I had a new natural gas water heater installed in 2016. The cost was $950. I expected my refund to increase by 10% of $950, but the refund amount increased $150. Why?

The tax credit is limited to a maximum of $150 for a qualified natural gas water boiler.  The tax credit is limited to $300 for a qualified energy efficient natural gas water heater.  See this website for eligible items - https://www.energystar.gov/about/2016_federal_tax_credits

For HVAC, electric heat pump water heater, natural gas, propane, or oil water heater, and a stove that burns biomass fuel to heat your home or heat water.  Enter the cost in the box labeled Energy-Efficient Building Property.

See IRS Form 5695 Residential Energy Credits instructions, page 2 - https://www.irs.gov/pub/irs-pdf/i5695.pdf#page=2

• A credit limit for residential energy property costs for 2016 of $50 for any advanced main air circulating fan; $150 for any qualified natural gas, propane, or oil furnace or hot water boiler; and $300 for any item of energy efficient building property.