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.
Best Answer
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?