Denny - the knob is used to set the temp you want, the capillary control will increase/decrease water flow to keep it close to that without you having to change it as the water heats/cools
"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir "When we tug a single thing on an old truck, we find it falls off" ~ me Some TF series details & TF heater pics
