On hob boils I used to have lid 3/4 on as that was where it balanced best.
I still do this with my big 25l boiler as it allows me to get a good boil using the smaller 900w element to maintain the boil.
It does reduce the boil off rate a lot, but it's 1/2 the electricity cost of an open boil on the 1800w element.
I do a similar partly-covered boil. Reduces the electricity cost (With the lid partly-on and an insulating jacket around the metal boiler, I run my GF on about 60% power instead of 100%)
Does keeping the lid on cause you any issues with drips into the wort ? I was always under the impression that boiling with lid on ir part on was a no no ? Something to do with nastys that your boiling off recycling back into the wort as drips
Assuming that your lid is clean, then no, there should be no issues.
Boiling with the lid on being a no-no is a common homebrewing myth. If you think about it, all professionals brew "with the lid on" or "partly covered" because their kettles have tops with chimneys in it. This is no different to a partly-covered homebrew setup.
The fear of it being a no-no is from the fear that DMS (which you want to drive off as part of the boil, along with other potential volatiles that can impact flavour) would condense on the lid and drip back into the kettle. This fear in unfounded. During the first few minutes of the boil, when the lid is cold (under ~40°) DMS will condense on the lid, and drip back in. But after the first few minutes of the boil, the lid will raise to ~100°. This is well above the boiling point for DMS, so DMS
will not condense on the lid - it'll just escape past the lid. The liquid you see condensing on the lid will be pure water. This will drip back into the wort, but it's just pure water.
I have a condenser on my boiler, so lid is on all the time. DMS is pretty volatile. It isn't carried out by the steam.
"carried out by the steam" is something that podcasts/blogs etc mention, often repeating it without really understanding what it means and making false assumptions, so I'll try to clarify it here.
"DMS is pretty volatile" - entirely correct. The boiling wort is at 100° and DMS boils at 37.3°, but due to complicated chemistry involving solutions/mixed liquids just raising the temperature of your wort to 100° will not immediately boil off the DMS (in the same way that simmering your spaghetti bolognese won't boil off all the alcohol). What will happen is that at the surface of the wort, DMS will evaporate (or volatilise, which is just another word for the same thing) and escape the liquid wort. There is nothing magical about the steam here so you're completely correct in that "it isn't carried out [of the wort] by the steam".
Once the DMS vapours are out of the wort (and won't condense on the lid!) it'll be in the air space above your boiling liquid. At this point, it's mixed with other gasses (mostly steam, but a little bit of air and other volatiles from them boiling wort). The hot gasses rise (and some of the steam condenses in the air that you see as the cloudy water vapour often referred to as steam). The mix of gasses gets 'carried away' from the kettle by convection currents and any breeze/fan/wind you have. So in a sense, the DMS vapour "is carried away from the boil kettle by the steam/air/wind". So in a sense it "is carried out by the steam", but only because the steam is the bulk of the gaseous mix that contains the DMS. If there were no steam, and we lived in a pure nitrogen environment for some reason, you could say that the DMS "is carried out/away by the nitrogen". There's nothing special/magical about the steam, it's just what happens to be there that forms the convection currents to carry away the DMS.
Exactly the same happens during fermentation. The DMS will naturally evaporate/volatilise from the surface of the wort* and join the mix of gasses (mostly CO2) in the headspace of the fermenter, and when more CO2 is produced, the mix of CO2, DMS and other trace volatiles in the headspace will be pushed out of the airlock. There is nothing magical about CO2 in this scenario, it is just the bulk of the gas that the DMS diffuses into and is then carried away as a mix of gasses, not necessarily "by" the CO2.
*And in a small way, as the bubbles of CO2 rise through the fermenting wort, some DMS (and other volatiles) will evaporate/volatilise
into the bubble of CO2 as it rises. Exactly the same would happen if you bubbled nitrogen up through the fermenting wort. The DMS would diffuse into the bubbles of nitrogen a bit, get carried to the surface, then "be carried away by the nitrogen". This process is often referred to as the generic "scrubbed" word that is overused and under-comprehended on forums. People refer to "CO2 scrubs the DMS from the wort" as if some magical chemical neutralisation happens, when in fact it's just a generic carrier that DMS can diffuse into and be carried away.
