Miguel

Rinnai C85i. Temp fluctuates (similar to other thread) but I eliminated the faucets or any other H/C cross as the cause minutes after I got there.
I'm back at the office now after having my cell signal drop out twice while talking to Rinnai tech support.

Problem: Call for HW at faucet (measured @2.5gpm). Unit fires up and heats to 118° (MC-91 set at 120), then after a few seconds drops to between 98 - 110. A few seconds later drops quickly to 58°. The burner remains on the whole time and you can hear the inducer ramp up and the flame increase just before the temp drops. Burner remains on high for a few seconds more (these would be really long seconds if you were the one in the shower!!!) and temp jumps back up to 118° before repeating the same cycle.

No error codes.

I think it's the bypass valve and/or actuator. Tech support guy kinda agrees but had me swap the two thermistors to see if any change. (That's when we got cut off). Thermistors are both within range when tested with ohmeter; swapping them made no change in operation.

Unit is 4 yrs old and out of warranty. Unit is very clean and thermistors may even be new, they're so nice and shiny! I disassembled flow control valve and bypass ass'y. Everything is reasonably clean and seams to operate freely. Screen is clear.

Anyone have any idears???

Any ideas?

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Tankless

I have dealt with that a lot.

Did you measure gas pressures?

Did the unit error out and shut down, or just continued the issue?

Has the unit been flushed?

Dud you confirm this particular unit HAS a bypass servo valve? (some older ones didn't)

Was everything working fine than all the sudden there is this issue?

When the unit refired, how hot did the output temp get?

Can you pull the vent and examine the top of the heat exchanger (this will tell you how the burn is behaving)?

How hard is the water or is there a properly working softener installed?

If all that checks out, I would request a new mobo.

A note about thermisters......rarley do they fail. Also, you cannot test them accuratly when they are in a circuit.

If you just threw your test leeds across a molex plug or something, I promise you, you will not get an accurate reading. It may be within spec, but it will not be accurate and can very easily cause the heater to switch burn rates because by testing like this, it may cause the output to read 90 instead of 120 or vise versa....read 160 instead of the requested 120.

In my experience I'd be looking at the gas supply pressures.....heat exchanger condition and if it does have a bypass, take it apart and check for scale build up. That can hinder operation and cause issues similar to this one. It looks like this:

Photo702.jpg

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Tankless

ok, just re-read your post.,.....That really sounds like a starved gassing issue. I would expect to see that on a gas line that has rusted itself mostly shut or there is some major breach. Make sure both gas solenoids are opening and closing. you can test DC voltage across any plug or terminals you want.....I think 80 to 100 VDC is the range. I would expect that unit to fire the second state around 2 to 3 gpm so let that be an indicator for when you should expect the call for more fuel.

OK, I gotta get back to work. Let me know if this makes sunse to you. I must say it is rare to have a burner assembly not shut down AT ALL during this sort of issue. Gas line or mobo....but it's hard to diagnose from a few thousand miles away!!

Oh, is this on LP? I haven't had much experience on gassing systems for LP......dirty jet maybe?

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"It's now. You just go for it now and you worry about the future and the past later." -Steve Nash

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Miguel

Thanks Tankless!
It's NG. Supply press is 7". Manifold pressures measured were .9" - 3.3"
No difference even if I fire up the 100MBTU furnace at the same time. These are the only gas appliances in the house.

The thermisters were measured unplugged.

Your pic of the bypass is at the inlet, correct? Mine looks nice and clean at all ports. Not shiny, new clean, but very acceptable and no buildup other than typical biofilming. Popping the actuator off and manually rotating the valve stem feels smooth with no sticking or rough spots. *I took care to make sure the stem was in the same position and the splines lined up the same when I reassembled it but I wonder if someone else before me could have taken it apart and got it off a spline or two??

The tech support guy was very helpful but was also somewhat bamboozled. He agrees that the bypass is somehow flakey but seems to think there may be an issue with the board as well since nothing, I mean nothing can seem to point to a clear fault.

He's fast-freighting a new bypass ass'y to me from Atlanta.

I hate not being able to isolate the cause with certainty.

Thanks for your input, Tankless. I really appreciate it!

Btw, what's a mobo?

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Tankless

Mother Board

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"It's now. You just go for it now and you worry about the future and the past later." -Steve Nash

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Plasticman

mud daubers in the burner maybe? I will guess at anything hoping I'm right

Tankless

I will say this and take it for what it's worth. There is without a doubt....for a fact testing that can be done to learn what the exact problem is.

As I have said before, just because you can, doesn't mean it's a good idea. I used to spend hours testing the mobo's and I finally got tired of it. I changed my testing methods to simply isolate the different systems. This is of course on harder than normal repairs where the issue doesn't pop out in your face. If you can bleed it down to.....maybe 3 things it makes the job more manageable. From a cost perspective we (as in guys why service these things) must be able to accuratly diag. them and fairly quickley.

If you figure $100 per hr. X 3 plus another 100 for parts / shipping - tax your at 4 bills.....almost half the cost of a new one. This will leave a bad taste in their mouths and the good name of tankless water heaters sufferes, which may cost us potential jobs.

At least that's the way I see it.

One thing I would also have you note is the inlet pressure. 7" standing or 7" wide open?

One thing to look out for is what the manometer is doing during all phases of the error. Does the pressure ever drop below 5 or 4 (or more)? even for a split second?

The internal mani pressures look fine, A little high on both sides, but nothing to warrant a problem.

I don't think it's the servo....just doesn't make a lot of sense, but that doesn't by any means, mean that's not it. It is possible to do tests to determin if the servo is bad, but I can't type that up, and it is fairly complicated.

The one clue that has my wheels turning on this, was a subtle detail you gave. You said there were no error codes. When there is a pretty big malfunction you should get some kind of code. The fact that you don't plus all the other details of this fix still leads me to the gas or mobo. 4 years ago, Rinnai wasn't that good.

Once you get it running sell them a flush. If they are gonna have to spend money on this thing, may as well get it all done at once so they can forget about it and just enjoy hot showers!

__________________
"It's now. You just go for it now and you worry about the future and the past later." -Steve Nash

And when I leave come together like butt cheeks
-Grits n Gravy

Asparta

You will not be able to check a standing pressure test, as the tankless unit does not have a standing pilot. Instead, take the test at the furnace, with the pilot going. If the furnace does not have a pilot either, you have to take a test where you can have a minimal amount of gas flowing. Do not take a test with the burner going, and stay on the inlet side of the gas valve.
Have you checked the flow switch? Not sure if that model has a separate flow switch, but I know Navien models do. Could be hooped!

Miguel

I'm a little miffed that of all the installers around here there doesn't seem to be anyone willing to perform service work on these things. The original installer, whom I hold in high regard otherwise, came out twice and then wouldn't return their calls. This is generally a red flag but I also happen to know these people. I didn't really want to get into this one either actually , but I did so I have to solve it or get fired.

7" dynamic. Furnace running and unit firing w/ a garden hose (from the unit) wide open into a laundry tub. The piping is sized to handle 350cfm. I took the main pressure at the meter, tho. The furnace and tankless are both about 20' away. I used my digital mano which is very precise but I should prolly take one at a drip leg and just use a U-tube to check the manifold press. Like I said, the digital is very accurate, and just recently factory calibrated but with an analog manometer what you see is what you get.

Seems high? I thought it was low on high-fire. Low seemed a bit high but I reckoned that I just wasn't able to throttle the flow precisely enough to get a true "just on" reading. Again, the digital seems to have a bit of a lag. I'll try again with the U-tube.

I don't think it's the servo or the gas supply, myself. There's no sign of sooting or poor flame. In fact, when it's running that part of it seems to be doing it's job just fine. but the desired reults aren't being seen.

Well, there were no error codes generated while I was there. I don't know if it makes a difference or not but one of my first checks was to run down the previous errors. They were mostly 03 and 11 which is consistent with what the HO was telling me when he was playing with it (shut off the breaker prematurely, started it up after de-scaling but forgot to turn the gas on, etc)
There was an error 06 which I don't recognise.

That'll be a hard sell since one of the first things he said was, "And don't tell me it needs to be de-limed! I've had it up to here with the deliming!!!" and he looked alot like Jack Nicholson in "The Shining" when he spat that at me.
But if I get it going you can bet I'll be back at least once a year.

Thanks again for your time, Tankless!

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Tankless

Ummm, wrong.

A static pressure test is a baseline test only. If I measure 9" in a static line, than fire the heater to full blast, I am now able to see the total pressure drop. This pressure drop must fall within a certain amount. This test will yeild issues with an undersized gas supply line....or some other fuel delivery issue. it's just one more test to figure out the puzzle. I do this often to see if a unit is actually NOT burning enough fuel. I know what they should consume and can accuratly guestimate a gas piping system. It helps figuring things out.

IMO there is no need to use a real manometer, the digital ones are very good.

__________________
"It's now. You just go for it now and you worry about the future and the past later." -Steve Nash

And when I leave come together like butt cheeks
-Grits n Gravy