[gumby79]

I use the defrost as a temperature maintenance assistant. When the motor is cold I run the defrost. This duse 2 things .
1 it puts a load on the motor , because more load means more heat is generated per combustion event. Equals more heat being put into the coolent.
2 the heat productionin the freon from the AC compressor is dissipated directly in front of my intercooler and be hind my grill block witch in turn warms the intake air charge ,witch inturn warms the engine at a slightly faster rate . Allowing me to reach efficient operating temperatures faster. The hard part is to remember to switch the controls over to a non AC compressor setting when warmed up but cab heat is still needed. Forgetting tends to offset any potential.
Disclaimer.
I have an old non-computer diesle. So I can not verifie if this actually saves fule . But it duse take less time and miles to reach 100°f coolent. (The temperature my digital temperature gage starts to register. )

One mor tip I forgot. Get a data loger , ScanGauge, tork app with obd2 dongle. Seeing what a particular habitd use to fule consumption is the best way to retool the nut behind the wheel were we typically see the most gain in FE.

Dreem big chisel down to reality