A tankless water heater with no hot water is usually doing one of three things. It can’t light (a gas, ignition or vent problem). It isn’t seeing enough water flow to start. Or it’s shutting itself off because scale has made the heat exchanger run too hot. A flush fixes the scale problems. It won’t fix the others. The display code, if there is one, tells you which group you’re in.
No hot water at all
If every hot tap runs cold and the heater shows a code, start there. Ignition and flame codes are the most common:
- Rinnai lists code 11 as no ignition (the heater is not turning on), and code 12 as no flame.
- Navien’s E003 is ignition failure.
- Noritz’s code 11 is ignition failure.
- On a Rheem RTGH, code 11 means the gas shut-off valve is not fully opened.
A blocked vent or air intake stops the burner too. Rinnai’s code 10 is an air supply or exhaust blockage.
These are gas and venting faults. A flush won’t help, and a technician should do the diagnosis. The repair page covers what’s involved and HomeGuide’s cost ranges.
Hot, then cold, then hot again
There are two kinds of swing, and they mean different things.
The cold-water sandwich. You shower, turn the water off, then turn it back on. You get hot water from the pipes, a short burst of cold, then hot again. The cold slug is water that sat in the heater while it was off, before the burner fired again. It’s normal for tankless heaters and doesn’t mean anything is wrong.
Swings during steady use. If the water goes cold partway through a shower and comes back, the heater may be shutting down and restarting. Scale is a common reason. Rinnai’s code 16 is high outgoing temperature (a safety shutdown because the heater is too hot). Navien’s E016 means the heat exchanger is overheating. Bosch’s E1 says the hot water sensor is reading above 185 F, possibly due to scale build-up. When scale coats the inside of the heat exchanger, heat builds up in the metal, and the heater trips its safety limit.
Low flow causes swings too. A tankless heater needs a minimum flow to stay lit. A single low-flow fixture, or a clogged inlet filter, can drop below it. Rheem’s P1 code means there is not enough water flow to operate the unit.
Weak hot water flow
When hot taps run weaker than cold ones, look at what sits between the cold supply and the heat exchanger:
- The inlet filter. This small screen catches grit and scale flakes. Navien’s fixes for E030 (exhaust overheating) include cleaning it.
- Scale in the heat exchanger. Bosch says scale may result in lower flow rates. In the Battelle lab study, every heater on unsoftened water was eventually clogged with scale buildup.
- A flow valve stuck by scale. On some Takagi models, code 651 means a flow adjustment valve fault, and Takagi lists a locked motor drive due to scale buildup as a possible cause.
All three are things a flush visit deals with.
Boiling, kettling or rumbling sounds
A heater that sounds like a kettle is a classic sign of scale. Bosch’s bulletin says scale buildup in the heat exchanger may cause lower flow rates, A7 and E9 codes and boiling sounds. The noise comes from water flashing to steam at hot spots where scale has built up. Clicking during start-up, or the fan spinning up, is normal. A steady boil or rumble while hot water runs is not.
Signs your heater needs descaling
Put these together and you have a good case for a flush:
- A reminder code: Rinnai SS, Navien E760, Noritz C1# or Rheem 88
- A scale warning: Rinnai LC or Rheem 1L
- Temperature swings or overheating codes during steady use
- Hot water flow that’s weaker than it used to be
- Boiling or rumbling from the heater
- More than a year since the last flush, on hard water
Utah water speeds all of this up. American Fork tells residents to set softeners at 12.38 gpg. Herriman’s local wells measure 25–60 gpg. Spanish Fork’s water page listed total hardness of 231–349 mg/L. Rinnai also says scale is accelerated by high temperature settings, so turning the temperature up to fix lukewarm water can make scale worse. The flush interval guide has more on timing.
What a flush fixes, and what it doesn’t
| Symptom | Can scale cause it? | Will a flush help? | Who should look |
|---|---|---|---|
| Reminder code (SS, E760, C1#, 88) | Usually a timer saying service is due | Yes, that’s what it’s asking for | Technician, for the flush |
| Scale warning (LC, 1L) | Yes | Yes | Technician, for the flush |
| Overheating code (14, 16, E016, E1, E9) | Often | Often, after a technician confirms | Technician |
| Swings during steady use | Often | Often | Technician |
| Weak hot water flow | Often, or a clogged inlet filter | Usually | Technician |
| Boiling or kettling sounds | Yes | Usually | Technician |
| No ignition or no flame (11, 12, E003, E012) | No | No | Technician, for a repair |
| Vent or air blockage (Rinnai 10, Noritz 90) | No | No | Technician, for a repair |
| Circuit board fault (Navien E515) | No | No | Technician, for a repair |
| Leak code (Rinnai 79) | Not usually | No | Technician, for a repair |
A flush is maintenance. It’s the right first step when scale is likely, and the wrong one when the problem is gas, air or electronics. Being clear about that saves you paying for the wrong visit.
Safe checks before you call
- Check other taps. Is it one fixture or the whole house? One fixture points to that fixture.
- Look at the display. Write down any code, and whether it flashes or alternates. The error code hub explains the common ones.
- Check power. Tankless gas heaters need electricity too. Make sure the unit is plugged in and the breaker is on.
- Check other gas appliances. If the stove or furnace won’t light either, the problem may be the gas supply. Call your gas utility.
- Leave the cover on. Anything inside the cabinet, on the gas line or on the vent is a job for a technician.
If you smell gas, leave the house and call your gas utility from outside.
No hot water? Call now. A local tankless technician can tell you whether you need a flush or a repair, whether you’re in Saratoga Springs, Orem or South Jordan. If the heater is old and the repair is big, the lifespan guide helps you weigh replacing it.