They get home and dust off the toolbox. Never reading any thermostat installation directions, they proceed to change the thermostat. Some are successful at changing the thermostat and some are not. Those that are most likely read some thermostat installation directions or had someone read the thermostats installation directions for them. Finally, the ones that are not successful end up calling a professional in to install a new thermostat.

Successful Thermostat Installation Advice Article (opens in a new window)

Thermostat Installation Advice

The bottom line advice to most people is to call an HVAC professional if something is wrong. Whether it is with your thermostat or HVAC system. The problem may not be with the thermostat. And you may exasperate the problem which will cost more in the long run. Additionally, If you have a multi-zone system, a high-efficiency heat pump or even just a heat pump, a regular split-system AC and a boiler for heat (and you have one thermostat), or an Apollo-based system (hot water heated in a water heater) with a split AC system……. call a professional.

These systems can be very complex and may require special sub-bases so the thermostat control circuit will work properly. Additionally, for heat pumps, there are different controls, and wires for these controls run into the thermostat, they are multi-colored thermostat wires. These controls can be very complex. Finally, each wire must go to the correct terminal on the thermostat or the unit will not run correctly.

Questions You Need To Answer Before Changing the Thermostat

What do the thermostat wire colors mean? (See our thermostat wiring color page for help) How many wires do I need for a thermost? This depends on the type of HVAC System you have. Typically, a heat pump will require more wires than any other HVAC system. How do you hook up a thermostat? See our detailed instructions below. What does the old thermostat wiring mean? These are the wires you will use to wire the new thermostat. Take a photo with your smartphone while they are still hooked to the old thermostat. That will help with wiring the new thermostat. What wires are for heat on the thermostat? What wires are for cooling on the thermostat? For detailed wiring explanations, see our thermostat wiring colors chart or page.

Variations with Thermostat Installations

There are so many different variations of this High Performance HVAC will not attempt to describe them here. We will describe the common type (at least for this region (mid-Atlantic) and if you see that you have the system described you can proceed cautiously at your own risk. Furthermore, here is a list of possible thermostat wiring combinations beginning with the most common first:

5-Wire Thermostat Wiring

This is the most common and covers many central air conditioners with an air handler or gas furnace. Finally, the 5 wires likely cover Red for 24-volt hot, white for heat, yellow for cooling, green for the fan, and blue for common (common could be another color).

4-Wire Thermostat Wiring

This is typically for a thermostat that is battery powered or a heat only digital thermostat. It covers the same control or color features as the 5-Wire thermostat wiring above except for heat only. Furthermore, with heat only, you likely will have a 24-volt hot and common (red and blue), a white wire for heat and a green wire for a fan.

3-Wire Thermostat Wiring

This is common for a digital thermostat that controls a boiler. Finally, it has 24-volt hot and 24-volt common along with the wire for the heat which is likely white.

2-Wire Thermostat Wiring

This likely a heat only thermostat that is either digital or non-digital. If it is digital it has battery power to power the thermostat. The two wires likely are red for 24-volt hot and white for the heat.

If you have more than five wires then you have more control points or you have a heat pump. Refer to our pages about heat pump wiring on how to wire a heat pump thermostat or our thermostat wiring color page to see the additional control points for your thermostat.

Thermostat Wiring Color Code Table (opens in a new window)

Additionally, be aware that thermostats are equipped with heating and cooling anticipators. Cooling anticipators are not adjustable where heat anticipators are adjustable in mechanical thermostats. Setting the heat anticipator is important for your heating system. And for the thermostat to function properly (only if you have a mechanical non-digital thermostat).

It is set according to the amp draw on the control heating circuit. Additionally, make sure the heat anticipator is set properly. It is set in the thermostat or thermostats. Proper settings ensure you will get the best out of your heating system and your thermostats.

How to Wire a Thermostat - Selecting the Proper Tstat

AC split or package system with gas, oil, or electric heat

The first thing you should do before changing the thermostat is to select the proper thermostat for your system. Selecting the proper thermostat.

If you are retired or if someone is at home during the day you likely do not need a programmable thermostat. If this is the case the only benefit you will get from a programmable thermostat or thermostats is at night. Or I prefer to say in the morning because you can set it so the unit comes on just before you wake.

That way the house is at a comfortable temperature when you get out of bed. I will describe the advantages of the programmable thermostat in full detail in a different section below. Finally, once you have made the thermostat selection you can proceed to the next step.

Before You Begin Replacing the Old Thermostat

Helpful Page to help you select the right thermostat (opens in a new window)

Honeywell Thermostats Available Here

Thermostat Wiring and Installation Advisory……….If you decide to install your own thermostat you do so at your own risk. There are many incidences where the homeowner installed their own thermostat and was successful. There are also many incidences where the homeowner was unsuccessful installing a new thermostat. The people that were unsuccessful wasted part of their day, ruined a thermostat or two and caused the malfunction of an integral part of their system.

That is not mentioning the fact that they ended up calling a professional HVAC Technician to fix the problems. Problems caused by improperly installing a thermostat. The unsuccessful end up paying three to four times what they would have paid if a professional was called first. Finally, factor this into your decision and if there is any doubt call a professional. Call a professional to install and wire the new thermostat.

Before you begin to install your new thermostat turn the power off. For both the condensing unit and the air handler/furnace/or boiler.

Thermostat Installation Tools & Steps