Can anyone help with our preheat tank problem? We live in central Illinois. We have an Envision Water Furnace with closed ground vertical loops. We use the DSH plumbed into an electric A O Smith 50 gal water tank (no electrical use but was comparatively inexpensive) for the preheat tank. Then the hot water outlet goes to a Rinnai tankless water heater that goes to a Mini Bosch 7 gal (for circulating hot water) then there is an expansion tank and a dedicated hot water line and returns back to a Grundfos Aquastat/Timer circulating pump. The problem is that the A O Smith preheat tank has failed for the 2nd time, leaking from bottom. The whole system was set up in 2007. The first failed in 2011 and now again. We have city water and it is hard so we use a water softener which is first in line from the main water line and then water goes to the A O Smith preheat tank. Not wanting to just put in another A O Smith because the plumbing cost is not worth it if the tank leaks every 3-4 years and what a mess. Wondering if there is something else wrong with the set up before we put in a higher cost preheat tank like a Marathon 50 gal with no elements???