# Tankless question - Rinnai



## Miguel (Aug 10, 2009)

Hey all. I'm not totally stumped by this one but I have to admit that the baffle-meter is reading a little in the high zone.

Problem: Customer is experiencing the hot-cold-hot sandwich effect. That is, in the shower the water comes out nice and hot but there are periodic cold spurts. The unit is an older Rinnai C85i.
My first response was that it should be piped with a small storage tank (ala the bronze pump and check valves, etc) but his response is that it's worked just fine for 3 yrs and now it's a problem. The unit has been de-scaled twice now (once by the HO and again by me) and the thermisters cleaned, the CW in screen cleaned many times, etc. I didn't do the install but I know who did and it's a very nice, clean job. The DHW *IS *direct to the faucets (not via a buffer tank as I would have done) but it has worked to the HO's satisfaction until now.
BTW, the HO is a very capable person and has been very diligent in maintenance and keeping the unit in tip-top shape. It's a 3 yr old unit but looks brand new.

Personally, I'd like to put in a small 9 gal storage tank and repipe to suit as I'm sure that this would solve the problem. Trouble is that it has been just fine until now and it is almost rediculous to suggest having the HO spend the $$$ to cure a symptom. (I'm with him on this as it has been just fine until recently.) 

I'm just coming up to speed with the Rinnai units and the unit in question is not a far cry from the current models.
I'm thinking that the CW bypass that helps temper the delivered HW is swinging way out of range which is causing this hot-cold-hot problem. My thinking is that the scale that would have been flushed from the system isn't being flushed from the modulating bypass line during de-scaling flushing since there is nothing to activate the bypass during normal de-scaling procedures.

Any thoughts?


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## LEAD INGOT (Jul 15, 2009)

Is it just at the shower? May just be a cartridge on the faucet going bad.


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## Miguel (Aug 10, 2009)

No, we tried at several faucets and even at the laundry tub only 20' away (1/2" copper pipe) there is a full-port ball valve and a hose connector to test from. AT the unit, with your hand on the HW line you can feel the temperature change while watching the burner firing.
*The burner modulates and then shuts off before re-firing again. The cold slug is felt after it re-fires. It almost defies logic.


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## Tankless (Jun 12, 2008)

I got thoughts!!

I'll ASSume the unit is installed correctly in that it is an indoor install with concentric venting, so make up air is not an issue.

I will spare my fingers and not (re)write every single possibility but here's a condensed version.

You do not have a cold water sandwich. You have a malfunction. I see this often and it can be a number of things, but you are already on the right track (did I just say that?).

I learned this about a year after I started descaling these things about 5-6 years ago. During the flush process, if you just unplug the unit and simply start the flush, the solution (vinigar / lime away / clr) will go in, than go right back out and totally miss the heat exchanger. By default the units are designed to have the CW bypass open at 100%. Once the unit realized the temps and flow the computer will decide how much flame is needed for the temp of water requested (120-130 F) Once that has happened the bypass valve will start to close and send water through the exchanger.

So, if you simply unplug the unit and just do a flush you have done nothing.

You need to flush the unit with the power on with the gas off. The solonoids will click and clack as its trying to fire, but the CW will start to close and direct your solution in the exchanger. You're looking for a decreased amount of fluid coming off the hot iso tap. Now you have visual conf. that you are directing your solution to the correct place. flush it for a good hour at least and every now and than shut the pump off than on, to help get all the scale out of the servo valves (very important). The solution you use is dependant on a few things...The temp of water being requested, and the incoming temp (basically how much work is the unit doing) next is the hardness. is there a softener, if so what's the hardness post softener?

The other thing that leads to this sort of thing can be a faulty gas regulator. Is this a consistant issue...does it happen only in the shower? can you hook up a hose to the hot iso and monitor the temp for 15 minutes? 60% of my tankless service calls have nothing to do with the tankless unit....if the problem is only noticed in the shower and you have a constant temp out of the tankless, than you have a cartridge issue. Throw it out and put in a new one.

A standby tank is a good idea, but is a bit overkill for a SFD (single family dwelling). And is not the fix to your problem. If you are still unsure plan on spending some time troublshooting the system. I have spent years learning the fine details and tendencies tankless units have in my area and after many, unpaid hours of work, I finally have my own system to trouble shoot any system fairly quickly and always accuratly (that's the hard part). 

Molsen's are on you.


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## Tankless (Jun 12, 2008)

Miguel said:


> *The burner modulates and then shuts off before re-firing again. The cold slug is felt after it re-fires. It almost defies logic.


 
That does not defy logic, that is actually perfectly logical. Water is still flowing and the burner has shut down, than the burner re-fires.....that is perfectly expected.

My guess from where I sit, you have a thermal overload issue. heat exchanger is gummed up due to dirty air makeup and / or hard water. You all who read this must know...I am seeing an unbelievable amount of damage to these units from hard water. not only on Noritz units but Rinnai's and TK's as well. Scale builds up inside the exchanger right at the primary and secondary gas beds and that's where the majority of work happens. When that scale builds up, it causes the outside of the exchanger to develop overheating. This in turn causes the exchanger fins to warp and now your really F-ed. Air will never flow over the exchanger correctly again....flushed or not. I'll tell you this, Noritz isn't warranting that either. And neither is TK. TK are really bottom of the barrel units. I am amazed they work. They are like Pinto....one small rear end away from blowing up.

if you are good with these units and know how to get into the computer you can see the internal exchanger temps and compare them with what is normal....Chances are you are over on that value and the unit shuts down to preserve itself.....by design. Pull the vent, and look down the flue collar and examine it carefully with a flash light. This is where your issue will be, me thinks!


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## Miguel (Aug 10, 2009)

Thanks, Tankless! Two-Four of Molsen's coming your way, man!
Never thought to try it energized! But it makes so much sense.
We'll give that a go and see what happens.
I didn't realise that the bypass would be open 100%. I'd have thought the other way around but, regardless the idea is to flush the whole unit.

Thanks, man! :thumbup:


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## Miguel (Aug 10, 2009)

Tankless said:


> That does not defy logic, that is actually perfectly logical. Water is still flowing and the burner has shut down, than the burner re-fires.....that is perfectly expected.


Right, but the temperature change whilst monitoring it does seem odd. ie: burner firing full 180MBtuh feels warm (outgoing HW), then burner cuts out... temperature at outlet continues to rise until almost too hot... burner re-fires and comes on full fire (outlet temp still very hot) then feels cold enough to condense moisture from air, hten very hot again.

I'll try your suggestion as that just seems to make sense. Let you know how it works out and Molsen's on me.


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## Tankless (Jun 12, 2008)

Miguel said:


> Right, but the temperature change whilst monitoring it does seem odd. ie: burner firing full 180MBtuh feels warm (outgoing HW), then burner cuts out... temperature at outlet continues to rise until almost too hot... burner re-fires and comes on full fire (outlet temp still very hot) then feels cold enough to condense moisture from air, hten very hot again.
> 
> I'll try your suggestion as that just seems to make sense. Let you know how it works out and Molsen's on me.


 
I understand what your saying....simplist way to explain that is, the water does have a bit of a path to travel before it leaves the unit. Also, the bypass could be working overtime to protect the family from getting scaulded. Meaning, it's diluting the overheated water down, therefore it can cause fluxuations. Main thing with tankless units is start with the simple stuff. Rarley are issues really rooted and difficult to find. This is a good lesson for you, because chances are you will get this same call again with a different solution. That is what creates a need for you and if you enjoy doing this sort of thing you can make a name for yourself in your market. That's what I did, and every month my business grows because of it. However, it won't come free...I have spent hours dealing with things like this


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## Optimus Primer (May 20, 2009)

i've seen this happen with takagi's too. the filter screen might be clogged on the inlet side. check that.


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## 1703 (Jul 21, 2009)

Good info tankless.

Correct me if I'm wrong, but not all rinnai models had a bypass?


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## Tankless (Jun 12, 2008)

Colgar said:


> Good info tankless.
> 
> Correct me if I'm wrong, but not all rinnai models had a bypass?


That used to be Rinnai's claim to fame, that they didn't bypass therefor didn't waste heated water. That is until they realized their products couldn't compare to the rest of the industry standard. Than they changed their tune.

But you are correct, I don't know ALL the particular units that don't bypass. The OP's does!


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## rickmccarthy (Jul 20, 2009)

*Error codes*

Have you checked the controller for error codes there could be a code 00 which stands for LC or Lime scale or a possible 16 for over temp

BTW Damage done to the heater from lime scale build up IS NOT covered by the warrenty. It is improper maintenence.


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## Miguel (Aug 10, 2009)

Error codes have been checked and there's absolutely no error history.
I've been out of town again this week (just got back this evening) and there's a msg that buddy wants me to help him do "a proper flushing" even though he's done several de-scaling flushes as per factory reps advice. Seems he's a bit nervous about doing it with power on to the unit. (Gotta love a homeowner that is diligent yet knows his own boundaries!) I'll disconnect his pump (for the in-floor heating) and make sure the unit won't fire during the flushing prolly tommorow.

Thanks for your help on this, Tankless. I really respect your suggestions and I'll let you know how it works out.

Thanks to all others who responded, too.


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## Tankless (Jun 12, 2008)

Miguel said:


> Error codes have been checked and there's absolutely no error history.
> I've been out of town again this week (just got back this evening) and there's a msg that buddy wants me to help him do "a proper flushing" even though he's done several de-scaling flushes as per factory reps advice. Seems he's a bit nervous about doing it with power on to the unit. (Gotta love a homeowner that is diligent yet knows his own boundaries!) I'll disconnect his pump (for the in-floor heating) and make sure the unit won't fire during the flushing prolly tommorow.
> 
> Thanks for your help on this, Tankless. I really respect your suggestions and I'll let you know how it works out.
> ...


No prob, I really think you should install the isolation kit. It's worth its weight. Makes running the system FAR easier. I charge 250 for the kit, installed for a 200kbtu or less version. You can control the exact outlet flow to look for issues at different levels. Typically 2 gpm is the threshold for the next gas solonoid to start firing. Knowing if you have a primary or secondary (or even a third) issue can be a big help, that tech support can't help you with. If you don't know what ISO valves are just search some of my installs and you will see them. I have never done an install without them. Good luck. P.S. It's not a horrible thing if the unit does fire during the flush, just don't let it fire for more than 5 or 10 seconds. Don't need to cook the solution. What do you plan on using to descale?


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## Miguel (Aug 10, 2009)

Iso valve kit was installed from the get-go, and yes it makes a *HUGE* difference for performing maintenance, etc. :yes:
I just got back from cleaning the unit and it was amazing the amount of crud we flushed out! We just used white vinegar. Worked well and I reckon this is the first time the heat exchanger has actually experienced a descaling even though he's performed the procedure twice already. Neither of us realised that the bypass defaulted to full bypass when de-energized (myself, I'd thought it worked vica versa. Live and learn). Now that it's been fully cleared it seems as if the bypass solenoid is working more smoothly now. Perhaps that _n_th of a second delay betwixt opening/closing was all that needed to be rectified.
Pulled the thermistor sensors and wiped 'em clean and the unit has been working to spec after many tries. We'll let his wife be the judge next time she has a shower, but so far we've seen a remarkable difference in operation.


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## WestCoastPlumber (Jun 17, 2008)

I have had "cold water sandwiches" before. I ma not saying htis is your problem, but always something to look at.

less then 6 tenths a gallon of water will cause the unit to kick on and kick off, gummed up shower heads, low flow shower heads, and also low water pressure, Rinnai requires a min pressure of 50 psi....you should see how well one works on 35 psi....

Nortiz requires you to have the unit plugged in with gas off when you flush, Rinnai says to turn the power off when you flush.

Tankless is correct, I'm an asp for rinnai and noritz, you will spend a lot of your own time fixing problems and not be paid for it, but in the end, it will pay off because everyone will call you for their problems and you will make your money this way. I still work a lot for free sometimes. I am sure this will happen for a long time.


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## Tankless (Jun 12, 2008)

It gets better bud. Once you get a hand full of solutions for a given problem it makes things go faster. Low flow has been my biggest pain for a long time. That 6/10ths on off issue is a tell tale sign of improper gas mani pressures and a dirty flow sensor. A lot of things can be resolved with an aggressive flush. However the skills of a tankless tech can be challenged when things are not so obvious. Learning the tendencies of different models and manufacturers is a gold mine in the making. It just doesn't come easy because not many people have the time invested of real world, market experience. I don't claim to know everything about each version of unit, but I have my little process that I abide by to eliminate things. It's just the process of elimination for me. What is it not is what I do. Testing things that have a greater chance to be damaged. IMO thermisters rarley go out. I've been told by the manufacture to simply throw a meter on the leads and see what you see. Well, falling back on my engineering degree, I know that a resistor in a circuit (plugged in) cannot be accuratly measured with test leads simply across the resistor. Cheap meters have a load on them, you are trying to measure and your meter actually takes up a little amount of power / current. Not saying you need a 1000 dollar meter but a 100-300 Fluke meter will last and will stay in calibration for field level work. Down to component level....that's a different story.

This is just one of MANY things that take time to figure out and takes intelligence to learn from. There were times I would test every single circuit off the board looking for a bad component....never found it. This is when knowing the tendencies of a unit comes in to play. Low flow on an old Noritz 6.9? Man, I have prolly spent at least 30 hours troubleshooting that one. I finally got the head of Noritz engineering to tell me I was chasing a ghost. It was a design issue and there was no fix. (problem was unstable temps at low flow). Leaning to recognize an intermittent mother board....God what a PITA. my last one was 3 hours billed, spent 4-5 hours actually working. The unit was a week old.

I can go on and on.....Once a "plumber" crossed his lines to the heater. Of course he didn't tell me this. I get a call for a new unit not working correctly. I get there, sure as shyit it's not working correctly. I spent an hour on it, and than went in the attic where he relocated. I saw the two couplers side by side where he crossed. He jammed the flow sensor (he actually crossed the softener too, and debris got in the impeller). Again, at first this can be a ***** to figure out, but that's why I saw it takes intelligence to learn from all these experiences.

I am a normal contributor to Noritz on difficult situations and their solutions. I have corrected a number of issues that were not able to be simply trouble shot. Those component failures usually mean the parts get sent back to Japan where they really get into them.

I have a feeling as time goes on, the cheaper tankless units will start to yeild basic failures...fan speed slows...solenoids fail to open 100%...etc..etc... I also think it will come to a point where it will be cheaper to replace the entire unit than it will be to fix the existing. Guys like me, who have invested the time in learning will have value but soon enough my value on a repair and the clients value on it will eventually start to shift away. It's already happening with bad exchangers. 1 hr to diagnose...1 to 3 hrs to change the Xchanger, plus a few hundred for the parts....thats 400-600 on a 950 dollar unit? Mark that 6 years out, is it worth it? That's what I mean!

I like the challenge, when I finally get one. When it boars me, I'll find something else to exhaust my brain!


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## WestCoastPlumber (Jun 17, 2008)

I have 3 or 4 meters, Fluke is by far the best meter I have.


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## Miguel (Aug 10, 2009)

My main one has been a Greenlee DM-500 for enough years to convince me that it's tough enough to last, yet although it's highly accurate and pretty fast to get a good reading it's never been my fave. My little yellow Fluke (forget which model but it was a lower end unit) was always the best for me. I still place flowers on it's grave from time to time.


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