Past issues with Rinnai tankless water heaters leaking

Prism3370

I've had issues with two different Rinnai tankless water heaters leaking from the heat exchangers over the past several years. I'm now on my second Rinnai tankless water (an RL94i) within the period of only 9 years. The first unit, which was installed when the home was built, started leaking from the heat exchanger after about 6 years, and was ripped out and replaced (it was going to cost more to replace the heat exchanger than to do a full unit replacement).

Earlier this year, about three years after the first unit was replaced, my second unit (also an RL94i) also started leaking from the heat exchanger. This time around I worked with a plumber to secure a replacement heat exchanger from Rinnai (they wouldn't send it directly to me) and did the replacement myself. It was a long and tedious process, but saved a lot of money and learned a lot along the way. That was about four months ago, and the unit has worked fine since, but I'm concerned the new heat exchanger will eventually start leaking just like the others did, due to some underlying issue I'm not aware of.

When the first unit started leaking, the plumber said it was probably due to no condensate line being installed, so when the replacement unit was installed he added one. When the second unit started leaking, he attributed it to scale build up in the heat exchanger. I do have hard water in my area and no water treatment system (yet), but I had been flushing both units with vinegar according to Rinnai's flushing procedure about twice a year, so I'm skeptical it was due to build up in the heat exchanger from hard water.

After inspecting the old heat exchanger from the most recent replacement, it looks like the leak might have been caused by condensate or outside water (rain infiltration) coming into through the exhaust vent and dripping down from the top of the unit onto the heat exchanger.

I've included some pictures and was hoping to get some input from those with more experience. I'd really like to prevent this from happening again.

Pictures:
https://imgur.com/a/GbACPGn

In one of the pictures you'll see how the first condensate line was installed by the plumber using PVC pipe. I've since replaced that with a condensate loop using clear tubing. For venting on the unit, it is just a straight shot from that 90 degree elbow to the outside wall of the house, so not a long distance at all.

Big Ed_4

Is that a condensation line ? Seems to be connected to the air intake ?

Mad Dog_2

That picture app you use is not opening for me, but with Rinnai's the Flue should generally be pitched down to the outside.  Yiu should not be getting rain down the concentric vent.  If it is condensing, you absolutely have to have a condensate drip. Always follow the directions..as close as possible.

Up until a few years ago I had installed over 100 Rinnai's...Never had one leak like that.  I still see many of them I installed and they are chugging away 15 or more years later...Very good product.  Mad Dog 🐕 

Big Ed_4

Shouldn't that unit have a just incase main drip ?

Prism3370

@Big Ed_4 Thanks for replying! Yes, the clear tubing is the condensate line/loop and it is connected to a nipple on the flue connection assembly that contains a condensate trap. If by "just-in-case main drip" you mean a drip pan, there is one under the unit with a pipe that drains outside, it just might not be visible in the pictures I shared.

Prism3370

@Mad Dog_2 thanks for your reply! I've reposted the pictures if you're still interested in taking a look at things. I've got a condensate line connected to the condensate collector on the unit, but I still need to check and possibly adjust the pitch of the vent to the outside. The manual says it should be sloped 1/4" per foot away from the appliance.

Mad Dog_2

Yes...its critical  mad Dog 🐕 

Alan (California Radiant) Forbes

With a condensate drain, the manual says you should slope the venting towards the appliance.



Do you happen to have a hot water circulating pump on this system?