I have a Rinnai R85 tankless water heater. We recently had an issue with no hot water which was resolved by performing a flush. Since then, upstairs sinks and tubs are fine, but our two kitchen sinks have no pressure and one doesn’t have pressure on hot or cold. HELP!!! Nothing was changed on any lines - but there’s no pressure and only in those sinks.
An issue with no hot water being resolved by performing a flush suggests to me that there was a buildup -- hard water scale, ie mineral deposits -- on the inside of the heat exchanger tubing. I suspect that the deposits were not entirely removed from the heater during the flush. Some debris may have remained inside the plumbing. That debris found its way to some faucets and is now plugging them.
The first thing to check is the aerator. This is a screen assembly installed right at the point where water comes out. Unscrew it and clean out any debris. It may not be easy to remove, especially considering that you had scale problems in the water heater.. Soaking the end of the faucet in vinegar for a few hours may ease the removal.
If cleaning the aerator doesn't resolve the problem the next thing to check is the faucet valve. Remove it, check for damage or debris, and re-install.