[MODERATOR NOTE]: moved to a more appropriate forum.
I wanted to post in the thermostat section but it doesn't allow posting of new threads so I guess I post here.
The problem is how simply (and badly) the current crop of thermostats work. Most are really simple and, in the case of a furnace/heater, go on when the temperature drops below the setpoint, and off again when the temperature rises above the setpoint, with some hysteresis in-between. Some have a timer to prevent cycling too quickly (usually for the cooling function). OK so far.
I have a multi-stage heat pump and there the behavior of the thermostat becomes annoying. Here is an example:
The heating setpoint 70˚F. The first stage heating function doesn't come on until the temperature drops by 1˚F to 69˚F. If it is especially cold the temperature continues to drop until 68˚F, at which point the second stage kicks in. Now instead of maintaining the desired 70˚F, it is cycling between 68˚F and 69˚F. If the temperature drops further, I think by 4˚, then and only then does the auxiliary resistance heat kick in. I can attest to the fact that, while we are having a really bad cold snap here in Texas, I am getting to enjoy the wide range of temperatures in my house. The algorithm of using 1˚F change for each is just too large a hysteresis. What I REALLY want is either a PI or PID thermostat. If one existed and I could find it, especially if it had a interface that would work for Indigo, problem solved.
However, it might not exist and I'd have to make one. One could easily use Indigo to implement this if there was a good high-accuracy temperature sensor, probably on the order of 0.1˚C or 0.2˚F resolution. The necessary relays, i.e. reverse (heat/cool), first stage on, second stage on, aux heat on, is pretty easy (4 relays). As soon as temperature drops below the setpoint, first stage on. Get differential (rate of drop) and if negative, activate second stage. If differential is still negative, activate aux heat. At this point temperature is still close to the setpoint. As soon as the differential goes positive and temperature is approaching the setpoint again, switch off aux heat, then second stage, then first stage. What would then happen is that it would end up cycling between off and first stage, first and second stage, and second and aux-heat stage, while actually keeping the temperature at my setpoint.
So, I know where to get a relay module that Indigo can control (Request: more support of RS485 so I don't have to depend on Z-wave or Insteon) but I am looking for a much better stand-alone thermostat, or a good, high-accuracy and high-resolution temperature sensor.
BTW, the various multi-sensor modules are not useful because the temperature sensing is biased by internal heat generated by the components. It needs to be something with a free-standing calibrated sensor that truly senses ambient temperature and has a reasonable amount of thermal mass/inertia.
Anyone have any information and/or ideas?
Thank you.