Thanks to Mr. Love for this forum. I found it very helpful in getting my Noritz N-0751M-OD tankless water heater performance back to normal. To cut to the chase as to my steps to tankless hot water nirvana, jump to the end of this post.
I've had my Noritz Model N-0751M-OD since 2009 and I've given it a 1-hour vinegar flush every 6 months, per the Noritz maintenance recommendation for this tankless model (http://support.noritz.com/article/ho...hanger-48.html). For the first year it performed well. Hot and cold water volume and pressure was equal at all taps. Then its performance slowly began to degrade. In the end, I determined several reasons for this. Here's my tankless water heater maintenance odyssey.
I should note we have very hard, mineral-laden water in my area of Southern California (Garden Grove specifically; Central Orange County).
Scale is the enemy, as anyone using a tank or tankless water heater knows. I was prepared for the 6 month maintenance on my new Noritz tankless and religiously performed my semi-annual 1-hour vinegar clean and fresh water flush. As noted above, during the second year my Noritz was outputting still adequately hot water but low volume and pressure. During my clean and flush sessions, I noticed the inlet pressure was increasingly much higher than the outlet pressure. Further inspection at the taps showed the scale and mineral deposits were plugging up the faucet aerators and the particle screens/filters in the water supply lines. It seemed no matter what I did I could not get the heater and supply lines and faucets cleaned out enough to improve the hot water flow.
Now in the 3rd year, finally the hot water flow at two of three taps nearly stopped. The 3rd tap was brand new and, though flowing better than the other two, still not as well as when the Noritz was newly installed. It was time to take a holistic approach to my hot water problem...either that or just call Noritz and get a certified plumber out here to figure out what's happening. I decided to give it one last shot myself, though admittedly I'm not the most handy DIY plumber.
I talked to a plumber friend who I knew has a tankless water heater (Takagi; don't know which model). He lives in an area with somewhat softer water. He uses Lemi-Shine to clean his twice a year. So I tried the same, seeing more sediment/scale when flushing the Noritz than in the past. Still, the performance at the taps was poor. So I decided to use CLR (3 parts water; 1 part CLR) and pump it through my Noritz for 6 full hours. While the CLR was coursing through the tankless, I removed all kitchen and bathroom water faucets, disassembled and cleaned them, along with their respective supply lines.
Low and behold the supply lines were plugged badly with sediment but only on the hot side. Hmmm...what to do.
I installed a large sediment filter just before the inlet on the Noritz tankless, replaced all the supply lines, reassembled and installed the cleaned faucets, clean water flushed the Noritz for 10 minutes from the garden hose after its 6-hour CLR cleaning. This time, lots of scale and very good outlet pressure from the Noritz during the clean water flush. WAY BETTER than before. Progress at last.
Reconnected all faucets and new supply lines, removed all faucet aerators and shower heads, fired up the Noritz and let all sink and showers run only hot water for 10 minutes. Saw plenty of scale/sediment coming from the taps at first. After a minute no more; just hot water and in decent volume (remember, all three sinks and two showers were full open on the hot water side; my tankless is rated to serve any two hot water devices at a time; was still impressed with the improved hot water flow).
Turned OFF each hot water tap/shower one at a time to see how the flow/pressure improved at the succeeding tap/shower. I liked what I saw as I progressed. Once everything was OFF, I inspected all my fittings for leaks and found none. Yeah for me! Installed the aerators and shower heads and tested them individually. The full hot water volume was now equal to the full cold. NICE! FINALLY! What a fun way to spend a Sunday!
So here's the steps that finally got my tankless performance back to new:
1) Use CLR for 6-hour cleaning of Noritz tankless water heater
2) Flushed tankless for 10 minutes with garden hose after CLR cleaning
3) Cleaned/Replaced all water supply lines
4) Removed all faucet aerators and shower heads
5) Powered ON tankless
6) Ran hot water full ON through all faucets/showers for 10 minutes
7) Shut OFF faucets/showers and replaced aerators/heads
8) Checked hot and cold flow from all taps/showers and enjoyed equal volume/pressure from both
9) Cracked a beer
10) Cracked a second beer, which somehow tasted even better than the first
In closing I should add that, when my Noritz was newly installed, I installed bypass valves on both the inlet and outlet ports (no, my plumber did not install them then...he should have). The bypass valves make cleaning and flushing so easy...even I can do it.
Hope this helps someone in a similar situation. All the best to you DIY'ers and thanks again to Mr. Love.