[T0ASTERvan]

I believe (from our experience owning the 2015 TC) is that there is a power draw when turning the fan up to high as you've indicated.. this is common place for us. Unless theres a clunking type sound.. or a significant power drain as to actually stop the engine from running or if you see a leak on the passenger side floor board of the van.. I wouldn't be concerned.

As with any engine that has a heater core setup.. the heater system draws it's heat from the antifreeze etc.. So the system kicks in when you've turned the fan to it's highest position. thus forcing the engine to work harder in order to force the coolant into and through the lines to the heater core located at or near the van's firewall.