Uh oh, Sylvan, now you are thinking like a heating guy. You see, in a residential heating system during the heating season, the circulators run for some time during an hour every hour to carry bubbles away. But a residential water heater’s load is ordinarily for two periods a day – during the morning at get up and cook time, and come home and cook time – about 5 hours per day. During the rest of the time and especially at night, the water sits there warm to let bubbles collect at the top. If there is a non-diaphragm tank piped above, the bubbles can go up there.
Now we have had some posts in the past asking about the clotheswasher appliance makers warning about hydrogen produced with possible release after collection in the washer. Collecting hydrogen, the lightest gas, in a tank can be something to worry about. Perhaps that is why I have never seen or sold a non-diphragm tank (not the Extrols, they have diaphragms) used in a domestic water system. The only open tanks sold in plumbing supplies were the thin-wall expansion tanks used for heating (not counting the heavy-duty ASME vessels sold for higher pressures.) So I worry about this post.