Heat Pump Water-to-Air Curves - error on coefficient calculation

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The source-side temperature (EWT) does vary, but the return air temperature from the room does not. The performance data available from the manufacturer is for ASHRAE test conditions. In order to get other data points, we would need to contact the manufacturer to see if they also tested the equipment at non-standard conditions (which is not typical). Is there another way to generate reasonable performance curves from standard published data?

You might try finding other WSHP data and compare to the data you have. Plot capacity/power vs entering water temperature at discrete coil inlet air wet-bulb temperatures. A full data set will have multiple curves representing capacity vs water temp at various inlet air wet-bulb temps. If your data looks similar to the more robust data set you can use that data set. If not, then you might be able to expand your data set based on this comparison (this is risky since you don't actually know how performance changes for your system). I suggest contacting the manufacturer, they should have data.