Tankless Water Heater Not Heating? 7 Things to Check Before You Call
A tankless water heater not heating tends to fall into one of three stories: no hot water at all, water that never gets past warm, or water that used to be hot enough and no longer is. The cause is often small, and the first few checks take about ten minutes.
Work down the list in order. Stop and get a technician out as soon as a step would involve gas pipework, the vent pipe, wiring or the inside of the cabinet. If you ever smell gas, leave the house and call the gas utility from outside.
1. Power to the unit
Every tankless unit, gas or electric, needs electricity for its board and sensors, and on gas models for its fan. Check that it is plugged in, that the outlet or breaker has power and that the display is lit. A dark display with no code points at power. After an outage most units restart by themselves. If yours did not, switch it off at its power switch, wait a moment and switch it back on.
Electric tankless units run on large dedicated breakers. If one has tripped, reset it once. If it trips again, stop and call, because a second trip means there is a fault to find. Do not open the unit's electrical compartment.
2. The gas shutoff
A gas unit has a valve, a handle or knob on the gas line leading to it, that controls its supply. The handle usually sits in line with the pipe when the valve is open. Confirm it is open, and on propane confirm the tank is not empty. Then see whether other gas appliances work: if the range or the furnace is dead too, the problem is upstream of the unit.
That is as far as an owner goes. Regulators, fittings and gas pipework are for a licensed technician.
3. A code on the display
Read it before anything else. A code turns 'not heating' into a specific fault: no ignition (11 on Rinnai, Noritz and Rheem), flame failure (12), an air or exhaust problem (10), or lime scale (LC on Rinnai). Meanings vary by maker, so check your manual. The full list of codes explains the common ones and how to clear them.
4. The set temperature
Look at the setting on the controller. A temperature that was knocked down, or lowered to save energy, produces warm water that feels like a fault. Many households are comfortable at around 120 degrees, so a setting well below that is worth correcting. If the controller says 120 and the tap still runs lukewarm, the setting is not your problem and you can move on.
5. The inlet filter and the taps
The unit only fires when enough water moves through it, so weak flow often looks like weak heat. A clogged inlet filter, a crusted aerator or a blocked shower head can leave you with water that never gets hot. To clean the inlet filter, switch the unit off, close the water valves under it, take out the screen, rinse it and put it back. Expect a little water to spill. Unscrew the aerators and soak them in vinegar.
If hot water is poor at only one fixture, start with that fixture's aerator or shower head. The unit itself is probably fine.
6. The vent terminations
A gas unit needs air in and exhaust out. If something is blocking the exhaust vent, that alone can leave you with no hot water. Walk outside and look at the termination for a bird's nest, leaves, or snow piled against it after a storm. Clear what you can reach by hand. Do not open, cut or adjust any vent pipe.
7. Scale, the slow cause
If the water has been getting less hot over months, or it cycles between warm and hot, the usual suspect is scale on the heat exchanger. Hard water leaves it behind, and it blocks heat from reaching the water. Makers call for a flush every year, and every six months on very hard water. A unit that has never had one is often simply 'not as hot as it used to be'. Our guide to flushing a tankless walks through it.
When to stop checking and call
If power, gas, the code, the filter and the vents all look right and the water is still not hot, or an ignition, flame or exhaust code returns after one reset, it is time for a technician. A diagnosis visit is estimated at $100 to $250, and most part repairs fall in an estimated $150 to $800. These are estimates only: the price is agreed in writing before any work starts. A gas smell, water around the unit or a breaker that keeps tripping means call right away.
Quick answers
Why is my tankless water heater only getting warm water?
The usual causes are scale on the heat exchanger, weak flow from a clogged filter or aerator, or a unit that is too small for the number of taps running at once. Check the filter and the fixtures first, then think about a flush.
Why is my tankless water heater not as hot as it used to be?
A slow loss of heat over months usually means scale. A dirty inlet filter or a failing flow sensor can do it too. If the unit has not been flushed in years, a flush is the first thing to try.
What is the first thing to check when there is no hot water?
Power to the unit and the gas shutoff. Confirm the display is lit and the gas valve handle is open, then read any code on the display.
Why is my new tankless water heater not getting hot?
A new unit that never heated well points away from scale. Suspects include hot and cold lines crossing over somewhere in the plumbing, a unit too small for the fixtures it feeds, or a gas supply that cannot keep up. A technician can test each one.
Why does my tankless water heater not work after a power outage?
It needs power to run. Once power returns, most units restart on their own. If yours does not, switch it off and on at its power switch, check the breaker, and call if the display stays dark or shows a code.
Still no hot water?
A technician who works on tankless units can test it and price the fix in writing.
