Two Kettle HERMS Systems

How does a two kettle recirculating system work?  

The two kettle brewing system is a space- and cost-saving alternative to a conventional 3-kettle recirculating system. The combination of HLT and boil kettle in one kettle eliminates the need for the third kettle, the heating element and the additional valves.

In a HERMS system, the wort is recirculated through a heat exchanger in the HLT kettle, which ensures gentle heat transfer without the risk of burning. There is no heating element in the mash kettle, and the low false bottom stand can be used. Very important: you still need to pay attention to the flow rate, so that the grain bed does not compact and your lauter bottom or pump is damaged. However, the HERMS system is more foolproof than the direct fire system.

Remember, you'll need a controller to maintain the temperature of the mash.

Disadvantages of two kettle systems

There are, of course, disadvantages to combining two kettles into one. You can't sparge as efficiently as with a 3-kettle system. At the end of mashing, the hot water is slowly layered onto the heavier, sugary wort in the mash kettle. Once the boil kettle is emptied, you can drain the mash kettle into the boil kettle and begin mashing.

System efficiency and overall batch size will be lower than that of a similiary sized 3 kettle system. You may find that a two kettle system is just not big enough for high gravity brewing or for batches with a full boil kettle.

The Duo system has added design complexity because the pumps' inputs and outputs need to be switched frequently. Changing hoses would be very tedious. You need more valves and fittings to connect evertyhing together and make it more useable.

Finally, you have a HERMS spiral in your boil kettle, which makes cleaning difficult. With a third kettle as HLT, the HERMS spiral would only be immersed in water and would require minimal cleaning. 

Tips for ensuring an adequate flow rate

  1. Mill your grains at around a 1mm mill gap setting. This is fairly standard when ordering milled grains from your local homebrew shop, but it doesn't hurt to ask.
  2. If brewing with large percentages of high protein grains or adjuncts such as wheat, oats, or rye, consider adding rice hulls to improve drainage.
  3. Always turn off the pump when mashing in. Make sure your heating element is also turned off, to prevent overheating.
  4. Stir well when adding the grains to the mash kettle.
  5. Use a 3-5 minute grain rest period before turning the pump and heating element back on.
  6. Close the pump output valve almost all the way and turn the pump back on. Slowly open the pump valve. Doing this too fast can compact the grain bed. Lastly, turn on the heating element
  7. Don't walk away! Stay close and keep an eye on your kettle, the wort can thicken and slow the flow rate during the mashing process. if you ever notice the volume of the mash kettle rising, you are pulling a vacuum under the false bottom. Stop the pump and stir the grain bed.

    Design Considerations

    In a HERMS system, it is recommended to place the temperature sensor for the controller near the return point of the sparge loop, just before the sparge arm. This is the best way to prevent the temperature from overshooting or oscillating.

    The best way to layer the hot water into the mash kettle is to raise the sparge arm while filling. This is easily done with the height adjustable sparge arm. Using a fixed sparge arm is possible, but you must fill very slowly and be careful not to mix the contents of the kettle.

    Used properly, the height adjustable sparge arm not only provides better lateruing, but can also save cost and complexity. You can eliminate the need for a whirlpool arm in the kettle and reduce the additional tubing, fittings, and splitters at the pump outlet side. To fill the boil kettle, simply move the sparge arm from the mash kettle to the boil kettle. The same arm can also be used for whirlpooling with the 400 micron brew bag as a whole kettle filter. This works wonderfully to filter out hops and hot trub, and allows you to fully circulate the kettle contents and completely drain the kettle. You can order a second kettle handle clamp for the Sparge Arm separately in the store.

    There are many ways to connect the kettle outlets to the pump inlet. In the pictures below, you can see that the bottom ports of the kettles are connected to the pump inlet with a 3-way valve to make switching easier. Here we use two 15 cm spools to connect the kettles and a 10 cm spool to place the pump in front of the kettles. For two 115-liter kettles, take two 20-cm and one 15-cm spools instead.

    On the pump outlet side is a tee with sample valve, useful for taking wort samples during mashing, but also for draining the circuit. Planning the right drainage points in hte system will simplify your brew day.

    Don't forget that starting with the 85L size, I recommend switching to 1" fittings and 19mm ID tubing.


    When lautering into the boil kettle and during the whirlpool, you should connect the hose to bypass the HERMS spiral, which has already been cleaned on the inside from the sparge water. Shown here filtering the kettle after boiling:

    If you don't want to use the whole kettle filter method with Brew Bag, you'll have to rethink the design. You'll want to put a 45 degree racking arm in the front of the boil kettle (don't forget the valve). However, that means you lose the temperature measurement point for the kettle, making it harder to get the right sparge water temperature.

    You probably want a dedicated whirlpool arm in the boil kettle. You might also want to add another 3-way valve to the pump outlet to avoid changing hoses when filling the boil kettle.

    Finally, you should use at least one flexible silicone hose to connect the bottom drain of the MLT to the front port of the boil kettle:

    Or like this:

    Shown here with 85L kettles:

    And shown here with 115L kettles: