What is the ideal HERMS coil length? Theory and experiments


I read through this and the engineer in me wanted to investigate some of the simplifications you made so I set out to optimize the HERMs coil analytically.

Over here most people use an AC pump produced by march or chuggar and because the “optimum” length is so dependent on the pump curve I wanted to be able to produce a family of curves for any pump and tubing set up. The example below is for the March 815-SS pump (one of the most common brewing pumps in the US) and assumes you have two 4ft lengths of 1/2" dia silicone hose with 90 elbows and cam lock fittings, a ball valve that is all the way open on your pump, 1/2" OD stainless steel tubing for the coil, and that you can maintain constant temp in your mash tun (i.e. you can provide the same heat you are extracting).

One thing the article neglected is that you don’t necessarily want higher HERMs outlet temps. Heat transfer is maximized when the temperature difference between the two materials is large. What that means is that the closer your wort temp in the HERMs coil gets to the HLT temp the less heat transfer into that fluid you get for the same area of heat exchanger. Additionally they neglect the fact that you don’t want the wort to get above 168F at all or you will start denaturing all of your amylases. On that note, you really want to do all you can to decrease the heat load on those amylases; therefore, you really want to have as small of a HERMs outlet temp differential as possible with a high flow rate.

The first run I just spit out the power vs. coil length but neglected the outlet temp and you get the graph below, which just says you want as big of a temperature difference between your mash temp and the HLT temp with a coil length of about 20ft.

The second run I killed each curve when the outlet temp increased above 165F (74C) which is why you see that strange feather shape on the left. The straight line back to zero is just a bug in how I plotted it, the curve should terminate at the top. What this says is that your maximum “enzyme safe” temp delta is 9C and at that delta T you want about 18ft of coil (yields a flow rate of ~3.5gpm). You can technically get a higher power system with the same pump if you have a very short coil but because that slope is so steep you run the risk of easily mis-estimating the head loss and not getting much power transfer at all or cooking your wort. To me, the risk is not worth the marginal power increase from the 9C delta set up. Also, that would need a very high flow rate and I think that would probably just result in a ton of grain bed settling, restricting your flow and then cooking your wort.


Lessons learned:
-You don’t need the expensive 50+ foot coils everyone is trying to sell you
-Next time I see an article about brewing that relates to engineering I should probably just not read it for my own sake


Awesome, looks like you found similar results as we did.
A longer coil just increases resistance after a certain point and the optimum is around 25 ft/7.5m.

A recent modification that we made to our coils is to make sure that there is some room between the windings. When the windings are pressed against each other, the kettle water doesn’t flow freely around the coil and this hurts the efficiency.

Another important point is to agitate the water in the HLT. If it is still, the water around the coil just cools down and isn’t replaced by hot water fast enough for an efficient temperature exchange. A whirlpool in the HLT increases efficiency.



I have the old 40cm HERMS coil and I’d like to upgrade to the new High Flow coil.

The 40cm has a 16 cm spacing and the new coil has a 10 cm spacing.

Do you have any suggestions for sealing the spare hold left by the old coil?



Something like this maybe in combination with our sealing locknut?

SS brewtech also has a hole plug and so does blichmann. The SS brewtech plug is 17mm, so it is probably too small.

You could also stretch the new coil to the old height, but this has the downside of having to put more water in your HLT to fully submerge it, which will result in having to do smaller and slower temperature swings in the HLT.

Maybe you could also install a float switch in the hole.



I might just put a separate return port with a tap on it.


Great work :slightly_smiling_face:

However I think you didn’t mention what do you mean with bigger kettles. What is the size that you would recommend the 3- way paralel flow?


When you go over 40-50 liters, I would recommend the high flow version.

Consider these assumptions:

  • Your HLT is 5 degrees hotter than your mash tun
  • Your pump rate is 8 liters per minute.

You are moving: 5C * 8L/60s * 4200 J/(L*C) = 2800W from HLT to MT.
(Temp difference * flow rate * heat capacity of water)

Moving 8 liters is already optimistic through a long HERMS coil, grain bed, tubing and valves.

So you can have a very big element in your HLT, but if you cannot move the heat due to low flow, you cannot heat up for a mash quickly.

A powerful pump and reducing friction as much as you can will get your flow rate up. The friction of the high flow coil is about 3x lower.

About 3000W is good enough for a 20L brew. For a 40L brew, it is already on the slow side.


Thank you for the answer @Elco .

However, I am not entirely convinced with why 3000W is not good enough for a bigger brew.

For a final batch of 57 L (15 gallons), we typically use around 14 Kg of malt grain with let’s say a water-to-grist ratio of 3 L/Kg. Assuming a heat capacity for the malt being 0.4 * Water Heat Capacity. We have

\delta E = 14 Kg * 3 L/kg * 4200 J(L* C) + 14 * 4200 * 0.4 = 200 000 J

This means we require 200 000 J of energy to heat our mash for 1 ºC. With a power of 3000 W we have

\delta t = 200 000 J / 3000 W = 66 s.

This means that we take 1 minute to increase the temperature of the mash of 1 C.

For a single rest infusion, this seems perfectly fine since we do not have losses of about 1 C per minute, especially with a moderately insulated mash kettle.

For a multiple rest infusion we might want to increase the temperature of the mash for 10 C which would take 10 minutes.

To conclude with, in my opinion for a single rest infusion the small coil should be enough for a 50 L batch (final volume) but for a single mash infusion might not be the best. Would you agree with this conclusion or there is something I should also worry about?


I agree that 1 minute per degree is acceptable. But if you don’t want to overshoot, you will have to lower the temperature difference between HLT and mash tun when you are close the setpoint. Or you need to stop recirculating.

With higher flow, you can use a lower temperature difference for the same heat exchange.
Another way to limit overshoot is to make your HLT a lot smaller than the mash tun. Because the 3-way coil has 3 coils at the same height, you can use less water in the HLT (while mashing).

Also take into account that you have temperature losses in the system.
Once I have my setup back up and running (we moved and I am rebuilding the brewery), I’ll come back with more data.


You can dramatically shorten the tube needed per cal transferred by really stirring the water in the HLT. The more movement of the water over the coils the more heat that is transferred. In our system it almost doubled heat transfer


This is great I’ve been collecting parts for a HERMS setup for a while now and actually learning about heat exchange in my thermo class. This is more fun than text book problems!