Work for a battery manufacturer do you. I just replaced my
thermostat battery when a problem with my furnace appeared.
The battery had no affect on the problem and was still
functioning. But it was 28 months old. Replaced the
battery in my battery operated smoke dector last week
thinking the tweet came from it. Nope it was a CO monitor I
forgot about, but left the new battery in the smoke detctor
anyway. The old battery was dated 01/24/01. The previous
battery last well over 2 years also. Several years ago I
started putting a piece of masking tape on all batteries
with the replacement date, so I would have some idea how
long they lasted. Well, know I know. Changing ever year is
a waste of time, and changing every 6 months is a waste of
resources.

If the guy is worried, he should change the batteries in
critical equipment before he leaves. I don't, but then I
check the date on the battery and the operation of the item.

Joe Bobst wrote:

I want my house to be more proofed for things like temporary power outages,
etc, in case we are not around.

Do what everyone else does with these thermostats - put in new batteries when
Daylight Saving time rolls around. Why get all worked up about something that
is your own responsibility? You can get control systems that are fail-safe and
fool proof, but unless you have a seven figure income they may be priced beyond
your budget. Odds are your batteries had gone way beyond their stated effective
life, so if you tend to be forgetful, get the highest quality replacements on
the market.
Maybe a warning tag on your furnace filter compartment would be helpful for
next year. HTH

Joe