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Discussion Starter · #1 ·
I'm at wits end and need some guidance on this one...

Just replaced a 20 year old Aquastar propane tankless 180,000 btu/hr
on demand water heater with a Rinnai RUC90i 180,000 btu/hr one. Old
heater used indoor combustion air and had pilot, not much in the way
of electronics, had a Maxitrol RV48L in front of gas input. Rinnai
has own regulator; RV48L is not present in current installation.
Rinnai uses concentric combustion air / vent pipe.

When the hot water is turned on, the heater ignites properly.
After less than a minute the burner shuts off and I get a "12"
(No flame) error.

Only dip switches on are:

SW1 in bank 2 (LPG/NG) is off (LPG).
SW7 in bank 2 is on (RUC90i indicator, I think)

SW1 in bank 1 on or off (long vent); seems to make no difference.
SW3 in bank 1 on (2000-5200' elevation; I'm at 4200')
SW6 in bank 1 on or off (120 deg / 140 deg) makes no difference

I have hooked up a pressure sensor to the test port at the heater gas
fitting; when it cuts out the heater is re-igniting and that process
results in a momentary dip in gas pressure to about 2.4 kPa = 9.6 WC.

The unit is *supposed* to work from 8" to 14" WC.

I calculate the "resting" gas pressure is from 10" to 14" WC, depending on ambient. I say "calculate" because I am working from a single graph of absolute pressure, and ambient varies over the period from ~88.50 to ~89.10 and I have to subtract ambient to get the relative values. Unfortunately I don't have two gas sensors so I can't track ambient along with the gas pressure.

I've tuned the output of the house 2nd stage up to 13.5" - 14" WC.
That seemed to help but does not solve the problem.

Rinnai installation:
Exhaust:
Ubbink condensing concentric
1.25' vertical 1.25
45 deg 3.
1.0 ' straight 1.
45 deg 3.
(ubbink 223177PP)
90 deg (part of 223177PP) 6.
1.75' horiz (part of 223177PP) 1.75
End fitting 20. ???
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36.0 ft equiv
What's the right number for the terminator, and does that include
the 90 deg and the 1.75' pipe as well as the end fitting?

Gas line:
House 2nd stage regulator:
Rego LV4403 B66R (~13 WC output) approx. 18 yrs old
1.0' 3/4" black pipe 1.0
3/4" tee tee to stove; straight through 1.0 (guess)
4.0' 3/4" black pipe 4.0
90 deg 5.0
6.0' 3/4" black pipe 6.0
90 deg 5.0
0.5' 3/4" black pipe 0.5
3/4" ball valve 1.0 (guess)
1.0' 3/4" fitting, 1/2" flex CSST 2.0 (approx 3/4" pipe equiv)
3/4" tee bullheaded in (trap) 5.0
0.5' 3/4" black pipe 0.5
90 deg 5.0
3/4" black pipe union 0.5
90 deg + NPT adapter CU 5.0
0.5' 3/4" CU to heater input 0.5
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47.0 ft 3/4" pipe equiv

The rinnai has this problem whether the stove is operating or not; the
stove has no pilot.

Questions:

1. Can anyone tell me what the equivalent length of the terminal
fitting on the vent is for overall vent length calculations?
It's not clear to me from the installation guide, since it
shows 20 ft equiv but the actual fitting comes with a 90 deg
elbow and two different lengths.

2. Any thoughts on what's happening? It appears to me when the
unit turns on in the middle of a combustion cycle (after having
previously turned off because of high heat in the combustion
chamber) there is a sudden drop in inlet gas pressure which is
causing the unit to shut off. Is this because of vent back
pressure? or slow response at the house regulator?

3. Is the 1' length of 1/2" flex a big issue? My order of 3/4"
was shipped wrong, but by my calculations I should be within
limits for gas piping; the I.D. of the pipe is about 1/2".

4. Is this a situation where the house 2nd stage regulator
should be switched out from a low pressure one to a 2 psi one
and another installed ahead of the heater?

5. The installation instructions mention "gas pressure setting"
as a last resort. Exactly what does this do?

6. In the gas pressure setting mentioned in #5, it instructs one
to connect a manometer to the gas sensing port on the burner.
No mention is made of the manometer subsequently, except to
disconnect it. No adjustments are mentioned in the calibration
step #10 and #13; they read like the unit is self calibrating.
So what is the manometer for? There is a table (Table 1) showing
"Forced Low" and "Forced High" pressures, but what does one do
with this information in the calibration procedure?

7. In step #10 it says:
Calibrate "Forced Low" combustion using switch A (up) and
switch B (down)"
I assume this means leave pushbutton A in the up position and
hold pushbutton B down for some short period of time?
What does this do? Is one supposed to read the manometer while
the pressure goes up and down and release the button when the
pressure is at the proper point according to the table?

8. Should I even go through the "gas pressure setting" procedure,
or is this a gas supply starvation problem?

9. Is this likely an issue with a worn out 2nd stage regulator being
stiff and not able to react fast enough? It was mfg in 1999, so
it's about 18 years old.

Thanks for any and all suggestions.
 

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is the unit in a sealed room? how big is the room? if so when you are testing unit, is the door to the room closed? could be you are running out of combustion air and the unit goes off on safety...I had that problem with a takagi unit..just needed a bigger vent into the room..or pipe in fresh air from outside..the older units didnt have that type of safety or wasnt as sensitive. the new ones with all the sensors and computer are much more finicky...
 

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Discussion Starter · #3 ·
is the unit in a sealed room? how big is the room? if so when you are testing unit, is the door to the room closed? could be you are running out of combustion air and the unit goes off on safety...I had that problem with a takagi unit..just needed a bigger vent into the room..or pipe in fresh air from outside..the older units didnt have that type of safety or wasnt as sensitive. the new ones with all the sensors and computer are much more finicky...
The room is in the basement and the door is open. However, since this unit uses outside combustion air sucked in through the outer tube of the concentric exhaust, it shouldn't matter. If it was an air supply issue, it should give an error code 10 instead of 12.

In any case, just for fun, I opened up the separate combustion air intake on the top of the unit so it could suck air from both the room and the outside. The behavior is the same -- flame extinguished after igniting, resulting in a code 12.
 

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ah, I read the top of your post that it was the old unit that used inside air...the problem with these new units is all the computer junk in them, you might have a bad board, but unless you can test it who knows,,,,
 

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Discussion Starter · #5 ·
Ugh. Problem solved. The unit was damaged -- the exhaust manifold, black plastic at the back of the unit, was cracked on one side and broken with a piece missing on the other. When unpacked all I could see was a dent in the bottom which was cosmetic, so I straightened that out. The broken off piece was wedged and there were no obvious pieces or damage. I removed the fan to check it and discovered the damage. Apparently when the cover was on the unit and it ignited, the exhaust pressure into the unit instead of out the exhaust caused the drop in inlet pressure and ignition failure. When tested with the cover off, it ignited fine. But eventually did a code 12 and stopped.
 
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