Tankless Water Heater Not Heating? Common Causes in Tampa
Published 2026-10-09 · Mr Water Heater Team · Florida Master Plumber Licence CFC1433539.
A tankless water heater that is not heating usually has one of four causes: the water flow is too low to switch the burner or elements on, mineral scale is choking the heat exchanger, the unit is not getting fuel or failing to ignite, or a sensor has failed. In Tampa, scale is the one to suspect first, because the city's water is hard enough to coat the narrow passages inside a tankless unit.
A tankless unit heats water only while it flows, so when something interrupts that process you notice straight away. The good news is that most of these faults are repairable. Here is how to tell them apart, and what is safe to check yourself.
Check the display first
Most modern tankless water heaters show an error code on the controller when they shut down. Write it down before you reset anything, because the code is the fastest route to the fault. Manufacturers use different numbering, so look the code up in your own unit's manual rather than a generic list. The codes usually point to one of the four causes below.
Cause 1: flow too low to start the heater
A tankless water heater only switches on once enough water is moving through it. Rinnai's Value Series installation and operation manual gives a minimum activation flow of 0.4 gallons per minute. It also notes that when you get no hot water, or the water alternates between hot and cold, it can be "due to the water flow being below or close to the minimum flow rate."
You will notice this most at a single low-flow fixture, such as a bathroom tap opened only a little. If the kitchen tap runs hot at full flow but the bathroom sink stays cold, flow is the likely culprit. Clogged aerators, a blocked inlet filter on the heater, or a partly closed service valve can all cut the flow.
Cause 2: hard-water scale in the heat exchanger
This is the most common tankless problem in Tampa. The City of Tampa's guide to water hardness puts the city's water at about 140 to 300 parts per million, or 8 to 17 grains per gallon, depending on the season. For comparison, the same Rinnai manual lists a maximum total hardness of 200 mg/L (equivalent to parts per million) for the water supplying the heater. Tampa's water can exceed that for part of the year.
Scale coats the inside of the heat exchanger and the temperature sensors. Water heats unevenly, the unit may overheat and shut itself down, and output slowly falls. Rinnai's units display an "LC" code when scale builds up, and the manual's instruction is to flush the heat exchanger to prevent damage. It adds that repeated LC codes "will eventually lockout the water heater."
The fix is a descaling flush through the service valves, plus regular water heater maintenance so it does not return. In very hard water, a water softener system in front of the heater cuts how often it needs flushing.
Cause 3: no gas, no ignition, or no power
On a gas unit, a cold-water fault can be as simple as a closed gas valve, or a propane tank running empty. It can also point to something more serious: a gas line that is too small for the unit, a failed igniter, or a blocked vent. Rinnai's troubleshooting table marks most of these checks "licensed professional only."
On an electric unit, check whether a breaker has tripped. Electric tankless heaters draw heavy current, so a breaker that trips repeatedly needs a licensed professional to look at it, not a reset.
Smell gas? Leave the house and call your gas utility first. Do not try to relight or work on the unit.
Cause 4: a failed sensor or control board
Tankless heaters rely on flow and temperature sensors to decide how hard to fire. A sensor coated in scale or failing outright can stop the unit heating even when the flow and fuel are fine. These faults usually show up as specific codes, and they call for testing the sensor, cleaning it, or replacing it.
When a repair stops making sense
Most tankless faults are repairable. Replacement is worth discussing when the heat exchanger itself is cracked or leaking, or when an older unit keeps failing. If the household has outgrown the unit, a larger model may solve a problem no repair can. Our guide to whether a tankless water heater is worth it in Tampa covers sizing, and our tankless water heater installation service handles replacements.
Get your tankless water heater repaired in Tampa
Mr Water Heater provides tankless water heater repair across Tampa and West Central Florida, from Carrollwood and Odessa to New Port Richey and Spring Hill. Mark Lowrie holds Florida Master Plumber Licence CFC1433539, and the team is licensed and insured.
Call Mr Water Heater at +1 727 203 8201 or contact Mr Water Heater to book a diagnosis. If you have no hot water, 24/7 emergency service is available.