Rinnai RUC80i water doesn't hit target temp at fixtures

rubberduck

Hi all, I have a 10 year old RUC80i in a 3 story townhouse that I've had issues with since taking possession of the property. Disclaimer that I'm completely new to home ownership so I may be missing something obvious here.

All of the fixtures in the house only get to ~104f, regardless of whether the heater is set to 120 or 140. I read a bunch of information online, and started off with a 3 hour vinegar flush which was well needed given that the vinegar was completely opaque black by the end of it, and we live in an area with moderately hard water (I've installed a softener but the unit ran for nearly a decade on hard water). Unfortunately I saw no improvement, so I replaced the flame rod and electrode.

This stopped some fluctuations in temperature, but otherwise the main issue persisted. I noticed that the unit would short cycle when set to 140f: I checked the diagnostics and observed that the unit temp would peak very briefly at 140 before the unit shut off and let the temp fall to ~100, then again repeat the cycle of hitting 140. I replaced the bypass servo assembly, and that stopped the short-cycling - the unit now maintains a stable ~140-145f under diagnostic #2. HOWEVER, this still did not resolve the issue of the temperature at my fixtures capping out at 104f. I have no clue why every fixture in the house is maxing out here, and would appreciate some thoughts.

LRCCBJ

The Rinnai is limited to 140F.

The only question that begs is how long is the piping from the HWH to the fixtures?

If you run the water at the fixture for five minutes………….what's the output temperature?

Additionally, is there any possibility that the unit was piped with a tempering valve to limit the temperature to the fixtures to 120F? These are noted to calcify with time and need to be removed and cleaned with citric acid or replaced entirely.

If none of the above, you need to determine if the Rinnai has an output of 140° at the HWH. This is best done with a infrared gun pointed at the copper feed from the HWH. You need a piece of black tape around the copper pipe or you will get erroneous readings from the reflection of the copper.

Larry Weingarten

Hi, I agree with @LRCCBJ , You really need to know what the heater is putting out at the heater itself. Lots can happen in the piping system. Without knowing that output temp, you don't know where to go hunting for problems.

Yours, Larry

rubberduck

I think @LRCCBJ is onto something with the tempering valve. There is indeed a Zurn Aqua Gard valve regulating the output temp, although it's seized up really good - I can see sediment "seeping out" by the key. Will report back on the result when/if I manage to turn it.

The unit itself is reporting a stable output of 140f when I go into the diagnostic mode (down button + on/off → diagnostic #2). The output at the heater would fluctuate wildly before I swapped out the bypass servo assembly, but that problem seems to be solved.

Larry Weingarten

Hi, A quick test is to put a finger on the cold supply to the tempering valve and have someone run hot water. If the line goes cold, you know it's mixing in cold water. If you can measure the temps of the three lines to the valve, you'll know if it's the problem.

Yours, Larry

LRCCBJ

Here are the typical Zurn maintenance instructions. Make your best efforts………they are quite costly.

https://files.zurn.com/care-cleaning-install-warranty-sheets/iszw1070xl.pdf

hilltown

on the hot side flush valve, shut off hot water valve to the house, attach a hose to the hot side flush port and run water thru the flush valve, This isolates your plumbing system and the tankless. At this point you can determine a tankless issue or a fixture in the plumbing system bleeding cold water into the hot side. Does the system have a external recirculating line?

rubberduck

Thanks all, based on the tests suggested here I am almost certain it's the tempering valve. Unfortunately the key on the valve is corroded in place, and my attempts to turn it have only resulted in disfiguring the brass key. I can actually see white sediment on it. Unfortunately I'll have to replace the tempering valve, then I'll report back

SirPlumber

Tempering valves only last about two years on hard water systems. Also for future reference the shower cartridges (when aged) are also a known cause when your gas tankless is sandwiching from hot to cold.

rubberduck

I'm thrilled to report that after swapping the seized valve, we now have hot water, hitting 122f at the faucet with the heater set to 130.

I was able to turn the adjustment key freely with a wrench per the instructions all the way to one end without applying much force at all. Unfortunately, the key seemed to seize up when I tried readjusting it after testing the water, and began to strip again. I stopped before I caused too much damage to this brand new $500 CAD part. I'll need to find a better tool for adjusting the valve, or just live with it set to whatever it is currently - which seems to be about 122.

While swapping out the valve, I did notice what looks to be a bit of a piping hack job, at least to my untrained eye. I also noticed a gap between two pex pipes, wanted to check in with the folks here as to whether I should contact a plumber to redo this whole setup. Pictures attached of the gap and the overall plumbing.

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hot_rod

soaking a valve with hard water scale in vinegar can often clean them out, use it for a spare

It looks like a heating air separator on potable water?

I don’t think that is low lead or NSF approved?

Slimpickins

What Hot Rod said… I'd get rid of the air eliminator and you don't need the mixing valve unless your code requires them in your area.