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Old 09-27-2009, 01:53 PM   #1
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Eliminate Dead Batteries in Thermostat


My 24Vac thermostat requires 2 AA batteries in order to work. I want to eliminate these and create a purely hardwired system. Is there a way I can accomplish this without harming the system? Possibly an AC/DC converter and a resistor?
Any suggestions would be great, thanks.

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Old 09-27-2009, 03:13 PM   #2
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Eliminate Dead Batteries in Thermostat


Radio Shack & others sell "battery eliminators".
If you cut the AC power and put a milliammeter in series with the batteries you'll know how much current this eliminator must supply.

Or the manuf. may tell you how many hours the 'stat is expected to run on the batteries and then use the mAh rating of the AA cells [". . .from 1700 mAh to almost 3000 mAh" ] to figure the current draw. E.g., if they last 20 hours the drain could be from ~90 mA to 150 mA.

Or, drop the 24vac supplied by a bell 'former [probably you can't use your HVAC 'former] to 3vdc. You'll need an IC voltage regulator, 1 or 4 diodes, 3 resistors and two capacitors; $3 from mail order places.

Or find an old wall adaptor/charger that puts out more than 5vdc and use the regulator circuitry above, without the diodes.

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Last edited by Yoyizit; 09-27-2009 at 05:38 PM.
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Old 09-28-2009, 11:29 AM   #3
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Eliminate Dead Batteries in Thermostat


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Originally Posted by Codywookman View Post
My 24Vac thermostat requires 2 AA batteries in order to work. I want to eliminate these and create a purely hardwired system. Is there a way I can accomplish this without harming the system? Possibly an AC/DC converter and a resistor?
Any suggestions would be great, thanks.
You know that it a REALLY good idea! I spend a small fortune on batteries...

And this gives me a new idea....

That would be to wire a house with separate battery power wiring for all the gizmos which use batteries. Smoke detectors, carbon monoxide detectors, thermostat, security system, clocks, digital thermometers, computer UPS, phone answering machine, etc.

Then use just 1 large battery for everything!

So there could be separate low power DC outlets for battery power.

Two ways to do this...

One way would be to provide every possible DC voltage. Maybe use cat 5 type phone jacks and then just use the appropriate wires for the voltage needed. (Have voltage regulators at the main battery.)

The other way would be to provide just one voltage to all DC outlets. Say 12 volts DC. Then at each device use a voltage regulator to reduce the voltage for the specific device.

Then you would have just one large rechargeable battery for all this stuff (like used in a computer UPS). These last many years. Then to replace it, just replace that one battery and you are good to go for several more years!
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Old 09-29-2009, 01:08 PM   #4
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Eliminate Dead Batteries in Thermostat


Just get a small transformer that will take 120 volts and drop it to 24 volts. Then you would need to find a way to hide the XFMR.
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Old 09-29-2009, 01:46 PM   #5
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Eliminate Dead Batteries in Thermostat


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Originally Posted by J. V. View Post
Just get a small transformer that will take 120 volts and drop it to 24 volts. Then you would need to find a way to hide the XFMR.
A good place is in the Boiler Room. Then run an extra 2-wire Thermostat cable, or a multi wire (Six wire, it also controls A/C) to the 'Stat.
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Old 09-29-2009, 06:25 PM   #6
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Eliminate Dead Batteries in Thermostat


I agree with you that replacing batteries can be a pain. I wonder though, if it's worth all this engineering and labor just to avoid replacing a couple AA's every couple of years. My t'stat is 4 years old and still on same batteries.
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Old 09-29-2009, 09:30 PM   #7
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Eliminate Dead Batteries in Thermostat


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I agree with you that replacing batteries can be a pain. I wonder though, if it's worth all this engineering and labor just to avoid replacing a couple AA's every couple of years. My t'stat is 4 years old and still on same batteries.
But when they go, life stops. Whether it's 100o outside and your A/C dies. Or Zero Degree and your heat stops. Yes. some (or most) 'stats have a low battery warning sign. But most people tend to ignore it. They (we) figure. "Let's squeeze out one more day. I'll change 'em tomorrow... or the day after! And then it happens. (No matter what) Don't Drink and Drive, Ever.!!!
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Old 09-29-2009, 10:23 PM   #8
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Eliminate Dead Batteries in Thermostat


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Originally Posted by Billy_Bob View Post
You know that it a REALLY good idea! I spend a small fortune on batteries...

And this gives me a new idea....

That would be to wire a house with separate battery power wiring for all the gizmos which use batteries. Smoke detectors, carbon monoxide detectors, thermostat, security system, clocks, digital thermometers, computer UPS, phone answering machine, etc.

Then use just 1 large battery for everything!

So there could be separate low power DC outlets for battery power.

Two ways to do this...

One way would be to provide every possible DC voltage. Maybe use cat 5 type phone jacks and then just use the appropriate wires for the voltage needed. (Have voltage regulators at the main battery.)

The other way would be to provide just one voltage to all DC outlets. Say 12 volts DC. Then at each device use a voltage regulator to reduce the voltage for the specific device.

Then you would have just one large rechargeable battery for all this stuff (like used in a computer UPS). These last many years. Then to replace it, just replace that one battery and you are good to go for several more years!

That would be neat but I wonder how much loss you'd get along the way, as low voltage degrades faster then high voltage. Suppose you could use higher guage wire like 14 or even 12. You could use a transformer so you always have constant power, or a battery bank so it works even if the power goes out. This would be great for during power outages too, you could have some 12 volt lights and maybe even a 12 volt fridge ready. A car battery would be the best solution for this, I think, though at 12 volts, you'd still need to step it down for most uses so maybe even 6 volts would be better, or a combination. Maybe use 14/3 so you have multiple legs. Don't need an actual real ground so you have 4 wires to play with.

Maybe something like this: (there may be a better way to do it so you can get more voltage combinations)
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Old 09-29-2009, 11:34 PM   #9
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Eliminate Dead Batteries in Thermostat


I hate buying batteries, I buy rechargeables now
I wish a lot of stuff (wall packs) were standardized & 12v power was available in the house for smaller items
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Old 09-29-2009, 11:40 PM   #10
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Eliminate Dead Batteries in Thermostat


Change the batteries when you change the clocks.
My t stat uses batteries for the display only.
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Old 09-30-2009, 02:07 PM   #11
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Originally Posted by J. V. View Post
Just get a small transformer that will take 120 volts and drop it to 24 volts. Then you would need to find a way to hide the XFMR.
Doesn't the thermostat only switch the 24vac? It is actually a DC relay I believe for the coil. If it uses 2 AA batteries would that be about 3VDC? So basically this would mean switching the 120 VAC to 3VDC in order to work properly...I think.

Please correct me if i'm wrong. Any great suggestions for this? I'm beginning to rethink this idea, but batteries are expensive in the long run.
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Old 09-30-2009, 09:48 PM   #12
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Eliminate Dead Batteries in Thermostat


Sorry if that was confusing. I will try to explain my thinking more clearly, but my guess is that it is not correct :s...

The 24V AC thermostat is rated "24VAC" because it switches a load line of 24 V which runs the HVAC system. This switching is actually done by the battery circuit...correct??? (I don't know for sure) This device is not listed as a power stealing thermostat, so I figured that the battery power was used to switch a small relay. The input or coil of the relay are switched by the battery DC power, but the contacts or output is a 24VAC line. Sorry if this still doesn't make sense.

So in my thinking, since we already have the 24VAC power line to the HVAC, I only need to replace the battery power. I believe these batteries provide 1.5V DC a piece...correct???? That would mean I need to supply the system with 3V DC somehow.

Electronics are complicated in words
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Old 09-30-2009, 10:31 PM   #13
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Eliminate Dead Batteries in Thermostat


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Originally Posted by Codywookman View Post
Sorry if that was confusing. I will try to explain my thinking more clearly, but my guess is that it is not correct :s...

The 24V AC thermostat is rated "24VAC" because it switches a load line of 24 V which runs the HVAC system. This switching is actually done by the battery circuit...correct??? (I don't know for sure) This device is not listed as a power stealing thermostat, so I figured that the battery power was used to switch a small relay. The input or coil of the relay are switched by the battery DC power, but the contacts or output is a 24VAC line. Sorry if this still doesn't make sense.

So in my thinking, since we already have the 24VAC power line to the HVAC, I only need to replace the battery power. I believe these batteries provide 1.5V DC a piece...correct???? That would mean I need to supply the system with 3V DC somehow.

Electronics are complicated in words
It's nice that you share your thoughts with everyone else, here. But if we (or you) would follow this method of "Trial and error", there would be lots of buildings (and private dwellings) flooded, blown up, burned and otherwise damaged in the learning process. A much better method is to open up a book or attend courses + manual labor in the field that we like to specialize in! (Eliminate confusion) through Education! Don't Drink and Drive, Ever !!!

Last edited by spark plug; 09-30-2009 at 10:35 PM. Reason: missing word!
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Old 09-30-2009, 10:34 PM   #14
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It's nice that you share your thoughts with everyone else, here. But if we (or you) would follow this method of "Trial and error", there would be lots of buildings (and private dwellings) flooded, blown up, burned and otherwise damaged in the learning process. A much better method is to open up a book or attend courses + manual labor in the field that like to specialize in! (Eliminate confusion) through Education! Don't Drink and Drive, Ever !!!
Thanks i guess...that's great advice. No one likes to admit they don't know the answer I guess...
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Old 09-30-2009, 10:42 PM   #15
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Eliminate Dead Batteries in Thermostat


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Originally Posted by Codywookman View Post
Thanks i guess...that's great advice. No one likes to admit they don't know the answer I guess...
Lots of folks know the answer, I guess...! Like pro installers and pro DIY'rs. But, large doses of humor (as were detected in your original post) would take away from the "Mission" of this website. Which is to help each other and share and acquire knowledge and experience!.... (Not Really)!!!

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