Installed the heater yesterday. Initially got an error 99. Tech support had us look for the two small tubes to connect to inlet elbow. They were not connected so we corrected that problem. Now we get an error 11. Gas is on and unit acts normally for ten seconds or so then shuts off with the error 11.
If you set the thermostat to "H" and start it several times in a row the top of the unit gets warm but no air comes from the louvers.
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i am not an expert. this is how i fixed my rinnai code 99. my rinnai 100 4f would run for few seconds then the red light would continue to flash. code 99 would come up.i took the unit out of service. i notice that the fresh air hose had two very small hoses located on it's elbow had been switched around when i took it to be serviced. the unit worked well. how ever. i changed my mind when i realized the service was not need. anyway the two small hoses on the elbow one office is larger than the other and they must be put on the correct one,i switched them around the unit flamed up. and now i have sweet heat again.no code 99.
This fusible link may be open. Try jumpering the two leads and see if the unit operates. If it does replace the fusible link. Do not operate the unit without the fusible link for an extended period. It is a safety device and is there for a reason. Hope this helps.
CODE DISPLAYED FAULT REMEDY
11 Ignition Failure
Check that gas is turned ON.
Service call if repeated.
12 Flame Failure
Check that gas is turned ON.
Service call if repeated.
Service call if repeated. 
16 Room Overheat Lower room temperature to less than 104°F (40°C).
Room Temperature Sensor Faulty Service call.
Overheat Temperature Sensor Faulty Service call.
49 Sensor Breakdown Service call.
53 Sparker Failure Service call.
61 Combustion Fan Failure Service call.
70 Faulty ON/OFF Switch Service call.
71 Faulty Solenoids Service call.
72 Faulty Flame Rod Service call.
73 Communication Error Turn heater OFF, then ON again.
99 Flue Block Check around the flue terminal for blockage. 
 Remove any obstructions. The flue needs to be kept clear to expel exhaust gases. If the appliance fails to operate contact a
qualified service agency.
 If the fusible link needs replacement, it must be done by a qualified service agency. In addition, the cause of the overheat
needs to be determined. The fusible link is a one use safety device that breaks to shut off the appliance.
Code 11 is that the unit is trying to light but does not sense a flame after 5 seconds. Usually means you have run out of gas if you are bottled gas. Check your bottles are turned on if you are on LPG, you might need to change over manually. Have you anything else on gas?
The fitting on the back of the unit is 1/2" MPT. Wouldn't suggest messing with it if you are not familiar with propane or natural gas. Make sure the unit is set up for LP if you are going to use a portable supply tank. These units are very reliable. Check all fittings with a diluted soap and water solution before turning on the unit. If it is not connected to it's venting system, you would have a forced induction source of ignition if you had a leak in your piping. These units should only be installed by a licensed plumber or gas fitter, one simple error could allow carbon monoxide (CO) into your house
I believe the 12 is the error code for "incomplete combustion". In this case you will want to start with the thermocouple. If you are not handy at these things I would call a service tech. You can find a service person on their website.
A code 11 also means that it might not be getting any gas. On thr right side of the unit you will find a small door. open it up and make sure the valve is in the open position. If it is, you should contact your gas company to come and diagnose what is going on. The Rinnai is not a do it yourself fix it type of heater.
This heater needs some back pressure on the exhaust to work so while it is on your bench, take the reducing coupling found on your vent system and put it on the exhaust pipe to restrict the outlet size. Now cycle the heater with the front cover off and with both of the small rectangle viewing ports off so you can see the flame. The heater fan will start to turn closing the contacts on that little square pressure switch. If it does not close you will get this error code. Make sure the small hoses are attached to the switch and the exhaust system. If you hear some clicking going on at the gas valve you can assume that the pressure switch is closing. The gas valve will energize and send gas to the burner. Look for flame. There is a small probe that will sence flame within a few seconds and keep the unit on. If it doesn't, it will turn the unit off within 3-5 seconds. I suspect that a spider has set up shop somewhere within the heater and either restricting the flow of air to the pressure switch, the burner or you may have a bad pressure switch. If you need a switch, I can send you one for $15. Contact me at "email@example.com".