On Tuesday, February 7, 2017 at 12:40:26 PM UTC-5, wrote:
On Tuesday, February 7, 2017 at 12:12:28 PM UTC-5, wrote:

The thermostat on my wall, a modern 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

As it applies to two-wire heat-only stats:

a) Imagine a programmable thermostat that wiped-and-reset at each power failure.
b) Furnace/boiler power can vary from micropile DC up to 24V AC.
c) Imagine an expensive device that could be spiked very easily from many sorts of causes.

Hence the need for batteries. All the on-board relay does is switch the AC to the unit. Furnace/Boiler power is entirely isolated from the thermostat power.

Not to mention the additional complications with 5 and 7 - wire heat-pump systems.

Peter Wieck
Melrose Park, PA

Small correction/clarification: The *micropile* (thermocouple driven by the pilot light) fires a *millivolt* gas valve. Usually, such devices operate at/around about 0.700 VDC or so. And the typical thermocouple puts out about 0.750 DC - or *just enough*.