Water level drops during steam cycle activating LWCO

New Hampshire DIYer

I have a Slant Fin TR-40 installed in 2014, sized appropriately for the radiation, radiators properly pitched, cast iron near-boiler piping with drop header, three Gorton #1s on the return main etc. In short, I think I've got the system set up correctly and it generally works fine but seems to have two potentially-related problems.
First, when the water level is set per Slant Fin's manual (middle of sight glass), the level drops during the cycle enough to cause the boiler to shut down due to low water. As a result, I usually keep the level higher. There is also a sticker on the boiler indicating a higher water level so that's confusing as it's at least two inches higher than the "25-1/2" from the manual. And second, I am losing water slowly over time. Perhaps as much as a gallon a week (I don't have a water meter on the system). The main vents close properly and I don't have any wet returns, unless you count the last 4' of the return as it connects back to the boiler. The Slant/Fin has only 8.8 gallons in it, so perhaps that's one reason it seems really sensitive to water level. Is there any problem maintaining a higher water level? Should I flush the wet part of the return? Any suggestions welcome.

mattmia2

Is the water level stable when it is steaming or does it bounce? It looks like there is oil on top of the water in the sight glass. There are some things wrong about your return piping but I don't think that is your problem. Is there even a skip port installed on the boiler? I see a nipple with a 90 that might be one.

New Hampshire DIYer

The water level does gently bounce or bob up and down--about an inch or less top to bottom. The drop in water level while steaming is below the level of the bounce. I do have a skim port (photo attached) and have skimmed the boiler several times. I can certainly do it some more.

ethicalpaul

I will say that's one short gauge glass