Why Steam Outdoor Reset?
In building heating, steam, unlike water, is very difficult to control. Initially, it takes a long time for the boiler to produce steam. Then, the steam needs to circulate through the system and heat all the cold pipes and radiators. See Steam Piping and System Sensor Location. When the space thermostat is satisfied, it shuts off the boiler. However, the boiler will keep producing steam for some additional time before the boiler cools off. This residual heat effect when combined with the residual heat stored in thick cast iron bulky radiators will cause the space temperature to overshoot. The space thermostat will keep the boiler off until the space temperature drops 1 or 2°F below its setting. By that time, the piping and radiators will have cooled off significantly before starting this scenario starts again.
To reduce this oscillation in space temperature while increasing the overall energy savings, Heat-Timer developed the steam outdoor cycling concept. This concept was developed over 60 years ago using electromechanical controls. The concept relies on inputting enough energy into the building based on the outdoor temperature.
Outdoor Reset Cycle Concept
By monitoring the outside temperature, the control anticipates the building heating need. Each CYCLE period (usually 60 minutes long but adjustable depending on the type of radiation) is divided into a Cycle-ON period and a Cycle -OFF period. The length of the Cycle -ON and Cycle-OFF periods varies with the outside temperature. The colder the outside, the longer the Cycle-ON period and the shorter the Cycle-OFF period. The control constantly checks the outside temperature using an outdoor sensor located on the exterior of the building. At the same time the control monitors the building heating system temperature using a heating system sensor. The heating system sensor is located at the furthest location in the building (or the hardest to heat area). Based the combined sensor data, the control sends instructions to the heating plant to control the heat level in the building. Before staring a heating cycle, the control checks the heating system sensor temperature. If the temperature was above the requierd set point, the control will not start another heating cycle. That is because there is enough heat in the system. However, if the temperature was below the set point, the control will start the heat until the system sensor temperature reaches the set point then initiate the heating cycle starting with the Cycle-ON period.
Outdoor Reset Heat Adjustments
Each building has different heat-loss characteristics requiring different outdoor reset cycle settings. Heat-Timer developed 16 outdoor reset curves that satisfy every building type. Each of the curves is charted based on the Outdoor Cutoff setting less the current outdoor temperature. This value is then used to give the length of the Cycle-ON and Cycle-OFF periods. A drop in the outdoor temperature will cause this Outdoor Cutoff less the outdoor temperature value to increase. This will result in an increase in the Cycle-ON period. Using a 60 minute Cycle example, an outdoor temperature of 30°F and an Outdoor Cutoff setting of 55°F (55-30 = 25) will cause the Cycle-ON period to be 10 minutes. This will cause the Cycle-OFF to be at 50 minutes. A drop in the outdoor temperature to 15°F will cause the Outdoor Cutoff less outdoor temperature value to increase to 40 (55 – 15 = 40). This will increase the Cycle-ON to 20 minutes while reducing the Cycle-OFF to 40 minutes.