I want to flush my tankless water heater (DIY).

I've never read plumbing schematics before.

Which one of these is the "V1" value they are referring to in the doc?

(And have I correctly identified all the values??)

Manual and my system

P.S. The values I circled look like threaded ports, but the diagram leads me to believe V1 is to to the left.

EDIT: Another view

EDIT2: This is how I flushed it and it worked!

My setup

  • Are there any other shutoff valves for the hot and cold water close by?
    – JACK
    Commented Feb 1, 2020 at 22:12
  • where does the vertical pipe in the back go? ... the one that runs to the bottom of the picture
    – jsotola
    Commented Feb 1, 2020 at 23:03
  • @JACK Probably up in the joists
    – Marinaio
    Commented Feb 2, 2020 at 2:37
  • @jsotola See new pic
    – Marinaio
    Commented Feb 2, 2020 at 2:41
  • @jsotola It is above the floor. Normally a small bucket sits under it.
    – Marinaio
    Commented Feb 2, 2020 at 2:43

2 Answers 2


The schematic is most directly applicable to a heater installed with ordinary two-way valves. Your heater is installed with a service valve kit which has three-way valves and it doesn't quite match the document. Still, the flush procedure is the same.

Please refer to the photo of my heater's valves below. The yellow handles are the valves; the round pieces on the front, which you had guessed might be V1 and V2, are only caps to keep debris out of the flush ports.

Rinnai water heater service valves

Valves V1 and V2 control flow through the service ports. These should normally be in the closed position (handles pointing to the left) -- don't rely on the service port caps to prevent water leaks! Valves V3 and V4 are three-way selector valves. When the handle is oriented vertically the heater is connected to the house and supply water piping. When the handle is oriented horizontally the heater is connected to the flush ports.

If your valves are like mine, V3 and V4 are extremely difficult to turn. On mine I would rotate the lower part of the handle toward the wall to change from normal operation to service/flush mode. You can confirm whether yours turn the same way by checking the position of the limiter tab where the silvery steel handle touches against the cast brass valve body.

  • Thanks Greg. This is interesting...your filter is easily accessible. I didn't even notice I had one.
    – Marinaio
    Commented Feb 3, 2020 at 17:53

From the pictures and the diagram, the best I can tell is V3 and V4 are the two valves with yellow handles further back on the sides of the two valve bases in your picture and they cut off the water inlet and outlet. V1 and V2 are the valves you have labeled as V3 and V4. They open the threaded ports so you can do the flush. A closeup picture from the right would help.

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