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Rinnai malfunction

We have a Rinnai tankless heater that is being used to supply both domestic hot water and to heat water for a hydronic heat system. It was installed 8 years ago and has been working pretty well since.

However, there were a couple of leaks in the copper plumbing, and since I'm very skilled in plumbing (not hydronic systems however) I decided to fix them myself, which I did. The problem arose after the repairs, during which a large amount of air was introduced into the system.

I'm not sure but think that may have caused the issue, which is:

When the system was turned back on after the repairs, and the pumps fired up, the Rinnai unit kicked on, and seemed to want to work, but shut down after about 30 seconds, even though the pump was still running and the system was calling for heat.

I unplugged the power to the pump and plugged it back in, the Rinnai fired back up but then cycled back down after about 30 seconds. I cycled the pump on and off again several times, and a couple of times the Rinnai stayed on for longer, up to a minute or more, but then each time cycled off again.

There are no error messages, and the unit was working fine prior to the plumbing repair.

I'm stumped on what to do.

I assume I need to purge the air from the system but not sure how to to it because it is a hybrid system (sort of), which uses an Metalpres J series tank in which the water for the hydronics is pumped through a coil in the tank, and the domestic hot water fills the rest of the tank, so in a way the two are separate and I'm confused on how to purge the hydronic system, if that's even the actual cause of the issue.

Any help would be greatly appreciated.


  • rick in Alaska
    rick in Alaska Member Posts: 1,362
    Will need pictures of your system to see how to purge the air out, but most likely air is the problem.
  • neolithic
    neolithic Member Posts: 2
    Thanks for responding! In the meantime, we discovered that air was in fact the problem so you are correct. We figured out how to purge the system and did so, and alot of air came out. I then re-pressurized the pressure tank to 12 pounds as stated in the manual, and the system began operating normally, and it's been working fine since!