My 1 year old Rinnai 556 direct vent heater is possessed. It's been serviced on-site, pulled and serviced at the shop, tested, declared fit, and still cuts out for no reason. Rinnai has a great rep. I haven't found anyone complaining of the same problem on a new heater.

I'll wake up in the morning to a cold house with the heater flashing "99" which indicates a blocked flue. Not.

Rinnai told me that I needed a UPS (uninterruptible power supply). I bought the best one I could afford with AVR that senses line changes and automatically activates. No effect on the problem. I know from the UPS that my line in is fine. No surges or unhealthy power problems, or faulty ground.

So far, they have replaced the fan motor and logic board. They have replaced the gaskets twice (once for recall, once because of damage done by other service). Then it stated flashing 12 sometimes, and 99 other times. The have completely replaced the vent, just in case there was some unseen blockage. The checked to make sure it was set for NG rather than propane. Yep, it was set right.

They took it in to the shop and tested it for three weeks. At the shop, it works fine. The loaner the installed to keep me from freezing to death (Vermont) worked fine at my house for three weeks. The only difference is that the loaner has one-setting thermostat. My 556 has a 7-day programmable.

After replacing the logic board on Friday morning, they promised that if it still cutout, they put in a new one. When I came home from work at 5, it was still running! By 8 pm. it cutout, flashing 99 again.

Since then, it's worse. It cuts out every 20 minutes or so. Sometimes it flashes 12 (flame failure), sometimes 53 (sparker problem), mostly 99 (flue block). The 53 is entirely new.

I'll call Monday a.m. to see how fast they can replace the unit, but I still want to know what's wrong.

Has anyone every had a similar problem? What was the solution?