In early days it was temperature which caused most of my problems. What I had not considered was a stick on temperature stick will show some where between ambient and brew temperature, there is nothing to insulate from ambient, using a sensor you can insulate with a simple sponge so very close to brew temperature, the other point is 40 pints takes a lot of heating and cooling, if the brew when up and down with room temperature then a room at 20ðC during the day is great, but where the room is 20ðC in the day and 16ðC at night the brew will sit some where between the two, and 18ðC is just on the limit for most kit beers.
So when the beer was fermenting well, there was some heat from the fermenting so brew likely at around 19ðC but as it got into the cleaning stage it was dropping to 18ðC which is just about warm enough, but at that temperature very slow, just 1/2 a degree at these temperatures can mean 5 days difference in brewing time. It will not likely stop, but it will really slow down.
I found a body warmer was great, the air lock sticks out of the neck, and a stool lifting the fermenter just a foot off the ground also helped, but still on the edge, I had an 18 watt demo underfloor heating tile, just the right size to fit under the fermenter, this was slow to heat the 40 pints so I could manually switch it on then 4 hours latter switch it off again and I had got about 2ðC raise, however the problem was, I would forget, and end up with a 8ðC raise which was far too much.
This manual Heath Robinson control resulted in not really knowing if what I had done to the brew had altered the taste better or worse, or if simply down to temperature. The moving to thermostatic control was more by accident than something carefully planned, the old fridge/freezer was condemned faulty insulation however the ice build up it caused will not happen using it as a brewing fridge, so moved from kitchen to garage. It was only then I realised what was happening.
At 18ðC in the kitchen the brew seemed to start very well, but in the brew fridge it stalled, I needed 19ðC for it to run well, this had me scratching my head until I realised the stick on was showing 1ðC lower than the sensor to start with, but the same temperature at the end of the brew, this was when I realised the stick on is more of an average between the ambient and brew temperature. There was however a down side, I expected except for heart of summer not to need cooling, however the insulation on the fridge/freezer may be faulty, but it still stopped most of the heat from fermenting from escaping so first 5 days needed cooling.
I will guess your problem has nothing to do with water, and is more down to temperature. It caught me out, so seems reasonable it will do the same with others.
