A HERMS is a heat exchanging system where you pump your mash wort through something that's in a warm water bath. The idea is you can heat the wort, thereby the mash, either periodically or continuously throughout the mash. There are many ways of accomplishing this simple task, but I'm just covering my system and the thought process that went into it.

Unfortunately, I had to work within some restrictions. I'm not much of a DIY guy, so I have limited abilities and confidence when it comes to wiring an plumbing work, but I'm creative, so hense the off-the-shelf design. Second, my garage and even a part of my house is on the same circuit. My breaker box is full, so running a dedicated line for big amps loads wasn't possible. I figured I had enough to run 1500 WATTs with enough left over for lights, a radio, and my pump.

My primary goals were:

1. Make it easier to ensure I get my desired mash temp every time
2. Maintain the desired mash temperature for the duration of the mash
3. Get more even temps across the entire mash, and get a good reading of the mash, not just the area around the thermometer
4. Streamline the process by setting the grainbed with a pump

First, I decided the best approach was to recirculate the mash wort continuously. By constant recirculation, I should be able to obtain a good temperature reading from the mash. Also, by recirculating it should be easier to get a consistent temperature throughout the mash. These items are not critical for making great beer, but I believe they are important for consistency between batches. This then makes it easier to consistently make great beer. An added benefit of recirculating with the pump, is it makes it easy to get very clear wort from the mash, and since you're recirculating during the mash, as soon as the mashing period ends, you're ready to sparge.

So now that I decided to recirculate continuously, this sets me up for some more things to consider. First is the mash-tun design. The most practical way to recirculate continuously is through a false bottom. The faulse bottom evenly supports the entire surface area of the mash, which greatly helps to maintain even mash permeability. Using some other method will likely promote greater channeling of the wort and worst yet, compaction of the grain bed and then a suck mash.

The other factor to consider is the mash return. There are many ways to sparge or recirculate water/wort on top of the mash. I took a lead from Sabco and just used a simple hose. This hose coils along the side of the tun on top of the grain bed and below the wort surface. When water or wort comes out, it creates a gentile whirlpool on top of the grain bed. This does a two important things, one limits mash aeration, and two creates an even sparge/wort distribution over the mash, which restricts channeling. This is just stupid simple and works great.

So now we have a way to get even mash temps throughout the mash, and a vessel capable handling such a function, but now we have a new problem. By constantly removing the mash wort from the MLT, there will be an inevitable temperature loss, which brings us back to the HERMS. In my system, the HERMS primary function is temperature control. It's not very good at heating the mash, but it's great at maintaining the mash temperature. This is really a key of most HERMS or RIMS systems, is it's not terribly efficient at raising mash temperatures.

So, now we have a system where it's possible to get accurate and even mash temperatures, while maintaining those temperatures, even in an uninsulated tun. Now is the big decision on which way I wanted to go. Did I want to put my heat exchanger in my HLT or a dedicated HERMS vessle? Putting it in the HLT is a popular approach, and to fully automate, I would need to plumb together a couple solenoid valves to direct the pump's flow. The valves work together to go through the HERMS or bypass the HERMS and recirculate directly to the mash. Since the HLT will be much warmer than the mash temp, you will only want to run the HERMS circuit to boost up temps. One big advantage here is it makes step mashes easier. The disadvantages as I saw them are I'd have to regulate my HLT temps to maintain whatever temp I wanted for the entire mash. While I could get a gas solenoid for that, I wasn't thrilled with leaving gas to regulate itself unattended, particularly if I built it. So, since I skipped the gas option, electric wouldn't work either. Not enough power for an uninsulated vessel. Plus, I figured these heating elements aren't food-grade, and while good enough for farm animals, not sure I want to drink from it after it started rusting. Also I needed to devise a way to agitate the HERMS water, and the simplest options also pose some food-grade risks. So, I scraped the HLT idea and looked towards making a stand-alone HERMS. In the end, I just opted to go the simple route and skip the solenoid valves and extra electronics to keep it simple since I thought it would fit my top priorities.

HERMS Design

I have no interest in wiring something up and tossing it into water. Water + electricity = bad has been burned deeply into my head. Combined with my garage's wiring, I can't even run a big element even if I want to. I found a 1000 WATT bucket heater at Tractor Supply Company for about $35. It's submersible and has a built-in guard on the element. This was great because I wouldn't have to wire anything myself. In an insulated container, the heater can warm about 4 gallons of water 1.5-2.0 degrees per minute, which isn't too bad.

For my exchanger, I had an old 25-foot 3/8 immersion chiller. I'm glad I kept that round! Copper prices have gone through the roof. I think when I originally bought this it was like $25, but now will probably run you closer to $45. You might be able to save some cash if you can find a coil cheaper at the hardware store or scrap yard. My only complaint is the ID of the tubing is pretty small and once in a while will clog on me if I got a gummy mash. Someday I may consider getting a 1/2 ID coil for greater flow and heat exchange.

With the heart of the HERMS out of the way, I looked for a container. My old mash-tun was a well insulated 10 gallon round cooler, which worked perfectly. I drilled some holes in the lid for the in/out of the coil and power cables.

For the brains of the HERMS I needed a temperature controller. I considered wiring one together, but because I'm only using 1000 WATTs, I could just use a standard Ranco controller. This controller pulls double duty for fermentation control, so I was a freebee for me, but I originally got it for $80 at a homebrew store.

The final piece of the HERMS is something to agitate the HERMS water. Without agitation, the element will really only heat the water immediately near it, and the water nearest the coil will quickly cool down. Adding this made a huge difference in the system, so good water agitation is really important. I considered a number of options, but on a whim I bought a small submersible pond pump from Harbor Freight. It was $15 and fit well in the cooler. I just stick it in there and let it jet the water along the side of the cooler. This creates a modest whirlpool and works very well. I was concerned about how well it will work in hot water, but so far, no problems. I may pick up a spare just in case it dies because this seems to be my permanent solution. Obiously you don't want to use this in your HLT with your sparge water!

System Control

There are different ways of running a HERMS. The control point greatly depends on what you're doing. If you have your coil in the HLT, you will want it at your bypass to read the wort temp as it goes through. If you're controlling your pump, you need it in the mash to detect when it needs to fire.

I didn't like the idea of controlling from the mash because there's a significant lag between what mash reads an applied heat. It seems intuitive to control from the actual mash temp, but if you do you'll have increased swings in temps. For example, when the mash drops from 152 from 153 it will probably continue to drop while you'll start to apply heat. The mash wort might be significantly warmer than the mash, so by time the mash reads 153 and the heat stops, the mash will continue to rise as all that energy equalizes.

I didn't like the idea of using the pump as my control point because it didn't fit my goals of temp consistency, also I found with my MLT things just go better if it's always recirculating. If I stop the pump and restart, the grainbed might not be set and I run the risk of pumping a glob of grain to get stuck in the copper coil.

An issue with either method is I felt ideally I didn't want the wort to heat up beyond my mash temp. So, for example, often a HERMS will be heated to 170-180, the wort is sent through it, then reintroduced to the mash. If I want the mash to be say 153, when I sent it through the HERMS it may be heated to say 165. So, the only way to have a hot HERMS and recirculate continously is to rig a bypass as already described or else the mash would quickly overheat. A bypass could be automated with solenoid valves or done manually, by sending the recirculation either through the HERMS or directly to the mash. I wasn't about to go this far to have a manual system, so that meant solenoids. This was my backup plain if my "simple" system didn't work.

So, as a result, I took a maintenance approach. My idea was I would maintain the mash temp by constantly providing it with wort at that temp. The idea is if the tun begins losing heat, I could increase the flow of the pump, and so long as my heat element could maintain the heat exchange, this could be accomplished.

The next decision was where to place the control point. Initially I had it in the HERMS itself, in the picture above of the HERMS insides, you can see a corny keg dip tube I used as a thermowell. I still use this, but only to heat the HERMS up to my desired temperature for startup. How this worked was I would essentially guess how hot to make the HERMS. The only problem is that temperature was dependent on my degree of thermal loss, or essentially the weather. This took some tinkering to maintain, and instead decided to put the control point where it matters most, as it returns to the tun. This way, the controller would simply turn the heating element on to get my desired result. So, for example, if I want a 153 mash, I simply set the controller to 153, the wort is pumped through the HERMS and past the thermowell-probe. If the temp is 152, it turns the element on until it returns 153. Piece of cake!

The final piece of the puzzle is the mash thermometer. The mash thermometer simply tells me what the mash temp is. Looking at the mash thermometer, I can instantly verify that everything is working as intended. Usually I just check in on it every 15 minutes as I walk by doing other things.

Observing the mash temp closely, it seems things stay pretty level between 1-degree below my set temp and maybe 1/2-degree above. That's probably about as good as it gets, considering.

Lastly, I greatly recommend fixing a dial thermometer in the mash-tun. Being in a fixed location I think helps give consistent performance from batch to batch. The dial thermos though are durable and when calibrated against a lab thermometer, very accurate. I've had nothing but trouble from digital thermometers, where my dial thermos have been rock solid.

Limitations

The biggest limitation is the heating element. I've yet to use this in the deep winter, but it does work in the 50's and above just fine. Unless I'm doing a step mash, I only use the burner to heat the strike water. In this case, I could easily wrap the tun in a blanket if things get rough in cold weather. Or I could just heat the garage. ;)

Another limitation is this doesn't work well for step mashing. This is really geared towards temperature maintenance. If you need to increase the temp of the mash significantly, it will take a long time to reach your next step with only the 1000 WATT element. I've experimented with using the burner while recirculating, but it's really tough to get the HERMS to work in concert. After the step, you just can't get both the mash where you want it to be and the HERMS heated enough to run temperature control. You would have to tinker with the burner to keep boosting the mash to maintain until the HERMS caught up. The alternative is to bypass the HERMS for the cooler steps, but that pretty much negates much of what I've accomplished since there will be no automated temperature control on the lower steps. I'm toying with the idea of bringing in a second power line for a second heat stick that I could manually turn on during the ramping period.

Another issue is the possibility of channeling in the mash. This is an issue with consistent recirculation, particularly if you open up the pump wide. Generally though I don't seem to have any problems. I consistently get around 80% efficiency with single infusion mashes. If I were experiencing significant channeling, I assume that number would be lower.