Good evening. I posted previously about my coaire tankless water heater issues and these forums were a great help. I had a pressure reducing valve start leaking/losing pressure. With the help of these great forums we replaced the PRV and added an inline pressure gauge to help monitor the pressure. 2 months later (now) the new PSI gauge sprung a big leak. I replaced the gauge again and all is well. This got me thinking. Why did a PSI gauge completely fail 2 months in? Could my expansion tank be bad? the psi guage never reads above 19 psi. Other than the gauge leaking the unit functions properly. I shut off the water going to the unit and drained the unit by manually lifting the pressure release valve until the unit had zero pressure at the gauges, I took my pressure gauge and measured the PSI at the expansion tank shrader valve and it measured ZERO. Does this mean my internal expansion tank failed? Did I test it properly? If the answer is yes, and the tank has failed. I would like to know if its possible and how to plumb in a external expansion tank. ( if it is possible it will be done by a journeymen gasfitter/plumber) dont worry! Please note this tankless unit also heats my house. Attached are some detailed pictures of the unit. The expansion tank is circled in red and I also attached a picture of the gauge that was leaking. hopefully this is enough info. Thanks so much.