What size thermostat wire do most manufacturers recommend?

Most manufacturers recommend using 18 AWG thermostat wire for HVAC systems, including heat pumps. This wire gauge strikes a balance between adequate conductivity and flexibility, which is essential for low voltage applications such as thermostat connections. 18 AWG is capable of handling the low voltage signals (typically 24 volts) used to control heating and cooling equipment without significant voltage drop over reasonable distances.

Using a wire that is too thick, like 14 AWG, may not provide any additional benefit for such low-voltage applications and could be less flexible and harder to work with in tight spaces. On the other hand, using a wire gauge that is too thin, like 20 AWG, could result in voltage drop issues that may affect the performance of the thermostat and connected systems, potentially leading to unreliable operation or failure to control the HVAC system effectively. Therefore, 18 AWG is the ideal choice for ensuring both performance and ease of installation in most cases.