We recently installed a Navien NR240A tankless and installed it per the installation manual. We do have a return recirculation line hooked up to it, but will probably rely on the internal recirculation (just avoiding cold water sandwiches to our intermittent water sipping high efficiency dishwasher and LG washing machine rather than giving us fast hot water that the external circulation does). We keep the valve to that return line closed.

Since we are doing a whole house remodel, a valve was installed in a back shower before the trim arrived and the plumber left the valve open which resulted in the hot and cold lines being connected unknowningly. The Navien with internal recirc on then ran nearly continuously and got the water too hot. I called Navien and the Navien tech was very good and immediately diagnosed the likely cause (hot and cold lines connected) and based on that our plumber quickly found and fixed the problem valve.

However, the Navien tech also advised putting a check valve on the cold inlet after the water softener. The install manual does not specify this (it does for multi-unit installs, but not single tankless installs). He also suggested putting a second check valve on the return line in case we do use it.

Should we install a check valve? Will it impair water pressure?

Our install:

If I do use the hot water return recirculation I'd rather put it on on-demand rather than use the pump and timer in the Navien. If I replumb the return recirc to use something like a chilipepper or metlund manual pump, should an expansion tank also be put in? I see that the chilipepper pump has a check valve internal to it.