This unit is in an apartment building in NYC. Each room's unit is self
contained (not central heat/air).
I was not able to find any low voltage controls inside of the unit. There is
a wiring diagram I could post a link to if anyone thinks that would help.
I thought about wiring into the thermostat, but this would leave the fan
"Byron A Jeff" <byron@xxxxxxxxxxxxx> wrote in message
In article <SmVfg.5082$ci.2432@xxxxxxxxxxxxxxxxxxxxxxxxxx>,
HS <N@xxxxxxxxxx> wrote:
I want to be able to switch an HVAC unit on/off. The unit is 220V AC.
the compressor on the unit draws about 8 amps.
There's no need for additional circuitry for this operation.
Because of the high voltage/current I want to make sure that this is
So the best thing to do is not add any addition circuitry for this
1. Is this project a bad idea in general,
or is switching through the relay a safe way to turn AC/heat on and off
(rest of circuit will be optically isolated)
Not the way that you're proposing to do it.
2. Assuming that I don't toggle power too often, is this switching method
'bad' for the AC unit (eg. for the AC give it several minutes before
it on again, if it has just been turned off)
AC works better when it runs longer. That's why it's a bad idea to
The cycle times become too short.
3. If I use a relay rated at 220V 10A will that be sufficient?
4. Can anyone recommend a relay with above specs (coil DC 6V, 9V or 12V),
with not too loud a click?
You cannot use a relay with the above specs.
Consider this. Your thermostat turns your AC unit on and off. How does it
that? There's no 10A 220V relay in the thermostat, right.
The way that it works is that there's a 24VAC circuit that controls the
contactors that are already in place. So instead of trying to insert yet
another contactor into the loop, you simply control the 24VAC circuit.
When I replaced my thermostat with a PIC based controller, I used 12V
automotive relays to drive the 24VAC low voltage circuits. Since they
were in the basement, I didn't worry too much about the click.
Be aware that the you'll need to drive both the AC unit and the fan
While the furnace will cut the fan on automatically, the AC unit won't.
Please stay away from the high voltage, high current circuits. There are
low voltage circuits that are a lot easier to manage. This is what John
be not fooling with anything if you're asking these kinds of questions.