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A HERMS (Heat Exchange Recirculating Mash System) is one of the most capable all-grain brewing setups available to homebrewers, it uses a heat exchanger coil submerged in the hot liquor tank to heat recirculating wort back to mash temperature without the risk of scorching that direct-fire RIMS systems carry. I built an electric HERMS after outgrowing my single-vessel BIAB setup, and the precision and repeatability it provides makes every brew day more consistent. The build is achievable with intermediate DIY skills, a welder or an angle grinder, and a few weekends of work.
System overview and components
A HERMS system uses three vessels: Hot Liquor Tank (HLT), Mash Tun (MLT), and Boil Kettle (BK). The HLT holds strike and sparge water heated to precise temperatures by an electric heating element. A coil of stainless tubing runs from the HLT bottom, up through the HLT water, and back to the mash tun, wort from the mash tun bottom is recirculated by a pump, passes through this heated coil, and returns to the mash tun top, gently and precisely raising or holding mash temperature.
Required components
- Three vessels: Typical homebrew-scale HERMS uses 15–20 gallon kettles for HLT and BK; 10–15 gallon cooler or kettle for MLT. Stainless sanke kegs converted to kettles are a popular economical choice.
- Electric heating elements: Low-watt-density (LWD) elements (800 W/in² or lower) for the HLT to prevent scorching. A 5500W 240V element heats 15 gallons from 60°F to 170°F in roughly 30 minutes.
- PID temperature controller: One PID per heating element, with a thermocouple probe in the HLT. The PID maintains precise temperature by cycling the heating element on and off. Auber Instruments, Inkbird, and Mypin PIDs are popular homebrewer choices.
- March or Chugger pump: One pump minimum for HERMS recirculation; two pumps for a more capable setup (one for HERMS recirculation, one for transfers). Head pressure rated to at least 6 feet.
- HERMS coil: 25–50 feet of 1/2″ OD stainless tubing coiled to fit inside the HLT. The coil submerges in HLT water, wort passes through the coil’s interior while HLT water heats the coil exterior. More coil length = more heat transfer surface area = faster mash temp adjustment.
- Tri-clamp fittings and valves: Ball valves on all vessel outlets, sight glasses on all vessels, cam-lock or tri-clamp connections for flexible reconfiguration.
Step-by-step build sequence
- Drill and weld kettle ports: Each vessel needs at minimum a bottom drain port, a thermometer port, and a liquid-level port. Use a step drill bit for thin-wall stainless; TIG weld stainless fittings (or use weldless fittings with gaskets if you don’t have access to a welder).
- Install heating elements: Cut element ports in HLT and BK side walls using a hole saw. Install low-watt-density elements with appropriate gaskets and fittings. Wire to 240V with proper circuit protection (30A breaker minimum for 5500W).
- Build the HERMS coil: Coil 1/2″ stainless tubing around a PVC pipe form to the diameter of your HLT interior. Use a tube bender to prevent kinking. Connect coil inlet and outlet to fittings on the HLT wall (or run hoses in/out through the lid).
- Connect PIDs and temperature probes: Wire PIDs to heating elements with solid-state relays (SSRs) rated for the element’s amperage. Install thermocouple probes in HLT and MLT for temperature feedback.
- Build the brew stand: Frame from 1.5″ square steel tubing welded or bolted to support all three vessels at staggered heights (HLT highest for gravity assist, MLT mid, BK lowest).
- Test water-only first: Fill all vessels and run the full recirculation loop with water before brewing. Check for leaks, verify PID control, confirm pump operation and flow rates.
Common Questions
What’s the advantage of HERMS over direct-fire RIMS?
HERMS heats wort indirectly, the hot liquor tank water acts as a thermal buffer that prevents the localized overheating that can denature enzymes or scorch wort in a direct-contact RIMS tube. The HERMS coil’s large surface area distributes heat gently across a large liquid volume. RIMS (Recirculating Infusion Mash System) applies heat directly to wort flowing through a heated tube, which is more efficient but risks thermal gradients that can denature enzymes if flow is interrupted or the element cycles hard. For most homebrewers, HERMS provides better temperature control with lower risk at the cost of requiring a larger water volume in the HLT.
