Cold Water Sandwich on a Tankless Water Heater

You open the tap, get hot water for a second or two, get a slug of cold, and then it goes hot again and stays. That is a cold water sandwich, and it does not usually come with an error code.

The part that is normal

Some of it is physics. When the previous user closed the tap, the water sitting in the pipe between the heater and the fixture was hot. It cools while it waits. When you open the tap again you get that leftover hot water first, then the slug of cooled water still in the line, then genuinely heated water once the burner has lit and caught up.

A short sandwich in a house with a long pipe run is the system working as designed. A recirculation loop is the cure for that version, not a repair.

The part that is not normal

A sandwich that arrives every time, or lasts more than a couple of seconds, usually means the heater is cycling on and off when it should be running continuously. The documented causes are worth knowing because several are free to fix:

• A clogged inlet water filter. Restricted flow drops the unit below its minimum firing rate, so it cycles. Manufacturers tell technicians to clean this filter on every service call, which tells you how often it is the answer. Rinnai’s own instruction is that you should have at least 1 GPM at every fixture; below that, clean the filter and then the aerator or restrictor at the tap itself.
• Improperly installed or flapping check valves. A swing check valve should be installed with the hex cap horizontal, facing upwards. Fitted wrong, it flaps and disturbs the flow the heater is trying to measure.
• Debris in the fan blower wheel, which makes the unit ramp its output up and down.
• Inlet gas pressure moving around because the meter, regulator, gas line, or propane tank is undersized. Check this when other gas appliances fire — a heater that sandwiches only when the furnace runs is telling you something specific.
• Fluctuating water pressure, typically on a well system or in an undersized plumbing system.
• A faulty valve diaphragm or check valve elsewhere in the house letting cold bleed into the hot side.

What to do about it

Start with the two free ones: clean the inlet water filter, and check the flow rate at the fixture that behaves worst. If a low-flow tap is the only one affected, clean its aerator before you suspect the heater at all.

If the sandwich happens at every fixture, at every flow rate, and cleaning the filter did nothing, the next steps — gas pressure, check valves, blower inspection — are a technician’s. Mention when it happens and whether other gas appliances are running at the time; that detail saves diagnostic hours.