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Old 12-11-2017, 02:55 PM   #17
Curious.George
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Join Date: Nov 2011
Posts: 1,565
Default Re: Looked at new thermostats, nah I'll DIY this

Quote:
Originally Posted by redwire View Post
I see you have knowledge of the control systems theory but in actual practice, my point is home thermostats perform quite poorly. PID control is doomed for home heating/cooling for many reasons. We could argue but it doesn't benefit the thread.
PID really only makes sense when you have finer-grained control over the "actuator" . Most home HVAC's are either on or off. Maybe you'll have some (automatic) control over blower speed...

Most plants are only crudely sized to their dwellings and the emphasis seems to be more on how quickly the "living conditions" can be changed rather than how efficiently.

The controls typically are only concerned with simple things like time of day variation -- or, perhaps, "remote" control (phone/internet). Rarely smart control -- despite the fact that the thermostat has very little to do all day long, every day long!

Quote:
OP's thermostat has good use even for basic logging.
I have seen data logs of a thermostat/furnace predict maintenance problems.
A dirty flame-rod, causing sluggish ignition or false flame-out trips, a data log shows the heat taking too long to come up. You can actually forecast HVAC trouble, if your software is smart. Wouldn't it be good to know ahead of time your furnace or A/C is struggling.
I've instrumented the HVAC, here. So, I (i.e., my controller) can "notice" ignition trouble, clogged filters, calcified pads (evap cooler), reduced cooling/heating efficiency (i.e., how much energy it requires to make a given change in indoor temperature given knowledge of outdoor conditions), etc.

But, most folks wouldn't appreciate those capabilities when making a purchase (which is when they'd have to rationalize the additional cost/complexity). It's only after the controls start offering advice that their true value can be understood and appreciated!
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