Arkansas HVACR NewsMagazine November 2021

HVACR NewsMagazine November 2021

Tech News

turning on. Whether that is heat or cool is actually dictated by whether or not the O/B terminal is energized. That is why, on many old thermostats, you would jumper Y1 and W1 in a heat pump application. W2 – Means second-stage heat. It could be the first stage of heat strips in a common southern heat pump, the gas furnace backing up the heat pump in a modern “hybrid heat” application, or just a second heat strip bank in the case of a straight electric system. W2 is generally called on based on a temperature differential between setpoint and space, outdoor temperature, or run time. W3 – This is just the next stage of heat after W2. E – Is emergency heat, usually just a way to manually drive on what would normally be the secondary form of heat without stage 1 heating. Emergency heat only makes sense when there is some sort of secondary heat source. Even then, it only helps if the secondary heat source is sufficient to heat the space, as in the case when the secondary is a furnace, Hydronics, or a large heat kit. In Florida, most of our units have 5KW auxiliary heat, which will never

be sufficient to heat a home in an “emergency.” Many of these other terminal designations are a holdover from a time when all the controls in the thermostat and defrost board were electromechanical. Much of it was for indication/trouble lights, and some of it was for the thermostat to perform staging based on outdoor temperature because run-time logic was not available. So, for your X2 question, have a look at the thermostat and diagram below.