Rinnai RL75i - bathtub faucet full on - hot.. unit starts up... approx 10 secs - hear a load rumble then flame goes out - code 12. What are probable causes?

SOURCE: Rinnai Integrity 2635 demand hot

Here's a link to the manual.
http://www.rinnaiuk.com/media/pdf/Manuals/26iandHD50iInstmanualUK.pdf

SOURCE: moen single handle bathtub faucet

Something is caught in the stem. Make sure the stem is free of debris. Also check that the valve or cartridge is intact.

SOURCE: Rheem Eco-180dvn. Error code 12

actually, does it give you a big rumble? if it does, then your problem is that the meter's btu's are not high enough and there's too litle gas getting to the heater so it's blowing itself out, basically. you'll need to call your gas provider and ask for them to bring in a higher btu meter. around 220,000 btu meter is best, so your furnace and gas stove(if you have one) dont take to much gas themselves, meaning dont get a 180,000 btu meter, because it would only be able to supply the heater.

SOURCE: bathtub faucet when turned to the left fot hot, no

If this is a moen fixture, which it sounds like because they are notorius for this problem. But its easy to fix, sorta.
Moen only- First make sure the water main to the home is off and relieve the pressure by opening some faucets verify at the fixture you are working on the water pressure is drained.
-Use an alan wrench to remove the handle. There may also be a handle adapter you have to remove as well depending on the model but its just a screw that holds it on, if so remove it too.
-Remove the screws attaching the large trim plate to the valve/wall.
-Pull off the small, about 1-1/2 dia., metal sleeve that slides over the valve body. Its round with a raised channel on the top.
-Now you should see a little u shaped cotter pin that has a tab that protrudes thru the top of the valve body. Use a pair of pliers on this tab pulling it up and out of the valve body. This cotter pin holds the cartridge in place which is what you will possibly need to replace.
-Use a pair of pliers, carefully, grab a hold of the cartridge on the stem (the part the handle attaches to) now pull the cartridge out of the housing. There may be hard to do sometimes there in there prtty good, but they do come out. If you have trouble you can get a tool made for this at most hardware stores or maybe even rent one, like from home depot.
-once you have it out insect it. It may just be clooogged with debris. If so try to flush it out. The cartridge is 2-pcs One slides out of the other but make sure you put it back together.
-Or, the cartridge is probably defective.
-Take the cartridge to a plumbing supply house and ask for replacement, it should be a free "moen cartridge exchange" this is a program they implimetted yers ago, but i know they have been getting more and more fiesty about it. So you may have to buy one. I believe most hardware stores also carry it but may not exchange it.
-Go back and put the new cartridge in and re-assemble. Turn water on it should be fine now.
-Make sure to lightly caulk the large trim ring to the wall after installation.

-If this is not a moen it is still most certainly a cartridge problem.
-All cartridges are basically remove the same way the only difference is instead of a cotter pin there is a large bonnet that screws onto and over the valve body and holds the cartridge in place. But in thiese cases its usually debris or replaceable washers on the cartridge that cause the problem. Sometimes these washer just get josteled out of place.
Hope that helps! If not or you need more help come back and ask and include manufactor. It should be on the fixture.

SOURCE: no hot water flow from the upstairs bathroom

yes it can I recently had a service call where the integral shut off valve was completely clogged with debris and no water came out