May 22, 2012 AT 8:59 am

ASK AN EDUCATOR! “How can I maintain temperature of my home-brew while it ferments?”

Kyle asks:

I’m just getting in to the world of homebrewing my own beer. My basement is a little too cold for proper fermentation, so I’ve constructed a simple box out of foam insulation and I want to add a small heat source.

I have sensors measuring the temperature of the fermenter and the ambient air in the box. I’d like to use an Arduino to turn the heat source on and off. My plan was to buy a small space heater (Lasko My Heat ceramic heater for example), and use my Powerswitch Tail to turn it on and off. I’m worried about the life of the Powerswitch Tail, since the heater isn’t a pure resistive load (it has a small fan motor).

Should I just use a a 40-60 W incandescent bulb instead? I remember seeing an app note about how an incandescent load needs to be derated even more than an inductive load. What’s the best way to safely add a controllable heat source that won’t result in me having to replace my relay switch too often?

Mmmmmm…..theres nothing better then brewing your own and I will see if I can tackle all of your questions.

First off, you are absolutely right in questioning the type of load you are putting on your Powerswitch Tail. Lady Ada has referenced a good document that describes the different types of loads commonly connected to relays and why they can affect the overall lifespan of the relay. The document describes 5 different load types, and as you had suggested, your system would be working with either incandescent or motor+incandescent loads.

As the document describes, because the filament changes resistance based on temperature, the contacts on the relay are more prone to wear due to the high inrush current when the filament is cold. The document suggests de-rating your relay by 10% or using a series resistor to current limit the load. This would apply to both your heater and your bulb ideas and your 1800W Powerswitch Tail is now capable of safely switching 1,620W.

Your space heater also has a fan, which would be classified as a motor load (although the fan is probably pretty small). Like the inconsistent incandescent load, there is a large inrush current when starting the motor. This is usually taken care of by attaching a starting capacitor to AC motors. The result is similar to the incandescent and requires derating your relay by ~20% and your 1800W Powerswitch Tail is now capable of safely switching 1,440W….which is a little low for a space heater.

Another concern with the Powerswitch Tail as a heater control is its life expectency. Adafruit states that the Tail should be able to switch 100,000 times with a 15A resistive load. As an example, if you system constantly switches your heater on and off every 30 seconds:

30s * 100,000cycles = 3,000,000s of operation
3,000,000s / 60 / 60 / 24 = 34.7 days of operation

Depending on how long you plan on fermenting and how accurately you want to maintain temp, this might be a bit low. You might want to look at some more industrial controls or solid state relay‘s to do the job.

Also, an alternative to the lightbulb/space heater idea would be to use a silicon heater blanket. I used to work in a lab and we used these all the time to maintain temp on bio-reactors. They are much better at distributing heat then point sources like the bulb/space heater (also are a lot more safe due to their max temperature!) This, in addition to a SSR and a thermal switch, would be a pretty stable system.

I hope this has helped answer your question and good luck with your brew!

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  1. A heating pad under the carboy works quite well, you can set them at various temperatures to keep the brew at a more or less happy temperature for the yeastie beasties.

  2. Smarter control could also help reduce switching frequency.
    Brett Beauregard and RocketScream have developed an open source PID controller that Adafruit reviewed earlier this year:


  3. Given the thermal inertia of a batch of beer in a well insulated enclosure, having the heat source switching on and off every 30 seconds seems an unreasonably high estimate. Also, programming some hysteresis, into the system (say +/- 1 deg C), or a minimum time (say 10 mins) between changes in on/off state will further reduce switching frequency of the relay.

  4. I thought a decent amount about this too. I am half thinking about using this:


    which i made anyway to play around with the cooking technique. I ended up retrofitting it with a solid state relay and output control to an outlet rather then the heating coils directly. I was thinking that submerging the primary in a large container of water with this running would do the job nicely and allow me to control the temp of fermentation very precisely = +/- .1C! Probably overkill but it was one of the directions i was thinking of going.

  5. An inexpensive source of a durable heating mat would be the seedling heat mats used to germinate seeds. One caution, the beer carboy is heavy enough to cause damage to heating pads or mats, so you might consider putting the heating pad/mat under a rack which the carboy sets on.

    Hysteresis is critical in heating/cooling situations. Even 0.5 F provides a huge reduction in number of cycles.

  6. Perfect timing on the question! I just finished up my “prototype” version of a fermenter thermostat. I created it out of a PIC18 microcontroller, a temperature reading IC, and a relay (well, a couple relays), among other components. It has an LCD screen and a user input in the form of an up/down/menu set of buttons.

    Fermenting lagers usually requires a more advanced temperature profile throughout the fermentation process, including slowly raising and lowering the temperature of the fermenter. My thermostat can store programs to accommodate such a program.

    I wrote a blog about it, like to hear it? Here it goes…

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