I recently had a plumbing company install a tankless hot water heater in my crawlspace. Everything with the install works and looks great, but we are experiencing a very strong chemical smell (like nail-polish remover or a strong sharpie marker) that comes out of our faucets when we call for hot water. It usually lasts for about 10 seconds then goes away. I notice it whenever we haven't used the hot water for awhile. For example: the smell is very strong first thing in the morning when turning on the shower.
The plumbers used PEX pipes to install a hot water recirculation pipe that is soldered to the original copper pipes. Other PEX pipes were soldered where they needed to connect to some other areas of the copper pipes including a new Pressure Reducing Valve.
Does anyone have any insight or advice on what could be causing this issue?