I replaced 2 HVAC units in 2024 but tax credit cap is only $2000, can I defer one of these to 2025? Does Fortified Roof upgrade qualify for any credits?

No.  These credits are only available in the year that you purchase and install the units.  Your HVAC replacement credit, unless it was solar, wind or a Geothermal Heat Pump, would not be able to be used in any year other than 2024 since you replaced them in 2024. If you did replace your HVAC system with a Geothermal Heat Pump, then the total credit is worth 30% of the cost of the replacements with no cap.   If you do not have enough tax liability in 2024 to use the entire credit, then you would be able to carry it forward until 2032 or it is used up whichever comes first. 

No.  There is not a federal fortified roof upgrade credit.  However, some states such as South Carolina do have tax credits for upgrading for hurricane protection. 

The Home Improvement Energy Efficient Credit

Worth up to 30% of costs with a maximum credit $2,000 for

• Air source heat pumps
• Heat Pump Water Heaters
• Biomass Stoves or Boilers
• Worth up to 30% of qualifying costs for a maximum combined total of $1,200 for the following:
  • Doors, Windows and Skylights
  • Insulation
  • Natural Gas, Oil Propane Water Heaters
  • Furnaces
  • Boilers
  • Central Air Conditioners
  • Electrical Panel Upgrades
  • Home Energy Audit (up to $150)
• You can combine the $2,000 credit with the $1,200 credit for a total of $3,200
• This credit is a is a nonrefundable credit. This means in order to claim the credit you must have a tax liability to offset it.  If you do not have a tax liability equal to the credit the credit will be reduced to match your liability or down to $0. 
•  Use it or lose it credit.  If you do have enough tax liability in the year you buy and install the item, you will not be able to carry the credit forward.  This credit is only available for the year that you install the item. 
• Doors, windows and insulation must be installed on your primary home.
• The home must be in the US

The Residential Energy Credit is also a non-refundable credit, however this one can be carried forward to future tax years until this credit goes away which is currently set to happen in 2034

This credit is worth up to 30% of the cost of the equipment, with no maximum limit.  The items that fall into this category are as follows:

• Geothermal Heat Pumps
• Small Wind Turbines
• Solar Energy Systems
• Fuel Cells
• Battery Storage Technology

To qualify

• The product must be installed on a home you live in, whether it is your first or second home
• You must have purchased the product new.  It cannot be used.

In order to claim the credit you must

• Live in the home you installed the item in
• You can not rent this home to others if you do not also live in the home
• it must be your main home
• It must be in the United States
• You cannot use the property solely for business. 
  • If you use the home partially for business but less than 20% you can take the full credit.
  • If you use the home more than 20% but less than 100% for business, you can take a prorated cred