tekmar 269 One Stage Steam Control - Manual (Page 2)

Copyright © D 269 - 06/00 2 of 12
Most residential and small commercial steam systems can only produce
heat at a fixed rate and are either on (100% output), or off (0% output). When
100% output is not required (warmer weather), the boiler must be cycled on
and off in order to prevent overheating. Figures 1 and 2 illustrate the
operation of two heating systems with different heating cycle times.
Cycle times are selected based on the operating characteristics of each
individual heating system. Figure 1 shows a system where a 30 minute cycle
is required, and Figure 2 shows a system where a 60 minute cycle is required.
In both examples the heating plant output is exactly the same at 50%.
As the outdoor temperature changes, the % on time of the boiler should
be changed to match the changing heat losses of the building. Figure 3
illustrates how Outdoor Reset of a steam boiler changes the % on time from
0% at the Warm Weather Shut Down point, to 100% at design outdoor
conditions (coldest day of the year). Applying this chart to the system
represented in Figure 1, we see that the outdoor temperature would be at
30
°F with the 50% on time that is illustrated, and if the outdoor temperature
warmed up to 50
°F, the system would operate with a 25% on time as
illustrated in Figure 4. Using outdoor temperature to reset the % on time in
this manner can help minimize the overheating and wide temperature swings
found in many systems that use only room temperature control.
Before adding this control to a steam system, it is very important that the installer examine the complete system and how
it works. In order to properly install and setup this control, the installer must know: the optimum cycle length and minimum on
time of the system, the outdoor temperature at which 100% boiler on time will be required, and the best locations to install
sensors. In addition to examining the boiler operation,
all other system components such as steam traps, air vents, radiators,
piping, etc. should be examined for correct operation to ensure that the system is working at its peak efficiency. If there are
existing faults that degrade the operation of the system, it is quite likely that the addition of this control could allow those faults
to have an even greater effect, making it important to correct such faults or at least point them out to the end user.
Outdoor Reset Strategy
30 minute cycle
15 minutes
on time
15 minutes
off time
15 minutes
on time
15 minutes
off time
15 minutes
on time
15 minutes
off time
15 minutes
on time
15 minutes
off time
Boiler
on
on
Boiler
off
off
off
Boiler Operation
30 minute cycle 30 minute cycle 30 minute cycle
on off on off on
Figure 1. 50% on time with a 30 minute heating cycle
60 minute cycle 60 minute cycle
Boiler on
on
Boiler off
off
30 minutes
on time
30 minutes
on time
30 minutes
off time
30 minutes
off time
off
Boiler Operation
on
Figure 2. 50% on time with a 60 minute heating cycle
Outdoor air temperature
Percentage of boiler on time per cycle
50
(10)
30
(-1)
10
(-12)
-10 °F
(-23) °C
20%
70
(21)
0%
on time
10%
90
(32)
30%
40%
50%
60%
70%
80%
90%
100%
-30 °F
(-34) °C
Boiler is turned on 100%
Boiler is cycled on/off
WWSD
Point
Boiler is turned off
Design
Outdoor
Temperature
Figure 3. Outdoor Reset of boiler % on time
60 minute cycle 60 minute cycle
45 minutes off time 45 minutes off time
15 minutes
on time
15 minutes
on time
Boiler
on Boiler
off
off
Boiler Operation
on off on
Figure 4. 25% on time with a 60 minute cycle time