At least swimming pool equipment is mostly just turning things on and off. If you look at the controller board for any given pool "timer" it's just a bunch of relays (for the pump, lights, and valves/servos).
Temperature sensors are all standardized for the most part (well, they don't seem to be anything special) but I'm not sure about chlorinators... Mine has a strange (electrical) connector and 100% proprietary threads on the PVC connectors (that were easy enough to reverse engineer in OpenSCAD: https://www.printables.com/model/24144-t-cell-cleaning-stand).
Fortunately there's plenty of 3rd party competition for things like that. Even though I had a Hayward system I was able to purchase a compatible chlorinator off Amazon for a fraction of the price Hayward was charging.
Temperature sensors are all standardized for the most part (well, they don't seem to be anything special) but I'm not sure about chlorinators... Mine has a strange (electrical) connector and 100% proprietary threads on the PVC connectors (that were easy enough to reverse engineer in OpenSCAD: https://www.printables.com/model/24144-t-cell-cleaning-stand).
Fortunately there's plenty of 3rd party competition for things like that. Even though I had a Hayward system I was able to purchase a compatible chlorinator off Amazon for a fraction of the price Hayward was charging.