Scratching head - Rheem rtgh-95dvln tankless making 'whomp' noise on startupp

[CPP]

3 year old Rheem rtgh-95dvln direct vent condensing tankless.

Since new, this unit has made a 'whomp' noise on startup. Its a low frequency bass drum kind of noise, and is slightly more pronounced when the unit is cycled on and off in quick succession (on-off-5 seconds-on kind of timing).

I had the unit installed by a authorized Rheem installation plumber. I;ve had two other plumbers inspect it while they were onsite doing maint on my furnace. None the 3 of these plumbers of them know whats wrong, but all have said they can hear/feel the 'whomp' and it doesnt sound right. I've tried Rheem customer support, but the 'whomp' is barely perceptible when I've taken a recording on my phone and they have just shrugged. Believe me, the noise is very noticeable in the house - I can feel/hear the 'whomp' anytime hot water flow is cycled on from anywhere in the house.

I'm 99% sure the source of the noise is the ignition. In fact the plastic critter screen that is installed in the exhaust pipe on the exterior of the house gets blown out of the exhaust pipe on a regular basis. The screen isnt a tight fit and was never glued in place during the original install.

Exhaust and intake piping are no longer than 4-5' and are 3" in dia. I, and 3 plumbers who have looked at the system, all have concluded the intake and exhaust piping is well within the spec laid out in the Rheem install guide. Piping was also approved by the local gas inspector. I had also sent Rheem support pics of the exhaust piping and they indicated it was perfectly fine

Unit is fed by 3/4 black pipe that is less than 15' run from the meter. Gas pressure after the meter (ie: inside my house) is 6.5-7" of WC and has been verified by all 3 plumbers. Piping is correctly sized for this unit per all 3 plumbers and Rheem support.

I'm posting here in the hopes *maybe* someone else has seen or heard of such a problem with Rheem tankless water heaters. The 'whomp' is very unsettling and has been bugging me for the 3 years since the unit was installed. I've been around enough machinery, heaters, and furnaces to feel that its not quite 'right'

 

[WorthFlorida]

I do not know this system but finding the grate blown out sounds like a delay ignition. Ignition could be occurring after some gas has accumulated, its a controlled explosion. The Rheem dealer techs should be calling their product support. In all my years working at troubled customer sites for telephone systems, in general, most installers get intimidated by problems. The dealer needs to send a maintenance tech that know how to troubleshoot and knows who to all at Rheem.

If this doesn't work out I would call Rheem back and ask for the engineering department. I'm sure a technician or engineer has seen this in their catastrophic failure tests.

 

[Sponsor]

 

[Jadnashua]

Many of these systems use a prepurge fan to first exhaust any potential combustible gasses in the system, then, unless it's a spark type system, they turn on the ignitor (hot wire type), so that when the gas is turned on, it gets ignited immediately rather than letting some accumulate first.

So, depending on the type of ignitor in your system, it's either mal-adjusted, or the timing is off, or may have a loose connection so it doesn't provide a reliable ability to ignite the gas when it should. Or, the fan is not coming on and purging the burner of any residual gas.

I suppose it's also possible that the gas valve has a slight leak, and there's gas in the burner when there shouldn't be. You should be able to smell that near the exhaust, though.

Unless it's a design fault, it sounds like a safety issue to me. THat initial explosion puts a lot of pressure on things that is likely to shorten the life of the heater.

 

[Fitter30]

Sounds like delay ignition of one burner. Heres a parts book with exploded diagrams. Looks like the burner needs to be pulled apart for in inspection.
https://s3.amazonaws.com/WebPartners/ProductDocuments/668FDB62-5723-40D1-9DFF-9198730D3D2A.pdf

 

[RamKiss]

I am having the same problem - 3year old tankless water heater. Wasn't noticeable 1st year. Have it serviced every year by licensed service dealer and they tell me that is normal. It is annoying and I am just concerned that it could result in more damage to the unit over time costing big $$ in parts and labor and shortening the life of the unit.
Seems that Rheem hasn't been helpful in resolving. posting to see if more people have this issue and something can be done to force Rheem's hand to find a solution.