Rinnai Help

midwest0003

Is Jack (represents Rinnai) still on this forum? The reason I ask is because I am having a problem with a Rinnai ruc98i and I saw an old thread in which Jack seemed to be highly knowledgeable. I have read through many threads of people having the same problem but have yet to see one where the problem was resolved (or at least where the resolution was shared).
So, here goes another thread on an old problem. I WILL post the resolution when done.

5 year old Rinnai RUC98i, propane. Installed indoors. Vented through 3" PVC and vent length well under the maximum allowable (taking into account vent length equivalents for fittings and termination). Drawing combustion air from inside the house: large house with plenty of fresh air. Water heater stops and restarts constantly during showers. NO ERROR CODES. Gas system is sized adequately. Burner goes out but flow rate never diminishes. Inlet filter is clean. Water meter is spinning freely with no buildup. Water is conditioned and always has been. Inlet LP pressure is what it should be. I have not checked the working pressure yet as I think there would be error codes present if that were the issue. Same with flame rod. The unit never fails to light. It just cycles off then on about every 2 or 3 minutes while operating with NO ERROR CODES ever setting. I have also tested resistance across the gas valve solenoids, water flow control device, bypass, etc. All check out ok.

EBEBRATT-Ed

Don't know but it sounds like a water flow sensing issue

midwest0003

I would think so too but the unit never stops reading flow. Unless somewhere between digital display and computer there is a lack of communication, but I dont know if that is even possible.

Alan (California Radiant) Forbes

Have you called Rinnai [(800)621-9419]? They are very helpful.

midwest0003

Yes, I agree. Nice folks and very helpful.

Tom_133

Have you tried running another faucet simultaneously? The unit may be a bit large and cant get enough heat out of itself so its shutting off to cool, then you get a bubble of cool water as it fires back up and heats. I would try running the shower and bathroom sink on hot at the same time and see if it stays going.
Caution: make sure the sink bathroom doesn't over flow during test

midwest0003

Yes, I connected a hose to the service valve to increase flow rate and it still shuts down. Although it may take longer to shut down. I spoke to Rinnai yesterday and they suggested it is hitting high limit at HX outlet, shutting down until it cools, then restarts. This due to scale build up. I will check the HX temp tonight.  If that is what it's doing it would make sense why a higher flow rate prolongs the burn before **** down. 

A question I asked though was, if the HX is overheating to the point of needing to shut down to cool off, why isn't it setting an "LC" code? And what input is the computer basing an "LC" code on if it isn't HX temp? The Rinnai tech wasn't sure. He thought maybe there was a point at which the rate of heat accumulation due to scale buildup is slow enough that it doesn't set a code. Tonight I will clean the HX and see if that solves it.

midwest0003

Not sure why my post says "**** down". Must have had an obscene mistype. Sorry forum moderator. 

Erin Holohan Haskell

@midwest0003, I think you meant to say "shut" down. Thanks.

rick in Alaska

PS. Jack retired a couple of years ago, if I recall. Very knowledgeable and nice guy.
Rick

midwest0003

I flushed the HX with vinegar for one hour. When I next test fired it the water heater still shut down at 2.7 gpm (only the shower running) but I found that if I increased the flow I could keep it fired up without a shut down. It took at least 5.5 gpm to keep from overheating. I will continue to clean the HX and hope that I can get this unit back to normal. Does anyone know what temperature the unit is trying to maintain (at the outlet of the HX and before the bypass)?

One thing I noticed that has me worried is that as the temp approached 190 degrees the blower seemed to ramp up at which point the temperature rapidly rises until shutdown at 205-206 degrees. It is as if the burner control is not reacting quick enough to avoid the shut down. But, that being said, with the obvious improvement just one cleaning made I think we are on the right path.

midwest0003

Ok. This will probably be the last post in this thread. Rinnai technical help is sure that the problem is a dirty heat exchanger (water side), thus the HX overheats, shuts down to cool off, then fires back up and repeats the cycle. Because cleaning the HX improved the situation, I agree that, at the very least, the condition of the heat exchanger is a major contributing factor. The only reason I say it is only a contributing factor is because the fan ramps up and boosts the temperature when it seems to me it should be letting off. If your familiar with the Rinnai tankless you know that water leaving the heat exchanger mixes with cold water to produce the set point temperature. In the industrial world, I am more familiar with steam fired water heaters. Leslie makes a line of water heaters that are engineered to produce 195-200 degree water at the HX outlet then a mixing valve mixes it to the desired set point temp. I asked Rinnai what temperature the HX is trying to maintain at the outlet and he wasn't sure. He said I should see a HX outlet temp. of around 25-30 degrees above set point temp. This Rinnai is consistently maintaining a temperature of 65-85 degrees above set point temperature. All that to say, after cleaning the HX repeatedly, I was finally able to get it to remain in operation without shutting down if I lowered the set point to 115 degrees. I feel pretty confident the HX is as clean as I can make it and I pulled, cleaned and checked resistance of the HX outlet thermistor, so I am not 100% convinced that a dirty HX is the only problem with this unit, but it certainly seems to be part of the equation. I guess the only way to know for sure would be to replace the HX but the customer is at the point where the next dollar they spend will be on a new water heater.

I want to give credit to Eric, the Rinnai tech., who helped me with this issue. He is extremely knowledgeable and was happy to "talk shop" with me to try and figure out this issue. I wish the Rinnai techs could spend some time out in the field troubleshooting. That would really make them bullet proof.

I am also grateful to all of you who commented on this thread. Thanks!

I am a little disappointed that this problem was not completely solved but am super happy to have learned so much. I hope this conversation helps another tech who will deal with the same issue.

midwest0003

I am surprised that I had one last chance to work on this heater. The customer asked me to make one last appeal to Rinnai before he bought a new heater so I called technical help one more time. I explained the issue once again and told them that we had thoroughly cleaned the heat exchanger, outlet thermistor and checked the thermistor for proper resistance at various temperatures. The Rinnai tech said that the only other thing that could cause shutdowns without setting a code is that the bypass valve was malfunctioning. He sent me a new valve under warranty (even though this heater was technically out of warranty, we had started the troubleshooting process when it was still inside the warranty period). Turns out the bypass valve was the problem and the heater is now working like a champ. The bypass valve was sending to much water around the HX causing a lower flow through the HX thus allowing excessive heat build up in the HX. The unit then shut down to cool off before restarting. By monitoring the HX outlet temp. through the diagnostic menu I was then able to see that the HX temp stays around the 25-35 degrees above set point temp. that the Rinnai tech said it should.

One thing I learned from this process: Every tankless heater should be cleaned annually (even if it is on a water softener) and thorough maintenance records should be kept. This helps Rinnai cut to the chase during troubleshooting and keeps the warranty in tact. The other thing I learned is that we should thoroughly learn the theory of operation, what each component does and how to test them, and how to use the diagnostic menu in the heater.

Overall, this was an excellent experience and I hope this helps others in the future.

TomS

Thank you for all the detailed troubling shooting. I think we all learned something here.

Alan (California Radiant) Forbes

Most excellent! So glad you got to the bottom of it. Persistence makes all the difference. And thanks for getting back to us.

How do you like the Rinnai user interface? I like their products, but found it cumbersome and non-intuitive to go through their menus.

midwest0003

I agree. Without the service manual, the diagnostic codes are very difficult to make any sense of.

EBEBRATT-Ed

@midwest0003

Good Job!

Thanks for posting all of this