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
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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.
