A UPS for the Pi is only useful for reporting power failure, which may be important for properties left unattended during cold winters. Off may be a critical failure state that needs attention ASAP.techpaul wrote:I would agree that in general multiple systems and multiple UPS for real failsafe.
In this case as this is one HVAC system, which will have inbuilt modes that put the unit safe when power fails (no doubt always off), powering one or more Pis from the same mains feed as the HVAC is adequate as neither can do anything to the HVAC during power fail. Running the Pi(s) from completely
separate mains feeds would be pointless and introduce more points of failure.
Adding a UPS to report main failure and time would be useful but it depends if you are trying to make a complete failsafe(r) system or just certain aspects.
The application is a thermostat so I assume the system can be remotely switched between modes to turn heating or cooling on or off to keep the property temperature within limits.
Here's an idea for what could be a simpler "fail-safe":
Connect a standard thermostats in series or parallel with the Pi controlled relays. If a thermostat is set to the max and min limits the Pi is restricted to control within that range. The HVAC may have such a device already.
Wire it so that a dumb thermostat <TH_MIN heat control is in parallel with the Pi controlled relay so that when the house starts to freeze and a dead Pi fails to turn on the heating the dumb thermostat does the job.
Connect a >TH_MAX N/C thermostat O/P in series with the Pi heating relay so that the Pi cannot maintain heating-on above the dumb thermostat limit.
You could use something like these:
http://www.screwfix.com/p/siemens-raa20 ... stat/81184
You may need a pair to limit heating and a pair to limit cooling
Force on T < TH_MIN (parallel N/O contact)
Force off T > TH_MAX (series N/C contact)
Force on T > TC_MAX (parallel N/O contact)
Force off T < TC_MIN (series N/C contact)
So, if the Pi goes crazy it can't heat the house beyond TH_MAX or cool it below TC_MIN.
If the Pi fails to act the dumb thermostats will heat the house to keep above TH_MIN and not go above TH_MAX.
In between these limits you have programmable control by the Pi.
Any single thermostat can fail without crisis. If the TH_MIN fails to short, turning on the heating when it shouldn't, the Pi can report the temperature is above set-point and the TH_MAX thermostat will cut heating before damage occurs. Similarly for cooling.
Is it worth £40 for some basic thermostats? Will the HVAC provide such a function anyway?
It's interesting to think about.