Our water heater is having issues. A couple months back it stopped giving hot water, so I looked up some simple DIY stuff, and found out about a red switch that resets it.

Code 12 is a flame failure.
First step in diagnosing the root of the issue is to check for any separation, obstruction, or failure in the vent system.
Second step is to use the isolator kit under the heater to flush the heat exchanger with vinegar or rectorseal calci-free.
These two things are a great starting point to working on any tankless water heater. Flame failure can be as simple as the heat exchanger being clogged not allowing the sensors to work properly or the exhaust being mixed with the air intake. If the problem persists I would suggest having a Rinnai Certified Tech look at the heater.

The hot side of the faucet is most likely plugged with something that came up thru the water system. The average thankless waterheater requires at least 1/2 to 3/4 gpm flow rate for the unit to sense flow and turn on. If there is not enough flow it will not fire and heat the water. Not sure about this particular unit but a few require at least one full gallon per minute flow. So chances are that if you fix the faucet, you fix the hot water issue at the same time

Its probably time for small claims court and a new company to install good equipment. Have a couple different companies/contractors give an estimate and possibly expert advice/witness you would then be able to use in court so you can get reimbursed from a bad company. Hope it all works out.

Had a similar problem and found that the plumber who had replaced the recirculating pump had removed the plastic pipe hollders. These are spacers used to hol the pipes firmly as they pass through the wood studs. Even with only two spacers missing the hammering sound passed through the whole system. Once new spacers were installed the hammering sound stopped.

Water pressure is blowing everyting loose with new heater,give your system a good flush,it will work it self out. RichieRich

I had the EXACT same problem (installed my tankless last winter and worked fine, now this summer it fluctuates, have to turn the shower completely off and then on again to get any hot water and then it only lasts for a couple of minutes!) Argh - it is frustrating!

Here's what I found: the demand for hot water is lower during the summer (hot) months because the "cold" water coming in is already quite warm. You don't need as much hot water to make the shower temp comfortable. SO, my tankless was shutting off due to low flow demand. I've removed the flow restrictor from my shower head and it helped alot but didn't completely solve the prob. Next step is what has been recommended to me by several people but is much more expensive - change the pressure valve on your shower to a temperature valve. Or just wait til winter and it should work ok again :)

the 90 error code will be caused by one of two things:
1. the incoming gas pressure is low
2. the unit is dirty and needs to be cleaned

make sure that it is pushed into the flame of pilot light good because if not it can cool off and go out.and make sure that no drafts are getting in to heater to blow pilot.like when you close or open the front or back door have someone open them while you watch pilot light to see if it flickers .

If you are not familiar with the element, I would get someone that is familiar with one and have them do it. It's not something I would recommend you do based on your question. There's electrical, pressurized water tank, and a whole lot of water that may cause you to have a worse bad day.

The red button you pushed on the hot water heater was probably what is called a "piezo-electric" ignitor. It's normal for it to make a snapping sound and then pop back out. It sounds like you just lost the pilot light.

That being the case, do this:

Turn the gas valve on the hot water tank to the "off" position - it should be silver in color and down low on the tank. You may have to push it down to do this
Leave it off for about two minutes or so
Rotate the valve back on to the "Pilot" position
Hold down the pilot gas valve - it should be red and immediately to the left of the main gas valve.

While holding the pilot gas valve down, push the other red button to the right of the main gas valve - this is the piezo ignitor - push it several times in a row quickly

The gas pilot light should light
You can stop pushing the ignitor now, but continue to hold down the gas pilot valve (the one just left of the main gas valve). Let this valve up after about another 20 seconds or so.

Now rotate the main gas valve to the "on" position

Listen carefully - you should hear the main gas light and feel heat at the water heater flue outlet on top - don't touch it with your hand - you could get burned

If no luck, repeat the process until the heater re-lights

TIP - the instructions for re-lighting the pilot should be written on the heater

Just a potential diagnosis - the thunder clap, if as close as you describe, may have been intense enough to create what is known as a "draft inversion" that may have snuffed out your pilot light. The air pulse came down the flue pipe.