The heating can and by some is done with an immersion heater really designed for fish tanks, they do work with a fermentor so I am told.
The cooling is more complex, the problem is the fermentation produces heat, this means if you insulate the fermentor it will over heat, some way some how heat needs to be removed, be it simply into the low ambient temperature of a room, or to the low ambient temperature of a refrigeration unit or to the low ambient temperature of a cooling liquid.
It is possible that you could have a solenoid, which as the temperature rises allows a cold water supply to flow through a heat exchanger within the fermentor. The problem is getting a cold water supply which is cold enough to do the job year round. Although in some areas the supply may be cold enough, it is not always cold enough. So even is circulating cold water is used, that cold water supply needs to come from a refrigeration unit.
Clearly you don't want to be cooling the centre of the fermentor only to have the heat return through the outer skin of the fermentor. So a fermentor using a heat exchanger would need an insulated wall, but that fermentor would be no good where you had no refrigerated water supply. So it would be a rather specialised unit, and not cost effective for a 40 pint batch, as size of the fermentor increases so the surface area per gallon decreases so likely you would need a heat exchanger with a large fermentor. But not really with DIY size.
So next look at methods of refrigeration, there are in real terms three methods, Peltier effect, absorption, and the compressor. The Peltier effect, and absorption are not very economical to run, OK for short term use, few hours in the car, or few days in a caravan, but for long term use it has to be the compressor. I do remember in the late 1950's early 1960's my dad buying a motor with an ice box designed to be built into a fridge, and even today you can get commercial units not supplied with a ready made insulated box, but to get one small enough for use with a fermentor of the size used with beer kits, means buying a fridge or freezer and converting it in some way. There is no reason why you should not use an air conditioning unit, but these are much bigger than the simple fridge or freezer.
Although you could have a liquid container inside the fridge or freezer with a pump to circulate as required, that way a small 32 litre fridge or freezer could be used, it is far simpler to just put the whole fermentor inside a fridge or freezer. So for a system that will work anywhere in UK with reasonable efficiency then it is the fridge or freezer. However I am sure there are places where the cold water is always very cold, and in those areas one could consider a simple solenoid allowing water to flow when too hot. But to market such a system would be a problem, one you are wasting water, and two it will only work in some areas.
You can get Peltier units which will cool or heat a liquid, so you could build a unit using the Peltier system. It would need a STC-1000 to control it, plus a pair of four pole relays, two poles to swap positive and negative to heat or cool and one pole to interconnect the two relays so both can never work at the same time and one pole to switch on pump. It would need the Peltier unit and also a pump plus a heat exchanger (length of pipe inside the fermentor) and a tank or reservoir to keep enough liquid in the system with expansion and contracting. And likely a 12 volt DC power supply. I would estimate a unit like this would retail at around the ã150 mark ready built which is more than it costs to by a fridge and convert it, so the demand would be low. With such a low demand I would not expect anyone to try marketing one ready built. It would be a cottage industry at best. And not as economical as a converter fridge.
As you can likely see I did consider the idea.
Even the fridge or freezer is not perfect, with the ambient temperature outside at over 14ðC you have to use the refrigeration option to maintain the brew at 19ðC because the insulation of the fridge or freezer will not allow the heat to escape. I considered automatic methods to open the fridge door to allow the heat to escape, and close it once the ambient air temperature raises. Then I used an energy meter to measure how much power is used. And decided not worth the effort. Yes in the winter months you could use an insulated box and fans which only turn on when required. But why bother, easier just to use a brew fridge.
