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Old 12-08-2017, 12:35 AM   #13
Curious.George
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Join Date: Nov 2011
Posts: 1,565
Default Re: Looked at new thermostats, nah I'll DIY this

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Originally Posted by redwire View Post
Honeywell mercury-switch tube, the skinny flex wires heat up, up to 10 degrees at 0.2A. Took me long time to figure out why the temperature was never consistent. see pic
The "legacy" thermostats employ a length of nichrome acting as an "anticipator" (i.e., "feed forward" term). While heat is being called for, the anticipator heats up to accelerate the thermostat's idea of the current indoor temperature. In this way, the thermostat stops calling for heat before it would, otherwise, thereby reducing overshoot (improving damping) in the control loop.

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The Pi and relays can make heat too.
Anything that dissipates power can be a source of heat. But, you can also make allowances for that in your control algorithm.

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Another house I owned, the wall had drafts and cold air blowing through the junction box. That made the temperature move around too.
You can actually use a device's self-heating as a means of sensing air flow. E.g., by knowing how much power you are intentionally dissipating in a device, you can calculate how much you expect it to heat up. By comparing this to the "actual" temperature in a device that doesn't have the same "operating constraint", you can determine how much heat is being carried away from the device -- by air in motion.

[This is a cheap way of designing "fan monitors"]
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