What size furnace or AC do you need? Load calculations explained
Updated · KeystoneProGuide editorial team
How is the right size for a furnace or air conditioner worked out?
By a load calculation for your house. ENERGY STAR says your contractor should verify the proper system size using Manual J, a calculation of how much heating and cooling your home needs to stay warm in winter and cool in summer. It warns that a system too large or too small for the space will struggle to keep you comfortable.
Manual J is published by the Air Conditioning Contractors of America (ACCA), the industry's trade association. ACCA describes it as the ANSI-recognized standard for producing equipment sizing loads for detached houses, row houses, townhouses, condominiums and small multi-unit buildings. ACCA also says a proper load calculation by this procedure is required by national building codes and most state and local jurisdictions; that is the association's statement, and the guide has not checked it against your town's code.
What goes into the calculation?
The house, not the old equipment. ENERGY STAR says a contractor should size the system based on the size of your house, its insulation and its windows, and should spend significant time inspecting the home and the present system. Its quality-installation page says to use the actual characteristics of your home and not just rely on a "rule of thumb".
ACCA's companion standard, Manual S, covers the next step: choosing equipment that meets the calculated loads, using the manufacturer's performance data and the local design conditions. ACCA says its size limits are meant to prevent short cycling.
What goes wrong with an oversized system?
ENERGY STAR says oversized equipment may cycle too frequently, resulting in less comfort and a shorter lifespan. "Bigger is better" is the opposite of its advice: in its hiring tips it says plainly that a bigger system isn't always better.
There is a second reason size matters in summer. ENERGY STAR links poor operation and inadequate equipment to humidity problems, with air that is too humid in summer or too dry in winter. A load calculation is how a contractor matches the equipment to the house so that it runs as designed.
Does the duct system get sized too?
It should be checked at least. ENERGY STAR says a good contractor will inspect the duct system for air leaks and insulation and measure airflow to make sure it meets the manufacturer's specifications. Its list of signs that something is wrong includes a noisy system, which it says may point to an undersized duct system. If a new, larger air conditioner is going onto old ducts, ask whether the ducts can carry the airflow it needs. HomeAdvisor's cost survey lists ductwork among the extras that can be added to an installation.
What else is part of a quality installation?
ENERGY STAR's quality-installation guidance lists four things to ask the contractor about:
- The equipment is properly designed and sized for your home.
- System airflow is optimized to meet the manufacturer's performance specifications. Poor airflow can mean reduced efficiency, higher bills, overly damp air and a shorter equipment life.
- The right amount of refrigerant is installed, with the charge tested and adjusted, which reduces energy use and the risk of failure.
- The ducts are evaluated, with an inspection, a leakage test and any necessary repairs.
The stakes are high. ENERGY STAR says improper installation can reduce system efficiency by up to 30 percent and may shorten the equipment's life. For trade-accredited installers, it points to ACCA's list of contractors in its Quality Assured Residential Services and Installation program.
Does the same method apply to heat pumps and mini-splits?
Yes. ENERGY STAR gives the same Manual J advice on its pages for ducted heat pumps and for ductless mini-splits, and adds that in a cold climate the contractor should work out the size together with any back-up heating the home will need. ACCA says the latest edition of Manual S adds expanded size tolerances for variable-capacity heat pumps that allow for sizing to the heating load. Where a heat pump is meant to heat the house through the winter, ask the contractor which load, heating or cooling, set the size it proposes.
Does Philadelphia ask for sizing paperwork?
Its forms suggest so. For one- and two-family homes, the city's Mechanical Permit application lists an HVAC Equipment Design Form, and the field inspector is given an HVAC Equipment Certificate and a Duct and Envelope Testing Certificate. Outside the city, ask your municipal building code office what it requires with a permit application.
How do you know the calculation was actually done?
Ask for a copy, and ask the contractor to explain how the proposed equipment matches it. Some practical checks:
- The contractor measured or asked about your rooms, windows and insulation, not just the floor area.
- The proposed size is explained, not simply matched to the old unit.
- If two companies propose very different sizes, ask each to show you its numbers.
Do not wait for a breakdown to start this conversation. ENERGY STAR suggests having a professional evaluate any system that is more than 10 years old or not keeping the house comfortable, which leaves time for a proper calculation before anything has to be replaced in a hurry.
Questions to put to bidders are in questions to ask an HVAC contractor, and what a heat pump needs in this climate is in the heat pump guide. Installers are listed on the HVAC contractors directory page.
Sources
- Air-Source Heat Pumps, ENERGY STAR
- HVAC Quality Installation, ENERGY STAR
- 10 Tips for Hiring a Heating and Cooling Contractor, ENERGY STAR
- Heat & Cool Efficiently, ENERGY STAR
- Manual J Residential Load Calculation, Air Conditioning Contractors of America (ACCA)
- Manual S Residential Equipment Selection, Air Conditioning Contractors of America (ACCA)
- Get a Mechanical Permit, City of Philadelphia
- How Much Does an AC Unit Cost?, HomeAdvisor
- When is it time to replace?, ENERGY STAR