Hi, I went in our mechanical room on Saturday and found water running down the sides of our 85-gallon Marathon water heater. The Marathon is original to the house, built in 1998, and we have never had an issue with it. A little over three months ago, we installed a Series-7 Water Furnace with the DSH and added a 50-gallon pre-heat tank. I'm wondering if the two events could be related in any way. The system had been working great, until the Marathon started leaking. Initially, the water appeared to be leaking where the pressure-relief valve connects to the top of the water heater. I called the company that installed our geo system as I thought it might be their issue. The tech that came out had helped install our system, but he was not a plumber, so he called one of their plumbers for advice as he was not familiar with the Marathons. I could hear both sides of the conversation, and the plumber told the tech that we would need to turn off the DSH and run hot water out of the taps until the Marathon's tank had cooled off as letting air into a hot tank could crack the tank's fiberglass liner. We started doing that and the leak got worse by the minute. By then we could tell the water was leaking out of all three connections on top of the Marathon (hot line, cold line and pressure-relief). At that point we determined the tank had probably cracked near the top. There was a little water coming out of the side access panels, but almost all of it was pouring out of the top. The tech said there was nothing more he could do, and recommended I contact Rheem, especially since he was on over-time since it was the weekend and we were most likely going to be charged for the service call. I contacted Rheem, who authorized a warranty replacement on the water heater. The rep I spoke to told me I needed to try and get in touch with the original installer to have them come out and remove the existing water heater. She didn't really go into detail on what happened after that, but did say if that is not possible, I'd need to call back on Monday as they have a department that handles warranty replacements. We are still in contact with our builder, so I called him and found out the plumber that did the install in 1998 is no longer in business. So I'll be calling Rheem back today. Has anyone had a warranty replacement with Rheem, and if so, how did the process work? We are two days into taking a luke-warm bath with water heated on the stove, and that's already two days too many! Hopefully we'll get a replacement tank shortly and get everything back to normal. I'm assuming Rheem won't pay anything for removing the broken heater and installing the new one, so I'm guessing we are going to be out several hundred dollars for the plumber alone. I've seen Marathon mentioned as both the main water heater, and as a pre-heat tank, in a number of posts, so I assume they are fine to use in geo installations. However, the timing makes me wonder if the broken tank could in any way be related to the install. I'm 99% sure the original installers did not cool down the Marathon before they switched the plumbing over to the new pre-heat tank. Which makes me wonder if that could have caused a hair-line crack or other issue that took over three months to appear. Or perhaps it was just coincidence that the 18+ year old tank finally had issues. Any thoughts on that? I don't know how warm the pre-heat tank should get. I had bought a cheap indoor/outdoor thermometer with a probe for the outside temp, and just inserted that into the input side of the pre-heat tank, between the pipe and the insulation. When the Water Furnace is running at a fairly high level, which it has been with our current weather, that temp has been running as high as 110 degrees, which again, isn't going to be really representative of the actual temperature, or the average water temp in the tank itself. Anyway, the water feeding into the Marathon should be considerably warmer than it ever was before. I would think that should in no way harm the Marathon. Sorry for the long post; a lot of things wandering around in my head right now. Thanks for your help!