How much temperature regulation is needed with large MLTs?

Forgive the massively Noob-ish question, but it seems to me that with a 25 gallon MLT, even if you're only making about a 15 gallon batch, I most likely won't need a RIMS or HERMS system unless I feel like getting fancy, right? That much water must have enough heat retention potential to only drop about a degree or two over an hour or 90 minute mash. Is that true or is HERMS seriously the ticket?

And on that note, I'm trying to upgrade my system to 15 gallon batches. I want a 25 gallon boil kettle because that way I can have a starting volume of 18 gallons or so and plenty of headspace to prevent boilovers. Would everyone recommend having a Mash-Tun and HLT of that size as well, or could I save some money and go with a 20 gallon pot for those two?

For any mash, the trick is to reach your mash temperature as fast as possible without over shooting and without stratifying the temperature in your mash. Apply heat to the bottom and your mash will be hotter at the bottom than the top & try to stir in hotter water from the top causes stratification in the opposite direction - your mash will be hotter at the top. And the difference can easily be 10 degrees F. The only really effective way to do a large mash is with some type of liquid recirculation system - a RIMS or HERMS system.

https://www.homebrewtalk.com/wiki/index.php/Recirculating_mash_systems

Good info, thanks! In my 10-gallon cooler I was very good at just mashing in the strike water and then it would never lose more than a degree or two in 90 minutes. I was hoping that with a larger system I could do a similar thing, but it makes sense to has the herms system. Guess I'll need to get another burner when I build my brew stand.

Any thoughts on the size of pots I'll need? I think I've got a handle on it but I like to double check.

My favorite HERMS pots are 14.5 gallon converted SS draft beer kegs but one of these would be too small for you. I suggest looking at Polar Ware - they manufacture a 100 quart (25 gallon) pot that would work great for 15 gallon batches. The only problem with their 25 gallon pot is it has no fittings installed but I suspect you will want to install them where you want to anyways. It's a nice wide pot that will help conserve gas.

And BTW I still mash in a 10 gallon round cooler. I'm trying to find time to put together a RIMS tube because I found that even in a cooler the temperature can stratify unless I do a very thorough job of mixing my mash when I adjust the temperature with hot water. I finally have all of the parts together including a MARCH pump. When it's done I'll post a link from www.instructables.com to this site.

Were you thinking that pot for the HLT or the MLT or both? Although I guess just getting three of them would make sense. I might be shooting for a taller, skinner boil kettle though to keep boil-off in check a bit. I really like the 90-minute boils but I don't like loosing 3 gallons!

I was thinking of the 100 Qt pot for both and I had not considered boil-off. Polar Ware's 100 Qt brew pot is 21" wide X 21" tall. Blichmann's 30 gallon is 19.7" wide X 23.6" tall, so the boil-off won't be much different. Blichmann makes great stuff - I just think you can get better value elsewhere.

I'm definitely looking to save money on this so building my own is part of the plan. Especially since drilling some holes and attaching weldless fittings is just about the easiest work you could ever ask to do. Still haven't decided if I want to be cheap enough to go with aluminium or go fancy stainless steel or not.

Aluminum is deceptive - you can buy a cheaper aluminum pot but the aluminum will not be near as strong as the same gauge stainless steel pot. And an aluminum pot that's as durable as Polar Ware's or Blichmann's stainless steel brew pots will cost you more. I guess it all depends on how much brewing you plan to do. I personally have no issue with aluminum - I just don't want to buy anything twice.

I know, and that's where I'm stuck. Both options have pros and cons and neither one is a clear cut winner in my book. I know a 1mm stainless pot will be a bit stronger than a 4mm aluminum pot, but the aluminum pot is literally half price, and in reality, how much of a beating is a brewing pot really going to take? I'm not going to be throwing it around and pounding on it with a hammer, so hopefully denting it is a complete non-issue.

The one that I WILL be going SS on is the boil kettle because I know aluminum and copper are oppositely charged ions (Al+ and Cu-) so I could see my wort chiller actually causing some issues and causing precipitates that may impact flavor when direct contact is made.

Do you think I can get away with a 20 gallon MLT and HLT for 15 gallons batches that won't restrict my OG capabilities too far? I'm sold on the 25 gallon BK, but trying to save money wherever I can.

Stick with a stainless steel water heater element and you can get Cu (copper) out of the picture, stick with a SS brew pot and now you have a all stainless brew pot.

If I were going to design my own recurculating system your size it would be RIMS with a RIMS tube and a 20 gallon Gott cooler - I'm actually going to do a scaled down RIMS with a stainless steel RIMS tube and my existing 10 gallon cooler. My HLT and boil kettle will both be a Polar Ware 321BP 32 Quart Stainless Steel Brewpots like this one I already converted.
http://www.instructables.com/id/Installing-a-water-tank-heating-element-in-a-Polar/

Mine will be gravity feed HLT > Mash Tun then Mash Tun > Boil Pot. My plan is to bring my HLT close, dump the water into the grain for dough-in then make the final temperature adjustments & maintain with the RIMS tube. Then I'll refill the HLT and heat for my sparge. This should also make step mashing easy - I do some step mashes now but they are a royal PIA with the "add more hot water" method. I know some raise their mash temperature with a heat stick or hand held charcoal starter but I don't trust either with a plastic cooler.

But I understand HERMS and they really work great. I just can't justify the cost of putting one together for the size brews I do.