Rinnai RUC98i Going Cold After 5 Minutes

GoremanX

We had a tree fall on some power lines recently, which fried our old tankless water heater. Insurance company paid for a replacement, and we had a Rinnai RUC98i installed about a month ago. Everything ran fine (just like our old QuietSide did) right up until today. We've taken multiple loooong showers over the last month without issues.

Starting today, the unit is turning off mid-shower for no apparent reason. The water suddenly goes cold after about 5 minutes, and it doesn't ever get hot again until I turn the water off completely and turn it back on. I thought maybe the flow was insufficient (the RUC98i is way over-specced for our humble home), so I tried turning on the hot water faucet in the sink while I shower. That did not help, the water still went dead cold after about 5 minutes. There's no way I'm not getting enough flow through the unit, so this makes no sense.

Coincidentally, we just had a new furnace installed yesterday because the old one was also damaged by the falling tree incident. The installers (different ones than the people who installed the water heater) did have to make some minor modifications to the gas piping at the furnace, but nothing that would affect the gas flow to the water heater. Also, furnace activity seems unrelated to the timing of the water going cold.

Leon82

Look at what the reworked with the gas line. If they restricted it it is possible the heater can't draw enough gas.

It could be shutting down because of abnormal flue temps too.

Any error codes?

GoremanX

Near as I can tell, the only modifications were done at the outlet near the furnace to accommodate the different angle of the gas intake. The main line was not changed. Also, the new furnace has a MUCH smaller capacity than the old one (56k vs 100k btu) and is infinitely more efficient (generic Goodman from 1989 vs brand new Carrier 96.5% efficiency unit), so I can't imagine it's drawing enough gas to make a difference in the flow.

My old QuietSide had a remote display and temperature control that I could see in the bathroom, which made it easier to diagnose any issues, like when it was overheating due to too little flow from calcium buildup. The Rinnai doesn't have a remote display. It's in the crawlspace, which makes it very difficult to figure out what's going on. I just went to check it out and there were no error codes displayed, just the current temperature setting. I dropped it to 120f from 130f to see if that will help, but that still doesn't make much sense to me. 130f isn't THAT hot, and I was running way more volume than the unit should need. Also, it's cold out and the incoming water is freezing. I know... I felt it in the shower.

GoremanX

I just ran the shower until it got cold (it's inevitable). Then with the water still running, I went into the crawlspace to check the water heater.

It was beeping repeatedly and flashing "25". Turning the water off and on (there's a handy ball valve for my shower right there) reset the unit and started it up again.

From what I found, error code 25 refers to some condensate drain issues. I'm checking the drain hose now to make sure it's not blocked... the new furnace is also a condensing unit with a drain hose. I wonder if the installers monkeyed around with my setup to route their drain hose.

Leon82

I would check it out.

Is the drain sealed or is there an airgap? Some heaters call for an air gap on the condensent drain

GoremanX

I checked inside the unit. I'm confused about how the drain system functions.

There's a small plastic tank at the very bottom of the unit that the drain hose is hooked up to. This plastic tank is fed from a larger metal tank or pan immediately above it. That plastic bottle is clearly half full of water. I disconnected the drain hose, and the tank didn't empty all over the floor like I expected it to, it's still half full. Is there a valve in there that allows the water level to reach a certain point before it opens? The instruction book refers to a "self-priming drain", which I assume means there's supposed to be a valve or float in there.

Leon82

That's the trap to prevent flue gas from coming inside

Leon82

It should just flow over a lip after it reaches it

Leon82

How does your drain attatch to the heater?

Eric_32

Hopefully you don't have an external trap on that condensate line. It is already internal and adding one outside on the piping will cause issues.

GoremanX

The hose attaches straight to the condensate trap fitting coming out of the heater. There's nothing between the heater and the main drain except for a 25' length of 3/4" hose, and all of the hose is well below the level of the condensate drain.

I tried running the hot water with the drain hose disconnected and holding a bottle under the drain fitting. The condensate trap fills up relatively quickly (about 5 minutes, what a coincidence) to about 3/4 full, then drips out of the fitting over a lip in the trap, just like Leon82 mentioned. I was able to run the hot water indefinitely with the bottle catching the condensation, at both 120f and 130f.

I checked where the drain hose empties, and the end of it was in the water at the bottom of the sump pit. I think the furnace installers pushed it down too far accidentally when they were placing their drain hose. It's possible air got trapped in the drain hose and this caused it to back the water up into the condensate trap. I'd have to run the hot water for ages to find out if it's working properly now, so I guess time will tell.

Also, I'll go buy a new drain hose tomorrow. The one I have now is opaque, and I'd like to have a clear one so I can spot any potential obstructions. I can blow through the hose just fine without restriction, but it doesn't take much to block gravity-fed water.

On a side note, after hearing the water heater run with the cover off at 120f and 130f, I've decided to leave it at 120f. That extra 10f causes a TON more noise, which I assume translates to a ton of wasted energy. I only like it at 130f so I can have a 50/50 mix of cold/hot when I shower, which results in higher water pressure than 40/60. But I'll take the slightly lower pressure for the extra energy savings.

Leon82

That's good.

Look thru the manual. My manual for my tankless shows an open connection which would prevent the hose from creating a vacuum

GoremanX

Oh there was definitely a vacuum in the hose. Even with the end of the hose out of the water, I couldn't get it to drain until I uncoupled it from the heater. I figured it was because the condensate trap is sealed somehow.

I went through the manual and couldn't find any diagrams showing how the drain hose should be attached. There's a bunch of do's and dont's, but no actual info.

And to correct myself, the drain hose is 1/2", not 3/4". It's exactly the same size as the condensate trap fitting.

GoremanX

I think I need to add an air gap where the hose connects to the unit. It doesn't say that anywhere in the manual, but that seems to be the case anyways. There's a slight upward slope that the drain hose has to travel to get to the drain, and that seems to cause a vacuum to form at the condensate trap, thereby preventing water from draining properly.

Leon82

This is mine just the next size up so the water can drip in

GoremanX

Thanks! Would most hardware stores have this? Or do I need to find a plumbing supply shop? All the ones I'm coming across online are 1.5" inlet or bigger, which is much too large. Maybe I'm just not using the right search terms...

BobC

Be mindful of legionnaire disease, 120f is good breeding ground for it. You don't want to be standing in a shower with legionnaire spraying all over you and your family.

Bob

Leon82

You can probably assemble a fitting from a hardware store. They should have a small inventory of plumbing supplies

GoremanX

BobC said:

Be mindful of legionnaire disease, 120f is good breeding ground for it. You don't want to be standing in a shower with legionnaire spraying all over you and your family.

What? It's a tankless system. The water stays in the unit for almost no time at all, and it doesn't maintain a constant 120f inside. It only warms it as it goes through then the water in the unit quickly cools back down to regular temperature once the water is turned off. There's like zero chance of legionnaire's disease in such a setup. In fact most of these units come pre-set from the factory at 110f to 120f.

gschallert

Hmm, I never really thought about legionnaires with an on demand tankless. I just figured it was only a problem when storing water. What is a "safe" temp for tankless DHWS? In laws are getting a combi boiler and will have a tankless setup when all is said and done. They are older and generally more at risk for these types of infection sources. I just googled for an answer and am getting wildly conflicting "facts"... anybody have a definitive answer?

Jack

The water in the tankless is actually heated to a higher setting than the touch pad and then mixed down to that temp. Being a direct fired unit and no accumulated organic, I've never seen or heard of Legionellas on a properly installed tankless. To the OP, your unit is double trapped. Rinnai's are internally trapped. Put an air gap at the outlet of he unit. The unit will then run fine. You could move the touch pad upstairs if you choose. Actually the wireless control will be introduced the first of the year which will allow you to see what is going on with the unit from your phone or mobile device. That will cover all Rinnai water heaters back to '06. With a crawl space install, that I think will be the hot set-up for you