How to Clean the Inlet Water Filter on a Tankless Water Heater

There is a small screen where cold water enters the heater, and everything your plumbing sheds ends up on it: pipe scale, solder debris left over from installation, grit from a well or from a water main repair down the street.

When it loads up, flow drops. That matters more on a tankless unit than on a tank, because a tankless heater needs a minimum flow rate before it will fire at all. A partially blocked inlet filter is why a low-flow bathroom tap runs cold while the shower is fine, and it is on Rinnai’s own list for recirculation low-flow faults and for cold water sandwiches.

How to do it

1. Close the cold and hot water isolation valves below the unit.
2. Put a bucket underneath. The heater still holds water and some of it will come out with the filter.
3. Unscrew the filter at the bottom of the unit, on the cold inlet.
4. Rinse the screen clean of debris. If it is torn, deformed, or collapsed, fit a new one — a damaged screen passes exactly what it was installed to stop.
5. Refit the filter and open both isolation valves.
6. Run a hot tap and check for drips around the filter housing before you walk away.

How often

Once a year is a reasonable baseline, and always as part of a descaling flush — Rinnai’s own flush procedure includes clearing this filter of the residue the flush loosens.

Clean it sooner if any of these apply:

• A tap that runs at low flow has started running cold
• The temperature swings during a shower
• Any plumbing work has been done on the house recently
• The unit is on well water

If it clogs again quickly

A filter that is full again within weeks is telling you something upstream is shedding. Old galvanised pipe, a failing water heater elsewhere in the system, or a well without adequate filtration will all do it. Cleaning the screen repeatedly treats the symptom; a whole-house sediment filter treats the cause, and it protects everything else in the house too.