On Tue, 07 Feb 2017 09:12:34 -0800, wrote:

The thermostat on my wall, a moden programmable type, also has
batteries and will not work without them. I don't understand why it
was not made to be powered by the furnace. It has a mechanical relay
inside, a reed relay I think. I can hear it when it switches on and
off. The 3 AAA batteries will power it for about 3 years.
Eric

My two AAA batteries lasted 4 years and 3 months. I checked my paperwork
for when that 'stat was installed. Like I said, I never knew it even had
batteries or I would have changed them long ago.