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Joined: Fri Jun 28, 2019 12:34 am

Monitoring HVAC

Fri Jun 28, 2019 12:42 am

hey guys, need some help... trying to learn to use the gpio on rpi3... running a new hvac system with a zone system running 2 zones, dual stage ac/furnace.. not impressed with information i am recieving from thermostats... would like to find a way to hook the hvac to a pi to MONITOR the hvac system... it looks like the hvac system utilizes 24vac on different cables to control hvac... so it seems like i would like to take several 24vac circuits and convert them to 5vdc to input into rpi so i can log info to a database...

i was reading about relays and they sound like a good system to accomplish what i am trying to do because it keeps the 2 circuits separated... sounds safer to me.... i can find 5vdc to 24vac relays but can’t seem to find the reverse?

i have been googling for a couple weeks now and cant find a SIMPLE solution that doesn’t require a bunch of calculus to accomplish.... does anyone know of someone that has done something like this? do you have any suggestions on how to go about it...I appreciate the assistance!

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Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: Monitoring HVAC

Fri Jun 28, 2019 5:17 am

You need to convert to 3.3V to connect to the Pi GPIOs, not 5V, otherwise you will fry your Pi.

Any relay that has a 24VAC input would be sufficient, the contact voltage rating is not important as long as it can handle the required voltage or higher. The 3.3V would be supplied from the Pi anyway. The important thing is to make sure the inputs to the Pi are fully isolated from the 24VAC circuits.

An alternative to relays would be to use AC Opto Isolators with a suitable resistor on the input to limit the current. The issue you may get with those is that they will be be returning a signal that turns on and off at your local mains frequency (100 or 120 times a second), so you may need either a capacitor on the output side, or software that can handle it.
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Joined: Fri Jul 06, 2018 2:13 am
Location: USA

Re: Monitoring HVAC

Sat Jun 29, 2019 2:20 am

From what you have said, it sounds like all you want to do is monitor the states of various 24VAC supply/signal lines that run throughout your HVAC system.

To reiterate what rpdom had stated, the Pi GPIOs can't accept a 5V signal, it can't be more than 3.3V.

A relay is a good solution, but there is one thing that you will need to be careful about, and that is the loading of the relay coil on the line that you intend to monitor. A relay coil will present somewhat of a load (this will depend on the actual relay used) on the line, and if the 24VAC signal line is loaded too much, there my be unanticipated results. Use a relay that has a small coil current, such as a signal relay (versus a power relay), which have coil currents in the 50ma vicinity. 50ma shouldn't be a much on a HVAC signal line, and probably won't present a problem.

Also, there is an AC line monitor optocoupler available that will accept an AC input on one side, and provide a logic signal output. More involved than using a relay though. The open collector output can be pulled up to 3.3v. These optos aren't cheap, but it is designed for this purpose and would provide a clean and reliable logic level output for connection to the Pi GPIO. The chip is the Fairchild MID400 optoisolator, available at both Mouser and Digi-Key.

If I were doing this, I'd go the signal relay route, versus the opto for simplicity sake.

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