Met with the wife and proceeded to diagnose the issues with the Navien Combi boiler. I placed the boiler through its paces and ran the hot water throughout the home placing the boiler under high demand. The boiler produced heat throughout but failed to generate hot water. Prior to my diagnosis, I called Navien’s tech support hot line for confirmation of my findings. Navien and I reached the same conclusion. The boiler had a faulty three way valve in need of dealing with a descaling flush. I broke down the boiler by removing all internal components such as electrical circuit board, circulator, wiring, plumbing and replaced the 3way valve. Once installed, I reassembled the boiler turned on the water and checked for leaks. After no leaks was confirmed, I restored power, fired up the boiler and confirmed proper operation with heat and domestic hot water throughout the home. Finally, I shut down the boiler and connected a bucket with descaling solution and flush the heater as recommended by