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# Biquadratic Curves for HPWH

I'm trying to generate a biquadratic curve for a heat pump water heater (HPWH Heating Cap Function Temp). In the EnergyPlus CurveFitTool the INPUT tab asks for Indoor Air Dry-Bulb Temperature and Outdoor Air Dry-Bulb Temperature as the independent variables. Obviously Indoor Air Dry-Bulb Temp doesn't make sense in this case, can I just use the EWT? Or should it be something else?

When I use the values I have all the coefficients are zero, but I'm wondering if it's because I have too many constant values? If anyone has any suggestions I'd really appreciate it. Thanks!

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## Comments

I have the same question. The Indoor Air Dry-Bulb Temperature does not apply to a WaterHeater:HeatPump, can we set the supply water temperature and receive reasonable results?

( 2019-08-27 09:30:17 -0600 )edit

## 2 Answers

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The curve fit tool can be used for any biquadratic curve, you just have to enter the data according to the fundamental equation.

For the Coil:WaterHeating:AirToWaterHeatPump:Pumped object:

A9 , \field Evaporator Air Temperature Type for Curve Objects
\type choice
\key DryBulbTemperature
\key WetBulbTemperature
\default WetBulbTemperature
\note Determines temperature type for heating capacity curves and
\note heating COP curves. This input determines whether
\note the inlet air dry-bulb or wet-bulb temperature is used to evaluate these curves.
A10, \field Heating Capacity Function of Temperature Curve Name
\note Heating capacity modifier curve (function of temperature) should be biquadratic or cubic.
\note Biquadratic curve = a + b(ta) + c(ta)^2 + d(tw) + e(tw)^2 + f(ta)(tw).
\note Cubic curve = a + b(ta) + c(ta)^2 + d(ta)^3.
\note ta = evaporator inlet air [dry-bulb or wet-bulb] temperature (C).
\note tw = condenser inlet water temperature (C).


So you would enter:

ta - evap inlet air temp in the first column (regardless of what the column heading says)
tw - cond inlet water temp in the second column (regardless of what the column heading says)


The fact that the Total Heating Capacity does not vary with changes in tw will cause the regression to fail, however, I would think you would get a constant of 185.8 with all 0's, not all 0's. But I haven't done this in some time. Try adjusting the Total Heating Capacity just a little to see if the 0's problem disappears.

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## Comments

@rraustad thank you very much for your response. I was not aware that the equation inputs could be found in the Energy+.idd file. Very valuable information to know.

( 2019-08-27 10:21:16 -0600 )edit

I first check the idd and if not there go to docs. Remember though that the idd will be replaced by a JSON equivalent although the same information should still be available.

( 2019-08-27 10:36:27 -0600 )edit

@JustinShultz I think I did eventually figure out the issue here with some help from a colleague. The spreadsheet is calculating an EIR =f(T) curve rather than a COP = f(T) curve. For the HPWH component, the curve is COP vs. temperature, while for a normal DX cooling coil the curve is EIR vs. temperature.

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Asked: 2019-07-11 17:22:40 -0600

Seen: 262 times

Last updated: Aug 28 '19