A tankless water heater may go cold in the middle of a shower because demand rose above what the unit can heat, the water flow dropped below what it needs to keep running, or the unit shut off because of a fault. Scale buildup and installation issues can also play a part.
Tankless units heat water as it flows through them instead of storing it. That design works well in many homes, but it also means the unit reacts quickly to changes in flow and demand. Knowing the common causes can help you describe the problem when you call for service.
When Too Much Hot Water Is Needed at Once
Every tankless unit can only heat a certain amount of water at a time. If someone starts the dishwasher, the washing machine or a second shower while you are in the shower, the total demand may be more than the unit can handle. The water may turn lukewarm or cold.
This is more noticeable in winter, when the water coming into the building is colder and the unit has to work harder to heat it. A unit that keeps up in summer may struggle in January.
When the Flow Is Too Low
Tankless heaters need a minimum amount of water flowing through them to turn on and stay on. A low-flow shower head, a partly closed valve or a mixing valve set mostly to cold can drop the flow below that point. The burner or elements may shut off, and cold water comes through.
Some people also notice a brief burst of cold water after turning the shower off and back on. This can happen as the unit restarts and heats the new flow of water.
Other Possible Causes
A few other issues may lead to cold spells:
- mineral scale building up inside the unit over time
- a clogged inlet screen or filter
- venting, gas supply or electrical problems that cause the unit to shut down
- a unit that is undersized for how the home uses hot water
Some of these can trigger an error code on the unit’s display. Note the code, but do not open the unit or try to adjust gas, venting or electrical parts yourself. These systems involve safety controls that should be handled by a professional.
What You Can Note Before a Service Visit
Write down when the problem happens, which fixtures were running and whether it is worse in cold weather. Note the make and model of the unit if you can see the label. These details can help narrow down whether the cause is demand, flow, maintenance or a fault.
If your tankless unit keeps leaving you in a cold shower, our tankless water heater service can check the unit and how it is set up. Call (929) 209-6183 to set up a visit in the Bronx.
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