• We’re currently investigating an issue related to the forum theme and styling that is impacting page layout and visual formatting. The problem has been identified, and we are actively working on a resolution. There is no impact to user data or functionality, this is strictly a front-end display issue. We’ll post an update once the fix has been deployed. Thanks for your patience while we get this sorted.

Why in the world are thermostats battery powered?

Page 2 - Seeking answers? Join the AnandTech community: where nearly half-a-million members share solutions and discuss the latest tech.
Jaepheth said:
Don't you have top run cables from the A/C unit to the thermostat anyway? Why not just have the thermostat tap into that power?
Click to expand...

NetWareHead said:
Because then you would need a transformer at the AC unit side to supply the power for the thermostat.
Click to expand...
Every residential central A/C sends 24VAC to the thermostat.

NetWareHead said:
With this design, the AC unit will only work with the voltage requirements of this particular thermostat. I mean sure you can have flexible power outputs and this way the AC could work with differing thermostat models. Again, I'm sure there is a cost reason why it is not done this way.
Click to expand...
There's no need for flexibility since 24VAC from the A/C is universal.
 
All furnaces send 24vac to the thermostat from the transformer inside them. Old houses didn't have the blue common wire in the t-stat wiring it wasn't needed back then but is needed for the non-battery powered t-stats. As an HVAC contractor I stock universal battery powered units for simplicity and price point.
 
One time cost vs. recurring cost.


Give a man a fish he eats for a day. Teach a man to fish and you can sell him rods, reels, lures, a boat and trailer, and a truck to tow them with. 😉


.
 
NoTine42 said:
Get a Classic looking round Honeywell T87 , it only has a (mostly non-replaceable) button battery with a 10 year life, and for the most part, that's not even needed as long as the thermostat is getting power to keep the capacitor charged.

http://yourhome.honeywell.com/Home/Products/Thermostats/Manual-Non-Programmable/T87+Round.htm

I don't have a T87, so, Yes, its dumb that my I occasionally come home to a cold home because my "smart" programmable thermostat refuses to heat because its 2-AA batteries are dead :/
Click to expand...

Wait those need a battery? I thought they were 100% mechanical/passive? Two different metals stuck together in a spiral and the temp makes it expand differently making the dial turn inside. At least that's how I thought they worked.

Cool to see those can still be bought, they are classics! Where I work they are actually used a lot for temperature alarms. you set it to the threshold and if it gets too hot or too cold (you need two of em) it will close the loop.
 
EagleKeeper said:
Consider the cost to implement/support external power.

Much rougher UL requirements and safety features.

Increase costs to install.
Click to expand...

Increased cost to install an additional +24v power only line alongside the 8-9 lines that are already there?

Although I think UL standard has something to do with it as well.
 
The thermostat that was here before never used a battery, but I had to replace it and that does use a battery. The battery goes out once a year and right around when we need to turn it on. In fact we just turned on the heat and the low battery light was blinking. It's stupid that you have power for a simple door bell but not a thermostat.
 
Red Squirrel said:
Wait those need a battery? I thought they were 100% mechanical/passive? Two different metals stuck together in a spiral and the temp makes it expand differently making the dial turn inside. At least that's how I thought they worked.

Cool to see those can still be bought, they are classics! Where I work they are actually used a lot for temperature alarms. you set it to the threshold and if it gets too hot or too cold (you need two of em) it will close the loop.
Click to expand...


I think the only purpose of the battery (in our case) is for the relay. In others it could be for the display.
 
There are plenty of batteryless thermostats out there. Most at the store have a battery because nearly all homes less than 5-15 years depending on the area don't have common wire to make that 24VAC able to supply the stat. The stat is a switch. You can't hook the stat to both sides of a switch and draw any useful power. So the batteryless stat would shut off as soon as it closed the circuit because as soon as the relay closed, power drops to 0VAC. If you want power at the stat, install a 10 wire cable and hook brown up as common (typically) and use that.
 
John Connor said:
The thermostat that was here before never used a battery, but I had to replace it and that does use a battery. The battery goes out once a year and right around when we need to turn it on. In fact we just turned on the heat and the low battery light was blinking. It's stupid that you have power for a simple door bell but not a thermostat.
Click to expand...
The battery's got enough power left by that time to run the idle thermostat, but then the relay's coil needs to be energized in order to close the contacts, and that coil likely draws a lot more power than the rest of the circuit combined. So the terminal voltage then drops down enough to trigger the "low battery" notification.

<sad trombone>
 
imagoon said:
There are plenty of batteryless thermostats out there. Most at the store have a battery because nearly all homes less than 5-15 years depending on the area don't have common wire to make that 24VAC able to supply the stat. The stat is a switch. You can't hook the stat to both sides of a switch and draw any useful power. So the batteryless stat would shut off as soon as it closed the circuit because as soon as the relay closed, power drops to 0VAC. If you want power at the stat, install a 10 wire cable and hook brown up as common (typically) and use that.
Click to expand...

Or I could just pop in a AA battery every few years. I for sure haven't changed it more than twice in the six years I've been in my house.

skull said:
All furnaces send 24vac to the thermostat from the transformer inside them. Old houses didn't have the blue common wire in the t-stat wiring it wasn't needed back then but is needed for the non-battery powered t-stats. As an HVAC contractor I stock universal battery powered units for simplicity and price point.
Click to expand...

I'm going to now start saying "t-stat" so I can sound like I know what I'm talking about. 😉
 
Red Squirrel said:
Wait those need a battery? I thought they were 100% mechanical/passive? Two different metals stuck together in a spiral and the temp makes it expand differently making the dial turn inside. At least that's how I thought they worked.

Cool to see those can still be bought, they are classics! Where I work they are actually used a lot for temperature alarms. you set it to the threshold and if it gets too hot or too cold (you need two of em) it will close the loop.
Click to expand...

I think they needed the battery to perform what the mercury used to accomplish
http://www.maxitinc.com/uploads/Sample_Announcement.pdf
 
NetWareHead said:
Because the expense in running a power cable to power a device with such tiny electrical requirements such as a thermostat is far greater than the cost of a battery. Especially when said battery could power a thermostat for maybe 2-3 years. Additionally, to power such a device from household current would also require building a transformer into the thermostat to step down the power to appropriate levels.

In short: its cheaper to use a battery.
Click to expand...

Close enough.

Because thermostats use a decades old standard for functioning with residential HVAC equipment. Remember that 50+ years ago thermostats where essentially mechanical mercury switches with temperature sensitive springs to kick the HVAC on or off.

There is also no residential standard for low voltage supply and ground. There should be some sort of standard... ala 5v or 12v but until home automation and so forth becomes far more common place, you won't see one.
 
WackyDan said:
There is also no residential standard for low voltage supply and ground. There should be some sort of standard... ala 5v or 12v but until home automation and so forth becomes far more common place, you won't see one.
Click to expand...

24VAC for control is the standard for homes to industrial HVAC. It has been that way I think going on 80 years now?
 
techs said:
Yup, that's the one I put in a couple of years ago. Never fails. Had a recent problem with the heat not coming on. The service guy took one look at the thermostat and said the problem must be in the basement at the heater. Yup, it was.
Click to expand...

How do you know that the battery will last 10 years? I just bought a CT87K and it mentions that it contains a lithium battery, but doesn't say how long it will last. Moreover, it doesn't explain how it helps the thermostat to work.
 
AustinHook said:
How do you know that the battery will last 10 years? I just bought a CT87K and it mentions that it contains a lithium battery, but doesn't say how long it will last. Moreover, it doesn't explain how it helps the thermostat to work.
Click to expand...
Too early. Need to wait until Oct 13, 2023.
 
I just had a realization that my custom hvac control system has been running for at least 10 years now. It was suppose to be a temporary solution until I come up with something more modular. Temporary solutions that work tend to be permanent I guess. 😛 I do small updates to the code once in a while and that's about it. The hardware itself has not changed.
 
You must log in or register to reply here.

TRENDING THREADS

Back
Top