Thermostat wiring reference chart - simplest case two-wire thermostat (C) Daniel FriedmanBackup Power Batteries for Room Thermostats
A dead battery can cause heating or cooling thermostat malfunction

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Heating or cooling room thermostat batteries & backujp power sources: What is the role of the battery in a room thermostat? How do we diagnose and fix a blank thermostat display?

Page top photo: a basic room thermostat showing the red and white wires being switched by the thermostat and a place where it's easy to check for voltage at those terminals.

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Backup Batteries & Default Programs for Digital Room Thermostats

Blank thermostat display problems

Some programmable digital room thermostats include an internal battery that will allow the device to remember the program that you have set even if there has been a power outage.

All programmable digital room thermostats are designed to return to a "safe" or "standard" temperature program to be sure that heat or air conditioning will continue to work in some reasonable range in case a power outage or dead internal battery causes the thermostat to forget how it was programmed by the user.

Watch out: some room thermostats require a battery to operate at all. These wall thermostats do not use power from the 24V thermostat wiring system and its low voltage transformer, and typically these thermostats will have larger batteries than those using battery power only for temporary backup.

Question: where to put batteries in our Hunter 44422 Thermostat - Hunter Fan Company Programmable Thermostat Instructions

Can't find where to put batteries on Hunter 44422 help - Jeanie 8/29/12

Reply: How to Change Batteries in the Hunter 44422 Programmable Thermostat

Jeanie, the Hunter 44422 you are discussing is a Hunter Fan Company programmable thermostat. You can get product information and instructions at the company's website or contact the company directly at Hunter Fan 2500 Frisco Ave. Memphis, TN 38114 888-830-1326

Quoting the company's product literature for the Hunter 44422 thermostat,

This thermostat is designed to work on the following heating and cooling systems:
Up to 2 Stage Heat/2 Stage Cool Millivolt Systems
Heat Only or Cool Only Systems 2-wire Hydronic Systems
Single and Multi-Stage Heat Pumps AC or Hard-wired
Gas, Electric or Oil Systems
This thermostat is not designed for use with 110V/220 V systems. If you are unsure of what kind of heating and cooling system you have, please contact a qualified HVAC Technician for assistance.

The batteries in this unit, or at least the one that I examined, are in the back of the hinged, removable thermostat assembly. You'll have to un-clip the thermostat from its top, swing it down and out on its hinge, and you'll see that using a plug connector the entire assembly is removable to make it easier to change the batteries without breaking something.

The company's instructions describe changing the batteries but omit a nice illustration of how to find them in the bottom-back of the assembly.

The CHANGE BATTERIES warning indicates two stages of battery power shortage.

When the batteries are weak, the CHANGE BATTERIES warning flashes until three new AA alkaline batteries are installed. If the batteries become too weak for normal operation, the thermostat enters the second battery power shortage mode.

When no battery power is left, the BATTERIES USED UP warning flashes on the display and the system turns off. The system remains off until new batteries are installed. The thermostat resumes normal operation after new batteries are installed.

See Files/Support/Owner_Manuals/44002.pdf for the manual for this thermostat.

Reader Question: Our digital thermostat is blank - what causes the thermostat LCD screen to go blank and refuse to respond when I press thermostat keys or controls?

When we came home from the store the digital thermostat was blank. We tried to take off the face of the thermostat to look for a battery as we were doing that the thermostat kicked on. Problem was it started to blow hot air. We went back to the thermostat and when we fiddled with it it went off but then came back on this time it blew cold air. Everything fine right?

Not so fast. In the middle of the night I woke up drenched with sweat. For some reason the AC started blowing hot air. I simply touched the thermostat and it went blank. I haven't touched it since.

Help..its hot and we have three kids. - Pearl in Annapolis

Reply: things to check if the LCD display on the room thermostat is blank or does not respond to touch

Whtie Rodgers Series 90 touichscreen thermostat Pearl:

First make sure that your thermostat has power. When the display on a digital thermostat goes blank that means that power has been lost to the unit. Power for a digital TT comes from one or at most two sources:

The low-voltage transformer mounted somewhere, often near the electrical panel or near the heating or cooling equipment supplies normal power to the thermostat; if one of those wires is disconnected, the transformer burned up, or power to the equipment where the transformer is mounted is turned off, the thermostat will go blank

Some thermostats also use a backup battery to remember settings when someone has turned off power or there has been a power outage. If the batteries are dead that'd be a second reason for a blank display. This is a less serious concern, as thermostats have a fall-back default program that will get things running after a power outage - which takes us back to reason #1 - power is off to the thermostat.

Next, check the thermostat setting mode and physical switches. Trane gives this advice if the display on their TCONT800 series Touch Screen programmable thermostat (illustrated at above left) is blank or not responding to touch:

Check to determine if the screen is in the Locked or Clean mode. See Screen Locked or Cleaning Comfort Control Screen sections.[21]

For example on this particular thermostat, a screen lockout function is provided under user setup number 0670:

0 — All keys are available. Screen is unlocked.
1 — Screen is partially locked. All key functions are locked except the Temperature Up and Down keys and the Cancel key. Enter Advanced Setup to unlock the screen.
2 — Screen is fully locked. Enter Advanced Settings to unlock the screen.

If the display is not entirely blank, but does not respond to touch, check the thermostat's Advanced Settings to see if the device was put into locked mode.


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