HVAC Service Calls and Thermostats
A very common occurrence with service calls is the complaint that the thermostat isn’t working properly. Sometimes this is true, but often it is something entirely different than the thermostat. Because some people believe the only problem is with the thermostat. Finally, they go down to the local hardware store and purchase a brand new thermostat.
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.
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.
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.
