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Old 12-04-2017, 04:45 PM   #11
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Join Date: Nov 2011
Posts: 1,260
Default Re: Looked at new thermostats, nah I'll DIY this

Originally Posted by evilkitty View Post
I designed the software do support a desirable temp range, under the concept lets warm the place up a little and shut off a while; the offset value is the desirable range, so if the unit is set to cool with a Target of 75 with a offset of 5 the system will not turn on till 80 and run till 75
You typically have a variety of such "fudge factors".

Almost always, you want a certain amount of hysteresis in the loop -- turn the heat on at X and off at X+delta so the plant doesn't get commanded back on, in short order (the temperature must fall to below X to do so).

Modern HVAC systems also have timeouts built in so the plant can't cycle back on too quickly (ACbrrr compressors don't like being commanded back on while they are still "under pressure" from the last activation; gas furnaces like to be able to vent any fumes before reengaging; etc.)

Ideally, you also want to factor in an "anticipator" to limit the amount of overshoot -- watch to see how far beyond X+delta the temperature climbs and use that, "next time", to shut off the plant before the actual temperature reaches X+delta (i.e., knowing that if you shut it off "early", it will still "coast" a bit and bring the house to your desired X+delta)

[This isn't as easy as it appears!]

In this way, you give the user (occupant) the ability to set the size of the "comfort zone" and YOU sort out how to control the plant to achieve those results!

If you add provisions for "programmability" (i.e., have the setpoint change over time so the house isn't kept warm while you're asleep, away, etc.), then you can also use your knowledge of the upcoming schedule changes to avoid unnecessarily heating the house -- when you know you'll be adjusting the setpoint lower, in short order!

[I've spent a fair bit of effort working on this problem :> ]
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