Full Boil Extract vs All Grain

Hopefully someone can clear this up for me, because I am confused. Why is it that you have to do a full boil when doing AG but it is ok not to with extract? I understand that the full boil is just better all the way around to keep from having any nasties in top-off water, but just don't understand why it is necessary with one and not the other. After all, isn't it all wort once you are to the boiling phase? (Sorry, I did try searching, but had no luck) thanks

There's nothing "wrong" with doing a full boil with extract - but most extract recipes are built around the idea that you're limited in the size of either your boil pot or stove.

You get better hop utilization when you're doing a full boil, but then you've got to cool down the entire 5+ gallons, rather than just boiling 2 or 2 1/2 gallons and topping off the rest.

Once you've got a big pot, the ability to bring 5+ gallons to a boil, and then a chilller to quickly cool off 5+ gallons.... it's just a small step to convert a cooler into a mash tun & save some money & increase your flexibility by going to AG.

technically you don't have too. but you'd have to have the gravity of the wort higher, i'm not exactly sure on the details, i guess i could figure them out if i had to. but its probably much easier to do a full boil with AG

In AG, you need to sparge to "rinse" sugars off the grains. Sparging takes a large volume of water. That volume is why you need a full boil.

With extract, all your sugars are in the extract so there's no need for all that volume.

Clear as mud?

Actually, it is perfectly OK to do with all-grain:

https://www.homebrewtalk.com/wiki/index.php/High_Gravity_Brewing

I've considered doing it to get 10 gal batches out of my 5 gal system, but eventually decided against it because I'd still have to boil and cool 5 gals of top off water. At that point, it's not that big of a deal to just brew another batch.

With all grain you are using your water to extract all of the sugars out of the mashed grain. Water can only pull of so much at a time, so you want to use as much as possible, therefor why you do a full boil. If you did a partial boil your OG would be incredibly low as you would not have extracted out as much sugars as you could have (this is referred to as Efficiency).

For Extract, the grains have already been converted to sugar and put in the Extract form. You are purely using the water to boil the hops and dissolve the Extract, so as long as you have enough water to do that then you are fine.

Also, for extract you usually do have some grains to steep, but the 2 gallons of water is fine for the 1-2 pounds of grain you usually have for steeping.

In the winter I do quite a few 5 gallon AG "Top Off" batches on my stove, and some of them have medaled. I've posted how I do it in great detail in this thread.

I've been explaining how the last several years on here.