How to Install a Thermostat for Split Air Conditioning System

Installing a thermostat for your central air conditioning or heat pump system can be a do-it-yourself task if you are handy enough. It can also be pretty simple given that you have the right tools. But before reading this and following through, make sure that you view our disclaimer as we are not going to be held responsible if you damage your equipment or injure yourself. If you still feel like you have enough common sense to follow this guide and install a thermostat yourself and save money, then read on…

Determine What Type of System You Have First

The following thermostat replacement tutorial is covering a split system, single stage compressor, using a 1F86-344 White Rogers digital thermostat. No zones. No heat pump, although if you have a heat pump system following this tutorial should work also, as there are only two extra wires that you would have to hook up which would be ‘B’ (brown) and ‘O’ (orange).

DIY Thermostat Replacement

Tools Needed for Easy Replacement – Small flat head screwdriver, needle nose pliers, wire strippers, volt meter, drill with Phillips head drill bit, drywall anchors.

Removing the Old Thermostat

First thing you want to do is turn the power off to the air handler or furnace. Then you are going to remove the cover from the old thermostat. Usually this is done by pulling from the top of the thermostat in the middle, towards you and the cover should pop right off. Some thermostats may have a small set screw so don’t go yanking it off the wall if it doesn’t come off with a little force.

With your small flat head screwdriver loosen all of the set screws from the wires and using you needle nose pliers, remove the wires from the set screws. Unmount the screws securing the old thermostat and then you should be able to pull the thermostat off of the wall.

 

Installing the New Thermostat

Pop the cover off of the new thermostat, note that some T-stats have a small set screw that you will have to loosen to get the cover off, but this should be covered in the installation instructions. The I will usually get a pen and a torpedo level and get an idea of where I am going to mount the thermostat. Once I got it flush against the wall with the wires through the hole I will make two marks with my pen. Pull the thermostat back from the wall and insert your drywall anchors.

Now just put the thermostat flush against the wall pulling the wires through the middle and mount the thermostat with the screws provided. Make sure that you get your colors right when hooking up the 24volt wires. Lots of times old wiring white can look like brown and vice versa.

The R (red) is the 24volt power to the thermostat.

The G (green wire) is for the blower fan.

The W (white wire) is for the heat.

The Y (yellow wire) is for the compressor and outdoor fan motor.

O and B would usually be for the heat pump.

If you have Y2 or W2 terminals are for multi-stage systems.

RC and RH terminals should have a red jumper connecting them. The only time this should be removed is if your cooling and heating have separate transformers, which in most cases they won’t.

The C terminal (usually black or blue) is for the common hookup, which is optional and requires a little more technical know-how to hook up, not really recommend for diyer’s as you can blow up transformers if hooked up wrong, and should be left to someone that has at least a little bit of hvac training as you will have to be able to locate the transformer (which is usually in the air handler in residential units).

If you have an AUX terminal (usually white wire) then this is usually for emergency heat. This would kick in when your heat pump can no longer keep up with the heating load.

 

 

:::Bonus Section:::

Hooking up the optional common wire will get power directly from the air handler with digital thermostats. Some advantages to this is that you will not need to be replacing the batteries every year. Another benefit from this is if you have an optional but oh-so-recommend water safety switch, and it happens to trip, it will make the thermostat blank.

A little help diagnosing an air conditioning problem because if you see your thermostat’s blank then you know that you are either missing 24volt power, or you are missing power from the air handler altogether.

Hooking up Common Wire in Digital Hvac Thermostat

First with the power off I connect the optional common wire up to the thermostat. Then I open up the air handler and locate the transformer. On the 24 volt side of the transformer follow the common wire to where it hooks up with the same string of wire that’s going out to you compressor, and this is where I tie the blue wire into. In the picture to the right (click to enlarge) I followed the common wire to the relay control board, and the common wire comes out at the C terminal (brown wire).

Or if you wanted to you can splice the common wire and tie it in where ever you want. Some air handlers have a terminal block similar to the one in your thermostat, if so just hook it up to the ‘C’ terminal. Just make sure that you hook it into the common wire and not the 24volt wire or you WILL short out you transformer.

Keep the discussion going if you have any questions about Hvac thermostat replacement the quickest way to get an answer is to leave a comment below.