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Rinnai R53 (Plus) - Interesting water heating issues. Please Help.

Hi, so we have a Rinnai R53 Plus water heater that has been troublesome for the past 4 months or so. We have done a lot of troubleshooting over that period of time and have narrowed down some key traits related to the issue.

The problem: Cold-Water Sandwich (Water temperature alternates between being hot and cold when taking a shower or using a sink with hot water )

We initially thought this issue was a gas related issue or venting issue. We had some plumbers come out and clean the water heater and troubleshoot with rinnai over the phone. This did not really solve the problem.

Eventually, we discovered that in the shower, if we took off the showerhead the problem would go away! We purchased a new showerhead thinking that our old one somehow became defective. The new showerhead worked well for about 1 month, but then the infamous cold-water sandwich problem came back. Then through trial and error we discovered that the coldwater sandwich problem was correlated with low water flow through the rinnai system. We are able to view water flow by pressing the Down arrow + Power button on the Controller board.

*****It seems that anything below a waterflow of about "22" has issues. To create a temporary fix in the shower we have removed the rubber regulator in the showerhead. This allows the showerhead to let out more water, and it allows a waterflow of about "29" through the rinnai unit. We have experimented with another showerhead that SOMETIMES has the coldwater problems. Of course these problems correlate with the waterflow. Sometimes the waterflow is around "21" when using it and sometimes it is "24".

We have been observing the water temperature fluctuation of the rinnai unit during one of these cold water sandwich problems. We can view water temperature by pressings the UP arrow + Power on the controller board.
This is an interesting part. It appears that the rinnai water heater starts to get out of sync as it heats up. For example:

If we set the max temperature to 125 degrees on the rinnai water heater and then start a shower, the water will warm up toward 125, then eventually overshoot it, reaching about 130 degrees. The sensors seem to tell it that 130 is too hot, so it tries to drop the temperature back down, and it will drop to ~120. Then the sensors will say that that is too cold so it shoots the temperature back up. This whole time, the flame inside the rinnai unit is out of sync with what the controller board is saying. (When the board says 130 degrees, the flame is very low. But when the board says 120 degrees the flame is very high) ****

This out of sync problem only seems to arise when the "waterflow" is below about "22". Does anyone what unit the rinnai waterflow is in? i cant imagine this is GPM nor is it PSI?

So does anyone have any ideas or experience with any of the issues we are experiencing? We can use any help we can get. thank you!



  • Snowmelt
    Snowmelt Member Posts: 1,416
    Yea get a new gas valve a PC board. That's what it usually is. Sometimes you have to adjust the gas pressure also.
  • mikey32230
    mikey32230 Member Posts: 7
    Does anyone else have any more ideas, before replacing expensive parts?

    @Jack‌ or @icesailor‌ ?
  • icesailor
    icesailor Member Posts: 7,265
    Snowmelt has hands on experience.

    The Board rules the operations.
  • Jack
    Jack Member Posts: 1,047
    It could be a scaled up HX. Check the inducer fan and when you remove it, take a soft brush and clean the fan blades. Get an inspection mirror and see if there is any debris above the inducer fan. You will see a series of small holes in the plate above the inducer and below the burner. Are they clear. Are there signs of moisture stains on the plate? That could indicate condensate from the vent returning to the unit and scaling the fins on the HX. Those points are inspection only and no cost. If necessary to go with a new board &/or gas valve. You have to program the gas valve to the PCB. That is a MUST!

    That unit should fire at a .4 GPM flow rate and hold operation to .25 gpm. Don't mean to pick a nit but what you are experiencing is not "cold water sandwich". Given you are having a problem with your water heater I guess it proper to give you a pass on the terminology. Check it out and report back