Losing my mind w/ Noritz tankless

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Master Plumber Mark

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We have the SAME nightmare issue- loud knocking noises in the pipes when the recirculating pump of the Noritz tankless WH turns on. No noises when hot water is running without recirculating pump on. It’s driving us crazy. Has there been any resolution? We have expansion tank, all kinds of check valves & the knocking noise is still there most heard in the wall of Master Bedroom shower. It’s not water hammering. The knocking noise is strictly limited to when the recirculating pump turns on (actually, when there is NO water being used!). Plz help. Several plumbers (including the installer) don’t know what’s causing it. I’ve already paid thousands of dollars. 3 Noritz WH were changed- all the same! :(((


This knocking is due to the line being held too tightly to the studs... The copper line or pex line is expanding length wise and must move slightly during expanding..... you have to loosen the pipe straps and it should go away,.....if it goes up a wall to the second floor its gonna be impossible to do , if your walls are finished in the basement you are gonna have to try to locate the area the noise is coming from and cut drywall to locate the little strap that needs taken out.....


good luck .
.
 

Muffichka

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This knocking is due to the line being held too tightly to the studs... The copper line or pex line is expanding length wise and must move slightly during expanding..... you have to loosen the pipe straps and it should go away,.....if it goes up a wall to the second floor its gonna be impossible to do , if your walls are finished in the basement you are gonna have to try to locate the area the noise is coming from and cut drywall to locate the little strap that needs taken out.....


good luck .
.
So, if I understand correctly, the recirculating pipes are SEPARATE pipes from the hot water pipes? We do not have any knocking noise when hot water is running, it only happens when the recirculating pump turns on.
 

Master Plumber Mark

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Yes you got it.. the recirc line goes usuallly to the very end of the hot water
system in either copper or pex depending on the age of the house,, normally its
a half inch copper line stretching out the full length of the system... When it heats
up from luke warm to extra hot then pipe attempts to expand,, and at those hangers it knocks and
causes stress in the lines.... normally it sounds like a woodpecker tapping on the house...
and then it passes once it cools back down....... Just find the hangers and loosen them to take
the stress out of the line and it should go away


some folks have just shut them down due to the noise and the wear and tear on the line and live with it
 

Muffichka

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Thank you for your explanation. I wish it would be so easy to find the loose hangers now that the house has been completely remodeled and closed up. The sound appears to come from second-floor high up behind the master bathroom shower area (the other side opens into the hall). I wonder if we tap & listen for the studs and then somehow go through the attic floor down “laparoscopically”, so to speak, without opening the walls and ruining tile & cabinetry? Although, I’m afraid, it won’t be possible :(
Do u think there is any merit in replacing all the “swing checks with spring checks” (whatever that means), as recommended by somebody in this thread? TIA for all ur help
 

Master Plumber Mark

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If the sound is strongest at the other side of the shower and it opens up into the hallway
the easiest thing to do is open the drywall on that side
and see what you find on that shower valve....
The recir line probably ties on and makes the loop back to the heater
at that faucet.... maybe you will get lucky and find the culprit

You have to tear up some drywall to do this , but its easy to hang a full length mirror over the damage
in the hallway or just re-drywall it.....
 

jjkyle

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The noises are far away from the tankless unit - most prevalent in the furthest (from the tankless) corner of the house which happens to be the Master BR. Perhaps we need air-column(s) installed in that corner of the house?

I'm not sure how the pipes were anchored throughout the house but we never (in 14 years) had this issue before. The only thing that changed was replacing hot water tank with tankless (and removal of external recirc pump).
Ricker, has this issue been resolved? In my opinion, the culprit is the new recirc pump. Compare the old and the new pump specifications. I bet the new pump is producing a higher flow rate than the old pump, and although you don't think the new pump is causing water hammer, I think it is. The velocity of the water in the piping is too high when the new recirc pump is turned on and since you probably have copper piping that is very rigid, the water momentum changes are causing the pipe noises.

A simple way to test this theory is to locate a valve in the system and close it most but not all of the way. Sometimes there are isolation (usually ball valve) valves installed at the water heater, be it tankless or tank. Close down on the supply or return valve while the recirc pump is running. Although this eats up energy and can be noisy at the valve, check to see if the noises go away in the piping where you're having the problem. If they go away, I would put your old external pump back in service. This means that the new pump is over-sized for your system. I don't believe the problem is the piping being mounted too tightly to the building. It should be mounted tightly to the building but have enough bends and u-shapes for expansion already. Pipe expansion is very slight and is usually only a problem for long straight runs, steam systems, and seismic joints. If the piping anchors are loosened, you'll end up with piping moving in the walls, which will cause more noise unless spring-isolation is added.

Good luck and let me know if this problem is already solved or if my suggestion helps solve the problem.
 

Muffichka

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Thank you jjkyle for taking the time & writing a detailed explanation Unfortunately, the problem has not been solved or fixed yet. I’ve had several plumbers come by and everybody is perplexed.
My husband & I have narrowed it down to the times when the recirculating pump runs together with the heater (flame symbol appears) - when the flames turn on, the heater makes those rather soft/faint knocking sounds inside the tankless water heater. We can hear them if we put our ear to the heater. However, these faint sounds somehow get GREATLY magnified, like in a megaphone, when they travel through the pipes & can be heard pretty much everywhere, esp in the master bedroom/bathroom areas. I have Noritz Tech rep and their Engineer coming tomorrow morning, but I’m afraid they will say that the water heater is functioning normally and my plumber says these noises are normal :((
None of our friends who have tankless water heaters have any noises heard in the pipes. We have a decent size home (~5,200 sq ft) & only 1 tankless water heater. I don’t think it’s oversized for our house. Originally, we were going to put 2. It’s been literally driving us crazy to live with these knocking noises for over a year now :(
Whenever I send the audio recording of those sounds, plumbers don’t know what those are. Are they so uncommon that nobody knows how to diagnose & fix the problem?
 

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However, these faint sounds somehow get GREATLY magnified, like in a megaphone, when they travel through the pipes & can be heard pretty much everywhere, esp in the master bedroom/bathroom areas.
Maybe it would be worthwhile to modify the piping with some resilient piping. Maybe a section of PEX or a zig-zag in the piping would reduce the sound conduction. Maybe a mass attached to the pipe would help. I am thinking of something like lead bars strapped to the pipe to dampen the conduction.
 

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I don’t think it’s reasonable to start doing exploratory demolition of the house walls. Before I start opening up walls, I’d like to know for sure the reason for the knocking noises. Are there any plumbing experts who’ve seen this type of problem with intermittent rhythmic (every 5-15 sec) knocking noises in the pipes?
 

phog

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Muffichka, is it possible to run the Noritz recirculation pump without activating the tankless burner? If you can recirculate only cold water, this would answer the question of whether this noise is thermal expansion of the pipes (one theory), or water hammer caused by the recirculator (the other theory). It won't solve the problem, but it will at least help eliminate some possibilities.
 

Muffichka

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Hi phog, from my measly knowledge of plumbing I thought the recirculating line is to take the cold water out of the pipes & bring it back to the tankless heater to heat. But I will ask the Noritz rep & engineer tomorrow. Thx for ur suggestion.
 

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Hi phog, from my measly knowledge of plumbing I thought the recirculating line is to take the cold water out of the pipes & bring it back to the tankless heater to heat. But I will ask the Noritz rep & engineer tomorrow. Thx for ur suggestion.

Yes that is exactly right. I believe the Noritz unit has a built-in recirculating pump. If you can have the Noritz tech activate the pump only, without having the burner come on, it will circulate cold water through the loop without heating it. If the noises don't happen while the pump is running cold water around, then you'll know that this must be a thermal issue related to hot water. On the other hand if you still do hear the noises even without the burner running, then you'll know the noises are something related to the pressure the pump is creating. Good luck.
 
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Muffichka

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I already know that the knocking noise occurs only when the burner turns on together with the recirculating pump. There is no noise when just the recirculating pump is running. So, knowing this how can I get rid of the knocking noises?
 

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Hello Muffichka, I am not clear what the circumstances are when you see the recirculator pump running without the burner. Can you tell a little more about that?

Are you saying that sometimes when the circulator pump kicks on, the burner does not turn on at all before the pump turns back off?

Or are you simply seeing a few extra seconds of circulation where the pump runs before / after the burner kicks on?
 

Muffichka

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I’ve witnessed the recirculating pump symbol appear without the flame symbol. I don’t know why or when or how this happens. Is this not normal? So, when that happens, there are no knocking noises.
The knocking noises occur when the recirculating pump AND the burner/flames turn on. The recirculating pump can run for awhile without burner turning on. I don’t know may be there are other issues with our Noritz unit, besides the knocking sounds. it also makes a short “wailing” noise when the hot water is turned on. Also, when we take a shower sometimes we have to wait 5-15 min for hot water & sometimes we have warm water right away for 30 sec & then cold water flows for a min & then it slowly warms up again - not a great feeling while one showers.
But the most frustrating issue are those pesky, unrelenting knocking noises heard inside the pipes which start at 5:15 am & sometimes are heard as late as12:30 am - nobody uses any water during those times :(
I hope this explanation helps u try to figure out the cause of the knocking noises. TIA
 

phog

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It is normal to see the recirculation pump running part of the time without the burner on. Typically the circulation system will kick on the pump for a few seconds first before the burner will start up. And then the burner will kick on & run for a little while as the pump stays circulating. And last, the pump might (or might not) continue running for a few extra seconds after the burner turns off.

The key point is that the burner is probably on at least part of the time, each & every time the circulation pump runs. My suggestion is to see if you can make the pump run without the burner turning on at all. If you can do this for a good long while, (at least as long as your recirculation pump usually runs and ideally a little longer), this can be a big clue to help figure out exactly what is causing the noise.

The other problems you have with your hot water being inconsistent can be helped by simply tuning the recirculation settings better. The delay in getting hot water can be eliminated by having the recirculation pump run more frequently. The hot / cold / hot effect is called "cold water sandwich" (Google it for lots of helpful articles). It can be tougher to address.

As you may already realize, the reason you are not hearing the mystery noises during certain hours overnight is because the recirculation system is programmed to turn off overnight. At the very minimum, tomorrow you can adjust the hours so that it is at least not running while you're sleeping. For example make the recirculation turn off at 10pm every night instead of 1230am.
 

Muffichka

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That’s the mystery- the recirculating pump runs for awhile (like 5 min or may be longer) WITHIUT the burner turning on & when that happens there are NO knocking sounds. Also, the recirculating pump in this latest Noritz unit is supposedly “smart” & learns our pattern. Supposedly, but it doesn’t happen. We NEVER use any water at 12:30 am or 5:15 am, yet, like a clock thepump turns on. Weird.
So, presumably, we have a water expansion issue - the burner & recirculating pump is on & we have knocking noises. What do we do to fix this?
 

jadnashua

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The unit has nearly a 200K BTU burner...the heat exchanger will heat up and the metal expands when that happens. As you noticed, it's pretty quiet in a well designed system, but it's there. As to the pipes, they will expand as well when they are being heated by the circulating hot water. The longer the line is, the more the actual length will change. The fact that you notice it at the furthest point seems consistent in that that end will have to move the most. As we've said before, if there's any spot where the pipe is tight, whether that's a hole or a clamp, once the pressure of the expanding line exceeds the clamping force, instead of just bending, the pipe will jump through that tight spot. It will do the same thing as it cools off, jumping the other way. FWIW, copper expands at 16-16.7 ^-6 mm/degree K (same size as C). that's 16-16.7 micrometers per degree. (0.000167mm/degree).

Say 30M of pipe, raised 20-degrees C, 30000*20*0.0000167 = 10mm, or a bit over 3/8" FWIW, the expansion of PVC is lots more, and drain pipes tend to be a more common source of noises when draining hot water.
"
 

phog

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Based on this description it sounds very likely that these are thermal expansion issues. As several others have said, the first suspect would be a pipe that is growing slightly in length when it heats up, and "catching" against something. The noise comes from the "catch" suddenly releasing. It could also just be a stress releasing in metal (like how your car makes ticking sounds from the engine area as everything cools down after you park).

The problem can be easily solved by either loosening / freeing up pipes where they come into contact with other things; or worst case cutting out & replacing noisy sections. However it may not be easy to figure out the location (or locations) that the noise is originating from. They may be inside walls, and it may be difficult to "catch in the act" even where pipes are exposed.

I'm sorry I don't have a better answer for you. As a homeowner you may be able to use existing isolation valves, if they are installed, to shut off branches one at a time & narrow down the location. You can also use your ears -- it's likely (but not 100% certain) that the noise will be louder in the pipe sections closest to the source.

Finally i want to point out that you do have a last-resort option, which is to disable the recirculation pump & live without it. Without the pump running, you will only get noises when someone runs the hot water & never when it's not in use. However, you will then have the 30 second wait every single time you need to run the hot water. If you really hate the noise *that* much ;)
 
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