I have a tankless with a descaler on the cold water input. When I turn on the hot water at the faucet it takes a minute or so to get hot water. Then I'll hear some banging/knocking of the pipes.

However, if I leave the hot water running, it turns to cold water.

What could be happening?

update It is a Noritz NR981-OD-NG. Natural gas model. We just installed the unit a few weeks ago. I just started to use it this past week.

This is our only hot water heater and is rated for 2 to 3 showers.

The hot water is on full and the cold water is off.

update 12/29/16 Could the flow of water be causing the hot water heater to not activating to heat the water? I can hear the hot water heater 'working' even when cold water is coming out of hot water faucet. From the specs of the water heater: "Minimum Activation Flow Rate 0.5" How can I tell/measure what the flow rate is ???

update 1/3/2017 Resolved! The plumber and I changed out the water limiting spout cover and that did the trick. Our local gas company gave us a water savings kit and we used the water limiting spout (rated for 1.5 gpm) and that seemed to fix the problem.

  • What is the make and model of heater? Central or point of use? Electric or gas? How long has it been in service? Is the hot water flow on full when the water temperature turns cold? Dec 22, 2016 at 9:50
  • Hi @JimStewart : I just updated with info you asked...any guesses at what could be causing this?
    – milesmeow
    Dec 30, 2016 at 7:15
  • Is the digital temperature display reading a value that would correspond to getting hot water, but you are not getting it? Dec 30, 2016 at 17:33
  • You could check the flow rate by collecting the output of the shower or a tap in a bucket for a timed period. A flow rate of 0.5 gal/min is very low and it is highly unlikely that you are below that. Who installed the water heater and was it new from the box? Dec 30, 2016 at 18:19

1 Answer 1


We have a basic Bosch tankless, NG 117 kBTU/h, with a standing pilot light, no a/c power required, all mechanical controls, discontinued model I think, been in service 12 years. It worked fine until I changed to low flow shower heads (1.4 gpm). I took out the flow restrictors in the shower heads and the water heater worked again. So that is my experience.

Your Noritz NR981-OD-NG is larger and with more features so there are more things that can go wrong. If you remove your shower head and get full flow of hot water, does the water get hot? Or if you turn on all the hot taps in the house, do you get hot or even warm water?

Is the variable burner at fault? Is it stuck on 16 kBTU/h? Is the gas pressure and flow adequate for the heater? Can you tell from the gas meter if the heater is pulling the amount of gas required for maximum 200 kBTU/h heating?

  • How do I measure pressure and flow? Would it require disconnecting the gas connection to the heater? Before the install the contractors hooked up a meter to the extended gas line.
    – milesmeow
    Dec 30, 2016 at 18:01
  • What does your installation guide say about the gas supply requirements? The static pressure can be sufficient, but if the gas line from your meter to your heater is too small in diameter for its length, then the pressure would drop when the burner turns on. The installer should have verified that the gas line was sufficient. Tankless heaters have a high demand for gas. Dec 30, 2016 at 21:37
  • Our gas supplier in Dallas sets the meters to give a pressure of 7" of water column (0.25 psi). The specs on my Bosch for NG supply are (before the Aquastar regulator) min 7" WC , max 14" WC. Required at inlet tap with heater operating 5.7" WC and when operating at maximum 4.2" WC. What your the altitude above sea level? Dec 30, 2016 at 21:56
  • Looking through my notes it seems that our gas company can supply 7" WC (=0.25 psi), 2 psi or 5 psi from the meter. I am sure we have 7". The higher pressure supplies are I think for houses with extra regulators maybe one at the service entry to the structure or at the individual appliances. I don't know if it actually occurs in practice, but there is an opportunity for a mismatch. Do you know what your heater is set up for and what you gas supplier is providing? Dec 30, 2016 at 22:35
  • A spec sheet for your Noritz gives gas pressure requirements as 4" WC to 10.5" WC. Dec 30, 2016 at 22:46

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