HVAC | Air Conditioning | Gas Furnace System | Boiler hot water Heating
Under Construction.

 

Note: I couldn't find any web sites that had a good summary of this and I haven't had time to figure it all out myself. Here a cursory summary, but I need to spend more time to be comfortable with it.

Brands:
At Central air conditioner Brand Reliability | ConsumerReports they show American Standard, Lennox, Trane, and Carrier as the most reliable.
They had a 20-24% failure rate over a year.
The others weren't that much worse with 27-29% failure.

High end furnaces models are 2 stage variable speed. They normally operate at 65% efficiency and kick it up to high when needed. A variable speed motor allows SEER ratings over 16.
The fan can operate to circulate air even when the heating element is off to better distribute air in the home. They are also quieter when running at low speed.

A downside is the high end models have more components that could break.

See AFUE - Annual fuel-utilization-efficiency for different fuels below.

Cost - Savings
A 95% efficiency model will cost about $1,500 more than an 80% efficient model ($9,000 vs $7,500) for a 70,000 BTU, 2.5 Ton unit, with installation , without duct work. You will save about $250 per year in energy costs, more in the south and less in the north.
So the payback is about 6 years.

Sizing:
Note: The following is a simplified sizing guideline. A HVAC professional will take into account, insulation, construction, the size and type of windows, shade trees, etc.

For central latitudes MD, KY, KS,
Sq ft x 20 = A/C BTU/hour
Sq ft x 40 = Heat BTU/hour
12,000 BTU = 1 Ton
e.g. for a 1500 sf: 2.5 Ton, 60,000 BTU













US Department of Energy (DOE)
Zone Cooling Heating
CDD BPU/sf
<1,000 sf*
BPU/sf
>1,000 sf
HDD BPU/sf
1 <2,000 21 18-21 >7,000 50-60
2 <2,000 21 18-22 5,500-7,000 45-50
3 <2,000 23 19-23 4,000-5,499 40-45
4 <2,000 24 19-23 <4,000 35-40
5 ≥2,000 25 20-24 <2,000 30-35
* Because the ratio of volume (Cubic Feed) to surface area (Square Feet) is lower in small homes and heat is gained thru the walls.
It seems to me there should be a greater difference between the north and south.

Use a lower BTU number for a well insulated house and higer number for a poorly insulated house.

I don't know why DOE numbers for CDD aren't more specific, but hey, it's our government. See below for EIA data.

Based on tables at How to Size Your Air Conditioning System at AC 4 Life
Note: the zones at AC 4 Life are reversed 1 is south from those at DOE (1 is north)


Hygrothermal Regions 2009 International Energy Conservation Code (IECC)
2009 IECC Climate Zone Map - Gives minimum R-values.
ZONE
NUMBER
THERMAL CRITERIA
IP Units SI Units
1 9000 < CDD50°F 5000 < CDD10°C
2 6300 < CDD50°F ≤ 9000 3500 < CDD10°C ≤ 5000
3A and 3B 4500 < CDD50°F ≤ 6300
AND HDD65°F ≤ 5400
2500 < CDD10°C ≤ 3500
AND HDD18°C ≤ 3000
4A and 4B CDD50°F ≤ 4500 AND
HDD65°F ≤ 5400
CDD10°C ≤ 2500 AND
HDD18°C ≤ 3000
3C HDD65°F ≤ 3600 HDD18°C ≤ 2000
4C 3600 < HDD65°F ≤ 5400 2000 < HDD18°C ≤ 3000
5 5400 < HDD65°F ≤ 7200 3000 < HDD18°C ≤ 4000
6 7200 < HDD65°F ≤ 9000 4000 < HDD18°C ≤ 5000
7 9000 < HDD65°F ≤ 12600 5000 < HDD18°C ≤ 7000
8 12600 < HDD65°F 7000 < HDD18°C
10°C = 50°F; 18°C=65°F
SI - International System of Units; IP or I-P - Inch-pound units
See:
Climate Zones and Degree Days | Mapawatt
Cooling Degree Days
Census Areas 2,012 2,013 2,014 2,015
New England 564 540 420 557
Middle Atlantic 815 683 596 801
East North Central 974 690 610 730
West North Central 1,221 892 814 940
South Atlantic 2,162 2,000 2,001 2,392
East South Central 1,762 1,441 1,493 1,722
West South Central 2,915 2,536 2,474 2,745
Mountain 1,572 1,462 1,432 1,485
Pacific 917 892 1,068 1,075
U.S. Average 1,494 1,306 1,297 1,487
Source: STEO Data Browser - 9c. U.S. Regional Weather Data | U.S. Energy Information Administration (EIA)

See:
Do You Know Your Building Science Climate Zone? | energyVanguard.com
Commercial Buildings Energy Consumption Survey (CBECS) - U.S. Energy Information Administration (EIA)

At How to Figure BTUs for HVAC Sizing | Home Guides | SF Gate they say,
"At The biggest problem with sizing your furnace and air conditioner is that so few people know how to do it accurately. Even many contractors rely on a simplified rule of thumb (which is, incidentally, illegal) that translates square footage into a one-size-fits-all answer. The result is usually oversized units that cost more, use more energy and fail to maintain the temperature you desire inside your home. Professional calculation factors in many of your home's unique characteristics to obtain the correct HVAC sizing. Online calculators can often get you close. So will calculating some of these measurements yourself. It may not be as precise as an engineer's, but it will ensure you can spot any gross inaccuracies."

See:
BTU Calculator
How to Size Your Air Conditioning System at AC 4 Life


AFUE - Annual fuel-utilization-efficiency


See Air Conditioner Efficiency


Terms:
  • AFUE - Annual fuel-utilization-efficiency - A percentage of how much fuel it can convert into usable heat, with a scale ranging from around 30-100
  • CDD - Cooling degree days
    Take the average of a day's high and low and subtract 65° F
  • COP - Coefficient of performance
  • EER -Energy Efficiency Ratio -The efficiency you can expect from the air conditioner at peak cooling time (in the midst of the summer) because it is measured at only one, higher temperature.
  • EIA - U.S. Energy Information Administration
  • HDD - Heating degree days
    65 minus the average of a day's high and low in degrees F.
  • IECC - International Energy Conservation Code
  • IP or I-P - Inch Pounds - US measurements - Fahrenheit, Inch, ...
  • IS - Système International d'Unités (International System of Units) Celsius, meteres, grams
  • PCL - peak cooling load (heat gain) in Btu/hr
  • R-value - A measure of conductance and resistance
  • SEER - Seasonal Energy Efficiency Ratio rating (10-26) - An average. It takes into account the highs and lows of a typical home's cooling pattern.
  • SHGC - Solar Heat Gain Coefficient - Measures how much heat from the sun is blocked by windows
  • U-factor - Measures how well a product prevents heat from escaping a home or building. U-factor ratings generally fall between 0.15 and 1.20. The lower the U-factor, the better a product is at keeping heat inside the building.
    . U-factor, takes into account more than conductance. It also is affected by the airflow (convection) around the window and the emissivity (radiated or reflected heat) of the glass.
  • VT - Visible Transmittance - measures how much light comes through a product

Links:
Room Air Conditioner sizing guide | ENERGY STAR
Central Air Conditioning | Department of Energy

last updated 10 Aug 2016