Rinnai tankless - intermittent hot water and no error code

lukekr12 · December 2018

I have a Rinnai tankless RC80HPi water heater. It was installed in my house back in 2009. It has been working flawlessly up until about 2 weeks ago. Then all of a sudden while trying to run hot water in either of the showers or in the kitchen sink, the hot water would start cycling between cold and hot. This cycling happens randomly and for various amounts of time. Sometime the cold water will last for 30 second, sometime it will last for a minute.
With the hot water at the kitchen sink and/or the shower turned on - if I am standing in front of my Rinnai water heater, I can see the water heater turn off and the In Use light go out. Meanwhile I can still hear water running through the water heater. The heat is just off. Then about 30 seconds to a minute later, the Rinnai water heater will fire up and start producing hot water again.
There are no error codes. The display consistently and always displays "120". I've checked to see what the water flow is through the water heater, it is 26 when I have the shower on and 11 when I have the sink on. I've checked to see what the outlet water temperature is, it is between 108 and 110 always.
I regularly flush this water heater, thus it is generally well maintained.
Twice in the past 3 days I have opened up the Condensate trap drain plug, and maybe a cup or two of water has flushed out. Is that normal?
Does anybody have any advice on what I can check next or what may be going on? I am at my wits end with this and want to get back to having an endless supply of hot water during showers or while doing dishes!

DZoro · December 2018

When you flush the water side are you using a cleaner?
Has the burner side been maintained annually?
There should always be some condensate inside the trap. When the system is running is there a steady flow of condensate running out?

lchmb · December 2018

go into the control and check your gpm. I'd start guessing your flow servo is failing..

lukekr12 · December 2018

Thanks DZoro!
I didn't realize there was a "water side" and a "burner side". When I flush my tankless water heater annually I "flush the heat exchanger" with a solution of white vinegar and water as described in the Operation and Installation Manual.

lukekr12 · December 2018

Thanks Ichmb!
When you say "check your gpm"...is that done by pressing the up arrow for 2 seconds and then pressing the "On/Off" button? Or is there a different way to check gpm?
I've done the way I described...and with the shower on the digital display shows 26; with the kitchen sink on it shows 11. I'm guessing that is gpm...

Intplm. · December 2018

Might be that the fan and or the exhaust is dirty. Also, I'm not sure, but the gpm has to be just around 3 gallons per minute. (That's water flow rate feeding the unit.) If it fluxuates lower, the unit will tend to "hunt" for a reason to stay on which I think is what you are describing. If it stays on longer, you might find that it will show an error code and then shut down.
So. Try checking the fan and the fan housing for dust build up.Check if water use has changed possibly reducing the water flow to the heater.

RBDeRoo · July 2020

LukeKR12 - did you figure this out? I have the same problem

psb75 · July 2020

Make sure your faucet aerators and shower heads are clean and free of debris that may be restricting flow.

HVACNUT · July 2020

> @RBDeRoo said:
> LukeKR12 - did you figure this out? I have the same problem

Are you sure there's no code?
I had a Rheem tankless last Thursday. The client reset it so it was ROA running on arrival. He said there was no code, just displayed a temp of 79°. I called tech support and 79 is a code not in the Customer Care Manual. This code was a blower motor fault. And of course they recommend replacing the board as well.
I don't come across too many tankless water heaters. This one was for a pool house so no emergency. He's gonna wait until it's dead dead.

Intplm. · July 2020

Is the gas main of adequate size?? It will shut down if the gas isn't flowing as needed.

jonl123 · September 2020

I assume you got to the bottom of the issue?

Same exact problem here. No code. Tried flushing the unit but that didn’t help.

Unit makes a loud rumble (very loud) when it kicks on. This was the only issue for maybe a week or so. Today it started kicking on/off while calling for hot water. So the water cycles hot/cold/hot/cold...

Flow when kitchen sink is on is 10-11 (I assume gpm, but logically it's not moving that much water of course, so maybe it's liters?... It’s the number you see when pressing up and power on the controller).

We’ve had a lot of things done recently that could have caused something to go wrong: roof replaced, water treatment system installed, flow turbine in hot water heater replaced, condensate neutralizer installed (this is over a 2-4 week span).

jonl123 · September 2020

Wanted to update that I checked the exhaust pipe up on the roof and it all looked fine and clean. Tested today and off one of the bathroom sinks it ran perfectly for 10-15 minutes. Turned it off, turned on kitchen sink and the problem started happening again. It happens with other faucets too.

Watching the unit, during those 10-15 minutes, I didn't see any condensate dripping out, so I'm wondering if something is going wrong there.

AggieOilman · September 2020

Did anyone find a solution to this issue? Dealing with the exact same and have a repairman coming.

BradSmith · December 2020

My Rinnai tankless water heater exhibits this sort of behavior. Cleaning the squirrel cage exhaust fan of bugs, which causes an imbalance slowing airflow, remediates this intermediate hot water flow behavior.

Tutorial video demonstrates the issue and resolution to restoring regular hot water.

rick in Alaska · December 2020

If your water heater is showing 108 degrees output while it is running, and you are not getting hot water at the fixture, then you have a cross connection somewhere. You should still be getting 120 degrees however, so you might be dealing with more than one issue. But first, check your plumbing and see if there is a place where the hot water can get back in to the cold water.
Rick

jonl123 · January 2021

BradSmith said:

My Rinnai tankless water heater exhibits this sort of behavior. Cleaning the squirrel cage exhaust fan of bugs, which causes an imbalance slowing airflow, remediates this intermediate hot water flow behavior. Tutorial video demonstrates the issue and resolution to restoring regular hot water.

I had high hopes for this fix, but unfortunately it didn’t do the trick for me.

jcormier · February 2022

Did you ever get this resolved? I'm having a similar issue with my RU98i. No error codes and it runs for several minutes but when demand gets too high it shuts down. It's almost like it's experiencing a flame failure due to not enough gas. I wonder if the burner assembly needs cleaning or if one of the gas valves are not opening. Note: The fan has little to no vibration and appears to be working very well. I took it off to verify how clean it was an all was good.

wasillaguy · April 2022

I have the same issue with my Utica-Dunkirk

mattmia2 · April 2022

Are you checking the gas supply pressure under load when it happens?

scubasteezz · September 2022

jcormier said:

Did you ever get this resolved? I'm having a similar issue with my RU98i. No error codes and it runs for several minutes but when demand gets too high it shuts down. It's almost like it's experiencing a flame failure due to not enough gas. I wonder if the burner assembly needs cleaning or if one of the gas valves are not opening. Note: The fan has little to no vibration and appears to be working very well. I took it off to verify how clean it was an all was good.

Jcormier, I have the same model and same problems, did you ever get yours fixed? How did that go?

David Penz · November 2022

In my experience, "no code, no hot water" has always been symptomatic of a faulty flow sensor. The heater requires minimum flow to operate the burner. But zero flow can be a normal event. So a faulty flow sensor cannot trigger an alarm code. A considerate heater manufacturer would install two flow sensors in series, so that the controller could warn about this impending failure. But, oh well.

Replace the flow sensor. It may be a separate assembly or part of another assembly, such as a water control valve. Save the old sensor assembly, and see if you can disassemble it to remove the turbine wheel from its housing. Usually the wheel and axle/bearing become fouled with a black powdery substance, likely micron-sized manganese dioxide or iron oxide particles. You may be able to clean the turbine and axle by vigorous brushing, but don't breathe the particles! These are magnetically attracted to the wheel. You may need to ream the thrust bearing with a small drill bit. Before and after, test the action by rotating a small magnet around the turbine housing. Before cleaning, the wheel may not turn or may turn with a jerking motion. After cleaning, the wheel should spin freely. Keep this unit as a spare, for the next time needed.

JC09 · February 2023

Hello All

Posting this here as I found a solution to my problem, and it was an easy fix. I have an interior unit (RUC98) that randomly started going hot for 5-6 minutes, and then go ice cold. Rinse and repeat.

The issue was my condensate line. I have a small tube that runs directly from the unit into the water pump that my A/C unit also uses to expel excess water/condensation. The issue was that the tube had slipped too deep into the water pump reservoir. This was causing my water heater to kick on for a few minutes, but once the condensate began to occur, the tube was underneath the water level in my A/C drain pump but the water level was not high enough to activate the drain pump. Because my condensate line was sitting in the water, it wasn't able to drain properly which I believe caused my water heater to shut down as a safety precaution (similar to if the condensate line/trap was completely clogged).

Throughout the day, my A/C would be on, and I imagine the condensation from the A/C raised enough water in the drain pump to activate it. Water drains out, my water heater condensate tube would sit out of the water (and be able to drain condensate properly), my unit would kick back on and I'd have hot water again.

Anyways, the simple solution was to pull the condensate tube out an inch or two so that it would be above the drain pump activation line, and not be submerged.

I understand this won't help everyone here, but hopefully it will help someone. It's been a frustrating three days troubleshooting this thing.

Cheers.

bfullen · December 2023

I have a Rinai RL75I, same issue. I have changed the inlet filter, water servo, water turbine, heat exchange sensor, water servo sensor, cleaned the fan, descaled the unit. I am convinced that Rinais' design is faulty and its garbage. I am currently looking to replace mine with Navien or Rheem.

GGross · December 2023

bfullen said:
I have a Rinai RL75I, same issue. I have changed the inlet filter, water servo, water turbine, heat exchange sensor, water servo sensor, cleaned the fan, descaled the unit. I am convinced that Rinais' design is faulty and its garbage. I am currently looking to replace mine with Navien or Rheem.

Well you will find the design of tankless water heaters is very similar across the board, if you are struggling to troubleshoot this one and throwing the parts cannon at it, you will find the same experience down the line with the next brand. Just switch to a tank

EBEBRATT-Ed · December 2023

If you switch to a Navien you will have more issues.

Rinnai tankless - intermittent hot water and no error code

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