Why your tankless water heater goes cold, and what fixes it

The cold water sandwich is normal, to a point

If you shower, turn the water off briefly, and turn it back on, you may get a few seconds of cold water before hot arrives again. That is the cold water sandwich: hot water left in the pipe, then cold water that passed through while the burner relit, then hot again. It is a characteristic of tankless heating, not a fault.

It becomes a problem when it is long or frequent. Short-cycling from a scaled flow sensor, a flow threshold set too high, or a recirculation setup fighting the unit can all exaggerate it. A small buffer tank or adjusted settings can reduce it considerably.

Scale is the most common cause here

San Jose's water is hard; published guidance puts it at roughly 10 to 16 grains per gallon depending on the season and blend. Heating that water drops minerals out as scale, and in a tankless unit that happens inside a narrow heat exchanger and around the flow sensor. The unit then struggles to transfer heat, overheats locally, or misreads flow, and the result is lukewarm water, temperature swings or a scale warning.

The fix is a descaling flush through the unit's isolation valves, followed by cleaning the inlet filter. If the unit has no isolation valves, fitting them makes this a short job every year after.

Flow too low to keep the burner lit

A tankless unit fires only when water flows above a minimum rate. A low-flow shower head, a partly closed valve, a clogged inlet filter or a scaled aerator can drop the flow near that minimum, so the burner cycles on and off and the water swings between hot and cold. Opening the hot tap a little further often confirms it.

Cleaning the inlet filter and aerators, and checking that the stop valves are fully open, is the first step. If the house has very low-flow fixtures, the unit's minimum flow setting may need attention.

A gas supply too small for the burner

When two showers run and the water goes lukewarm, or the unit throws ignition errors under heavy demand, the gas supply is often the reason. A tankless burner needs far more gas than a tank heater, and units installed without upsizing the line starve when demand is high. A gas pressure reading at the unit while it runs at full fire shows it clearly.

The fix is a larger gas line sized for the unit and every other gas appliance. No replacement part solves this one.

A unit undersized for the house

Tankless units are sized by how many gallons per minute they can heat at a given temperature rise. A unit that handles one shower comfortably may not keep up with two showers and a dishwasher. If the water goes cold only when several fixtures run and the gas supply checks out, the unit is simply too small, and the options are a larger unit or a second one linked to share the load.

What to check before you call

Note any error code on the display; it narrows the diagnosis. Check that the gas valve to the unit is fully open. Clean the cold inlet filter if you can reach it safely with the water off. Try the shower with the hot tap opened a little further. If the problem continues, or you smell gas, call.