I have a home-built controller for a HERMS brewing system (Heat Exchanging Recirculating Mash System). The purpose of this controller is to drive liquid (wort) from a large mashing vessel (a cooler box), through a heat-exchanging coil that is submerged in a reservoir of heated water, in order to maintain the temperature of the wort in the mash vessel. This temperature control is achieved through the use of a PID, which takes a temperature reading from the liquid wort as it flows out of the heat exchanging vessel, and drives a 10A kettle element (through an SSR) to maintain the desired temperature. This setup works extremely well, but there is a problem that sometimes occurs that could, potentially, be dangerous: the wort is pumped around the system by a 12V pump. If, for some reason, the recirculation stops (e.g. the pump gets blocked with debris/airlock, or simply breaks), then the measured temperature (just outside the HERMS vessel) does not rise with the temperature of the water within the vessel. This causes the PID to keep driving the 10A kettle element, up until the point that the water inside the vessel boils, and steam starts venting through some pressure release holes in the lid.

This exact situation has now occurred twice. The first time, I was lucky enough to spot it early and switch the system off. After this, I added a new PID to the system that measures temperature within the HERMS vessel. This PID (from Auber) has an alarm feature that can be triggered when a certain temperature is hit. The alarm feature triggers a small, internal, SSR (240v, 3A), which is connected to an alarm unit that flashes and sounds a loud siren. This works, and saved me on the second occasion.

What I want to do now is go further than simply sounding an alarm, and have the kettle element circuit shut off completely if this alarm condition is met (I can't always be within earshot to turn the system off). I am not well versed in electronics, although I do try my best to wire everything I have up as safely as possible. The only thing I can think of is a normally closed relay, that is driven by 240v to open it when the alarm condition is met. This would work, but would only open the circuit for as long as the alarm sounds. I also have some concerns that running this normally closed relay at 10a for, potentially, many hundreds of hours at a time (between alarm events) could see it fuse shut? Is this a reasonable concern? What I would really like is a way of switching a circuit off, in much the same way as a circuit breaker - i.e. the circuit switches off, a physcial switch is thrown and I have to manually intervene to enable it again. Ideally this would happen whilst the alarm siren continued to sound.

Can anyone recommend a solution please? As mentioned, the easiest option for me is to use the already existing 240v alarm signal, but if another way makes more sense I am willing to consider it. Bonus points if the solution can be DIN mounted, as that is the way this thing is currently built.


This all sounds far too complicated. You need something simpler, like a thermal fuse. This is mounted on your hot tank, and goes in series with either your heating PID controller, or the kettle element.

In a few seconds with google and 'thermal fuse' I found a supplier of 73°C, 15 A, 240 VAC fuses for less than 1$ each.

  • \$\begingroup\$ Thanks for the asnwer - not something I had thought of. The immediate issue I can see here is that the fuse would have to be immersed in the hot water reservoir, so I'd need to run the cable for the element right through a hot water bath that normally operates in the range of 60-85C. I'm not sure how I would do that safely. The reservoir is insulated, so it doesn't get that hot on the outside. With respect to complication, bear in mind that I do already have the alarm system set up and working, so I am hoping there is just one (or more) additional component that I can add to that. \$\endgroup\$ – Matt Dec 29 '20 at 12:10
  • \$\begingroup\$ Get a length of copper or plastic plumbing pipe, put an endstop on the end, put the fuse at the end of a cable, push the cable in the pipe, dip the pipe in the tank. Temperatures of 84 and 93 also available. \$\endgroup\$ – Neil_UK Dec 29 '20 at 13:45
  • \$\begingroup\$ @Matt What I'm advocating is a very simple bomb-proof stand-alone safety cutout on the heater as a necessary minimum, even if you do have alarms to turn it off as well. \$\endgroup\$ – Neil_UK Dec 29 '20 at 13:53
  • \$\begingroup\$ thanks again. It does certainly make sense to have an ultimate fail safe like this built in, assuming that the fuse can hit the required temp in a quick enough time inside such a thermowell as you describe (reservoir is about 1.2 litres of water, so can go quite quickly from 90 to 100C). It would also be nice to have something a little easier to reset, in the case where e.g. the pump has just become blocked, and I want to get going again quickly. It did also strike me, since asking this question, that some sort of hot water thermostat might fit the bill? \$\endgroup\$ – Matt Dec 29 '20 at 14:07

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