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This is my first brew and im brewing a munich lager. What is the best temperature to be brewing this at? and does it effect the beer greatly if the temperature fluctuates.

Second to cleanliness, I'd rate temperature control as one of the most important factors in brewing a decent beer.

If the yeast is a genuine lager yeast then you'll want to brew around the 10-12 degree mark, if it is an ale yeast 16-18 would work well.

Stable temperature if preferential.

Cliffo is right on point.

The only thing to add is that 20-odd litres of wort takes a fair amount to move the temperaure all that much, so don't be too concerned about that unless you have some wild temperature swings where you are.

....and I'd suggest that temp control is more critical in the first few days of fermentation as the yeast adapts to its new environment.

Cheers,
TL

It should also be pointed out that a stable temp is more important than the actual temp you are holding. I suggest you try to hold a temp which is easily achievable with what you have available in your current situation. From there decide which yeasts to use and which styles of beer you can make with those yeasts.