I have a trouble call coming up and not sure what to look for. The customer says he had a new air cond system installed, a split system with a variable speed indoor blower unit. Now every time the unit starts, his flatscreen TV goes nuts with buzzing and lines on the picture. It didn't happen before the new AC was installed. The TV is NOT on the same circuit as the AC. The AC people came out and checked everything and said there is nothing wrong with their installation. The new AC replaced an old one that was in the same place, on the same circuit, and didn't cause any interference on the TV.
I can't think of anything except putting each on opposite legs of the service, if not already done, or installing a filter on the TV. (The TV is built in so he has not tried other circuits using an extension cord.)

What else can I look for, or will it be a total waste of time?

I bet the fan on the new AC has a VFD controlling it. We know how much noise they can generate. I think it would be better to place any filtering as close to the noise source as possible, especially since they will probably discover more things that it interferes with.
Joe

I second Joe's opinion. You may be able to do the HVAC guy's job by getting the model info from the air handler and beating up Google a little. There may be a known problem and a fix from the manufacturer. It is probably as simple as a noise filter kit.

I am guessing they use an antenna, not cable?

If it is cable/satellite, those guys may have something too.

It could be as simple as an unterminated TV cable somewhere.

And, lest we forget, GROUNDING. Make sure the AC has a good bond, from the unit all the way to the service.

... or a loose co-ax connection.

Thanks guys, turns out the guy called this morning and said he figured it out, he ran an isolated ground to the sattelite dish. I didn't think anyone was still using motorized dishes??

@$###$#!!!!

I don't believe it for a second.

What I DO believe is that he mucked something up during his DIY installation, then somehow got his errors to cancel out as be blundered about 'fixing' it.

When - not 'if' - you get called back, CYA. I wouldn't be surprised to find 10 amps running through that 'ground.'