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In this answer it is mentioned that boiling with the lid on or off may make a difference in final gravity. Which do you recommend and why?

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You want to boil with your lid off.

Part of the process of boiling is to remove dimethyl sulfide (DMS), which is a sulfur compound off flavor that tastes like cooked corn. DMS is formed by heating the wort. If you leave the lid on the kettle DMS won't evaporate with the steam and you'll have more of the flavor in your beer.

You might also run the risk of boilover if you leave the lid on, but DMS is the larger concern. This is also one of the reasons you want a real, rolling boil as it allows more DMS to escape.

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Interesting, never knew that about DMS but it makes sense that it can escape with the steam. Does the DMS affect the gravity, or is it more of a micro-substance that just affects flavor? – Greg Nov 12 '10 at 16:40
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I don't know about the gravity. That's a good question. Most grain has DMS precursors already in it; heating it causes it to form, so you can't escape having it. Some grains have more DMS than others. For example, Pilsner malt tends to have more DMS which is why you'll often see recipes with Pilsner malt recommend a 90 minute boil. You can read more here: homebrewtalk.com/wiki/index.php/DMS – sgwill Nov 12 '10 at 16:44
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What Greg said in the answer below. But to clarify. A typical pot geometry will lose 9-11% volume per hour. However, that is dependent on environmental humidity (and to some degree temperature too). Your evaporation rate will be lower when it is humid. Recipes are designed with this loss in mind and should have a higher starting volume. I.e. a ten gallon batch will start with about 11 gallons of extract at the start of a 60 minute boil. – thebeav Nov 12 '10 at 19:32
Yes, definitely keep the lid off; primarily for DMS elimination as sgwill said. @Greg, DMS won't affect the gravity; it's just a compound. Gravity is a measure of the dissolved sugar. – Morgan Nov 13 '10 at 0:33
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@Morgan - I believe that technically gravity is a measure of everything in the mixture. If the DMS is of sufficient quantity and with significantly different mass than water, it would affect gravity. But I think it's probably in such small relative quantity that gravity effects are negligible. – Greg Nov 13 '10 at 1:44
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I think the difference (regarding gravity) would just be water volume. Lid off = liquid lost to steam. You can compensate by adding more (or less) water after the boil.

The actual amount of liquid lost depends on the geometry of your kettle, but I think 1-1.5 gallons per hour is a rule of thumb. The easy way to know for sure is to measure before and after a boil.

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