Navien lists a short set of checks. Clean the cold water inlet filter, open the hot tap enough to pull at least 0.5 gallons per minute, read any error code, and look for cross plumbing between hot and cold lines. Confirm the valves are open and the temperature is set. If frozen, warm the heat exchangers.
How do I clean the cold water inlet filter?
The inlet filter is the first item on both of the cold water rows, so it is the first thing to try. Navien's steps for cleaning it, from Section 4.4, run as follows:
- 1Turn the heater off with the Power button.
- 2Close the gas valve.
- 3Close the water supply valve to the heater. If there is no valve, shut off the water at the main.
- 4Open the hot water taps to drain the lines, with a bucket under the heater to catch the water that remains.
- 5Remove the cold water inlet filter and rinse it with clean, cold running water. Scrub it with a brush if needed.
- 6Put it back and tighten the cap securely.
- 7Reverse the drain steps to refill, restore power and turn the heater on.
On the A models there is also a recirculation inlet filter to clean the same way, and Navien says to reinstall each filter and tighten it securely before refilling the heater. Once the heater is refilled, open a hot tap fully and let the air work out of the line before you judge whether the cleaning helped. If you are not comfortable with any step, ask a plumber. It involves the gas valve and the power supply, and a mistake can damage the heater.
A partly clogged filter is a good first suspect for a heater that heats but delivers a weak or cold flow. It also explains why this filter appears in the chart for E016, the overheating code, in our page on Navien error codes E012 and E016.
Why does the flow rate matter?
Navien's chart asks whether the hot tap is open wide enough to draw at least 0.5 gallons per minute through the heater. Navien treats 0.5 gallons per minute as the floor for the heater to work as it should. That is why a trickle from a low flow aerator or a barely opened lever can leave you with cold water.
Two practical checks follow. Open one tap fully and see whether the heater lights. If it does, and it goes cold when you back the tap off or when a second fixture opens, the flow is the issue, not the heater. And if only one fixture goes cold, look at its aerator or cartridge, which Navien's manual does not cover.
### What about cross plumbing?
Cross plumbing means a connection between the hot and cold lines. Navien lists it under the not hot enough symptom. If cold water reaches the hot line, it dilutes what the heater sends out. A plumber finds this by isolating fixtures one at a time. It is not something to guess at.
### What if I use a recirculation line?
The manual says the heater will not deliver hot water instantly at the tap unless a recirculation line runs from the furthest fixture back to the heater, and on A models the unit must be set to external recirculation mode. For an external recirculation system, Navien's chart also asks whether there is a check valve on the return line.
### Could the heater be frozen?
In the Valley a frozen heater is a less likely cause than a dirty filter, but Navien's manual covers it, and says freezing damage is not covered by the warranty, and that the heater's freeze protection function needs electricity. Do not unplug it, and do not close the gas valve except for maintenance. If you suspect a frozen heater, the manual says to turn off the Power button, close the gas valve, open the hot tap nearest the heater, and warm the primary and secondary heat exchangers with a hair dryer or portable electric heater. Check every few minutes for flow, and when it returns, check the heater and piping for leaks. If you find a leak or the heater misbehaves afterward, call a technician.
### Is the temperature set correctly?
The heater ships set to 120 degrees Fahrenheit. Navien's manual says to close all hot taps before adjusting the setting, because with a tap open you can only change it within 98 to 110 degrees. It also warns that water above 120 degrees can scald instantly, recommends the lowest comfortable setting, and says to install a temperature limiting or mixing valve if you set the heater above 125 degrees. The controller changes in 1 degree steps up to 120 degrees, and above that, holding the button for 2 seconds moves it in 5 degree steps. The setting is kept through a power outage. Do not raise it to solve a cold water problem before you know why the water is cold.
When should I call a plumber?
Call if the water is still cold after the filter is clean and the tap is fully open, if a code keeps returning, if you suspect cross plumbing, if the heater has frozen, or if you smell gas at any point. If you smell gas, do not light or switch anything, leave, and call Southwest Gas at 877-860-6020 or 911.
HQ Plumbing and Air is licensed and insured and works emergencies 24/7. Free estimates and upfront pricing come standard. For the codes themselves, see our list of Navien tankless error codes. For other causes of a heater that goes cold, see why a tankless water heater goes cold mid shower and common tankless water heater problems. If a code has been showing up, how to view or clear the Navien error history explains how to read the saved list.