I have a spare mini fridge that can hold inside my fermenter and i was thinking instead of spending water with my immersion chiller to rack my wort from my kettle and place the fermenter in to the fridge.It might take longer but since the lid will be on with airlock would it be too risky to do it?
Some brewers chill their wort overnight, using the No Chill Method, so in principle you could do variation of that, but use your fridge to get the wort chilled quicker.
For this to work you need to ensure 2 things:
Doing this with a regular carboy or bucket may be risky because of the large headspace. It will take the fridge several hours to chill the wort to pitching temperature, during which time it is exposed to the airbourne contaminants in the headspace. How risky this is depends the environment - it may work fine for one person but not for another.
I'd suggest racking the hot wort to a cube first and squeezing to remove the air. Then place this in the fridge to cool. Once cooled, you can then rack (noisily) to your carboy.
This has a few advantages:
A lot of guys where I live (Florida, USA), use an immersion or counterflow chiller to get the beer down to about 80-90F (which is about as low as it will go with 70F+ groundwater), and then rack to a carboy and put in a fridge for 6-8 hours to hit pitching temps. This makes me nervous, but they seem to have good success with the plan.
I couldn't tell from your question if you want to remove the chiller entirely, or if you just don't want to run it endlessly. But its definitely not a good idea to rely on a refrigerator to take a 5 gallon batch down from boil temps to pitching temps without any extra cooling help from a chiller, ice bath, etc.
This is because a carboy full of hot wort is incredibly well insulated and dense, and will take many, many hours to cool down to pitching temps. During that time, your aroma and flavor hops will continue to "cook" and become more bitter.
Also, once the wort drops beneath about 140F, its going to be spending many, many hours inside the temperature zone thats PERFECT for bacteria before you pitch. This is a serious risk to your beer.
I would suggest using the chiller to knock the temp down lower than 100F at least, and then using an ice bath to get the carboy to pitching temps, or putting it into your fridge for 6-8 hours before pitching, tops. I do the former, and 20lbs of ice will take a carboy from 90F-ish to 65F with some swirling in about 20min.
I have left 6 or 8 batches in the fermenter (sanitized) overnight before pitching with nary and ill affect.
All I do is capture about 8oz of wort and put it in a sanitized mason jar, put the rest in my plastic bucket with an airlock and set it down over night. in the morning, I pitch the yeast in the jar, give it an hour or so to get wet, then pour the whole jar through the stopper hole in the bucket lid and put the stopper back in. Its that simple. And sometimes I thik I have over thought that!
PS I should mention that I usually do a partial boil and add 2 gallons of refrigerated water to start the cooling process