the control, the
Room setting becomes a Room Target temperature. The
control measures the current air temperature in the room
using the indoor sensors. The control then automatically
shifts the heating curve so that the measured room
temperature reaches the Room Target temperature.