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I have a thermostat that is completely blank (no display). I'd like to troubleshoot it myself even if I do ultimately hire a pro! We're in sunny southern california, so it's possible it's had no display for sometime, but when I first noticed it was after we flipped all circuit breakers to the house for another reason (checking power consumption). I'm ignorant of heating systems, but having done research, it seems that one of two things are most likely:

1.  No power to main unit.  Main unit communicates to thermostat via 24 V or lower.
2.  Power to main unit, but no power from main unit to thermostat.  Possibly bad connections, etc.

I'd like some ideas from a kind and patient soul on how to troubleshoot both of these.
Starting with #1.... The main heater is plugged in, but how the heck does one tell "it's on"? I see no lights, etc to indicate it is on. I have tried the socket it's plugged into and it's powered. However perhaps there is an on/off switch? The model # is MPGA075B4B. Originally, I thought the make was Honeywell because I saw that huge name on the on/off gas switch in the unit. But as I did internet searches, the name "Ducane" kept popping up. Not sure if these heaters are made by one company and use Honeywell switch or what is going on. Anyhow, I can't find a manual for it, and I can't see how to know if it's "on".

Number 1 seems like where I should start. Assuming the unit is on, my understanding is that one can check at the thermostat if it's even getting power.
a) How does one do this? b) assuming it is not, what next? Do you check the Honeywell switch on the heating unit itself? The Thermostat in the living room (that is blank) is Venter, Model T1050. There are 4 wires coming out. Red, Green, W1, and C (which looks blue to me!).

Finally, any other ideas? And I can post photos or more information.

I appreciate the help. Dave

  • Do you have a voltmeter or multimeter? – Tester101 Dec 1 '15 at 18:46
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    Some thermostats have a battery. Does yours? – Yehuda_NYC Dec 1 '15 at 18:51
  • Is the thermostat for A/C or Heat only (not both) ? – JPhi1618 Dec 1 '15 at 19:53
  • Yes I have a multimeter. As far as I can tell, the thermostat does not have a battery, and the manual does not mention a battery. I don't know if the thermostat could handle A/C, but in my house, I have just a heater. – Dave Dec 1 '15 at 20:53
  • Blank LCD on a thermostat definitely means "not getting tne right power", unless it means "dead" – keshlam Dec 1 '15 at 23:12
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Since you have a multi-meter, you can check the voltage of the wires at the thermostat. Make sure the meter is set to AC (Alternating Current) in this case, and if there are ranges, something safe for 24v. Probe the Red and Blue wires and see if there is 24v. You can also check the voltage between the red and any other wire.

If there is no voltage, the low-voltage power supply for the furnace is bad or not getting any power. There is probably a breaker in the main cabinet for the furnace, but there will probably be a breaker located on the furnace/air handler as well. Make sure they are both on and recheck the voltage.

The power supply will typically look like this and should have two 120v wires and two 24v wires coming off of it.

enter image description here

If you can find it, check the voltage going to it to see if 120v is present. Troubleshooting will move on from here if there is not 120v.

Thermostat information

Assuming the wire colors you have are standard...

To turn on the fan, the thermostat makes a connection between the red and green wires. If you place a jumper between these wires, the fan should run.

To turn on the heat a connection is made between the red and white wires. The call for heat will always be accompanied by the call for fan.

The blue wire doesn't signal anything - it just supplies voltage to the thermostat in conjunction with the red wire. To draw a parallel with your house wiring, the red wire is like the black "hot" wire, and blue is like the white "common" wire.

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Use your multimeter to check for power, working your way backward from the thermostat. You should find 24VAC between the red thermostat wire, and any other thermostat wire. If not, move to the furnace control board. Test for 24VAC between the R terminal, and C terminal (if present, or any other terminal).

If there's no power on the low voltage side of the board, check for line voltage. Check for line voltage (120VAC) between the HOT and NEUTRAL terminals on the board. If there's no power there. Check for line voltage between the line side of the door switch, and neutral.

Next, check for voltage at the servicemen's switch (if present). Finally, check for voltage at the breaker.

If you find proper voltage at all locations, try replacing the thermostat or thermostat batteries (if present).

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This answer appears to be incorrect. This "low power" thermostat uses "power sharing" to sip the few millivolts it needs to operate without the need for a separate C-wire. It can use a C-Wire but doesn't require it like a newer color display or WiFi thermostat. I'm leaving the answer for informational purposes.


The manual for your Venstar T1050 does indicate that the uint uses batteries, and has a non-volitile memory. It does not say that the "C" wire is required, which is a good indicator that it uses only the batteries for power.

Modern "smart" thermostats normally have repeated warnings and explicit instructions about the C-wire because that's what they use for power.

Based on the above, I'd say grab a set of fresh batteries and you should be good to go.

Edit: Hmmm. This Amazon question says it doesn't have batteries even tho the manual specifically mentions that it has a memory backup when there is no battery power... Looks like I could be wrong.

  • Thanks for the answer. I read the manual you link to and the only thing it says about battery (as far as I can see) is: "In the event of a power loss, the thermostat will retain the stored program settings without external power or batteries." To me, this means there are no batteries. I certainly don't see any batteries. – Dave Dec 1 '15 at 20:55

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