If your unit has a black and red thermistor (newer model) this is not the correct part, Click on the link above or search THRM S103 for the correct set of thermistors.
If your unit has double red plugs, red on the inlet and on the outlet, these are for older units and unfortunately are no longer being manufactured.
Description: Thermistors w/ O-ring
Model #: THRM QC MEDLRG
Compatible With: models 18kW, 24kW & 27kW (Residential units)