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  1. "Efficient Natural Gas".American Gas Association, 2021, https://www.aga.org/natural-gas/efficient/.

  2. "Water Heater Key Product Criteria".Energy Star, https://world wide web.energystar.gov/products/water_heaters/residential_water_heaters_key_product_criteria.

  3. Delforge, Pierre et al. "Very Absurd: Estrus Pump Water Heaters Save Free energy And Money".NRDC, 2016, https://www.nrdc.org/experts/pierre-delforge/very-cool-heat-pump-water-heaters-save-energy-and-money.

  4. "Natural Gas Water Heaters".American Gas Association, 2021, https://www.aga.org/natural-gas/in-your-dwelling house/water-heaters/.

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