I want to use a clock timer switch to control a window AC unit. I purchased a 15 amp timer and made preparations to replace the current on/off wall switch that controls the AC unit but when I opened it up I was surprised to see a double pole switch wired to both poles controlling the AC unit. I tried disconnecting one side and the unit would not start. I then reconnected that side and disconneted the other side and the unit would not start. Since I have a single pole timer I got worried that I don't understand what I am doing. Any advice?

What you're describing sounds like the AC unit is powered with 220 volts - is that right? In this case you should use a timer that is specifically designed for 220 volt operation, which will indeed open and close both legs of the line, since both legs are "hot."

Good point, window units come in both 120 and 240 vac, which do you have? If you are not certain tell us what your plug looks like

Thanks for the tip. I will look at the plug when I return to the store later this week. I suspect that since all 4 wires are the same color that the unit is 220V which is surprising because it appears to be a very standard sized (although old) window AC unit. I will let you know later this week. Thanks!

Just called and confirmed it is 220V. Thanks again for your guidance.

By the way, do you know if anyone makes a 220V timer that will fit in a standard switch box? I can only find 220V timers that go between the AC unit plug and the receptacle which will be difficult to use because the unit and its plug in receptacle are 18' up on the wall above some windows. Any advice? Thanks again...

Are you in the USA, most likely 240vac. Intermatic HB112C - Heavy Duty Plug-In Timer 240 Volt - AquaCave

Why do you want a timer on an AC unit? Maybe we can figure out a different way to accomplish the same thing.

We would like a timer because the shop has east facing windows and the sun beats in from about 6AM to 10AM each morning. We would like to cool the place before we arrive in the morning without leaving the AC unit on all night. A remote thermostat would be better but I thought that a timer would be simpler.

The AC unit does not have a thermostat?

Ceilingfan, it's a window unit. Wes, you can get all sorts of stuff like an X10 or a telephone remote control system but the time I gave you earlier is about the cheapest route.

A contactor can be placed anywhere you have access to the wiring like near the breaker box. The timer just controls the contactor. If N is not available, a control transfomer can provide 120. You could also make the controls 24 VAC and use a programmable HVAC thermostat. Use one with a remote sensor for better control.

My window units have thermostats. I thought they all did.

I would think installing a thermostat would get you better cooling for lower energy costs, than a timer. The timer doesn't account for cool or overcast days.

There are window AC units with temperature displays and digital thermostats, but older models just had cooler/warmer and that's it.

Pulling the plug (relay) may not work for the fancier AC units, but a regular programmable thermosat should.

These are available programmable from the internet or phone.

Thanks very much. I think we will go with the plug in timer and call it quits for now. I very much appreciate your help.

Good inexpensive move

OK, I think this may be my last question! The 220 plug from the AC unit has 1 verticle prong and 1 horizontal prong. Can you recommend a timer that can handle this? I can only find ones with 2 horizontal prongs on the plug. Many thanks.

The plug with one vertical and one horizontal is a 20 amp plug. The one with the 2 horizontal is a 15A plug. I would not use a timer that is rated for only 15A. The contacts will not be designed for the heavier load. You will have to go with a real timeclock with 2 pole switching. That won't be very cheap.

I have done the contactor/24v transformer/inexpensive thermostat solution already. If you are handy at electrical it isn't that expensive. A DP relay that will handle 40 amps (a relay from an old HVAC unit) and a 20va transformer (out of a shot gas water heater or that HVAC unit) don't cost much. The problem is the wiring, you can't put the 240 volt supply wires and the 24v control wiring inside the same enclosure. It can get fun. And most of those window AC units are very difficult to hook up internally with a tstat.

Good luck.