We installed a Navien 240S natural gas in our house, and it is working fine.

Except the short range for hot water valves in the showers. So, in other words, I need to turn shower valve almost all the way on to start the hot water. This leaves us a very small range to adjust hot water when taking a shower. My plumber explained that it is because the sensor is triggered only with a certain water flow. In general, the water pressure is high already. But there is just not enough flow until I turn the faucet almost all the way on.

Is there way to adjust the flow sensitivity for the sensor in the water heater?

My plumber suggests the following: He can install some flow controls on the cold water pipe in each shower and set them to desired temperature range.  

Is this a good solution?

I find it strange that I cannot adjust that sensor on the unit.

5 Answers 5


I have 19 of these units installed in the Hotel I am responsible for. The flow sensor isn't the problem. The valve adjusted by the flow sensor is the problem. I had to replace all eight in one of my buildings. The part is not user serviceable but they have a great warranty. Navien overnighted all eight replacement parts to use free of charge after using their tech support. There tech support is quite helpful and very savvy as to troubleshooting their machines.


  • The specific issue was the servo motor to adjust the flow was not functioning (not opening valve enough ). The tech had me pull the servo motor off and turn the valve manually to check valve function. The valve WAS working the motor wasn't turning it enough. This procedure required replacement of the entire servo motor/valve unit. They overnighted 8 for this particular installation immediately. You cannot remove and replace the motor unit. They are calibrated at the factory. Once you pull that motor off it HAS to be replaced. Call tech support for Navien. Their warranty is quite extensive. Aug 19, 2022 at 14:25
  • Wow, this is some great info. Unfortunately, 6 years late for the original asker, but should be very useful for someone else who may run into this issue.
    – FreeMan
    Aug 19, 2022 at 15:38

This is a common complaint for these types of water heaters.

Unfortunately, you have a model that seems to be slow to respond to water flow. I don't think any company has a way to adjust the flow sensor on their on-demand water heaters. This is not a customer adjustable function of these style of heaters.

If you're dissatisfied with this product I would suggest searching for a company/product that has positive reviews from people for the product's low-flow response rate. Some products respond even respond well to just running a hot water valve at a trickle.

Good luck!

  • My plumber suggested to install some flow controls on the cold water pipe under each shower and set them to desired temperature range. Do you think it can resolve the inconvenience ?
    – vlr
    May 9, 2016 at 6:52
  • I wouldn't pay him until he successfully achieves your desired goal. The water heater has a temperature setting on it so temperature control of the hot water is not the problem. It is the turn-on flow of the water heater that delays the hot water and the balance of hot to cold water flow that limits the range of your shower valve. It sounds like he is trying to reduce the cold water flow to the shower valve with another valve. This would then change the flow of hot versus cold water. This sounds like a reasonable approach to the problem of the limited range. It won't change the delay time.
    – ArchonOSX
    May 9, 2016 at 10:13

I find the very BEST fix on almost any tank-less hands down is to install dual handle faucets then you can turn the hot water on higher keeping the flow high and adjust the cold water to the perfect temp this also gives you a lot more water pressure and NO more hot water drop off. if you don't have that option then you have to turn the temp down on your heater so the water flow is not to slow for the flame. its all about flow if the flow is to slow then the flame could over heat the boiler coil and that can cause the coil to steam and explode so it shuts off to prevent damage to the coil. So lower flow = lower flame. I tried temperature flow control sensors and my tank-less has issues with them and still was getting the hot water drop off and low water pressure. hope this helps.

  • This might be fine in a sink but you typically want a thermostatic mixing valve for a shower.
    – Matthew
    Mar 11, 2019 at 13:49

Best solution is to rise the difference between primary water and DHW. Usually is not only the water flow to trigger the boiler, but a low temp on primary water. On most boilers primary water is kept 20°C above DHW.

My 15 years old Hermann can handle as low as 2,5L/min, so I think you have a setting problem on the boiler' computer.

EDIT: According to your boiler specs, the minimum flow rate for your boiler is 1,9 L/min.

I suggest you to lower the DHW temperature on the boiler, 55°C usually is enough, on your biler, so more hot water is required to flow trough the termostatic valve in the shower (and trough the boiler) so the boiler flow won't fall under the minimum.


Clean the shower head strainers or remove the flow restricted from the heads

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