Any downside to kegging 3 -gal batches in 5 -gal kegs?

Brewing Processes
1

I’m planning in getting into kegging and I’d like to figure out the best setup for me before I spend my money instead of  spending more later because of wishing I had bought something else the first time.

My considerations include:

Because of age, weight, and possible medication issues, I normally drink only 3-5 beers per week.

I will be refrigerating the kegs in a garage fridge that will also have to do double duty as a fermentation chamber.  Because I would be using the fridge for fermentation also, my plans would be to brew and ferment two different recipes at the same time in smaller batches say 3 gals each to minimize the time the fridge would have to be kept at ale fermentation temps rather than at keg serving temps.  Brewing two five-gal batches at one time would be too much for my normal consumption.

I can buy used 5-gal kegs much less expensively than the 3-gal kegs so I’m thinking about getting those.  I’m assuming that if you purge the additional head space with CO2 then that would keep possible oxidation at bay, and the 5-gal kegs would work okay.

Thanks in advance for your advise.

2

you are correct, any amount of less than 5gal in a 5gal keg will be fine, just purge the O2.

3

What’s the difference in your ale fermenting temperature and your keg serving temperature?
You could use a yeast that works at keg serving temperature to ferment your ale recipes.

4

I  plan on serving at about 40 F because I will be dispensing into room temperature glassware. My fermenting temp starts at 64 F ramping to 68 F and then up to 72 F to clean up before cold-crashing and then into the keg.