When asked about my favorite "brewing toy," I can easily answer, without hesitation, my Thermapen Mk4 thermometer. When asked about the most improvement I've ever made to my process, I can easily answer, without hesitation, the day I added a temperature-controlled fridge to my home brewery.
You know why? Because temperature really, really matters in brewing.
I'm amazed at the number of brewers I know who treat this as some kind of luxury. It isn't. It's essential. It may not be necessary, strictly speaking - you can brew beer without thermometers and temperature controllers - but if you're an even remotely competent brewer, you'll see significant improvement in the final product when you're tracking and holding temperature throughout your process.
Getting Hot In Here
From the get-go, you should be monitoring and maintaining temperature. Luckily, brewing software these days will give you a foolproof target for your mash strike water and sparge water temperatures, and if you use something like that Thermapen (I have two - I loved my brewing one so much, I bought one for the kitchen, too - INSTANTANEOUS and accurate measurements), you're going to hit them easily.
Once you start hitting those temperatures (rather than winging it and either hoping you got it right or adjusting with small quantities of boiling or cold water), maintaining them in the mash is easy, too. And you don't need a super-expensive HERMS or RIMS system, either - you just need a cooler. My Coleman Xtreme cooler has been plugging along for years, and it holds my mash for an hour with one degree of drop. And it's cheap.
What does accurate and steady mashing get you? Reliability. Efficiency should be roughly the same every time, your ratio of fermentables to not will be much more consistent, and those both mean that adjustments to your recipe are meaningful instead of creating variable effects.
Things really improve, though, when you get to the fermentation side of things.
Find a fridge or freezer. Any fridge or freezer. Seriously - even one that someone is throwing away. Why? Because we don't need it to work as whatever it is, we just need it to work well enough to act as a fermentation chamber, and that's a much lower bar! Cooling to 50F to make your lager fermentations work better is a pretty easy lift, and even a discarded fridge or freezer can often do the job for years.
Now, once you've borrowed, begged, or stolen a fermentation chamber, you just need to control the temperature inside of it - even refrigerators, at even their warmest setting, aren't likely to give you the temperatures you need (too cold), so you'll want a good temp controller. These used to be pretty expensive, but lately you can get a good dual-stage (warming and cooling, if you hook a hooded light bulb up to the warming side!) temperature controller from Inkbird for next to nothing (you can catch them on sale regularly for under $40, and sometimes as low as $25!).
There are all kinds of methods for where and how to run the temperature probe, including drilling through the fridge wall and/or running it directly into the beer itself, but that seems like overkill to me. Instead, I run mine through the gap in the door (the gasket will seal things up just fine) and into a growler filled with water. Why water? Because if I just let it hang in space, it's reacting to the air temperature in the fridge. If it's touching something like the metal wall or the glass of a carboy, it's getting a completely different temperature (especially if you have hot spots from coils or on the compressor hump). But that growler is going to be about whatever temperature the beer is.
Now, not exactly the right temperature - after all, the beer has active fermentation going on, which will make it a bit warmer, but don't obsess over that. Set the temperature to what you want it to be, then evaluate the finished beer. Seem like it was fermented too warm? Then drop the temperature a bit next time - what's important is that the probe itself stays in a consistent environment, because that way the relative change will be accurate!
Don't Sweat It
If you can make these kinds of additions to your brewery, then I recommend them highly. Space, money, or something else might make it impossible, though. If that's you, then do the best you can, and it doesn't mean you can't make excellent beer (it just might make it a bit harder!).
For water and mash temps, even a meat thermometer can get you close. In fermentation, you can probably find a cool-ish and temperature-stable basement corner or room in your house. But do what you can - it'll pay off.
Keep it simple.
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