Thermastat Potometer. Plugs into main printed circuit board. 50K Ohm. For the following Heat Surge models.( X5A, X5B Retrofit* only) X5C, X5C-S, X1. Ymada M5, M6 Retrofit* only, Jinhua X23A Retrofit* only. Winners W1, W5, W9, W10, W1A.