Comments by "ke6gwf - Ben Blackburn" (@ke6gwf) on "Thermostats: Cooler than you think!" video.

  1. The first two stage furnace I ever worked with was actually two identical furnaces, each half the size needed, set next to each other into a shared plenum. The interesting part was the thermostat. It was mercury switch, so a glass tube with a drop of mercury in it, and attached to the end of the coil spring. The tube was slight curved or bent, and it had 2 sets of electrodes sealed through the glass, so as the spring cooled off the drop would engulf the first set of contacts, and if it got colder it would tip far enough to get a engulf the second set of contacts.
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  2. One thing that you missed is that some thermostats, including some of the older programmable thermostats use a form of PID Tuning to reduce temperature overshoot. Basically when heat is commanded by the time the air gets to the set point at the thermostat and the heat is commanded off, the heater is still producing heat, and that heat will still get pushed out into the room, causing overshoot as the temperature rises well above the set point. This is especially true with electric baseboard heaters and such, where the heat gradient across the room to the thermostat can be steep. So the PID thermostats monitor how far the temperature exceeds the set point, and on the next cycle they command the heater off at a slightly lower temperature, and see how close it is, and continue reducing the set point until the final temperature is the desired temperature. This keeps a more even temperature, and saves energy by avoiding the spike. It also can effect how fast a room heats, and like with a two stage system, actually makes it where turning the thermostat up higher than desired will heat the room up faster, because you keep pumping heat in, rather than just letting it settle gradually.
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