On Jan 29, 9:16*pm, PeterD wrote:
On Fri, 29 Jan 2010 20:01:01 -0500, DGF wrote:
How do you verify that a battery-powered digital thermostat is working
like it should? From what I've read, it seems that it is supposed to
electrically bridge the connection between R wire and the W wire, when
it turns on heating. I detached a thermostat from the HVAC system and
tested the connection between the two terminals using a multimeter *when
the room temperature was well below the heating set-point*; the contacts
on the thermostat corresponding to the R & W wires didn't seem to allow
current to flow through. Given that it's an electronic device and may
not respond to the small DC voltage from the multimeter the same way it
may to 24 VAC, I can't tell whether the thermostat is good or bad.

Any ideas?

Like making a test circuit that provides the necessary 24 volts?

I think he's right. Most T-stats have "power stealing" circuitry,
meaning they depend on the 24VAC from the HVAC system.