HVAC · Lesson 10 of 13Advanced8 min read
Efficiency, Sizing and the House
The best system is sized for the house it serves, and the house comes first: insulation and air sealing shrink the load before any equipment is chosen. This lesson does the math.
Heat loss and heat gain
Heat conducts through every wall, ceiling, floor and window at a rate set by three things: the area, the temperature difference across it, and how well it resists heat flow. That resistance is the R-value. Physics books write it as the thickness divided by the material’s thermal conductivity; in the United States it’s quoted in ft²·°F·h/BTU, usually without the units. A 3.5-inch fiberglass batt in a wall is about R-11, and a 6.5-inch batt is about R-19.
Add up the losses through every surface, plus the heat carried out by air leaking through the house, and you have the house’s heating load. In summer the flows reverse and sunshine through windows and the heat from people and appliances add to the cooling load, which also has a latent part: the moisture the air conditioner must remove.
Q = A × ΔT ÷ R (Q in BTU/h, A in ft², ΔT in °F, R in ft²·°F·h/BTU)
Example: 1,000 ft² of ceiling, 70°F inside and 20°F outside (ΔT = 50):
R-11 → 1,000 × 50 ÷ 11 ≈ 4,545 BTU/h (1,332 W); R-49 → 1,000 × 50 ÷ 49 ≈ 1,020 BTU/h (299 W)
Heat flow through one flat layer at steady temperatures, Q = A·ΔT/R, ignoring framing, air leaks and sunshine. 1 BTU/h = 0.2931 W (NIST). The recommended attic levels are ENERGY STAR’s, based on the 2021 International Energy Conservation Code.
Why the first inches count most
Because heat flow goes as 1/R, each added layer saves less than the one before. Going from R-11 to R-19 in the example above cuts the loss by about 1,900 BTU/h; going from R-38 to R-49 cuts only about 300 more. That’s why an uninsulated or thinly insulated attic is often the best first project, and why the recommended level depends on the climate.
| Climate zone | Attic with no insulation | Attic with 3 to 4 inches already |
|---|---|---|
| 1 (hottest) | R-30 | R-25 |
| 2 and 3 | R-49 | R-38 |
| 4 through 8 | R-60 | R-49 |
Right-sizing: why bigger isn’t better
An oversized air conditioner cools the air fast and shuts off before it has run long enough to pull much water out. The Department of Energy’s Building America program explains that an oversized system quickly meets the temperature setting, and the water sitting on the coil can evaporate back into the airstream when it stops. The result is a house that’s cool but clammy. ENERGY STAR adds that oversized equipment may cycle too often, giving less comfort and a shorter life, and a cold-climate guide hosted by DOE warns that grossly oversizing heat pumps leads to excessive cycling, low efficiency and poor summer dehumidification.
Old rules of thumb, so many square feet per ton, are based on older construction, and Building America notes that even new code-built homes would likely be oversized by them. The industry method is a room-by-room load calculation, usually ACCA’s Manual J, followed by Manual S to select equipment and Manual D to design the ducts. ENERGY STAR advises asking your contractor to size the system from your home’s actual characteristics: its size, insulation and windows, not a rule of thumb.
| Rating | What it measures | Used for | Better is |
|---|---|---|---|
| AFUE | Share of the fuel’s energy that becomes heat over a typical year | Furnaces and boilers | Higher (up to about 98.5 percent) |
| SEER2 | Cooling delivered over a season ÷ electricity used (BTU per watt-hour) | Central air conditioners and heat pumps (cooling) | Higher |
| EER2 | Cooling efficiency at one hot peak condition | Central air conditioners and heat pumps | Higher |
| HSPF2 | Heating delivered over a season ÷ electricity used (BTU per watt-hour) | Heat pumps (heating) | Higher |
| COP | Heat moved ÷ energy used, both in the same units, at one condition | Any heat pump or air conditioner | Higher; electric resistance heat is 1 |
| R-value | Resistance to heat flow | Insulation and building materials | Higher |
| MERV | How well a filter catches small particles | Air filters | Higher catches more, but check airflow |
Installation counts as much as the label
ENERGY STAR says improper installation can reduce a system’s efficiency by up to 30 percent. A quality installation includes a load calculation, airflow set to the manufacturer’s specifications, a refrigerant charge that’s tested and adjusted, and ducts inspected and sealed. ENERGY STAR certified heat pumps and air conditioners can save up to 20 percent on heating and cooling costs, but only if they’re installed well.
Fix the house first
The cheapest heating and cooling is the kind you don’t need. The Department of Energy says air sealing can save 10 to 20 percent on heating and cooling bills, and that combining an efficient system with insulation, air sealing and thermostat settings can cut those bills by about 30 percent. Sealing and insulating first also shrinks the load, so the new equipment can be smaller and cheaper.
NoteCodes decide
Building and energy codes set minimum insulation and equipment requirements where you live, and they change over time. The local code and the authority having jurisdiction decide what applies to your house and what needs a permit and an inspection.
Check yourself
Tap an answer to check it, or open “Show the answer.”
If you double a ceiling’s R-value, the heat flowing through it:
Show the answer
Is cut in half. Q = A·ΔT/R, so doubling R halves Q.
Why can an oversized air conditioner leave a house clammy?
Show the answer
It reaches the temperature setting and shuts off before removing much moisture. Short runs mean little time for water to condense and drain; some can even evaporate back off the coil.
What is the industry’s standard method for calculating a home’s heating and cooling loads?
Show the answer
ACCA Manual J. Manual J is a room-by-room load calculation; Manual S and D follow for equipment and ducts.
Which rating describes a heat pump’s seasonal heating efficiency?
Show the answer
HSPF2. HSPF2: heat delivered over a season per watt-hour of electricity.
ENERGY STAR says improper installation can reduce efficiency by up to:
Show the answer
30 percent. Up to 30 percent, which is why airflow, charge, sizing and duct sealing matter.
Go deeper
- ENERGY STAR: Recommended Home Insulation R-Values ENERGY STAR’s full table of recommended insulation by climate zone.
- U.S. Department of Energy, Building America: Why Right-Size? Science versus Practicality (Consortium for Advanced Residential Buildings, 2009) DOE Building America on why right-sizing matters.
- ENERGY STAR: HVAC Quality Installation ENERGY STAR on quality installation and what to ask your contractor.
- OpenStax College Physics 2e, 14.5 Conduction Conduction and R-values in an open physics textbook.
Sources
- OpenStax College Physics 2e, 14.5 Conduction
- NIST Guide to the SI (Special Publication 811), Appendix B.8: Factors for Units Listed Alphabetically
- ENERGY STAR: Recommended Home Insulation R-Values
- U.S. Department of Energy, Building America: Why Right-Size? Science versus Practicality (Consortium for Advanced Residential Buildings, 2009)
- ENERGY STAR: HVAC Quality Installation
- NEEP: Guide to Sizing and Selecting Air-Source Heat Pumps in Cold Climates (2020), from the U.S. Department of Energy Building Science Education Solution Center
- ENERGY STAR: Ten Tips for Hiring a Heating and Cooling Contractor
- U.S. Department of Energy: Energy Conservation Standards for Central Air Conditioners and Heat Pumps, Frequently Asked Questions (October 2022)
- U.S. Department of Energy, Energy Saver: Furnaces and Boilers
- OpenStax College Physics 2e, 15.5 Applications of Thermodynamics: Heat Pumps and Refrigerators
- U.S. EPA: Residential Air Cleaners: A Technical Summary, 3rd edition (2018)
- ENERGY STAR: Heat and Cool Efficiently
- ENERGY STAR: When to Replace Heating and Cooling Equipment
- U.S. Department of Energy: Energy Saver Guide: Tips on Saving Money and Energy at Home (2022)
A free course from Teddy Rook Book. It explains how HVAC systems work in general and follows U.S. sources; your equipment’s manuals and the local code are the final word, along with your gas utility for anything involving gas.