HVAC unit controls light fixture

[Glenn Curtis]

Saw this one last week while inspecting a 1960-built home in La$ Vega$. Previous owner built a small office in the garage and put a thru-the-wall A/C unit in. Watch the video and try to figure out what's going on here.

YouTube - HVAC unit controls light fixture (http://www.youtube.com/watch?v=2MLpKjlqPCU)

Ideas??? I told the 'buyer' I wanted to be there when the electrician tackled this one:confused:

[Bruce Breedlove]

I would guess the circuit for the light fixture is in series with the circuit for the A/C. So whatever current goes through the A/C circuit goes through the light fixture circuit. It the circuits were in parallel they would operate independently.

[Jim Luttrall]

Since I never heard the a/c come on, I am thinking there is a neutral problems somewhere but I would have to think more on that one.

[Jerry Peck]

If the light were wired in series with the a/c itself, the load of the a/c would cause most of the voltage drop across the two in-series loads to be across the a/c, leaving little voltage across the light, possibly not even allowing the light to come on.

It is possible that there is a neutral problem.

It is also possible that whoever wired that tapped the light off the a/c switch (using the a/c as a junction box - which is a no-no) and instead of tapping the light off the hot to the switch they tapped it off the power from the switch going to the a/c itself - meaning that each speed of the fan would increase/reduce the voltage to the light as well as to the fan.

Glenn, you also said that the a/c was on a dedicated circuit/receptacle (forgot which word you used) but, in fact, it is not as the light is also on that circuit/receptacle. Just something to be aware of when choosing your words in a video like that.

[Vern Heiler]

Sumpin defiantly wrong! But even the most inept DIY electrician will not usually stop without the results they are after. With strange things like this the first thing I look for is 3-way switches. Notorious for being wired wrong and with the switches in the right configuration everything seems to work. FWIW