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Thread: Poor battery life with electronic thermostat

  1. #1
    Flashaholic* WildChild's Avatar
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    Default Poor battery life with electronic thermostat

    I got a Honeywell electronic thermostat for my appartment 3 months ago. It's an oil, hot water system with 24V AC going to the thermostat (2 wires). Battery life is supposed to be of 1 year but the last 2 sets lasted under 1 month. What can cause that? Except for this problem, the thermostat works flawlesly.

    Thanks

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    Flashaholic* Valpo Hawkeye's Avatar
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    Default Re: Poor battery life with electronic thermostat

    Is it backlit? Do you use the backlight a lot? If you have a common wire run to the stat, you can often run the stat off the transformer, with the batteries only being used/needed in power outages. Your stat has to be capable of this, though. You may also have a defective stat.
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    Default Re: Poor battery life with electronic thermostat

    Hum, now that you talk about backlight... I can see a really dim light in the corner. Too dim to be useable but it's always on. I'll check for warranty.

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    Flashaholic* Valpo Hawkeye's Avatar
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    Default Re: Poor battery life with electronic thermostat

    Depending on where you bought it, many places have liberal exchange policies. Regardless, the best way to hook up any electronic thermostat is using the common wire. They're called "power robbing" stats by most people in the trade. Unfortunately, a lot of installs don't have a sufficient number of leads in the stat wire that they pulled.
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    Default Re: Poor battery life with electronic thermostat

    Quote Originally Posted by Valpo Hawkeye View Post
    Depending on where you bought it, many places have liberal exchange policies. Regardless, the best way to hook up any electronic thermostat is using the common wire. They're called "power robbing" stats by most people in the trade. Unfortunately, a lot of installs don't have a sufficient number of leads in the stat wire that they pulled.
    The system was a 3 wires one and the valve motor has been replaced. Now, I can only detect voltage between 2 wires. The 3th seems unplugged. If I use only the 2 wires, plugged on the R and W slots of the thermostat, heating works correctly. The instructions says the thermostat doesn't use the C wire.

  6. #6
    Flashaholic* Valpo Hawkeye's Avatar
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    Default Re: Poor battery life with electronic thermostat

    OK. The fact that there are three wires may be, as you found, irrelevant. The only way to be sure what's going on is to go to the equipment and determine what the wires are hooked to there. In almost every system, red is hot and the other wires are different modes. For example, in 90% of the systems in my area, this is what you find:

    red = 24v to stat
    white = call for heat
    green = call for fan
    yellow (sometimes blue) = call for cooling.

    There are others too such as:
    orange = reversing valve for heat pump
    blue = second stage cooling
    various colors = second stage heating

    In your system, it sound like simple continuity between red and white gives heat. Your circulator is controlled by a relay at the boiler, so the stat has nothing to do with it. Therefore, if that third lead isn't used, you can connect it to the common terminal on the secondary (24v) side of the transformer to allow your stat to run off the transformer. However, it also sounds like you said that stat won't do it. Therefore, after a very long-worded reply ( ), my point is...

    Exchange the stat and see if it gets better.

    PS - Only mess with the wiring if you are comfortable doing so. Always kill the power to the transformer, usually by a switch or disconnect at the equipment, before messing with the control wires. If you have any doubts in your ability, call a professional. I don't want you to accidentally short a transformer, or worse, a control or ignition module, as those are $$$$$$.

    Hope it works out!
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  7. #7
    Flashaholic* WildChild's Avatar
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    Default Re: Poor battery life with electronic thermostat

    Strange thing... This thermostat is not supposed to be backlit.

  8. #8
    Flashaholic* Valpo Hawkeye's Avatar
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    Default Re: Poor battery life with electronic thermostat

    Hhm, that's weird. What's the model of the stat?
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    Default Re: Poor battery life with electronic thermostat

    Quote Originally Posted by Valpo Hawkeye View Post
    Hhm, that's weird. What's the model of the stat?
    RTH4000B. Only available in Canada. I tried to find similar problems without success.

  10. #10
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    Default Re: Poor battery life with electronic thermostat

    I am experiencing a similar problem with a RTH4300B over my parents house, battery need to be changed every month. Reading a number of post, I wonder if the thermostat should ever run from the battery, or the battery is only used when the house is out of power to keep the program alive on the stat? I have a feeling that the stat is not getting power from some other wire (if there is one) which suppose to keep it alive without battery. I will check the wires when I visit my parents, and suggestion?

  11. #11
    Flashaholic* Valpo Hawkeye's Avatar
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    Default Re: Poor battery life with electronic thermostat

    You need to be sure 1) the stat is capable of running off 24v from the furnace's transformer, and 2) that the common wire (usually blue) is connected.
    The only easy day was yesterday... now go run!

  12. #12

    Default Re: Poor battery life with electronic thermostat

    Some electronic thermostats require a common wire for power, while many lower power consumption ones use a latching relay (click to turn on, click again to turn off).

    Many thermostats don't use the common wire and if you don't have neutral coming to the wall, you're S.O.L.

    Get a thermostat that doesn't require a common.

    2 wire means you've got R & W with red being hot and W connected to heater solenoid then to common.


    R------------ R

    C--(solenoid)-W
    |
    ------------- C (optional)

    and in your case, you don't have the optional common line running back to the stat.
    Last edited by Handlobraesing; 05-26-2008 at 11:27 AM.

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