i dont think he really means another 'tier', but just a seperate HERMS pot somewhere (not the HLT).
what size batches are you doing? 5 gallons is probably going to be too big (er- atleast just
unnecessarily large, which has some downsides)... i do 5 and 10 gallon batches and use a 1 gallon HERMS pot and 2kW heater in it. and im building another one for larger batches that is in a 2 gallon pot and has a 4kW heating coil in it.
the reason for keeping it small (the same reason i dont like using the HLT as a heat source) is because you can more precisely control the temperature changes; specifically how quickly you see a change in measured temperature from the time you turn the heat on. that is the benefit of a RIMS where the element is directly heating the wort; there is no lag between when you turn the heat on and the time the wort starts heating. the more 'distance' between the heater and what you are heating, the slower your steps will be. if you arent stepping, but just maintaining temperature, the point is kind of moot.
you can sort of compensate for this 'distance' by adding more power, but obviously it gets difficult quickly. with 1 gallon and 2kw of heat; thats a ratio of 2000w per gallon. to get the same speed out of a 5 gallon HERMS, you would need 10kW of power... and if you used a 10 or 15 gallon HLT as the source... 20 or 30kW- thats not really possible.
Correct me if I'm wrong, but if I can't run the pump dry, doesn't quite a bit of it stay in the line between my MLT and BK?
its not that you cant run the pump dry, its that as soon as air gets in the pump it will stop pumping. that is what stops you from getting it all out. and yes, this is one of the few reasons its not a good idea to use excessively long HERMS coils, like 20 30 or 50 feet (ive even seen longer!). i have 8 feet of 3/8" copper in my small HERMS, and will have 10 feet of 1/2" stainless tubing in the new one. its constantly recirculating so it doesnt need to be longer. the liquid will spend almost the exact same total amount of time in the coil; it will just make more passes thru it.
you can run clean water thru the system at the end, and just stop it before it empties into your boil kettle. that way you can get more sugar out.
edit: +1000 words...
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old one: