I saw this old thread (started in 2009!), while looking for the procedure to reset the LC (lime scale build up) code on my Rinnai tankless water heater (model RUC98I).
Off topic: Hint, older units simply required you to unplug and wait 60 seconds to reset this code. Newer units have two buttons inside the unit near where the dip switches. They are tiny, and you're supposed to push both for 5 seconds to reset. LOL, I can cover both buttons with my little finger; the only way to reset them was to use one finger on both buttons. Only the troubleshooting (i.e. repairman's guide) shows how to do this, and a couple copies I found on the internet had that information blanked out, with "call Rinnai or your serviceman" added. Apparently, Rinnai doesn't recommend the descaling and reset procedure for homeowners. In any case, what the directions do not say is that you have to have the unit open and plugged in to reset the switches - so FYI.
Back on topic. My unit started sending calcification throughout my plumbing almost from the first couple of months it was installed. I first discovered the issue when my unit sent a code 63 (internal recirculation pump flow blocked). I checked the thimble sized filter and it was half filled with what I then thought was sediment or sand from the old city water plumbing (new home in an old lake area). After receiving the code and cleaning out the filter about every 10 days, I finally got a whole house sediment filter and thought the problem was solved. Then the shower head started becoming problematic, and I cleaned that out about once a month. Then the check valve separating cold water intake from hot water recirc return jammed due to particle blockage, resulting in cold water back feeding into the hot water line (i.e. delightfully cold showers, just long enough after you jump in to get your hair fully lathered up; that became a real source of ... ah ... domestic unrest).
I went through about three more months of trying different filters, and complained to the poor guys who made the unit about their "defective product". I finally got fed up, and replaced the water filter with a clear water strainer (50 mesh, 1" water main line). Then I flushed every appliance in my system.
Once again, I figured that I had finally solved the problem, and we enjoyed delightfully "instant" hot water - for about a week. Then the check valve jammed again. and then the shower head became filled with debris again, and then EVERY fixture started clogging up. That was when I finally figure out that (a) these units require periodic flushing with an acid based product (i.e. white vinegar), and (b) the problem was on my outbound hot water line.
I purchased a second, smaller 3/4" plexiglass strainer and installed it. Three seconds after activating the hot water pressure, the strainer was full of debris, confirming the problem. I cleaned it out multiple times, and cleaned and flushed the household faucets and figures. I had a pump and hoses on order to flush the system, it arrived today, and I saw my first "LC O" code. (For the Rinnai units, that code beeps continuously, LOUDLY, and obnoxiously, and the hot water ends. Apparently, this can be overridden by toggling the ON/OFF button 5x, and you'll have hot water for another 70 hours. Then the alarm will sound again with an "LC 1", the system can be temporarily reset again, for a number of times until permanent lock out. )
I flushed the system with vinegar (look it up on Google; there are video's out there fully describing this), reset the code, and we're back into the hot water game.
The bottom line for me, as far as lesson's learned, is:
a) Yes, install a 50 mesh strainer just AFTER the point where hot water exits the water heater. Not only will it protect the rest of your plumbing against sediment, but you'll see the sediment in the strainer, giving you a clue that it's time to flush and clean the system. Rinnai comes with a thimble sized filter/wire strainer on it's intake to protect the water heater (which with recirculation, fills with calcification sediment from the water heater itself). The second strainer I added ought to be standard for every tankless system. Sediment builds in a tank system, but it gathers in the bottom, while hot water is drawn from the top.
Any tankless system is going to have a sediment problem, regardless of water quality, because there's no tank to "hide" the sediment that every water heater (of any kind) generates.
b) I intend to flush this system every 4-6 months as preventive maintenance, pumping vinegar through it for about two hours, regardless of what I see in the strainer.
c) I'm going to have to get a water treatment system in eventually, even though the hard water here (Northern Idaho) doesn't seem as hard as where I came from (So California).
This is my first tankless hot water heater. My conclusions:
- It's solid technology. It's terrific when working normally.
- However, it depends on very clean, soft water.
- It's far easier to get a cold water block in a tankless system.
- Tankless systems require far more regular maintenance than tank systems.
- This works well for me, because I'm already on propane. I have a 500 gallon tank I use hot water, cooking, backup heat (30K BTU fireplace), and the clothes dryer. (I'm plumbed for a backup propane generator, to be added soon.) Primary heating is via heat pump/electric. We turn re-circulation on and off manually (versus scheduling it) when we want hot water, and have hot water at every faucet in under two minutes. Propane cost me about $18 a month last year, almost nothing, and we're on schedule to spend about $45 a month this year using the fireplace to heat whenever outside temps drop below 30F (i.e. heat pump is not efficient below that temp, and electric coil heating kicks in; our electric bill skyrockets 6x higher). The goal is to drop our $350 a month electric bill to about $150 a month, and increase our propane consumption/cost by $30 to $60 a month. Propane + tankless water heat = cheap hot water and cheaper energy overall (i.e. when propane is at $1.50 per gallon or less).
- I'd never install a tankless water heater just for the "instant hot water" purpose touted by the manufacturers. I'd only install a natural gas or propane, and propane only as an alternative to an electric tank water heater. Regardless of how well the technology works, it's just too much maintenance and trouble for something I've taken to be "automatic" all of my life, i.e. hot water heated in a tank by natural gas. The fact that I have to think about "hot water", makes it a failure. I'd install a water heater tank with natural gas, and add recirculation, if I had natural gas available.
Long winded post, but I hope it answers a few questions for people considering propane, or trying to solve their "sediment problem" on their tankless hot water system.