My kitchen goes from 16 at night to 26 in the afternoon and every ferment behaves differently, how much does that matter?

Before buying anything, go and find the most boring place in your home.

A ten degree daily swing on a kitchen counter is unusually bad, and there is nearly always somewhere much steadier a few metres away:

An unheated cupboard against an internal wall.

A basement or cellar, which in many houses is the most stable place in the building.

A north-facing room that is not heated.

A garage, seasonally.

Low down, since temperature stratifies and floor level is cooler and steadier than a counter.

Put a cheap thermometer with a minimum and maximum reading in two or three candidates for a week. You will usually find somewhere that varies by two or three degrees rather than ten, and that alone makes your results repeatable.

The other free lever is the amount of salt, for vegetable ferments. A slightly higher salt percentage slows things down, which partly compensates for a warm room, and a lower one speeds it up in a cold one. Not a substitute for temperature control and a genuine adjustment.

And for a starter specifically, the amount you keep and how often you feed it can be tuned to your kitchen - a smaller inoculation slows it, which is how people cope with warm summers without a fridge.

You have worked it out yourself: temperature is the main control and everything else is a detail.

The rates of everything happening in a ferment roughly double for each modest rise in temperature. So a ten degree swing is not a small variation, it is a completely different process running at a completely different speed. Your three weeks becoming nine days is exactly that.

It also changes which organisms dominate, which is why it affects flavour and not only speed:

Cool favours a slower, more complex succession. Kraut fermented cool is generally considered better and it takes much longer.

Warm is faster and can be one-dimensional, and it favours organisms that produce sharper, sometimes less pleasant results.

Hot starts causing problems, off flavours, soft texture in vegetables, and a greater risk of things you did not want.

So the timings in recipes are only meaningful with a temperature attached, and most recipes do not give one. A recipe saying two weeks is really saying two weeks at whatever the author's kitchen was.

Stop using days and start using signs. Taste, smell, bubble activity, pH if you measure it. That is the durable version of every recipe and it makes the swing survivable.

Nobody has mentioned the one that bit me: the swing matters separately from the average. My kitchen averages a perfectly reasonable 21 and the kombucha still went strange, because going 16 to 26 every day means the culture never settles into anything.

Moving it two metres to a shelf that sits at a flat 19 fixed it, and 19 is a worse number than the average I had before.

Cold and slow is the good kraut though. Two months is a feature.

Nine days in July and three weeks in spring is such a good detail. Explains my entire first year.

If you do decide to control it, the cheap version is genuinely cheap and it is what most people end up with.

An inexpensive plug-in temperature controller with a probe. It switches a socket on and off to hold a set temperature. Two configurations:

For cooling, plug in any old fridge. The controller overrides the fridge's own thermostat and holds it at whatever you want: fermentation temperatures are far warmer than a fridge normally runs, so a second-hand one is fine and it will barely work.

For heating, plug in a small seedling mat or a low-wattage heater inside an insulated box. This is the version for a cold flat in winter.

Either is a fraction of the cost of anything sold as a fermentation chamber and it does the same job.

The intermediate options, in ascending effort: a cool box with a bottle of frozen water swapped daily, a cardboard box lined with insulation to damp the swing, and a bucket of water around the jars: water has a lot of thermal mass and slows the swing considerably for nothing.

Start with finding the stable spot. For a lot of people that is the whole solution, and the controller is what you buy when you want a specific temperature rather than merely a steady one.