First steps with HERMS

Hi all,

only recently, about one month ago, I finished my HERMS setup and did my first batches with it. I was surprised how my feeling for thermodynamics and heat exchange were actually wrong. My setup consists of 3 vessels and two pumps. Mash tun capacity is 50l and I am aking batches of about 45l. I have temperature sensors in many places, mash tun, HLT, HERMS out, the inlet of the mash tun from the HERMS, sparging inlet and so on. In this post, I would like to share my experiences and also to ask some questions which I believe are interesting for other brewers:

  1. First, I couldn’t believe that heat exchange through the HERMS coil (I use 3-way version) works in that way that if I am stepping to 70 degrees, my how water is at about 76-77 degrees and that there is 6-7 degrees temperature difference of the wort coming out of the HEMS coil and the how water around the coil. I thought there should be plenty of time and area to actually exchange the heat. I am not disappointed, jut excited about my new discovery :smiley:.

  2. If I set my PID to let’s say 68 degrees, the hot wort from HERMS coil entering the mash tun will indeed always be 68 degrees, but the mash itself is always lower. This was the biggest surprise to me because the mash doesn’t reach the set point in some acceptable time frame. My pump is set almost at the maximum, which means that in only 3-4 minutes the whole liquid volume is pushed through the coil, so I was expecting after 10 minutes to have the main mash at the set temperature. Do you also experience this? Do you use a sensor inside the mash for controlling your PID or the output of the HEMRS coil? I use the output of the coil. If so, how is possible that the grans have such thermal capacity? And finally, if I am pushing the wort through the coil where it reaches the desired temperature, that could also mean that the mash conversion is actually happening there. So, what should be the reference point in this case?

  3. Brewing German type wheat beer was really stressful and basically impossible due to the stuck mash. I did all rests, at 38, 45, 51, 55, etc … but at some point especially about 60 degrees the mash bed becomes hard and impenetrable.

What do you think?

Hello Nemanja,

  1. I use the Mash_temp out sensor to drive the temp in the HLT. The sensor is mounted just after the ball valve at the bottom of the mash tun. That means that the temp of the wort that leaves the coil is higher than the temp in the MT so that will heat the mash. It takes some time to heat the MT. You use the three-way coil so that is more efficient than my one-way coil. In future, I will have a small(er) HLT with a high heating capacity and less water and a three-way coil because I think that this is the most efficient way to heat the MT.
  2. The pump speed I use is not at max. I am afraid that the pressure this causes compact the grain bed too much which could result in a stuck mash. I have never had a stuck mash, so far. Never used rice shells.
1 Like

I am getting a much lower difference. Are you recirculating/stirring your HLT so that the water around the coil gets refreshed continuously? This can make a big difference. I have a whirlpool arm in my HLT for this reason.

Our HERMS wizard sets up 3 PIDs:

  • One for the BK element, based on the BK temperature.
  • One for the HLT element, based on the HLT temperature.
  • One to dynamically change the HLT setpoint, based on the mash temperature.

The last PID is the magic block for faster rise times. The PID gain should be based on the volumes of the HLT and mash tun. Less volume in the HLT is prefered, so you can use a higher HLT temp without overshooting.


  • Mash volume equals HLT volume
  • Mash temp is 60, should be 65
  • HLT temp will be set to 70

Because the volumes are equal, both kettles end up at 70 eventually (assuming no losses).
The HLT setting is continuously adjusted, so when the MT reaches 63, the HLT setting will be reduced to 67. This ensures the fastest possible rise without overshooting.

Try running the HERMS wizard to set up to control scheme like this and ensure your HLT is stirred. I have a much lower difference between HLT and mash in my system…

I am getting a much lower difference. Are you recirculating/stirring your HLT so that the water around the coil gets refreshed continuously? This can make a big difference. I have a whirlpool arm in my HLT for this reason.

Yes, I do have a separate pump that recirculates water in HLT. Although I do not have a whirlpool arm but a kettle inlet and a hose.

I believe that Spark has a much more sophisticated control algorithm. I am currently using Craftbeerpi3 - which means only one PID, where I am controlling the temperature of the wort coming out of the HERMS coil.

The question for you all: Which temperature is your setpoint for the mash: is that inside the mash tun, out of the mash tun, out of the HERMS coil or something else?

One of the things in my system is that it is designed exactly as it should not be designed (as explained in the optimal coil length article) - I need at least 35-36 liters of water to cover my HERMS coil, while my mash volume is about the same volume.

Once again, in my case, I set the temperature to let say 68. The control works indeed well, and it really holds that temperature. Am I wrong to expect that after 3 - 5 (if we take exponential capacitor loading analogy) turns of the liquid part of the mash through the HERMS coil the whole mash must be very very close to the temperature of the wort coming out of the HERMS coil?

If I am wrong, the only possible solution that I see is to set the temperature to about 2 degrees higher and as soon as that temperature (out of the coil) is reached, I set the temperature I want and the innertia will help to reach it in the whole mash.

Due to temperature losses in the mash tun, especially if it is not insulated, there can be a 4 degree difference between the top and bottom of the mash. The output temp of the HLT shoud be adjust for this.
With a pump speed of 10 liters per minute, it takes 3,5 minutes for the mash liquid to go from top to bottom.

I think a sensor halfway would be most accurate, or better yet: in the middle of your grain layer.
But you can also measure your expected offset and adjust.

I actually do have a sensor in the middle of the mash tun and the mash tun is heavily insulated. Why I started this tread is because the temperature in the grain bed doesn’t reach HERMS coil out temperature in lets say 10 to 15 minutes counting from the moment when HERMS out temperature reached its set value.
What kind of dynamics do you observe on your system - does your grain bed temperature eventually reaches HERMS out temperature? If you set 68 degrees for example - does your HERMS out goes above that temperature or not in order to reach 68 in the mash bed?

No, there is a small difference. The integrator of the PID that sets the HLT will automatically adjust so that the HLT ends 1-2 degrees higher to account for losses.

Here is a typical mash step in my setup:

This was a Weizen using the Hermann method. The graph shows the step from 45 to 67.
You can see that the HLT is driven much higher initially for a fast approach (limit set at MT setting + 10C) and later the HLT and MT setpoint approach each other.

Open the image in a new tab for full size.

This is with a the TD5 PWM pump that we sell, our high flow HERMS coil and an SS Brewtech insulated mash tun. Brew volume is 40 liters, HLT element is 9000W.
You can see that the HLT and mash tun rise at the same rate, the element is running at full power most of the time.