pybyr said:
...
I've got an Azel 777 setpoint control with pulse duration modulation that's going to handle the return protection on my Econoburn (by switching between the main circ and near circ pumps if the return gets too cool).
Just need to start wiring it up with all the other relays, differential control, etc. Hopefully, that'll start next weekend, as I finally now have nearly all the parts on hand.
I'm going with a PID duty-cycle modulation controller for return temperature control on the wood boiler as well, since the wood boiler is not as simple, predictable, and controllable as the oil boiler, although I'm set up to run the boiler recirculator all the time instead of switching back and forth.
The weak point in cycling motors on and off is normally the relay contacts, which are typically rated for 100,000 cycles under load, unless they're overloaded and then they'll crap out pretty early in the game. So a longer cycle will of course lower the number of cycles over time, but at some point you lose tight enough control.
Just going by the looks of them I would recommend not using an Azel type unit to switch even a dinky motor; you'd probably be better off using the Azel to switch the coil of a socketed ice-cube relay that can be replaced easily and cheaply when it goes bad.
--ewd