Improving Partial Boil Beer
Experienced home brewers will tell you that when it comes to brew kettles, size matters. If you're brewing a 5 gallon batch of beer, the volume of liquid, spent hops, trub etc. will occupy 5.5-6 gallons of space. Then there's the 10-25% of the liquid will evaporate during the boil and the additional space to prevent/control boil-overs. So it seems the minimum kettle size for a 5 gallon batch would be about 7.5 gallons. Of course to boil that much volume requires more energy than is produced by most kitchen stoves. Your kitchen sink will not suffice as a wort chiller with a pot that big either.
You can make very good beers using malt extract with a few mini-mashed or steeped grains in a smaller kettle but there are two big reasons for not going small. They both have to do with the density of the wort in the kettle. First, since all the sugars for the entire batch are concentrated in less volume the smaller kettle there is a high probability for kettle caramelization which is not desirable in most beer styles. Second, hop alpha acids which create beer bitterness are isomerized during the boil. All beer recipe hop additions are calibrated for full wort boil and isomerization of the hops which create the hop bitterness decreases in wort densities above 1.050 specific gravity. So if your specific gravity of 5 gallons is to be 1.050 then the specific gravity of the 2.5 gallons of kettle wort would be 1.100. At that density you will need 25% more bittering hops the get the recipes suggested IBU's. There is a third reason for not using a smaller kettle. Denser worts are much more prone to boil over.
So what do you do if you don't have $500-$5000 earmarked for brewery upgrades? If you have two pots and burners you can split the extracts and use one pot for your bittering hop pot and the second pot for flavor and aroma hops. The specific gravity of the bittering hop pot should be between 1.040 and 1.050 with the remaining grains and extracts added to the second pot. The bittering pot must be boiled for the full 1 hour after adding the hops. The second pot must only be boiled for the length of time specified for the longest of late hop addition. Split your kettle finings (Irish Moss) between the two pots for the final 20 minutes of each boil. If you only have one pot or one heat source consider splitting the boiling process into two as described above. Another option is to reserve ½ of the malt extracts and add it the last 20 minutes of the boil. These steps should reduce kettle caramelization and deliver more bittering from your early hop addition.