Just make sure you have power off the systems before you start removing wires. Without seeing it, that is the only advice I'd give you, other than the cheapest one is not the best. If it's a simple 4 wire stat, you "might" get it done by a pro for around 100 dollars. Much cheaper than if you DIY and fry a transformer.

If your screen on your thermostat is blank, check to see if has batteries. If not, then it might not be the thermostat at all and could not be receiving the 24volt from the air handler/furnace. I would pull the t-stat. The wires to the thermostat should be color coded, red yellow white...and so forth. Dont worry about the wire colors. Look at where they are attached to the thermostat and there should be a letter next to the wire. You should have R (power), G (fan relay), W (heat relay), & Y (cooling relay). Write or take a picture of the wires and what terminals they attach to. Remove the wires from the thermostat. Now take the wire that went to the R terminal and touch & hold it to the wire that went to the G terminal. You should hear the indoor blower come on. You can also do the same with the R to Y to test the cooling & R to W to test the heat. I always do the R to G (fan) first. That tells me i have power to the airhandler/furnace, transformer isnt burned up & the t-stat wire itself from the airhandler to the thermostat itself is ok. If you hold the wires together and no fan comes on, or no outdoor unit kicks in when you touch the R to Y, its probably an equipment issue and would require a tech to come look at it.

** Even though its low voltage (24-27 volts), its still dangerous. Be careful or call someone who is qualified**

Thanks fluffybunnysui!
That is very helpful information and much better than what I hoped to get.

I really appreciate it and I will let you know what I find.

"Even though its low voltage (24-27 volts), its still dangerous."

Hardly unless your hands are soaked in sweat.

The biggest 'danger' is damaging things by shorting them out.

No real hazard to people.

Im just trying to make sure they understand that electricity, regardless of voltage is dangerous. Im sure they know that but just trying to cover all my bases,

Before you remove the wires on the old tstat, make sure to write down what color goes to which terminal. Though some if not most terminal designations and wire colors are standard, there are still variations that mess people up. Certainly true with heat pumps.

BTW: if this is a heat pump system, then the replacement tstat needs to be HP compatible.

Thanks for all the help folks. This was as simple as could be. I found 2 AA batteries inside the thermostat and replaced them. That did it, all is well. The only question I have is why in the world did they use batteries in this when there is 24 V AC available? at first I thought it might be to save settings over a power outage, but with FLASH devices costing pennies, you can store the settings for 40 years without power. Just curious.

Thanks again for all the help and I saved some of it for future reference.

PickyBiker

Keep on an eye on the thermostat. The ones with batteries usually have a low battery indicator.

The thermostat wiring has a wire for 24V, but in many cases there is no ground wire available. You need a ground wire in order to power the display and operate the electronics of the thermostat. This is the reason the batteries are used.

You are probably right that a small amount of Flash memory is inexpensive, but the problem goes back to the lack of the ground wire.

Thanks!

Isn't it great when something so unreasonable sounding suddenly makes sense when you finally understand what's going on?

Getting a renter to monitor the status of the battery in the thermostat (at least with my renter) would be nearly impossible. She will let me know about the batteries when the screen goes blank again.

Usually blank screen means bad batteries. A set should last 1-2 years. Use a good brand of Alkaline batteries.

Like mike said, definitely get good batteries, and as far as why they even use batteries, it is most likely a ground wire issue. I believe someone already said that but that would be my guess also. Batteries are always better then getting shocked!

A friend is rehabbing an older house, which had a 2-wire thermostat. Fine for the old mercury-type thermostat, but he put in an electronic one.

He has tenants also, and I raised the question of what will happen if/when the batteries run flat. It gets cold around here (BC, Canada) and if the batteries run flat, I presume the furnace won't run...which would be a disaster.

It may be the same case, not enough wires to power the thermostat. If that's the case, maybe you can run another wire if it's accessible? I think the minimum is 4 wires, but standard modern tstat wires have more...

Here is a link that might be useful: Tstat wires

Change the batteries when you change the ones in the smoke detector, once a year.

I change my batteries in the smoke detector when they start to make noise. That once a year thing is something dreamed up by people that sell batteries to steal my money.