Why in the world are thermostats battery powered?

[Lean L]

This has got to be one of the most puzzling things to me about the design of modern things. I can understand having a coin battery backup so that you don't lose programming settings but battery as the main source? How large is a power adapter? smaller than a double A nowadays?

Change the standard people...

 

[BrokenVisage]

Because you touch your coppertop at night.

 

[olds]

Remember to inflate your thermostats to the sidewall pressure rating.

 

[boomerang]

This has got to be one of the most puzzling things to me about the design of modern things. I can understand having a coin battery backup so that you don't lose programming settings but battery as the main source? How large is a power adapter? smaller than a double A nowadays?

Change the standard people...

What in the world are you talking about?

 

[NetWareHead]

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.

 

[phucheneh]

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.

That's why he mentioned DC power supplies.

Any stationary device with battery power that doesn't also have an input for a power supply can suck it.

 

[EagleKeeper]

This has got to be one of the most puzzling things to me about the design of modern things. I can understand having a coin battery backup so that you don't lose programming settings but battery as the main source? How large is a power adapter? smaller than a double A nowadays?

Change the standard people...

Consider the cost to implement/support external power.

Much rougher UL requirements and safety features.

Increase costs to install.

 

[vshah]

? my nest charges it's internal battery off of the 24v thermostat line. i guess it is still technically battery powered.

 

[boomerang]

Remember to inflate your thermostats to the sidewall pressure rating.

I save energy by underinflating two degrees. I can feel it every time the furnace kicks on.

 

[Meghan54]

Consider the cost to implement/support external power.

Much rougher UL requirements and safety features.

Increase costs to install.

You do realize that the OP would be the type to bitch about how much thermostats cost if they were wired in on the house's elec. circuit and muse about "Why aren't they battery powered? It'd be so much cheaper....yead, yada, yada...."

 

[Jaepheth]

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.

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?

 

[Mixolydian]

I thought most thermostats were powered from the furnace/AC they're connected to. The system supplies voltage through one of the wiring pairs to the thermostat.

 

[Jeff7]

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.

And a lot of those small power adapters, even the more efficient switchers, still use around 0.3-0.5W when they're just sitting idle. It's better than the heavy 60Hz iron-core transformer designs, but not exactly great, when you consider that the load of the thermostat itself is going to be tiny when compared to that.
So you'd basically be running a power line for the purpose of keeping part of the power adapter warm, with the side effect of also running a thermostat.

(Unless you specifically look for an adapter that uses very little power when idle. Some of the better ones can be less than 0.03W, but even that is a lot more than the thermostat is going to consume. Passive LCDs and a simplistic low-speed controller IC use very little power.)

 

[Red Squirrel]

When I bought my house it came with a programmable one and I asked myself the same question, why not just use the 24v power and just have a backup battery. Heck why not just save the settings on flash, if power goes out, who cares.

My current thermostat is basically a server so it runs off AC and only runs off batteries if the power is out, in which case the furnace wont work anyway, though I'd have to get a backup battery for that too one day.

Of course today everything is made as cheap as possible, so probably cheaper to use a battery than to have a AC-DC converter in there.

 

[NoTine42]

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 :/

 

[IronWing]

My thermostat is hardwired. Therefore Ironwing>OP.

 

[Jeff7]

When I bought my house it came with a programmable one and I asked myself the same question, why not just use the 24v power and just have a backup battery. Heck why not just save the settings on flash, if power goes out, who cares.

My current thermostat is basically a server so it runs off AC and only runs off batteries if the power is out, in which case the furnace wont work anyway, though I'd have to get a backup battery for that too one day.

Of course today everything is made as cheap as possible, so probably cheaper to use a battery than to have a AC-DC converter in there.

A cheap little low-power linear voltage regulator should be able to do the job - pull power from the 24V line, drop it down to 2V or whatever the control circuitry needs, and you're done. That's probably looking at a few dozen microamps of draw, so you'd get very minimal heat generation in the regulator, and those things are pretty cheap, especially if you don't need to drop a lot of power.

But, from what I've seen of consumer electronics, "cheap" usually means "scrape into the dirt beneath the bottom of the barrel."

 

[who?]

Battery holders are probably cheaper than an adapter or even a circuit to power off of the 24 volts. If your main criterion is purchase price the battery powered thermostats are probably the winners.

 

[NetWareHead]

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?

Because then you would need a transformer at the AC unit side to supply the power for the thermostat. 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.

 

[FeuerFrei]

My thermostat's just a coiled spring and a blob of mercury in a glass capsule. Old house.

 

[lxskllr]

My thermostat's just a coiled spring and a blob of mercury in a glass capsule. Old house.

Hard to beat simplicity.

 

[techs]

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 :/

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.

 

[Mr. Pedantic]

We don't have a thermostat.

Insulation solves all problems. No thermostat, no problem.

 

[drnickriviera]

Because then you would need a transformer at the AC unit side to supply the power for the thermostat. 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.

They already have a 24VAC transformer in them to trip the contactor relay at the compressor. That 24V goes to the thermostat as well. The thermostat determines what you want (fan,a/c,heat) then sends that 24v back on the proper wire. So the power is already there.

 

[Mixolydian]

With this design, the AC unit will only work with the voltage requirements of this particular thermostat.

Isn't 24v the de-facto standard?