A tankless water heater has no stored hot water, so any fault shows up right away as cold or lukewarm flow. The cause is usually one of a handful of things: scale on the heat exchanger, a clogged inlet filter, a flow rate too low to trigger the burner, a venting or gas problem, or demand that exceeds the unit's rating.
Check these in order, from easiest to hardest. If the unit shows an error code, look it up in the manual first, because a code often names the fault directly. When a check points to a repair beyond a filter clean, our tankless water heater service page covers what a plumber does.
Start here.
Fast checks before you do anything else.
- Check that the unit has power and the display is lit. Reset the breaker once if it is dark.
- Look at the temperature setting. A unit set too low delivers lukewarm water by design.
- Confirm the gas valve is open on a gas unit, and that other gas appliances work.
- Try a different hot tap. If only one tap is cold, the problem is the faucet or a cross-connection, not the heater.
- Read any error code on the display and note it.
Diagnosis.
Common causes and what they look like.
| Symptom | Likely cause | What to do |
|---|---|---|
| Water warm but never hot. | Scale on the heat exchanger. | Flush with descaling solution. |
| Water cuts in and out. | Flow too low to hold the burner on, or a clogged inlet filter. | Clean the filter, open faucets fully, check for low pressure. |
| Cold water suddenly in a shower. | Demand above the unit's rating, or a cold water sandwich. | Reduce flow or run fewer fixtures at once. |
| Hot at some taps, not others. | A cross-connection in a single-lever mixing valve. | Plumber diagnosis. |
| Unit will not ignite. | Gas supply, venting or a failed igniter. | Check the code and call a plumber. |
Maintenance.
How to descale a tankless heater.
Most tankless heaters have two service valves on the water lines. With them, you can circulate a descaling solution, often white vinegar or a manufacturer-approved product, through the heat exchanger with a small pump and two hoses. A typical run is about an hour. Afterward, flush with clean water and clean the inlet screen.
Skipping this step is the single most common reason tankless units lose output. If your unit has no service valves, a plumber can add them. Annual flushing is a standard recommendation, and manufacturers specify details for each model, so check yours.
Other causes.
Minimum flow and the cold-water sandwich.
Every tankless heater needs a minimum flow rate before it fires, often around half a gallon per minute. A low-flow faucet, a clogged aerator or a partly closed valve can drop below that, and the unit shuts off. Cleaning aerators and checking valves is a free test.
The cold-water sandwich happens when you shut off hot water briefly and then turn it on again. Water left in the pipe cools while the unit has shut down, so a slug of cold water arrives before hot water returns. It is a design trait, not a fault, and a small recirculation setup can reduce it.
Gas smell or soot.
If you smell gas or see black soot around the unit, shut off the gas, ventilate and call the gas utility or 911 before anything else. Do not try to light or reset the unit.
Tankless heaters in Salem winters.
Massachusetts winters bring incoming water down to temperatures where a tankless unit has to lift it much further than in summer. A unit that handles two showers in July may fall short in January, so a dip in output during a cold spell is not always a fault. The Board that supplies Salem water moves water between the Putnamville and Wenham Lake reservoirs, and its October 2025 update noted manganese causing color changes in some areas. Sediment can reach the inlet filter, so a filter cleaning is worth doing after a period of discolored water.
If your unit is a gas model in a closet or basement, check that the vent is clear of snow and ice at the termination, since a blocked vent will shut the unit down.
For homes across the harbor, we serve Beverly as well. If the unit is dead and you need to compare options, water heater installation covers the choice between a repair and a new unit.
Questions.
Tankless heater questions from Salem homeowners.
The most common causes are scale on the heat exchanger, a clogged inlet filter, a low flow rate or demand above the unit's rating.
Many manufacturers recommend yearly. Check your manual, and flush sooner if output is dropping.
Yes, but colder incoming water lowers the unit's output. A unit sized for peak winter use handles it.
It points to a fault such as ignition, flow, venting or a sensor. Check your manual for the code and tell your plumber.
If cleaning the filter and descaling do not help, if there is a gas smell or if the unit will not ignite.