Tankless Water Heater Not Getting Hot? Check the Flow First
A tankless heater only fires when enough water moves through it. A surprising number of cold showers come down to a trickle that never crossed that line.
Quick answer
A tankless water heater only fires above a minimum flow, about half a gallon per minute on most models. Open the hot tap fully, clean the faucet aerator or showerhead, and check that the heater has power, gas and water. If it cuts out mid-shower, clean the inlet filter and try lowering the set temperature to 120°F so you mix in less cold water.
- Difficulty
- Moderate DIY
- Minimum flow to fire
- About 0.4 to 0.65 gpm, by maker
- Quick test
- Fill a quart from the hot tap; time it
- Cuts out mid-shower
- Flow fell below the minimum
- Technician job
- Gas pressure, descaling, error codes
On this page
A tank water heater keeps a store of hot water ready. A tankless heater has nothing stored: it heats water only while it flows through, and it only starts when it senses enough flow. Below that minimum, it stays off on purpose, because it reads a trickle as “no hot water needed.”
That one design fact explains most complaints. Water that never gets hot at one faucet, a shower that turns cold when you turn it down, hot water that comes and goes: all of them are often the flow dropping under the line. So the flow is the first thing to check, before anything inside the heater.
If you smell gas, stop. Do not touch the heater or any switch. Leave the house and call your gas utility or 911 from outside. Do not remove the heater's front cover: gas pressure, venting and the parts behind the cover are work for a licensed technician.
The minimum flow, and a test with a jar
Makers publish the flow a heater needs to switch on. A quart is a quarter of a gallon, so you can test a tap with a quart jar and a clock: the faster it fills, the higher the flow.
| Maker’s figure | Flow needed to fire | A quart from the hot tap must fill in |
|---|---|---|
| Rinnai gas models, low end | 0.4 gpm | 37 seconds or less |
| Noritz, most models | 0.5 gpm | 30 seconds or less |
| Rinnai gas models, high end | 0.6 gpm | 25 seconds or less |
| Bosch Therm C 1210 ESC | 0.65 gpm | 23 seconds or less |
The times are worked out from the makers’ flow figures (15 divided by the gallons per minute). Bosch gives the same 23 seconds in its own troubleshooting steps. Rinnai adds that once lit, its heaters need 0.26 gpm to stay on. Your model’s figure is in its manual.
To run the test, open only the hot side of the tap, all the way, and time how long a quart container takes to fill. If it is slower than the figure for your heater, that tap will not fire it.
Check in this order
- The display.Safe check
A blank display means no power. Check that the heater is plugged in and switched on and that its breaker has not tripped. If the display shows an error code, write it down and look it up in the manual before doing anything else.
- Gas and water supply.Safe check
Noritz's first checks for a heater that shuts off are that gas, water and power are all supplied to it. Make sure the gas shutoff valve and the water valves at the heater are fully open, and that other gas appliances in the house are working.
- Open the tap fully to hot.Safe check
Noritz says to make sure the fixture is opened fully to the hot side; a very low-flow fixture that is only partly open may not create enough flow to activate the heater.
- Aerators and showerheads.Safe check
If one fixture is the problem, unscrew its aerator or showerhead and rinse out scale and grit. Rinnai lists cleaning or removing aerators, and replacing low-flow showerheads or fixtures, among the fixes for minimum flow problems.
- The shutoff valves under the sink.Safe check
Noritz says to make sure these are fully opened. A half-closed stop valve can be the whole problem at one faucet.
- The inlet filter.Safe check
Tankless heaters have a small screen on the cold-water inlet that catches sediment. When it clogs, flow drops at every tap. Noritz, Rinnai and Bosch all list cleaning it. Follow the steps in your manual, or ask a plumber to do it.
- Lower the temperature setting.Safe check
This one sounds backwards. Noritz suggests lowering the water temperature to 120°F, because you then use more hot water and meet the minimum flow. Rinnai gives the same advice, to avoid cutting in too much cold water.
- How many taps are running.Safe check
A tankless heater can only heat so much water at once. Bosch says its Therm C 1210 ESC will support two showerheads of 2.0 to 2.5 gpm at the same time, and that greater draws reduce flow at the taps. If the water turns lukewarm only when the washer, dishwasher and a shower run together, you have found the limit.
Match the symptom to the cause
| Symptom | Likely cause | First step |
|---|---|---|
| No hot water anywhere, display blank | No power | Plug, switch, breaker |
| No hot water anywhere, error code showing | The heater has locked out | Look up the code; call for service |
| Cold at one faucet only | That fixture’s flow is under the minimum | Aerator, stop valve, quart test |
| Goes cold when you turn the shower down | Flow fell below the minimum | Lower the set temperature to 120°F |
| Hot, then cold, then hot | Burner shutting off and relighting | Inlet filter, set temperature |
| Lukewarm everywhere | Setting too low, too many taps, or cold water crossing over | Check the setting; run one tap |
| Was fine, slowly getting worse | Scale, or a clogging inlet filter | Clean the filter; book a flush |
Cold water crossing into the hot side
A worn single-handle faucet or mixing valve can let cold water leak into the hot pipes. Bosch calls this a plumbing crossover and says it does two things: it cools the hot water on its way to you, and it pushes back against the flow through the heater, so the heater needs more flow than normal to fire.
Bosch’s test: shut off the cold water supply to the water heater, open all the hot taps the heater serves, and wait 10 minutes. No water should be flowing from any of them. Any continuous flow, small or large, means there is a crossover, and Bosch says to have a plumber find and correct it.
Leave these to a technician
- Gas pressure. If the gas supply is too small for the heater, it cannot reach full output. Bosch says a pressure reading is needed and that the installer or a certified gas technician must take it.
- Descaling. In hard water, minerals build up inside the heat exchanger. Rinnai says the unit should be flushed periodically and that this should be done by a professional installer.
- Error codes that return. A code that comes back after the heater has been switched off and on needs service.
- Anything behind the front cover.
A tankless heater that is serviced lasts a long time: Rinnai puts the life of its heaters at up to 20 years, depending on water quality, use and maintenance. If yours is near that age and the repair is large, the repair or replace calculator helps you compare.
Have a tank instead? See a water heater that is not heating, and how long water heaters last for what to expect from each type.
To see what a tankless heater costs to run on gas or electricity, see what a tankless water heater costs to run.
Frequently asked questions
Why does my tankless water heater go cold in the middle of a shower?
Usually the flow through the heater dropped below its minimum and the burner shut off. Noritz says this type of problem is typically caused by low water flow and suggests cleaning the filter and lowering the temperature to 120°F so more hot water is used.
What is the minimum flow rate for a tankless water heater?
It depends on the model. Noritz says 0.5 gallons per minute for most of its models. Rinnai says its gas units need 0.4 to 0.6 gpm to ignite and 0.26 gpm to stay on. Bosch lists 0.65 gpm for its Therm C 1210 ESC.
Why do I get hot water at some faucets and not others?
Noritz says that when some fixtures activate the heater and others do not, the fixture is probably not creating enough flow. Scale or debris in the faucet, a shutoff valve under the sink that is not fully open, or a very low-flow fixture can all cause it.
Why would lowering the temperature make the water hotter?
With a very hot setting you add a lot of cold water at the tap, so less water passes through the heater. Bosch explains that too much cold can slow the flow through the heater below its activation point and shut off the burners. A lower setting means you use more hot water and keep the flow up.
How often does a tankless water heater need to be flushed?
It depends on your water. Rinnai says the unit should be flushed periodically to remove scale and lime, that the interval depends on water quality, and that the work should be done by a professional installer.
Sources
- My heater shuts off while I am in the shower or using the sink in the kitchen, why? Noritz
- Why don't I get hot water at all my fixtures? Noritz
- Frequently Asked Questions Rinnai
- Troubleshoot Therm C 1210 ESC Bosch Home Comfort
First published October 8, 2026. Last reviewed October 8, 2026. Spot an error? Tell us and we will check it against the source.