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Jim Yanik

(GregS) wrote in
:

In article
,
Mikepier wrote:
On Jan 29, 9:16=A0pm, 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.

Most thermostats have a relay. One has to know the circuit to
troubleshoot it. Some of the best thermostats have power stealing
circuitry and will opperate without batteries, but most of the cheap
ones just have a small relay, and contacts could have some resistance
over time.

greg

I believe the batteries power the clock.
there's probably a diode to allow the 24VAC to power the clock until the
mains drop off,and then the batteries take over.But the batteries don't
power the relay that controls the system.

--
Jim Yanik
jyanik
at
localnet
dot com