This almost always caused by the furnace tripping on high temperature limit, or some other safety limit. When the temperature limit is exceeded, the furnace shuts off the burners and the 24VAC to thermostat. When the stat loses power, the clock stops. The furnace blower remains on, so the homeowner may never notice a problem, and when the temperature is back within limits, the burners come back on, and the power to the stat is restored. The clock will be slow by however many minutes the power was off to thermostat. Correction: Find the cause of the high temperatures - dirty filter, etc.

The thermostat only does this when there is no common wire. If the common wire is present, check it for continuity, look for a short, a broken wire, or a bad connection. If the equipment is a heat pump, or heat only or cool only, you MUST have a common wire. If the equipment is gas/electric, the stat will work without a common, but if it goes blank when starting the heat or cool, add the resistor (included with thermostat) at the equipment side (not at thermostat!) between W and C.

Cooling on heat demand, heating on cool demand - the heat pump setting is wrong - go to setup step 3 and change the heat pump setting. If the unit is a commercial heat pump, it may not require a heat pump thermostat, so step 3 should be turned OFF. In this case, electric heat (setup step 5) may have to be turned ON.

The heat pump setting is wrong - go to setup step 3 and turn HP to OFF.

Most likely the furnace is tripping on high limit or some other safety limit. The furnace will shut off the burners AND break the R leg to thermostat, so thermostat has no power and will go blank. Solution - identify the cause of the limit trips.

Check the house code on thermostat and on the receiver. They must match. The house code on the receiver is set using dip switches, the house code on thermostat is set in thermostat setup, step 4. Also, check batteries - these thermostats require AA Lithium batteries, alkaline batteries will not work correctly.

Make sure the batteries are AA Lithium batteries. Regular alkaline batteries will die rapidly and cause erratic thermostat operation. Also check thermostat backlight setting in setup step 9. If the backlight is ON, the batteries will run down very rapidly. If the customer really wants the backlight on all the time, use the optional transformer part # T0800 to plug thermostat into a wall outlet.

To get into setup on any of the 4 button thermostats, hold Mode and Fan (or Mode and Override) for SEVEN seconds.If you only hold the buttons two seconds, you can only get to step two. You must hold the buttons SEVEN seconds, even though the display will say setup after two seconds.

Most likely the keypad is locked. Press Mode, Up and Down simultaneously to unlock the buttons.

Turn electric heat on, setup step 5.

Some heat pumps create a large voltage spike when the unit goes in to defrost and/or when the supplemental heat comes on. This large voltage spike can cause the microchip in thermostat to reset, changing settings. Settings can be restored with a factory reset (see below), but if the problem is re-occurring, replace thermostat with a flush-mount model, a T1050 or with a newer Venstar which has additional high voltage protection.

Check that the degree symbol on thermostat is flashing - if it is not flashing, the remote sensor is not connected correctly. Check wiring, and make sure that you are not using shielded wire, this will throw temperature off. Do not reverse RS+5 and RS GND as this may damage the sensor! Note: Do not use an ohmmeter to test the sensor - these are digital sensors, not thermistors, and they can be damaged by an ohmmeter.

This happens when there is air blowing through thermostat from the wall behind it. Insulate the hole behind thermostat with insulation, spray foam, or even duct tape - whatever will stop the airflow from behind the wall.

This can be caused by poor pin connections between the backplate and thermostat, almost always because the backplate is flexed against an uneven wall. Loosen the screws that attach the backplate to the wall, allowing the backplate to flatten out, then snap thermostat back onto the backplate

Wireless thermostats: Make sure the model number of thermostat matches the model number of the receiver. If the model numbers do not match, the stat and receiver will not be compatible. When replacing a Venstar T1100RC with a Venstar wireless stat, you must replace the receiver with a Venstar as well. Also, at this time the Zone setting in wireless thermostat setup is not used. It should always be set to No.

Voltage outputs: To check your voltage outputs you must have a load on thermostat outputs and measure between R and the specific output. Your readings should be between 0.5 volts and 1.1 volts AC if the output is on, and between 21 and 30 volts if off. Also, you cannot read temperature from a remote sensor using an ohmmeter.

Dry Contacts: When CK1 and CK2 are energized, thermostat is forced into Occupied 1 (if the stat is in Program On mode - in any other mode, the dry contact is ignored). When CK1 and CK2 are de-energized, thermostat will revert to its regular program.

Ck1 and Ck2: Programmable stats - only works if the Thermostat is in Program On mode. Non-programmable stats - works in all modes except OFF.

Resistor: The resistor that is included with thermostat should ONLY be used if there is no common wire AND thermostat is not operating correctly (see number 2 in troubleshooting). Otherwise, the resistor is NOT USED.