Troubleshooting a digital thermostat

[DGF]

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?

 

[Stormin Mormon]

Put it back on the wall. Run the temp way up. Go see if
there is a 24 volt signal at W.

--
Christopher A. Young
Learn more about Jesus
www.lds.org
..


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?

 

[PeterD]

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?

 

[Gnack Nol]

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

That is correct thermostats electronic or mechanical always depend on the
power source in the HVAC system they control for power and are never self
powered.

The battery is just there for power interruptions to keep the thermostat
from resetting to a different temperature nothing more.

To test it you will have to provide the 24v source for it to operate from
otherwise nothing will happen.

Gnack

 

[GregS]

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

 

[GregS]

Battery operated thermostats don't have a conventional relay - ie one
which takes power all the time it's made. That would run down the battery
in short order. They use some form of 'motorised' switch, which only takes
power when it changes state. On mine you can hear it operate.

I always assumed a relay, and yes they are loud. On a soild state unit I could also
hear a click, but I think that was from current flow. If units use a switch, I hope
they have a safety overheat breaker.

greg