OUTDOOR LOW is also known as Design Day. This should be set to reflect the average cold day during the heating season.
SP @ LOW is the desired system water temperature when the outdoor temperature reaches Outdoor Low.
OUTDOOR HIGH is the warmest outside temperature when the system still needs heat from the boiler.
SP @ HIGH is the desired system water temperature when the outdoor temperature reaches OUTDOOR HIGH. Note that the calculated setpoint created by the curve will never drop below this value.
OUTDOOR TEMP is the simulated outdoor air temperature to return a CALCULATED SETPOINT
SH TEMP or System Heat Temperature acts as a high limit to the outdoor reset curve. The curve itself has no natural bottom, so the calculated setpoint could go to infinity depending on how far the outdoor temperature drops. This setting will limit the calculated setpoint so that no matter how cold it may become outside, the calculated setpoint will never go beyond this point. This can be very useful for zones where you desire the agressivness of a 1:1 ratio but also need to protect concrete or wood floors.
CALCULATED SETPOINT is the water temperature that the boiler will attempt to maintain based on the outdoor reset curve settings. This setpoint will range between SP @ HIGH and SP @ LOW while the outdoor temperature ranges between OUTDOOR LOW and OUTDOOR HIGH. This temperature will increase beyone SP @ LOW if the outdoor temperature drops below OUTDOOR LOW, until it reaches a maximum of SH TEMP.
