Re: voltage sag/ lights dim/UPC-battery backup clicks
If a heat pump was the source of the problem, I'd expect it to happen more frequently. Does one of your neighbors by chance have a large air compressor that they leave turned on? As air slowly leaks out, it will need to restart every once in a while to pump back up. Twice a day seems like it could be a common frequency for this sort of thing. You'd notice it more often if they were using the air.
I do computer consulting for work, and we've got one customer (a business) who, every 59 minutes, experiences a voltage fluctuation. It affects their computers in that the computers lock up during this time. We've been unable to find the source of the problem, and we've had to tell them that it's an electrical problem and outside the scope of our business.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.