Keeping thermostat backlit display on with no c wire

abekrausz said:

I pulled a wire from the system transformer to the thermostat and the back light comes on only when the system is on.

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If I understand you correctly, you pulled a new wire from the transformer to the thermostat and connected it the C terminal. If so, you don't need an additional transformer. I think you need to go into the thermostat configuration mode and select the back light on continuous option.

Thanks a lot for responding.

Correct, I pulled a new wire from transfo to tstat. Problem is backlight only goes mon when system comes on for either heat or cool. As soon as system shuts off because of no call for heat or cool, backlight goes off. Do I need to connect my new wire somewhere else in the system then direct to the transfo?

If you pulled a C wire it’ll be connected at the air handler and thermostat. After that the thermostat will be powered off the indoor units transformer.
What is the thermostat in question?

You need to connect the C wire to the circuit board in the furnace and to the C terminal.

Then there should be a setting in the thermostat for battery operation ( choose NO ) and there may be one for constant backlight.

If you have it on battery operation it may not turn the light on all the time. The batteries are good for backup in a power failure.

Some new units have a temporary memory storage for several hours but keeping batteries in is a good idea IMO.

If you have the C terminals (unit/thermostat) connected correctly as stated above, you need to configure your stat. Looks like screen reference #13 in the menu is your target.

https://www.manualslib.com/manual/1063779/Emerson-1f95-1277.html?page=5#manual


Edit: You might as well go through the whole configuration menu and fine tune your system. If you have questions regarding this let us know.

Some new units have a temporary memory storage for several hours but keeping batteries in is a good idea IMO.

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except old batteries forgotten in have a tendency to leak - so op, make sure you still change them every few years.

57TinkerMan said:

Edit: You might as well go through the whole configuration menu and fine tune your system. If you have questions regarding this let us know.

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Thanks again,
I already went through the complete config menu and it did help me fine tune the system.

It's exactly menu # 13 that bothering me. If you look at page 6 of the manual, with a c wire, I should be able to keep backlight on all the time. As it is now, the backlight only hours on when system calls for either heat or a/c.

user_12345a said:

except old batteries forgotten in have a tendency to leak - so op, make sure you still change them every few years.

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Or better yet, REMOVE THE BATTERIES AND GET RID OF THEM. Sorry for yelling. I'm an HVAC guy and this is a pet peeve of mine. For most thermostats that are hooked up to a C wire, the batteries serve no real purpose. Most stats have either a permanent memory in them to remember the settings when the power goes out, or they have a hidden button battery inside for this purpose. If you have a C wire, you don't also need the AA's.

Honeywell uses a coin cell battery to keep the time.

The settings usually don't get lost on modern stats even when power is cut but for programmables, the time is lost.

So programmable units without the backup clock battery should have regular batteries left in even with a common connected.

user_12345a said:

Honeywell uses a coin cell battery to keep the time.

The settings usually don't get lost on modern stats even when power is cut but for programmables, the time is lost.

So programmable units without the backup clock battery should have regular batteries left in even with a common connected.

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It's the Honeywells that I normally deal with, and like you said they usually will keep time even without the AA's in them, so that's why I don't put the batteries in.

Perhaps you're right and other makes will lose time if the power goes out and the AA's or AAA's aren't in there.

Thanks for correcting me.

Interestingly, on some of the Honeywell's, the coin battery is hidden and hard to get to and there's no mention of it in their owner's manual. Even some of the stat's that don't use a C wire (like some of the round T-87 versions) have this hidden battery in them. I believe these lithium batteries are supposed to last about 10 years, and then after that when it quits working Honeywell might be betting that you might be in the market for another new thermostat. :whistling2:

don't know about other makes, have only dealt with honeywell.

The old ones like the magic stats, chronotherm 4s to my knowledge don't have the coin cell battery and lose the time.

The really old ones like the chronotherm IIIs actually lose the programming. (although otherwise they are fantastic and were really ahead of their time)

Thanks to all for responding.
Issue not yet solved.
As mentioned, original older thermostat had only 3 wires and no c wire. Thermostat was recently replaced with the Honeywell F195-1277.
This tstat should have an option to keep the backlight on all the time with a C wire. I pulled a single wire from c terminal in air handler and connected it to c terminal in the thermostat. Now, the backlight comes on only when system is on. When system goes off, the backlight goes off as well.
I would really like to have the backlight on all the time. Any solution?

Those power stealing switches are HORRIBLE and I NEVER use them.

They will bleed a small amount of power down thru W or Y to make a circuit and will damage most modern circuit boards. I had one make the AC contactor chatter like a machine gun and blew it.

I like White Rodgers but those power stealing thermostats should never use that function IMO.

User/buyer beware.:hang:

57TinkerMan said:

You mentioned that you had only 3 wires in use before you pulled a common wire, is this a cooling only application?


View attachment 602377

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No, But this is a slave tstat to a master, where the fan is controlled by the master. System is using a 2 zone controller with master and slave tstats.
So this controls only a call for heat or a call for cool.