Noritz temp climbs beyond set-point at lower gpm.

[StuartNC]

Noritz NR71-od -ng. Incoming water psi is 65. Incoming water temp currently 81. All faucets are Moen single pull handle. Issue can be reproduced 100% at any faucet. Testing with one faucet on full hot, no issue. Temp is maintained precisely at 120 as expected. Gpm rate 2.0. Now, move the faucet handle towards the middle to get warm water. When gpm drops to about 1.2 gpm or less, temperature climbs to 130+. Once I hit 0.9gpm temp will climb even more.
Noritz had me de-scale, check the filter and adjust the manifold pressures which were a little high. Maybe this normal given 81 degree ground water and the challenge of single handle faucets messing with the flow. Advice appreciated.

 

[Bannerman]

With 0.5 GPM or greater water flow, your WH model will continue to keep the burner active to heat water.

See page 8 of the Noritz manual ( Noritz Manual) which shows 3 graphs indicating the hot water temperature at various flow rates for 3 corresponding inlet temperatures.

The graph on the far right is for 82°F inlet temperature. The graph seems to show the minimum flow rate possible to deliver 120°F is 1.1-1.2 GPM.

Since the specified minimum burner rate is 25,000 BTU/h, with 81°F incoming water temperature flowing at only 1.2 gpm through the heat exchanger, it seems the burner is currently supplying a greater amount of heat than the 25,000 BTU necessary to raise the water temperature from 81° to 120°F

As you are experiencing 130° at 1.2 GPM with 1° cooler incoming water, perhaps your incoming gas pressure is slightly exceeding specification, or maybe the WH's gas modulating valve requires a slight adjustment to allow it to further reduce the minimum gas flow rate to the burner.

As the chart indicates 1.2 GPM is the minimum incoming water flow at 82°F to provide 120°F hot water to faucets, reducing the flow rate below 1.2 GPM will then result in the water temperature exceeding 120°F.

 

[Sponsor]

 

[StuartNC]

With 0.5 GPM or greater water flow, your WH model will continue to keep the burner active to heat water.

See page 8 of the Noritz manual ( Noritz Manual) which shows 3 graphs indicating the hot water temperature at various flow rates for 3 corresponding inlet temperatures.

The graph on the far right is for 82°F inlet temperature. The graph seems to show the minimum flow rate possible to deliver 120°F is 1.1-1.2 GPM.

Since the specified minimum burner rate is 25,000 BTU/h, with 81°F incoming water temperature flowing at only 1.2 gpm through the heat exchanger, it seems the burner is currently supplying a greater amount of heat than the 25,000 BTU necessary to raise the water temperature from 81° to 120°F

As you are experiencing 130° at 1.2 GPM with 1° cooler incoming water, perhaps your incoming gas pressure is slightly exceeding specification, or maybe the WH's gas modulating valve requires a slight adjustment to allow it to further reduce the minimum gas flow rate to the burner.

As the chart indicates 1.2 GPM is the minimum incoming water flow at 82°F to provide 120°F hot water to faucets, reducing the flow rate below 1.2 GPM will then result in the water temperature exceeding 120°F.

With 0.5 GPM or greater water flow, your WH model will continue to keep the burner active to heat water.

See page 8 of the Noritz manual ( Noritz Manual) which shows 3 graphs indicating the hot water temperature at various flow rates for 3 corresponding inlet temperatures.

The graph on the far right is for 82°F inlet temperature. The graph seems to show the minimum flow rate possible to deliver 120°F is 1.1-1.2 GPM.

Since the specified minimum burner rate is 25,000 BTU/h, with 81°F incoming water temperature flowing at only 1.2 gpm through the heat exchanger, it seems the burner is currently supplying a greater amount of heat than the 25,000 BTU necessary to raise the water temperature from 81° to 120°F

As you are experiencing 130° at 1.2 GPM with 1° cooler incoming water, perhaps your incoming gas pressure is slightly exceeding specification, or maybe the WH's gas modulating valve requires a slight adjustment to allow it to further reduce the minimum gas flow rate to the burner.

As the chart indicates 1.2 GPM is the minimum incoming water flow at 82°F to provide 120°F hot water to faucets, reducing the flow rate below 1.2 GPM will then result in the water temperature exceeding 120°F.

This makes sense and the situation did improve a bit when I adjusted the manifold min max pressures. The Noritz chart shows a manifold min max value for cover on and cover off. Im not sure which one to use. Currently it’s using the values for cover off. Thanks!

 

[Fitter30]

Hot water mixing valve would take care of your problem . Btu to water flow .9x39x500.4 = 17564 btu . 1.2 gpm = 23419 btu's. Why is your entering water 81°?
BTU = Flow Rate In GPM (of water) x (Temperature Leaving Process - Temperature Entering Process) x 500.4*Formula changes with fluids others than straight water.

 

[StuartNC]

Hot water mixing valve would take care of your problem . Btu to water flow .9x39x500.4 = 17564 btu . 1.2 gpm = 23419 btu's. Why is your entering water 81°?
BTU = Flow Rate In GPM (of water) x (Temperature Leaving Process - Temperature Entering Process) x 500.4*Formula changes with fluids others than straight water.

 

[StuartNC]

I’m in charlotte NC and the entering water is around 80. Today using a thermometer it hit 82.5. It seems higher this summer than I recall. My issue with the water getting too hot began this Summer. We get most of our water from the Catawba River if that matters. It’s definitely working better with the manifold pressures set properly. Min and max were both too high. Still based on the controller I’m still flirting with 130ish. The wife says the water went cold for her in the shower and that’s because she keeps nudging the handle towards cold causing the burner to shut off.

 

[Bannerman]

I’m s0t8ill flirting with 130ish. The wife says the water went cold for her in the shower and that’s because she keeps nudging the handle towards cold causing the burner to shut off.

The left edge of the chart referenced above, indicates the water temperature exiting the WH will be approx 130° @ only 0.8GPM, and ~145° @ 0.5 GPM.

While a thermostatic mixing/tempering valve after the WH as Fitter30 suggested, is a common method to reduce the water temperature to the home's fixtures, I suspect that may not work well for your situation.

A mixing valve will blend an appropriate amount of cold water with the hot exiting the WH, to achieve the desired temperature to home fixtures. Adding cold water after the WH will further reduce the flow rate through the WH, potentially below 0.5 GPM whereby the WH's gas burner will shut down. Just as is already occuring, this will result in intermittent hot water being supplied to the mixing valve and to the shower. For a mixing valve to resolve your issue, would require a constant minimum 0.5 GPM flow through the WH to ensure the burner will remain active for the entire time hot water is being utilized.

Suggest discussing your situation with Noritz technical support to determine if there is any possibility to further reduce the minimum heating rate below the design specification for your model. Perhaps the minimum gas flow rate through the modulating valve can be further reduced which should then reduce the minimum water temperature.

One usual concern with reducing the gas input rate is the possibility of water condensation occuring within the heat exchanger and exhaust venting, but with 80°+ supply water and hot combustion air during the summer season, condensation may not be much concern.

 

[wwhitney]

While a thermostatic mixing/tempering valve after the WH as Fitter30 suggested, is a common method to reduce the water temperature to the home's fixtures, I suspect that may not work well for your situation.

Definitely.

If you want only 0.5 gpm at a temperature rise of 40F, that's 20 F-gpm, and apparently 1 F-gpm = 500 BTUs/hr. So you'd need to deliver only 10,000 BTUs/hr to the water flow, and at 80% efficiency, that would mean burning only 12,500 BTUs/hr.

If the burner won't go that low, and there's no storage in the system, the OP is out of luck. If the lowest burner input rate is 25,000 BTUs/hr, then the lowest flow rate at which you can get 120F hot water is 1 gpm.

Cheers, Wayne

 

[StuartNC]

So I just turned on the shower and moved the handle to the usual spot. I went downstairs and looked at the controller and the flow is .9gpm and the temp is 127. That’s really my problem. I already have the water saver removed from the Moen shower head but maybe I can find a model that will get me over the 1.0 threshold.
Also with the shower still running I turned on the sink faucet a very small amount to cross the 1.0gom and bingo temp is 120.

 

[Bannerman]

maybe I can find a model that will get me over the 1.0 threshold.

Since the shower will likely not be 100% hot water, what is important is the actual flow rate through the WH.