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Digital Thermostat out of battery

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  • Nov 3rd, 2011 11:15 pm
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[OP]
Deal Fanatic
Jun 7, 2005
6582 posts
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Digital Thermostat out of battery

If I remember correctly my Honeywell Digital Thermostat was showing on the screen that the battery is low, and I changed the battery inside.

I have a couple quesitons:

1) I thought the unit is wired to control the furnace. So, it should be somehow hard wired. How come it needs external battery ?
2) What happens if it runs out of battery while I am on long vacation? Would the furnace temperature go out of control ?
10 replies
Deal Addict
Mar 27, 2011
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Nowhere
Did you happen to notice all the connecting metal and wires when you took the faceplate off?
The digital front needs a battery and then it connects into the panel attached to the wall which is connected to the furnace.

I replaced my battery on weekend in fact. Main part of thermostat slides right out and then you line up the metal poles and shove them back into the base plate. Volia!


And dude relax. The battery lasts years usually.
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Nov 24, 2009
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A typical four wire furnace / AC setup has
  1. Common (RC+RH)
  2. Heat
  3. Cool
  4. Fan
This combination controls three relays
Power (24VAC) is not really available yet some thermostats can steal power from the voltage across the coils. So they use batteries. Typically batteries should last for a couple of years.

If your thermostats batteries are dead it won't do anything. The relays won't come on, the only real problem might be your water pipes may freeze if the temperature in the house drops below zero for a significant amount of time.
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Jun 14, 2003
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rdx wrote:
Nov 2nd, 2011 4:01 pm
If I remember correctly my Honeywell Digital Thermostat was showing on the screen that the battery is low, and I changed the battery inside.

I have a couple quesitons:

1) I thought the unit is wired to control the furnace. So, it should be somehow hard wired. How come it needs external battery ?
There can be a black out. The external battery saves the setting of the thermostat if that happens.
2) What happens if it runs out of battery while I am on long vacation? Would the furnace temperature go out of control ?

If there is a black out and it runs out of battery and you are in long vacation, most likely, it will use the factory setting.

EDIT: it appears not all the thermostat is designed the way I described.
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[OP]
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Jun 7, 2005
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gman wrote:
Nov 2nd, 2011 4:31 pm
There can be a black out. The external battery saves the setting of the thermostat if that happens.



If there is a black out and it runs out of battery and you are in long vacation, most likely, it will use the factory setting.

EDIT: it appears not all the thermostat is designed the way I described.

I don't mind if it goes by factory setting instead of doing nothing …………….
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Jun 14, 2003
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rdx wrote:
Nov 2nd, 2011 4:53 pm
I don't mind if it goes by factory setting instead of doing nothing …………….

May be you can leave the dying or dead battery inside to see what would happen.
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Deal Guru
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Mar 25, 2003
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rdx wrote:
Nov 2nd, 2011 4:01 pm
If I remember correctly my Honeywell Digital Thermostat was showing on the screen that the battery is low, and I changed the battery inside.

I have a couple quesitons:

1) I thought the unit is wired to control the furnace. So, it should be somehow hard wired. How come it needs external battery ?
2) What happens if it runs out of battery while I am on long vacation? Would the furnace temperature go out of control ?

Everything will keep runing, it just warning you the battery is low, so you know you have to change it
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Jul 26, 2010
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On mine I have only a certain amount of time to pull the battery and replace it without losing the settings.
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Mar 25, 2003
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38racing wrote:
Nov 3rd, 2011 1:01 am
On mine I have only a certain amount of time to pull the battery and replace it without losing the settings.

Don't pull all out at once. Take 1 out replace it, and then take next one and so on
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Blueroom wrote:
Nov 2nd, 2011 4:29 pm
A typical four wire furnace / AC setup has
  1. Common (RC+RH)
  2. Heat
  3. Cool
  4. Fan
This combination controls three relays
Power (24VAC) is not really available yet some thermostats can steal power from the voltage across the coils. So they use batteries. Typically batteries should last for a couple of years.

If your thermostats batteries are dead it won't do anything. The relays won't come on, the only real problem might be your water pipes may freeze if the temperature in the house drops below zero for a significant amount of time.
The above is correct except that some thermostats are powered by the common rail. Also, many thermostats don't have common hooked up by default because the cheap builder-spec thermostats don't require power. My thermostat is a Honeywell IAQ PRO that uses the 24VAC from the common wire. If the common wire is not hooked up, then the thermostat will require external power.
gman wrote:
Nov 2nd, 2011 4:31 pm
There can be a black out. The external battery saves the setting of the thermostat if that happens.



If there is a black out and it runs out of battery and you are in long vacation, most likely, it will use the factory setting.

EDIT: it appears not all the thermostat is designed the way I described.
Most thermostats will do nothing if the battery fails. You'll get no heat and the house will freeze.

This happened to my parent's place, so I had to rush over to change the batteries. That's why I always keep an old school mercury bulb thermostat on hand in case the thermostat is fubared.
rdx wrote:
Nov 2nd, 2011 4:01 pm
If I remember correctly my Honeywell Digital Thermostat was showing on the screen that the battery is low, and I changed the battery inside.

I have a couple quesitons:

1) I thought the unit is wired to control the furnace. So, it should be somehow hard wired. How come it needs external battery ?
2) What happens if it runs out of battery while I am on long vacation? Would the furnace temperature go out of control ?

You need to check how your thermostat is wired. My guess is that your thermostat uses a simple 4-wire install. Here's a guide on the function of the most common wire colours:
http://www.honeywell-thermostat.com/faq ... ostat.html
↑, ↑, ↓, ↓, ←, →, ←, →, B, A, START

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Apr 2, 2007
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This happened to me and you may want to read my thread.

What happens is the screen goes blank and the furnace shuts down.
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