How to replace a 4 wire thermostat, how to upgrade to a smart thermostat with four wires, and a step-by-step guide for fixing thermostat wiring.

We have a 19-year-old condo with an old Carrier mercury thermostat.We have an AC unit and a natural gas furnace.There are four wires going into the thermostat.The thermostat has wires connecting it to a number of labeled contacts.

We were given a new thermostat and would like to use it.I planned to use the conventional wiring because we don't have a heat pump.I think the jumper was for the old thermostat, and it seems like one wire is missing.

To identify the wires and where they go, we need to use our guide to thermostat wire colors and codes.

Understanding where the wires go can help you.Most thermostat installations require the installer to follow the conventional color code.

I assume this jumper is not used for anything.You won't have to worry about this on your new thermostat.Follow the instructions on the thermostat.That is good news.The not so good news now.With your new thermostat, you're going to need one extra wire, so you didn't mention it.

Your old thermostat was a mechanical thermostat and did not have a common wire.If you want the new thermostat to work, you need a 24 volt common to complete the circuit with the 24volt hot wire to provide power for the thermostat, because it is a state of the art digital wi-fi thermostat.You need a hot wire and a common wire.

If you don't find another wire in the bundle or run all new thermostat wire, you will have the wrong amount of wires for the job.

If you want to learn more and take it that far, you should read this article Running New Thermostat Wire along with the one All About Thermostat wire.Most people call in a professional to get the job done if they don't do it themselves.

This wire goes to the gas controls to engage the heat on a call for heat from the thermostat when you have it set.This closes the control side of the gas furnace and begins the process of starting it into a sequence of operation.Red is the hot wire which powers the relays or circuit boards for the air conditioner, the gas furnace, and the blower.This wire is only used for heat pumps.The reversing valve is inside the heat pump.This wire may be different on heat pumps.

The mercury switch thermostats were mechanical.The newer thermostats are digital.Some systems still have the older mechanical thermostats, which are typically 4-wire thermostats that control the same type of system.

The answer depends on the type of system and thermostat you have.The C-Wire is not required for mechanical mercury switch thermostats.These types of thermostats are no longer available because of the new laws.There is a breakdown of the types of heating and cooling systems they control.

Unless there is a battery backup, digital thermostats need a power source.Here is a breakdown for the type of systems they control.

Some boiler systems have multiple zones.Unless you have a digital thermostat system that can link with wireless sensors, you don't need a thermostat for multiple zones.

The ultimate guide to creating a comfortable, healthy, long lasting, and efficient home is the Home Comfort Book.

You can use a UV light to kill harmful viruses in the air handler.The researchers used the UVC to kill the viruses.It requires a professional installation to turn on and off the blower fan in your air handler, but it will offer the protection you need for you and your family from any viruses.

I own a thermostat with 5 wires.A basic thermostat only has hookups for 4 wires.What do I do with the extra wire?

The metal part should be covered with electrical tape.A small wire nut can be used with electrical tape.

I own a four wire system.I am trying to install a new thermostat.I have white, red, green, and blue wire, but no yellow.The blue wire was connected to the yellow spot.I thought there was a wire.

If the blue wire came from the yellow terminal, it will go on the new stat.

An additional wire is needed for the C wire.You will need to pull new wire if it isn't there.

There isn't a connection in the Y terminal.I found an extra blue wire in the package and connected it to Y and all five to the Sensi thermostat.My compressor isn't kicking on.The heating appliance works well.What should be done?Please help.

The thermostat in my house has 4 wires, but the bundle in the wall has 5 unused wires.The 5th wire is white and connects to the C terminal.The red wire goes from the heating and cooling unit to the outside unit.I'm wondering if I can hook one of the extra wires to the C terminal on the heating and cooling system where the white wire is and then hook the other end to a thermostat.I need a wire for the new thermostat.

You need an extra wire in the bundle for the thermostat.Attach the extra wire to the C terminal in the air handler and the other end of the wire will be used on the new thermostat.

I have wires at the furnace.I have four at the thermostat.There are two wires at the thermostat.Why?

Two greens in a bundle of thermostat wire are not normal.You will need to trace the wires from the source to the air handler.Either that or call a professional to figure it out.If you want to do it yourself, make sure you turn off the power before you start.

A 2 WIRE OUTLET HAS A 4 WIRE THERMOSTAT.I joined the two blacks and two reds before connecting to house power.

There isn't enough information.The uploader link is in the menu on the right.