I never understood the need to preheat mashtuns, unless you enter the preheated value ( and grain temp ) in your 'mash' tab to arrive at your strike water temp. brewfun has a post on how to determine your thermal mass. I posted a while back how to do it in promash. Follow the procedure once and you are set...till you get a new mashtun. This method gets you within a degree..forever.
Here's my process and rationale.
1. Have BS2 calculate strike temp without compensating for equipment temp.
2. Heat strike water to strike temp + 10F.
3. Transfer strike water to cooler MLT (48 qt. Cube).
4. Allow MLT to stabilize. Generally about strike + 5F.
5. Add grain. generally about mash temp + 2F.
6. Stir until desired mash temp is achieved.
Before you ask: Yes, I've calculated and measured the thermal mass of my equipment. Yes, I can hit "within 2 degrees" of my desired mash temp using "compensate for equip temp" option.
So, why?
Because of the "within 2 degrees" qualifier. That is a 4 degree variation in the mash temp depending on ambient conditions. If I'm targetting 152, 154 isn't a huge difference. But, if I get 150 on one batch and 154 on another...that IS a noticeable difference.
So, I add enough heat to the process to guarantee that I'm on the high side, and can drop the temp simply by stirring. Then my mash temp is dead on. It takes me about 15 seconds of stirring to drop my mash by 1 degree.
The alternative is to add more hot (boiling) water if I'm low (or decoct). Both alternatives take significantly more than 60 seconds unless I have a second burner of boiling water. Also, both processes have some impact on consistency (mash thickness variation --- not a big deal, melanoidin formation---a noticeable effect).