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Sizing a central AC system for a house in Hillsborough County means accounting for a short, compressed cooling window. Homes in Hillsborough County, NH run cooling in a single month of the year, July. The compressor accumulates little runtime wear on that schedule, which shifts the pressure onto the install itself. A coil that is correctly charged, a refrigerant line that is properly routed, and a duct system that matches the load will perform year after year with minimal degradation. A poor install has nowhere to hide.
Ductwork laid for a furnace-only system does not always move enough air for a central AC coil, and in much of Hillsborough County the ducts behind the walls were put there for exactly that purpose. Half the homes in Hillsborough County, New Hampshire were built in 1978 or earlier (2020-2024 ACS 5-year). A contractor quoting an installation has to get into the trunk lines and branches before a load calculation means much, because the system a homeowner is replacing and the one going in are not the same shape. Electrical service matters too, but in stock this age the duct layout is usually where the job gets complicated or does not.
What a Central AC Involves, Piece by Piece
The outdoor condenser unit sits on a pad outside the house and holds the compressor and condenser coil, the two components that do the heaviest work in the system. Inside, the air handler or furnace cabinet holds the evaporator coil, which is mounted above it in a plenum or coil cabinet and is the indoor half of the refrigerant circuit.
A copper refrigerant line set runs between the outdoor unit and the evaporator coil, connecting the two halves of the system. A condensate drain line runs from the indoor coil to a floor drain or outside, carrying away the moisture the coil pulls from the air. A thermostat on the wall ties it all together by telling the system when to run.
The drawing beside this text shows how these pieces relate to one another in a typical layout, and a licensed contractor determines exactly how the system comes together in your home.
Find a contractor in Hillsborough CountyHillsborough County, by the numbers
251 severe weather events were recorded in Hillsborough County, New Hampshire between 2015 and 2024.
NOAA National Centers for Environmental Information, Storm Events Database, bulk CSV export, events 2015-01-01 through 2024-12-31.Half the homes in Hillsborough County, New Hampshire were built in 1978 or earlier (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesHillsborough County, New Hampshire holds 177,734 housing units (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesOwners occupy 67.4% of the occupied homes in Hillsborough County, New Hampshire (2020-2024 ACS 5-year).
U.S. Census Bureau, 2020-2024 American Community Survey 5-Year EstimatesRules & Standards for Central AC Installation in Hillsborough County, New Hampshire10 sourced rules and standards
What the installation actually looks like
Split-system central air conditioners rated below 45,000 Btu/h manufactured on or after January 1, 2023 must meet a federal minimum of 13.4 SEER2.
Split-system central air conditioners rated at or above 45,000 Btu/h manufactured on or after January 1, 2023 must meet a federal minimum of 13.4 SEER2.
Split-system central air conditioners manufactured between January 1, 2015 and December 31, 2022 had to meet 13 SEER, the metric that preceded SEER2.
Single-package central air conditioners manufactured on or after January 1, 2023 must meet a federal minimum of 13.4 SEER2.
Space-constrained central air conditioners manufactured on or after January 1, 2023 must meet a federal minimum of 11.7 SEER2.
Federal efficiency standards for split-system central air conditioners and heat pumps are set on each certified combination of outdoor unit and indoor coil rather than on the outdoor unit alone.
Residential and light commercial air conditioning and heat pump systems assembled in the field, including mini-splits and unitary systems, may not be installed on or after January 1, 2025 using a refrigerant with a global warming potential above 700.
A field-assembled system whose specified components were all manufactured or imported before January 1, 2025 carries no installation compliance date under the refrigerant restrictions and may still be installed.
Licensing Rules for Hillsborough County, New Hampshire
New Hampshire's mechanical licensing board issues nine credentials — hearth system, four fuel gas endorsements, domestic appliance technician, and master, journeyman and apprentice plumber — and a licensee may not work outside the specialty their licence names. None of the nine is an HVAC licence.
New Hampshire does not license HVAC as a trade, and says so where it matters: the plumbing licence expressly does not reach a person installing heating, cooling, air conditioning or domestic water heating systems, whether solar, oil, gas or electric. The gas piping on a gas-fired system is a separate matter and needs a fuel gas fitter licence.
Severe Weather Events in Hillsborough County
Weather places real demands on HVAC equipment, and those demands vary by location. What happens in Hillsborough County shapes how hard a central AC system has to work and how well it needs to be sized and installed to hold up over time.
251 severe weather events, 2015–2024. Source: NOAA National Centers for Environmental Information, Storm Events Database.