I have a rinnai c53 tankless hot water water heater.I use it to run my hot water baseboard and regular water use. I recently converted some CPVC to copper because of leaks. After this change over, my kitchen and bathroom faucets are not firing the hot water heater up. The only shower in the house, washing machine and heat will fire the unit up. Sometimes if I let a faucet run long enough it will fire, but only sporadically.
A few notes:
Both faucets on simultaneously will fire unit.
All faucets are free of debris
Pressure gauge reads 50 then drops to about 38.
Water pressure much stronger than before, plenty of pressure at faucets.
Unit is 12 years old
Regularly de-scaled (water isn't hard anyway)and maintained

Any thoughts would be appreciated.

could be the flow switch ?

aerators now dirty restricting flow

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"Measure Twice & Cut Once" - Retired U.A. Local 1 & 638

I would suggest taking off the aerators and run the faucet to see if it triggers the unit to fire up. This will eliminate the aerator from the possible causes.

Have you checked/cleaned the inlet filter?