My tank is a 50 gal heat-flow:
https://www.heat-flo.com/products/indirect-water-heaters - in service since 2005 - sediment issue cropped up in the last year. The current model does show a sacrificial rod - black cap above the aquastat well - mine doesn't have this cap - perhaps it's under the outer cover. I have a call into the manufacture. This would be true for most people in eastern Ma since we all get our water from the quabbin reservoir. Since heat-flo is based in Ma, they must have encountered this before. Will call on Monday - they are closed on Fridays.
In the service manual has a procedure for descaling with 1 gal of commercial ice maker descaler.
I think my high Ph of 9.5 may be the culprit. The service manual also states:
Water Quality
Improper water quality will reduce the expected life of the water heater. Hard water, sediment, high or low Ph, and high levels of chlorides in the domestic water should be avoided. Sediment and hard water will eventually coat the heating coil inside the water heater and reduce the rate of hot water production and may eventually cause a failure. High or low Ph and/or high chloride concentrations will cause corrosion and eventually failure. A filter is strongly recommended where sediment is present in the water. A water softening system is recommended for areas with hard water. In an area where the water quality is not known, a water quality test should be performed.
WARNING:
Do not operate the Heat-Flo water heaters in areas where the Ph is above 8.0 or below 6.0, and/or with chloride concentrations greater than 100 parts per million (ppm). Heat-Flo’s standard warranty does not cover problems caused by improper water Ph or excessive levels of chlorides.