Best way of achieving a constant water temperature from a hot and cold feed?

Hardware
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Hi all,

I’m looking into developing an automated bath filler that takes the existing hot and cold feeds to a bath and can run it to a set temperature. I’m aware of a number of control techniques but not sure on the best approach. What I have so far:

  • Typically this is done with 0-10v 3-port valves that take a hot and cold feed in and then mix the temperature to provide a regulated temperature out. These would need to respond quickly to input to be able to make fine adjustments to provide a constant output temperature.

  • I’m aware of PID control and basic hysteresis temperature control, but not sure on the best methodology for control here. Is this something the Particle should handle, or are there existing ICs on the market that I can offload this control to?

  • I know I’ll need a temperature sensor for the mixed water outlet. I’m assuming I may need one to read the incoming separate hot and cold feeds also?

  • I’ve thought about heating the water in-situ opposed to using a hot feed, but I’d really like it to be designed to be retrofittable with an existing bath unit.

Thanks all. Looking forward to hearing your suggestions.

UPDATES:

27/06: Thermostatic mixing valves look like a good place to start. These traditionally (at least in the UK) use a fixed head which has a dial on used to set the temperature. You can get valve controllers that would fit on top of these mixing valves.

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Just a personal opinion, but I’d lean towards leaving the TMV set at a slightly higher temperature than your Target (say 110 F for example), but still in the “safe” zone.
Then only adjust your cold water feed for final temperature adjustment. That seems to be the safer way to prevent scalding in the event of a solenoid valve or control failure…assuming this isn’t just for your personal use.

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