Navien CH-240 knock/bang noises

[Giantsean]

Hey All,

As if I didn't have enough trouble with my system, today I was powering up after a power outage and the boiler starts making a banging noise the likes of which I've never heard. I am neither an HVAC pro nor a plumber, but I know machinery and this was NOT a normal noise. I literally thought an animal had crawled in there and was trying to get out. It was a loud knocking/banging which was shaking the whole unit. The strangest part was that it persisted after I powered it down. It seemed to be coming from the top of the heat exchanger (upper right) near what looked to be some sort of solenoid. Not sure if this was stuck or if it was trying to push the mother of all air bubbles through but long story it was not working, first firing off error 16 (overheating heat exchanger), 01 (water boiling in HE), and 46 (abnormal thermistor). I tried cleaning all filters (both the water filters were clean) as well as the air filter (which was not as clean). Eventually after several on/off cycles it began working normally again.

I will say for sure that I have had the DHW inlet turned off for a few days (the house is still under some renovation)

Typically the unit will do the "singing pipes" routine which from what I understand is just from expansion. This was definitely not that. It also did not sound like trapped air (I hope!) but I guess it's possible. It went through an air purge cycle after I pulled out the supply filter. As I mentioned it works now, just wanted to see if anyone has seen this and whether there will be anything to watch out for in the future.

Thanks for any info!

 

[Dana]

If the system pressure is too low you'll get a lot of banging out of the macro-boil on the heat exchanger collapsing as the water flows. I'm not sure whether the CH-240 has a pressure sensor to inhibit the burner from firing if the pressure is too low or not (?). Somewhere on the plumbing it's good to have a system pressure gauge, and it should read at least 12-15 psi.

 

[Sponsor]

 

[Giantsean]

The unit has a pressure display but at the time I didn't think to check it. It was definitely low on pressure once I started checking filters but it's to be expected as I opened a closed system, but again that was after. I am wondering if the pump just stopped or got stuck somehow when it powered back up.

Collapsing? That sounds pretty bad if it's a coil, unless I'm confused on the terms!

 

[Dana]

It is the bubble that is collapsing, not the equipment. When the pressure is too low or the flow too slow large bubbles form on the heat exchanger. As the water leaves the heat exchanger he heat flow that has been sustaining/growing the bubble is not longer present, and the bubble collapses quickly with a loud pop/bang.

 

[Giantsean]

so the knocking I heard which continued AFTER the unit was powered down was this process? And it stopped when the HX finally cooled down?

 

[Dana]

If the water was still flowing slowly due to convection or momentum after you brought the system down it could continue for awhile. Without an active flame it would be a fairly short while the cool down time with even modest flow through the heat exchanger is measured in seconds, not minutes, but could conceivably be as long as 10 seconds.

Assuming the thing is working OK now, just leave it until the heating season is over. Next time you're in plumbing-hack mode worth installing a pressure gauge to be able to independently monitor the system pressure. Checking/adjusting the air charge on the expansion tank every year or three is also worth it.

 

[Giantsean]

Thanks as always!

 

[Brian McDonald]

I have a NPE180 A that started making the banging but not everytime it comes on

any ideas?

 

[Dana]

I have a NPE180 A that started making the banging but not everytime it comes on

any ideas?

That indeed sounds a bit like flash-boil "kettling" on the heat exchanger, which can be a symptom of low water pressure. What's the water pressure? Sometimes kettling can be traced to lime deposits, which builds up over time in hot water applications, sometimes in under a year if you have hard water. Have you ever descaled it?

BTW: If it's a propane fired NPE180 it has been recalled. (Don't sweat it if it's natural gas.)

 

[Brian McDonald]

Thanks for responding so fast. I don't have a pressure gauge on my house but we are on a shared well. That could be the issue even though it just started doing it a week ago. I descale twice a year, last time was 6 months ago. Might be due for an intensive one! I was thinking flame rod assembly ignitor or water flow sensor or circulation pump but this would be easier.

It is a propane fired unit but six years old. The recall involves about 3,400 units sold at retailers nationwide from August through October 2018. At least according to the below forum https://boston.cbslocal.com/2018/12/20/navien-tankless-water-heaters-boilers-recall-carbon-monoxide/
Unless you know different?

I'll let you know how I make out after a thorough descale and cleanup.

Thanks
Brian

 

[Brian McDonald]

Did a 1 hour descale the other day and upon fire up still had some kettling. By the time I went to each hot water tap and purged the air from system it has left and not returned. Pressure has returned to so something must have gotten blocked. Thanks and Happy New Year.