Notes on today's flush of the same heater.
It was first power flushed for about ten minutes, which deposited about two quarts of (hydrated) sediment in the collecting pool. Then the T&P valve was removed and the PEX sprayer inserted. It worked well for a while and then the hose jammed. I believe the mistake was in not keeping enough water in the tank at all times. If the pressure is too low there isn't enough flow always to prevent sediment from piling up at any sort of restriction. Unfortunately we cannot easily see into the tank to assay the water level. After clearing the blockage (at the junction of the drain and garden hose) water was added to the tank through the cold water inlet in bursts until the flow out of the hose was "substantial" (much more than a dribble, but nowhere near the flow from a faucet) and then the sprayer was run 10 seconds each minute, which broke loose sediment, and that flow was maintained for 30 minutes. A few extra squirts were needed from time to time when the rate decreased.
What I really need is a clear bit of hose or pipe right at the WH drain to see if sediment is still going by, and how fast it is moving.
After all of this flushing the tank was inspected with a Tesslong borescope which has both forward and sideways cameras, and associated LEDs to provide light. The sideways camera showed that the anode was still in good shape. When it was lowered all the way to the bottom of the tank it showed that the "dome" had a thin crust of white crystals. That wasn't evident when it had been viewed from above.
One gallon of 5% acetic acid (from Smart and Final) was poured into the empty WH and allowed to sit for 5 hours. When it was drained the pH of the fluid in the bucket was roughly 6 and a substantial amount of gas pressure had accumulated (the T&P valve had been reinstalled during the soak, and no hot water valves were open.) 5% acetic acid is about .83 M and has a pH of 2.4, but pH 6 corresponds to 1 microM. So even after all of that flushing there was still around 3.3 moles (4 x .83) of "something" in the tank which had reacted with virtually all of the acetic acid. Calcium carbonate has a MW of 100 g/mol, so if that was the only material, there was at least 330g (.72 pounds) of unflushed sediment. There was also some rust and other material coming out though. The rust would have reacted to form iron (III) acetate, which isn't soluble (and no gas is generated).
When the WH was refilled and restarted it did not bump. Previously (see prior posts) flushes without a vinegar step resulted in a WH that did bump. But removing all the sediment using just vinegar might take a very large amount, since at least 3 quarts of hydrated sediment had already been removed from this WH before the vinegar went in.
