I have a Naiven tankless water heater that does a fantastic job! However, if you are in the shower and another hot water faucet is turned on, the water goes cold. We have 70+ psi pressure and 3/4” PVC and cove piping. Any thoughts?
Thank you Diehard! It is Naiven model NPE-240A NG. It has a built in recirculating pump. It is 199,900 BTU/Hr. It flows 8.7 gpm at 45F temperature rise. To answer your question, you can turn on both and they will be hot, but if you are in the shower and turn the other one on the temperature drops quickly (and uncomfortably). I have considered installing a temperature sensing cartridge in the shower.
Even at 67°F(36°C) Temp Rise, that unit has a rating of 5.6 GPM. So it would appear that the units size should not be the problem.
Although the shower cartridge could very well be the source of the problem, I would be inclined to try a combination of other fixtures running together, without the shower running, before replacing the shower cartridge.
There is a check valve built in to the NPE240 series, inside the unit near the bottom.
I have solved the exact problem you are experiencing countless times by having Navien send out a free replacement check valve (if you are in warranty, otherwise just buy one from a supplier). It is extremely easy to replace.
There is a o-ring on those check valves that with just a little age, can cause it to hang up and cause the symptoms.