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New RU199iN (199k BTU) delivering just around 161k BTU

Hello all,

I recently hired a plumber to install a Rinnai Tankless RU199iN. The heater seems to be functioning as expected, but it isn't delivering hot water at the maximum flow rate stated in the specifications.

According to the system’s specifications, the heater should be able to output hot water at 5 GPM (∆T=75F; with incoming water at 45F and set temperature at 120F). However, the peak capacity I currently observe is only 4.3 GPM, which is around 161k BTU.

To rule out water flow restriction, I changed water pressure to 50psi and 80psi from 60psi (my normal water pressure) by adjusting the PRV.
I measured the maximum flow rate. It remains the same at 4.3GPM at both 50psi, 60psi & 80psi.
I also noticed the water flow control servo is in mid position instead of being fully opened using the diagnostic menu when the heater is giving out 4.3gpm of hot water.
(Additional data: HX tempature is at 142F, bypass value is at 30°).

I suspect the issue may be related to gas supply rather than water supply.
The gas pressure measurements are as follows:
- Static gas pressure at 6.6" WC
- Full load gas pressure at 5.3" WC
They fall within the range of the required gas pressure (3.5" - 10.5" WC). However, these readings tend towards the lower end of the range.
Could it be the reason why my water heater is giving out less hot water? What's the easiest way to unleash the full power of my water heater?

Thank you.


    EBEBRATT-Ed Member Posts: 15,543
    Call Rinnai and ask them. Sounds like gas pressure could be the issue. Also talk with the gas utility about the BTU content of the gas before you call Rinnai.

    Doesn't seem to me that you are that far off. Your incoming water should warm up soon.

    Many don't like the tankless heaters lot's of issues.
    Intplm.Larry Weingartennatural_gas_src
  • natural_gas_src
    natural_gas_src Member Posts: 5
    Thanks. I will call my gas company and Rinnai and post an update later. I apologize for the double post. I could not delete the duplicate.
  • PC7060
    PC7060 Member Posts: 1,160
    Are you located above 4500 ft elevation?
  • hot_rod
    hot_rod Member Posts: 22,158
    edited March 25
    How are you measuring the 4.3 gpm?

    199k input X 90% efficiency would be 180k actual, so closer to you calculation

    I wonder if their numbers are from actual flow testing or calculations?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • natural_gas_src
    natural_gas_src Member Posts: 5
    PC7060 said:

    Are you located above 4500 ft elevation?

    My house is located at 800 ft.
    hot_rod said:

    How are you measuring the 4.3 gpm?

    I used the Rinnai's built in diagnostic menu to measure the flow rate. To find out the maximum flow rate, I turned on all the faucets (2x bath, 1x shower, 7x sinks) at the same time.