Inlet water filter
A small screen at the cold water inlet that catches debris before it reaches the heater. One of the two genuinely homeowner-serviceable parts on a tankless unit.
What it does
Everything entering the heater passes through this screen first: pipe scale, solder debris left from installation, grit from a well or from a water main repair. It protects the flow sensor and the narrow passages of the heat exchanger from anything that would restrict them.
What happens when it clogs
Flow drops. That sounds harmless until you remember that a tankless unit needs a minimum flow rate to fire at all — so a clogged inlet filter shows up as water that will not get hot at low flow, as temperature swinging during a shower, and as a cold water sandwich. Rinnai’s own troubleshooting for low flow and for the cold water sandwich both include cleaning this filter, and its guidance to technicians is to clean it on every service call.
Cleaning it
This one you can do. Close the hot and cold isolation valves under the unit, put a bucket underneath because the heater still holds water, unscrew the filter, rinse the screen clean, refit it, and open the valves again. Our step-by-step guide covers it in full, including what to check while you have the valves closed.
Replace rather than rinse if the screen is torn or deformed — a damaged screen passes exactly what it was fitted to stop.
What a replacement costs
$10-30 for the filter, and nothing but ten minutes if you clean it yourself.