heat-pumpthermostat
I would like to understand if three 3-relays are enough to control a heatpump. As I understand it, 24V AC is applied to:
When designing a network connected controller, are there any other circuits that should be controlled by a relay?
You'll want to connect the Y and C wires to the coil of the relay, so you end up with something like this.
When the temperature switch in the thermostat closes (calling for cool), 24 volts will be applied to the relay coil.This should pull the contact in the relay to the normally open (NO) position, allowing electricity to flow to the evaporative cooler (or whatever 120V device needs to be powered).
Yes, it is easy to control a swamp cooler with any thermostat. The problem is that in order to use a swamp cooler effectively, you need strategic openings (typically open windows) to let the air out. This is the part which is hard to automate. The best way to use a swamp cooler is to open windows in the rooms that need the cooling the most. You want as much windowspace opened as possible without any outdoor air coming back in through the windows. There are some automated devices I've seen (such as UP-DUX that vent into the attic but they cause additional resistance so they are much less effective.
Your drawing has a couple of problems...
Below is a drawing that would fix these problems but it is much more generic, the only specific part is that Packard contact delay because I am unfamiliar with a generic part name for that. If you fix 1 - 3, your design should work flawlessly. Again, the exhaust ventilation is your biggest problem.
I would put a nice 24v DPDT relay and a nice 24v 3PDT relay on the appropriate mounting sockets and mount them to a DIN rail. This will make it easy and clean. Put it all in a box with the transformer and the Packard and voila.
Best Answer
Don't forget the reversing valve!
While a normal air conditioner outdoor unit only needs a single control signal to it to turn the compressor (and condenser fan) on and off, a heat pump needs a minimum of two control signals: one for turning the compressor on and off, and a second for the reversing valve that selects whether the heat pump is heating or cooling. Furthermore, there's no set standard for which "way" the reversing valve signal works. Most heat pumps use an "O" signal, often keyed to an orange wire, that is energized to put the heat pump in cooling mode; however, Rheem/Ruud units use a "B" signal, often keyed to a blue wire (with a different color used for C), that is energized to put the heat pump in heating mode, instead.