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New RU199iN (199k BTU) delivering just 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.


  • Alan (California Radiant) Forbes
    Alan (California Radiant) Forbes Member Posts: 4,002
    edited March 23
    I don't think water pressure will make much difference. It's all about GPM unless flow restrictors are removed.

    Your static gas pressure (natural gas, not propane) should be around 7", so I would have your utility bump it up. And your pressure drop is kind of high. Check with Rinnai (800)621-9419 to see if it's within spec. Their tech. support is top shelf.

    All in all, YMMV. Also, other industries (cars) overstate their fuel efficiency.

    I like Rinnai better than I do Takagi better than I do Noritz. You made a good choice.
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab
  • natural_gas_src
    natural_gas_src Member Posts: 5
    I will call Rinnai to confirm and post an update. Thank you so much for your help.
  • Zman
    Zman Member Posts: 7,569
    Have you clocked the gas meter?
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein