Navien Tankless Water Heater cycles on and off due to a clogged faucet aerator, bad flow sensor, and a restricted cold water inlet filter.
Apart from these, clogged exhaust venting and defective 3-way valves can also cause the unit to cycle on and off every 8-10 seconds.
All the culprits I mentioned actually cut-off the minimum water flow your tankless unit requires to heat the water. As a result, your tankless water heater will cause intermittent short cycling.
Fortunately, I will walk you through every troubleshooting step to fix the water heater short cycling problem. So, stay in tune with me till the end.
Table of Contents
- Navien Tankless Water Heater Cycles On And Off [5 Easy Solutions]
Navien Tankless Water Heater Cycles On And Off [5 Easy Solutions]
This chapter will reveal the reasons that cause the Navien Tankless unit to cycle on and off every few seconds.
01. A Clogged Faucet Aerator
Every tankless water heater requires a minimum flow rate to activate the unit for heating water. If the water flow is getting lower than the recommendation, the tankless unit will not function properly.
And a clogged faucet aerator is the main culprit behind the low water flow. If the faucet aerator gets clogged, it will cut-off the minimum flow for your tankless water heater. As a result, your Navien tankless unit will cycle on and off.
Generally, the faucet aerator gets restricted by dirt, debris, and mineral deposits.
How To Fix:
Unclogging the faucet aerator will be a simple fix to this issue. So, start with removing the faucet aerator. Regarding this, use an adjustable wrench or channel-lock pliers to unscrew the aerator from the end of the faucet spout.
Then, turn on the faucet so that dirt, debris, or anything stuck in the spout comes out of the faucet with water. Next, use a paper clip to remove mineral deposits or sediment from the aerator. If needed, you should soak the aerator into the vinegar solution.
Lastly, rinse the aerator with clean water. Then, reinstall it at the end of the faucet spout and tighten it up with the channel-lock pliers.
02. Flow Sensor Issue
The flow sensor on a tankless water heater generally senses the presence of water flow and turns the unit on to heat the water.
If the flow sensor gets covered with mineral deposits or broken, it will work intermittently. Because of this, it will send wrong signals to the unit and your unit may fail to read no water flow.
As a result, the water heater will cut the flame but receive a signal again from the flow sensor. And it will cause your Navien tankless water heater to cycle on and off every 8-10 seconds.
How To Fix:
First off, locate the flow sensor on your tankless water heater and inspect it. If the sensor gets covered with scale formation, cleaning the sensor is a must. Regarding this, get an emery cloth and scrub the sensor gently.
If cleaning the flow sensor doesn’t bring any luck for you, you must replace it. The following video will help you in replacing the flow sensor on Navien Water Heater:
03. Cold Water Inlet Filter Gets Clogged
The water flow will slow down to a trickle if the cold water inlet filter gets blocked by dirt, debris, or mineral deposits. When the cold water inlet gets clogged, it will cause your unit to throw a flow sensor error code. In other words, your water heater will cycle intermittently if there is a water flow issue.
How To Fix:
You should inspect the cold water inlet filter periodically. If it gets clogged, release the blockage from the filter to fix the short cycling issue. I recommend you read this guide on Navien Tankless Water Heater Maintenance to learn how to clean the cold water inlet filter.
04. Exhaust Vent Gets Clogged
Sometimes clogged exhaust vent piping can also cause intermittent short cycling. The exhaust vent may get blocked by lint, dirt, and insects. So, you should clean the exhaust venting to ensure proper water heater operation.
How To Fix:
You need to disassemble each venting section or piping to clean the vent system. Otherwise, you can hire a professional to unclog the exhaust vent for you.
05. Problem With 3-way Valve
A defective 3-way valve is another culprit that causes your tankless water heater to cycle on and off.
Basically, the 3-way valve on the Navien Tankless Water Heater diverts water from the space heating system to the DHW plate heat exchanger, depending on the input from the DHW flow sensor and PCB. This is how the valve satisfies the call for domestic hot water. 
If this valve is at fault, it will create a low water flow issue. And this low water flow will cause intermittent short cycling.
How To Fix:
I highly recommend you contact a certified plumber to inspect and replace the 3-way valve. It’s because dealing with a 3-way valve requires disassembling the entire unit and detaching many internal piping. So, it will be better to let the professional handle this task for you.
Your Navien Tankless Water Heater keeps turning on and off because of low water flow. Basically, restricted faucet aerators, dirty cold water inlet filters, and defective 3-way valves are responsible for low flow rate.
Fortunately, I described above how to fix those issues. So, follow the troubleshooting steps I mentioned.
Besides, a bad flow sensor, clogged exhaust vent, and loose wiring can also cause the tankless unit to cycle on and off. To resolve this issue, you should fix the problems with these water heater components.
However, if fixing those water heater issues fails to solve the short cycling problem, don’t hesitate to hire a certified plumber.
Eric Alvarez is an HVAC guy based in El Paso, Texas, United States. He obtained his Bachelor of Science degree from the University Of Texas at El Paso. Years of experience in the HVAC field have taught him many lessons, not the least of which is that the value of quality and knowledge far exceeds any promised initial savings. He has a good standing reputation for superior skills in heating, air conditioning, hot water tanks, and indoor air quality systems.