AC Thermostat Wiring

Thanks.......will check it out and let you know how it goes.

Depending upon the age of the installation/knowledge of the installer, you may find that the wire is already there, just not hooked up (usually taped back).

This is because when thermostats were a set of mechanically switched contacts (usually a mercury bulb), the thermostat itself did not need power to do it's job. With newer thermostats, you need power and ground/common to run the electronics. Unfortunately, even today I have seen too many places where the installer did not hook up the common, and instead relied upon battery power to run the electronics. The most irritating thing about that, is that this is at industrial plants where the installer should damn well know better.

Interesting.

One of our 3 AC units at the office quit working, (during an Arizona summer) a year or so after we had programmable thermostats installed.

After some investigation, and a lot of luck, I discovered that the backup battery had died.

New battery and all was well.

What are the 3 wires on the existing set-up? There will be incoming live and switched live, but is the 3rd neutral or earth? Some (traditional type) stats have a booster heater, energised when the contacts close, which needs a neutral.

I'm slightly surpised the new one doesn't need 5 wires, live, switched live, neutral, earth and 24VAC. Do you have 24VAC available?

"residential AC Unit" Is this a stand alone AC system with no furnace for heat

Below is a typical T-sat wiring use in the US. Unless the contractor cutting cost ran only 18-3 (3 conductor wires), then you should 5 to 8 wires installed. Easiest way to verify how many wires were ran for the T-stat is to open your furnace cover where you see the T-stat wire goes into it the furnace and count how many wires are coming out the T-stat wire and note how many wires and colors that are not used and wrapped back around the insulation of the T-stat "wire". This is common with practice wrapping unused wires around the insulation, (for future use) at both ends of T-stat wires in both the furnace low voltage terminal block and at the thermostat. If there more than the 3 wires at the furnace, then there's the same amount of wires in the wall where the T-stat mounts. And, as previous mentioned, if you only have an 18-3 wire" in the furnace (?) then you'll have to run new T-stat. I doubt that you only 18-3 thermostat wire if you also have an AC. 18-3 used for furnace heat without AC

Remove the T-stat mounting plate and gently pull on the T-stat wires, if the hole is large enough in the dry wall then the rest of the wires should come out. If not you'll have to carefully enlarge the hole. 2 things thou, do not let the thermostat wire fall back into the wall!! And do not cut the T-stat wire if you have to enlarge the hole in the drywall! Then wire according the diagram.

Note, the common wire "C" terminal may not necessarily be a blue wire, the HVAC installer could have used any of the unused T-stat wires. And that can also be verified at the furnace's low voltage terminal "C".

Make sure all the wire colors are connected to the same terminals on the furnace and on the thermostat, red to "R", yellow to "Y" green to "G", white to "W"

Good luck, DJ

Thank you.......this is very helpful! I am a 'Do it your selfer', so I will take your advise and let you know how it goes.

Nice info! Your thermostat or programmable thermostat is an integral part of your comfort system. Homeowners also need to keep themselves updates with every bit of information about their heating and cooling unit. They need to know about the installation and wiring of the thermostat, which may help them in the emergency situation. A very common occurrence with service calls is the complaint that the thermostat isn't working properly. In any state of confusion, it is advisable to most people is to call a residential HVAC service NJ professional if something is wrong. Whether it is with your thermostat or HVAC system or AC unit.

All the Wi-Fi thermostats and many touchscreen ones draw too much power to be run off of a battery so as others have stated they need 24v AC power. Ideally you should have an unused wire and use the existing transformer in the system that can supply this. If you don't have this you would have to pull a new wire. If pulling a new wire is not possible, while not the best case I believe you can probably supply 24v AC by a separate transformer that is wired somewhere closer to the thermostat. Adding that transformer is likely more than a DIY job and you may need to hire someone.

Depending where in this world the OP is located, chances are the OP has 24 volts for the supply power, Unless they have a wall/ floor furnace that is a millivolt system, then they will have 2 separate thermostats, (millivolt systems will not operate properly on a higher voltage thermostat) and would explain why they have a 18-3 T-stat wire for a stand alone AC system. Anything other than a millivolt furnace, they should already have 24 volt, or what ever standard control voltage is for their country

Thats great. i was also looking for the same..