Hi, I am really hoping someone can help me figure out my problem. I am on day 2 trying to figure this out and I have searched, read, and watched many videos on how to flush my Rinnai tankless water heater but can't figure out what the problem is with this one. I have two Rinnai tankless water heaters. One is outside mounted and one is in the attic. The one outside is a smaller and different model. The outdoor one flushes fine with good water pressure coming out of the outlet hose during the entire flushing process. I also have a Rinnai tankless in the attic. This one is giving me problems. I do the same procedure I do for the outside unit but this one loses water pressure after about 30 seconds of flushing. The water is just slowly coming out of the outlet hose. The only difference between the two tankless heaters is one is outdoor mounted and smaller (R63L), and the other is in the attic R94L). I am using a 1/4 hp submersible pump that seems to put out plenty of water. I have tried with the tankless unit on and off but nothing seems to work. It is as if the flow control valve is closing during the flush but I don't know how to keep it open. If any one has a solution please let me know. Driving me crazy.

Ok so I figured out my low flow flushing problem and wanted to post just in case someone else had the same issue. Again, I only had a problem with the bigger upstairs 9.4GPM tankless heater. I had to let the unit run for about 30 seconds to a minute under my house water pressure then I was able to transfer over to my pump and vinegar bucket. I am sure there is probably a better way but this finally worked.

Here are the steps that worked for me.
Place pump in bucket with 3-4 gallons of vinegar
Connect one side of first hose to pump and the other to the cold inlet isolation valve
Connect the second hose to hot outlet isolation valve and place other end in a spare bucket (2nd bucket) I did this in the attic so I needed to make I didn't flood my ceiling.
Place pump cord near outlet so you can quickly plug in to outlet

Turn down temp to lowest setting on heater
Turn off Main Hot water outlet valve
Place outlet hose in spare bucket
Turn on hot isolation valve: Water should flow into bucket
Let it run for 30 to 60 seconds

Do the next steps quickly
1) Close hot isolation valve to shut off flow to bucket
2) Place hot outlet hose in vinegar bucket with pump
3) CLOSE Main Cold water inlet valve
4) Open cold-water isolation valve
5) Open hot water isolation valve
6) Plug in pump
7) Turn off tankless heater once flow is established
Once the system is descaled you need to flush the vinegar out
1) Flush system with clean water
2) Turn off isolation valves and open inlet and outlet valves
3) Turn heat back up

I think a better way would be to cycle plain water until good flow is established and then add Flow Aid to the water in tank. This way you do not have to transfer from one bucket to the next.

hey thanks for this, it just helped us - info we couldn't find anywhere else. my water heater would have gone unflushed without your advice. I just created an account here to say thx.

I just came here to say thanks. I was up until midnight trying to figure out why my flush wasn't working, until I saw your post.

You are a legend. No YouTube video covers this. Or the manual.