Heat Loss, R-value, U-value, & K-Value measurements:

This article defines Heat Loss, R-value, U-value, & K-Value measures of heating loss rate or insulation effectiveness and provides basic building insulation and heat loss guidelines including how to measure or calculate heat loss in a building, defines thermal terms like BTU and calorie, provides measures of heat transmission in materials, gives desired building insulation design data, and shows how to calculate the heat loss in a building with R values or U values.

Taking advantage of a trip to Italy we use the Venice lagoon temperatures as an example while we explain saturation temperature and subcooling temperature and similar heating, cooling, temperature and pressure terms.

Definitions of AFUE, BTUs, BTUH, and Calories for Discussing Building Heat Gain or Heat Loss Analysis

When we are evaluating the quality and effectiveness of insulation in a building or the adequacy of a building heating or cooling system, we need to use measurements that permit us to describe the rate at which a building loses heat under various conditions (such as outdoor temperature, wind velocity, how leaky the building is, the area of its windows and perhaps doors, and the amount of insulation in the building walls, floors, and ceilings.

Discussed here: definitions for BTU, Calorie, R U& K Values, Design Temperature, Degree Day, Tons of Cooling Capacity.

How to measure or calculate heat loss (or gain) in a building. How to measure heat transmission in materials: definition of R-values, U-values, K-values, BTU, calorie, and rates of heat loss or gain. Building design temperatures & how to use a home energy audit or heat loss analysis.

What insulation "R" values should be used in a building insulation?

[Click to enlarge any image]

At page top: Formula-R™ and Owens Corning™ which may be visible in the page top photograph of pink Styrofoam™ insulation boards are registered trademarks of Owens Corning® and were photographed at a Home Depot® building supply center.

A few of these critical definitions is given just below, followed by some simple formulas used to calculate the heat loss in a building. Sketch at above left is provided courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

Article Contents

• HEATING, COOLING & INSULATION TERMS, DEFINITIONS
  • AIR ADMITTANCE VALVES AAVs
  • AIR BLEEDER VALVE FUNCTIONS
  • AIR FILTER EFFICIENCY MERV HEPA
  • AIR FLOW MEASUREMENT CFM
  • AIR FLOW RATES in HVAC SYSTEMS
  • AIR FLOW RATE TYPICAL CFM
  • AIR VELOCITY MEASUREMENT & STANDARDS
  • AIR VELOCITY RATES TYPICAL FPM
  • AFUE
  • AFUE, DEFINITION in DETAIL
  • BOILER COMPONENTS & PARTS
  • BTUs BRITISH THERMAL UNIT DEFINITION
  • BTUH or BTUs per HOUR
  • CALORIE
  • CLIMATE ZONES for the U.S
  • COMPLETE COMBUSTION, STOICHIOMETRIC
  • CONVERT BTUs to TONS of COOLING CAPACITY
  • COOLING CLIMATE ZONE BTU REQUIREMENTS
  • DEGREE DAY HEATING DEGREE DAYS or COOLING DEGREE DAYS
  • DESIGN TEMPERATURE for BUILDINGS
  • DRAFT REGULATOR, DAMPER, BOOSTER
  • ELECTRICITY USE per HOUR
  • EER
  • HARTFORD LOOP DEFINITION
  • HEAT FLOW DIRECTION
  • HEAT LOSS R U & K VALUE CALCULATION
  • HEAT PUMPS - includes heat pump terms, definitions & terminology
  • HEATING CLIMATE ZONE BTU REQUIREMENTS
  • HIGH SIDE in AIR CONDITIONING & HEAT PUMPS
  • JOULE
  • LATENT HEAT & LATENT COOLING
  • LOW SIDE in AIR CONDITIONING & HEAT PUMPS
  • K-VALUE
  • R-VALUE & RSI VALUE
  • SEER
  • SENSIBLE HEAT & SENSIBLE COOLING
  • SPECIFIC HEAT
  • STEAM VENTS
  • SUBCOOLING DEFINITION
  • SUPERHEAT DEFINITION
  • THERMS
  • TONS of COOLING CAPACITY
  • U-VALUE
  • R-VALUES - HEAT LOSS PER HOUR
  • WATT & WATT-HOUR

Definition of AFUE

AFUE is an abbreviation for Annual Fuel Utilization Efficiency. In short, the AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat. Higher AFUE is better.

See AFUE DEFINITION, RATINGS for details & examples.

Definition of BTUs and BTUH

Complete details about the definition of BTUs and BTUH, and examples of uses & calculations using BTU numbers can be found at BTUs DEFINITIONS OF where we include more detailed definitions of BTUs and BTUh.

Also see THERMS - used as a measure in some natural gas meters.

One BTU is also equal to 252 calories. See CALORIE

A BTU is one "British Thermal Unit" which is defined as the quantity of heat that would be required to increase the temperature of one pound of water by one degree Fahrenheit.

Definition of Calorie or Calories

A calorie is defined as the quantity of heat needed to raise the temperature of one gram of water by one degree Centigrade

Definition of EER

See SEER RATINGS & OTHER DEFINITIONS

Except:

Energy Efficiency Ratio (EER): This is a measure of the instantaneous energy efficiency of cooling equipment.

EER is the steady-state rate of heat energy removal (e.g., cooling capacity) by the equipment in Btuh divided by the steady-state rate of energy input to the equipment in Watts.

This ratio is expressed in BTUh per Watt (BTUh / Watt).

EER is based on tests performed in accordance with ARI 210/240.

Also see BTUs DEFINITIONS OF

Definitions of R-Value, U-Value, K-Value

R values and heat loss

The "R" value of a material is its resistance to heat flow through the material. When buying various insulation materials you will almost always see an "R" value quoted for the material. In general, higher "R" means more resistance to heat loss and therefore lower heating or cooling bills for the building.

Mathematically, "R" is simply the reciprocal of the two measures discussed in more detail below:

• U - the measure of heat transfer (the ability of a substance to conduct heat) discussed above and also at "U-VALUES"
• K - the coefficient of heat transmission discussed at "K-VALUES"

"K" (R = 1/K) or "U"

(R_{(whole building)} = 1/U)

As you'll read below, the heat transmission coefficient "K" measures the heat flow through an individual substance and "U" measures the overall building heat loss by adding all of the various areas and substances together.

Reader Peter J. Collins has noted in clarification of the definition of "R" value that

The R-value of a material is a measure of its thermal resistance.

The U-value (or U-factor), more correctly called the overall heat transfer coefficient

We add that R-value measures the resistance of a material to transfer heat (in any direction). Higher R-vales are more resistant to heat transfer.

When we are discussing building insulation, an insulation with a higher R-value would be expected to resist heat loss more than one of a lower R-value, if all other factors such as air leakage or heat radiation are the same.

And as Mr. Collins elaborates,

R-value = resistance to the movement of heat

U = the ability to transfer heat, obviously an inverse condition, to resistance, or in other words, or to allow the transfer of heat

But as we elaborate below at DEFINITION of U-VALUE - Definition of U value or U-coefficient of heat loss resistance, the NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the U-Factor (U) as follows:

U-Factor measures how well a product prevents heat from escaping a home or building.

U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-Factor, the better a product is at keeping heat in. U-Factor is particularly important during the winter heating season.

This label displays U-Factor in U.S. units. Labels on products sold in markets outside the United States may display U-Factor in metric units.

Definition of Insulation R-Values or Building R-Values: Rate of Heat Loss Per Hour for a Building

How to Calculate the R value U value & K value for a Building & How to Use These Numbers

If you like, read below this section to see our details about "K" values, "U" values, and "R" values as measures of heat movement in buildings. Actually calculating a building's actual rate of heat loss is pretty simple - it's a "cookbook" process that uses the following formula:

Also see HEAT LOSS in BUILDINGS

Heat Loss using "R" values:

(Building Heat Loss in BTU's per hour) R = (Building Total Surface Area in sq.ft.) / (Surface Area "R" value) x (Temperature Difference)

Temperature Difference = the difference in temperature in deg F. on the two sides of the building surface, typically indoors and outdoors

Surface Area "R" value = the "R" value of the surface area being evaluated (say an insulated wall).

Heat Loss using "U" values:

(Building Heat Loss in BTU's per hour) U = 1/R, - or in other words - the inverse of R:

(Building Heat Loss in BTU's per hour) U = (Building Total Surface Area in sq.ft.) x (Surface Area "U" value) x (Temperature Difference)

Also see BTUs DEFINITIONS OF

How to Convert Between R-Values & RSI-Values

It's easy to convert North American-used R-values to SI Units to obtain an RSI Value as following:

R-Value / 5.678= RSI

or

RSI x 5.678 = R-Value

 

More considerations when measuring home energy use or heat loss

But there's more work to do for a complete answer to building heat loss. We need to make up a simple table which will contain the total surface area of each type of material (since each will have it's own "R" value) and then plug in the area's "R" value and the temperature difference.

Usually we assume the same temperature difference for all of the areas of the building though this might be a simplification since that may not be exactly true.

How to include the effect of wind on home energy use or heat loss

We're also missing, from this simple calculation, the effects of windOn a building's heat loss, though a more sophisticated version of this approach might simply adjust the temperature difference to include the wind factor.

For example, you could use a wind/temperature chart to derive the effective outdoor temperature when it's also windy. In cold conditions, adding a wind velocity will lower the effective outdoor temperature and thus it will increase the temperature difference across the building wall.

Use any "wind chill factor" chart for this data. Still more sophistication of measures of heat loss are possible by adding the effects of moisture on heat loss from a surface, but while this is important for a (sweaty) human in cold conditions it is generally ignored when considering building heat loss.

Using a spreadsheet to accurately calculate building heat loss or heat gain

This is a perfect application for an Excel or similar spread sheet, listing each building surface type (wall, window, door), it's R, K, or U value, and its total area. Adding temperature difference across these surfaces permits a calculation of the heat loss (or gain) through each surface type. These are simply added together to represent the entire building's heat loss or gain.

Heat loss vs. heat gain in buildings: applying the simple laws of thermodynamics

You may have noticed we keep talking about heat loss and then we add "or heat gain" in the same sentences or headings.

That's because heat loss analysis works just fine for both building heating and building cooling. The only differences between looking at heat loss and heat gain for a building are the direction of heat flow and the fact that we may be using different equipment with different equipment efficiencies (a heating furnace or boiler versus an air conditioner).

If we're in a heating climate and are in the heating season, heat will flow from the building interior to the outdoors.

If we're in a cooling climate and are in the cooling season, heat will flow from the outdoors to the building interior. Just remember that (according to the laws of thermodynamics), heat (or energy) always flows from the warmer (or more exited state) into the cooler (or less excited state) area of a building.

Definition of the K value or K-coefficient of heat transmission

A building's K value or K-coefficient of heat transmission is one way to express the heat loss in a building. "K" is defined as the number of BTU's of heat moving through any material with these details:

• Per square foot of area of the material
• Per degree Fahrenheit of temperature difference
• Per inch of thickness of the material

So "K" takes a lot of variables into consideration and gives us the rate of heat loss per square foot of building surface, per inch of thickness of material in that building surface, per degree of temperature difference in Fahrenheit, in BTUs per hour.

By "degree of temperature difference in Fahrenheit" we mean that we are taking into consideration the difference in temperature on the two sides of our building surface. For example, if the indoor temperature in a building is 68 deg. F. and the outdoor temperature is 48 deg. F., then we have a 20 degree temperature difference on the two sides of the building (wall or roof for example).

This temperature difference on the two sides of a surface, say an insulated building wall, for example, is very important in understanding how a building loses heat (in the heating season) or gains heat (in the cooling season). That's because the rate of heat transfer through a material increases exponentially as a function of the temperature difference. This is intuitively obvious and is confirmed by physicists.

For example, if the temperatures on either side of a building wall were the same, there would be no heat loss or gain through the building wall. As the temperature difference on either side of that same wall increases, say from one degree of difference to 20 degrees of difference the rate of heat transfer increases.

An interesting version of this heat transfer theory was shared with the author in a class on how to minimize building heating costs when the instructor told us that "the thermal conductivity of finned copper heating baseboard is exponentially greater at higher temperatures".

He was saying that if we ran heating water from our heating boiler through the baseboards at 200 deg.F. we would see much more efficient heat transfer from the heating baseboards into the building.

There are other factors involved in heating system efficiency such as the length of boiler on cycle (longer is more efficient), so there was more to think about, but the instructor was applying classic heat transfer theory that is reflected in the "K" values of building insulation as we've discussed here.

Definition of U value or U-coefficient of heat loss resistance

U-value measures the ability to transfer heat, an inverse condition, to heat movement resistance, or in other words, or U-value measures the ability of a substance to allow the transfer of heat.

Formula for U-Value - U Coefficient of Resistance to Heat Loss

U = 1 / Rt

U = thermal transmittance expressed as (W / M³ =-K)

Rt = Total thermal resistance of one or more building elements coposed of layers of a given size expressed as (M2²x K / W )

and

Rt is the sum of these components:

Rt = Rsi + Rse +( R₁ to R_n )

Rsi - Thermal Resistance of Interior Surface (varies by climate zone)

Rse - Thermal resistance of exterior surface aera (varies by climate zone)

R_{1 +} R₂ + R₃ etc. up to+ R_n to obtain the total thermal resistance of all of the layers that make up the building element (such as a wall or roof), and where each R_n is further calulated as

• R_n = D / λ where n = 1, 2, 3 etc. corresponding to R1, R2, R3 etc and
• D = Thickness of material Rn
• λ = Thermal conductivity of material Rn

The NFRC (National Fenestration Council) in discussing solar heat gain at windows, describes the U-Factor (U) as follows:

U-Factor measures how well a product prevents heat from escaping a home or building. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-Factor, the better a product is at keeping heat in. U-Factor is particularly important during the winter heating season.

This label displays U-Factor in U.S. units. Labels on products sold in markets outside the United States may display U-Factor in metric units.

Computing "K" values (discussed above) tells us the heat loss rate for a specific material, thickness, area, and temperature difference but while we need to be able to calculate "K" values, those alone don't tell us what's going on in an actual building.

We need to be able to combine all of the rates of heat loss (or gain) across all of the types of surfaces, insulation, and building material for the whole building - at least for all of its external or perimeter surfaces including roofs, walls, and floors as well as windows and doors. That's where the "U" value makes its appearance.

A building's "U" value or U-coefficient of resistance of heat loss is a related measure of resistance to thermal energy or heat flow out of a building (if it's warmer inside than outside) or conversely the same concept works in a warm climate where air conditioning is in use, except that we expect outside heat to be flowing into the building.

A building's "U" value is much more complete, and therefore useful than "K" values alone because a building's "U" value combines the "K" factors for all of the building's surfaces and materials.

In other words, we add the effects of heat loss (or gain), still expressed in the number of BTU's per hour per square foot of area, and still expressed per degree of Fahrenheit of temperature difference and still expressed per inch of thickness of material (just as with "K" values), for all of the substantial areas and surfaces of the exterior of a building's floors, walls, windows, doors, ceilings, or roofs (if cathedral ceilings are present).

To calculate the "U" value, or overall heat loss (or gain if we're air conditioning) for a building, we need to add the "R" values for each material in the structure, and to factor in the total area of each material in the structure. We discuss this procedure in more detail below at "Calculating Heat Loss for a Building".

Definition of SEER

For details about SEER please see SEER RATINGS & OTHER DEFINITIONS from which we quote:

Seasonal Energy Efficiency Ratio (SEER): This is a measure of equipment energy efficiency over the cooling season. It represents the total cooling of a central air conditioner or heat pump (in Btu) during the normal cooling season as compared to the total electric energy input (in watt-hours) consumed during the same period.

SEER is based on tests performed in accordance with ARI 210/240.48

Definition of Design Temperature for buildings and Building Insulation?

The "indoor design temperature" for a building refers to the assumed target indoor temperature that the building owner or occupants want. Typically 70 deg.F. is used unless the owner specifies something different.

The "outdoor design temperature" for a building is (for heating purposes) assumed to be the average lowest recorded temperature for each month between October and March (the heating season in most climates). If we are specifying a "design temperature" for cooling climates we'd use the average outdoor highest recorded temperature during the heating season, perhaps April through September.

Watch out when calculating building or room heating needs. In a recent review of the number of linear feet of heating baseboard needed for a New York building addition we tried out an excellent heat loss analysis program provided by SlantFin.

The program considers most of the key variables you'd want examined for an accurate and reliable heating design. But we found that our building had properties not considered by the heat loss software, including

• The specific R-value of the insulation we used in a cathedral ceiling and in a floor as well as in 2x6-framed walls
• The effects of the surprise by our plumber who installed 1/2-inch diameter heating supply and return piping instead of the 3/4" diameter pipes we anticipated, and his assumption that the flow rate was 1 gph through the system would be OK instead of the design point of 4 gph.
• A designed variation in placement of windows to provide more solar gain on South and West walls and less glazing (and heat loss) through the building's North wall
• Effects of using an insulation method and other building design features that minimized air leaks (foam insulation, and detailing around window and door openings, for example)

Fortunately simpler rules of thumb analysis by consultants at our heating equipment and parts supplier indicated that the modified design would adequately heat the space.

Definition of Heating Degree Day or Cooling Degree Day

Some building insulation designers and architects look at the number of "degree days" as an easy way to get at the average outdoor temperatures for an area and a season. A Degree Day is the daily average number of degrees Fahrenheit that the outdoor temperature is below 65 deg.F.

The number of "degree days" during a heating season is easy to obtain: call your local oil delivery company or utility company.

These energy providers keep close tabs on degree days for their area since this number is used in planning for the automatic delivery of energy. It's the number of "degree days" that have occurred in a given period, combined with a building's historic rate of heating oil use, for example, that tells an oil company when to schedule that building for an automatic delivery of heating oil.

Air Conditioning, Refrigeration, & Heating System Standard Definitions

What is a Watt Hour or Wh?

What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home). For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.

See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.

How do we calculate watts, volts, and amperage for an electrical device like an air conditioner?

Watts (W) as used in a simplified manner here and by electricians, is a measure of electrical power and is expressed by any of the formulas shown below. [All forms of power are measured in units of Watts, W, but this unit is generally reserved for real power (see definitions further below.]

DC circuits: W = V x I (this is a simplified formula and is technically exactly correct for DC circuits. For AC circuits,

• V*I=VA  not watts.   In an AC circuit, things are more complicated. An electrical load in an AC circuit will typically use both real power - P - and reactive power - Q - (definitions below).

AC circuits: Watts   W=V*I*PF  where PF = power factor

See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.

Also see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".

Reader Daniel Mann adds that "Watts is correctly shown as Watts-Voltage times Current times power factor. 

Since power factor varies all over the place,..." [W = V x I] "perpetuates misinformation". We include additional more technical explanation of power factor, real power, apparent power, complex power, and reactive power as we elaborate

at DEFINITIONS of ELECTRICAL TERMS.

Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too.

See A/C DATA TAGS for details.

Definition of Latent Heat

Latent heat is defined as the amount of heat absorbed by a substance with no change in a temperature - such as when a substance changes state (from water to steam, for example)

In other words, heat that is absorbed by a substance with no change in temperature is latent heat. For example when a substance changes state (liquid to gas) latent heat is or may be involved.

Illustration courtesy of Carson Dunlop Associates, a Toronto firm.

The latent heat of vaporization  is defined as the number of BTUs to raise one pound of liquid to a pound of vapor (to a varying degree per BTU depending on the type of vapor - this is "superheat").

The latent heat of condensation is defined as the number of BTUs necessary to change a state back from a vapor to a liquid

The latent heat of solidification is defined as the amount of energy (or number of BTUs) needed to change a liquid to a solid (such as water to ice) while the temperature remains unchanged (at sea level, 32 °F).

A latent heat system is one that uses a cooling media that changes state - for example refrigerant that changes state beween a liquid and a gas form.

See AIR VELOCITY RATES TYPICAL FPM for an example of the use of latent heat.

Definition of Latent Cooling & Latent Cooling Load

"Latent cooling" is in essence a re-statement of the definitions of latent heat, with focus on cooling rather than heating an evironment. In both cases the definition turns on the state change of a substance and the resultant heat absorbed (= cooling effect) by a substance with no change in a temperature, such a changing a refrigerant liquid to a gas.

Latent cooling load has a more narrow definition focused on dehumidification of building air.

Latent Cooling is defined as the cooling capacity needed in a building to dehumidify the occupied space to the desired level when adding external cooling load factors such as the number of people occupying the space, air leakage into or out of the building, and the heat generated by equipment in use in the building.

The energy required to remove moisture from air depends primarily on the air moisture level and temperature.

When air gains moisture by evaporating moisture from indoor moisture souirces, the air is gaining in latent energy and its latent cooling load increases.

Whe air loses moisture (e.g. passing through an air conditioner cooling coil), the air loses its latent heat by the process of condensation of moisture out of the air onto the cooling coil surface.

Grundfos gives a succinct definition of the latent cooling load in a building:

Latent cooling load is a measure of the amount of energy that is necessary to dehumidify the air in a building, for example, regardless of the outdoor humidity. Cooling load needs to be considered when a cooling system is being dimensioned. Latent cooling load refers to the wet bulb temperature. - Grundfos Research & Insight, retrieved 2022/03/07 original source https://www.grundfos.com/solutions/learn/research-and-insights/latent-cooling-load#:~:text=Latent cooling load is a,to the wet bulb temperature.

See

• Shirey, III, Don B., Hugh I. Henderson Jr., Richard A. Raustad, UNDERSTANDING the DEHUMIDIFICATION PERFORMANCE of AIR-CONDITINING EQUIPMENT at PART-LOAD CONDITIONS [PDF] (2006), Florida Solar Energy Center, University of Central Florida / Florida Solar Energy Center 1679 Clearlake Road Cocoa, FL 32922 (321) 638-1000 CDH Energy Corp. P.O. Box 641 132 Albany St. Cazenovia, NY 13035-0641 (315) 655-1063 - fretrieved 2022/03/07 original source: http://www.bpi.org/sites/default/files/Ecostella_docs/FSEC-CR-1537-05.pdf
  
  Abstract excerpt:
  Air conditioner cooling coils typically provide both sensible cooling and moisture removal. Data from a limited number of field studies (Khattar et al. 1985; Henderson and Rengarajan 1996; Henderson 1998) have demonstrated that the moisture removal capacity of a cooling coil degrades at part-load conditions – especially when the supply fan operates continuously while the cooling coil cycles on and off.
  
  Degradation occurs because moisture that condenses on the coil surfaces during the cooling cycle evaporates back into air stream when the coil is off. This degradation affects the ability of cooling equipment to maintain proper indoor humidity levels and may negatively impact indoor air quality.
  
  This report summarizes the results of a comprehensive project to better understand and quantify the moisture removal (dehumidification) performance of cooling coils at part-load conditions.

See also SENSIBLE HEAT & SENSIBLE COOLING - removing heat that people can feel.

Definition of Saturation in HVACR systems

Saturation is defined as the specific point at which a state change takes place in a material such as water or refrigerant.

The saturation temperature of water - its boiling point - is 212 °F (100 °C) as long as we're looking at the Venice Lagoon in Italy or at any other sea-level location.

Actually on 20 July 2017 the temperature of the water of the Venice lagoon at San Croce was a tepid 77°F (25 °C)- nowhere near saturation temperature.

Definition of Subcooling

Subcooling, a condition that occurs only in liquids or solids, is any temperature in the liquid or solid that is below its saturation temperature.

Example: Since the saturation temperature of water at sea level is 212 °F (100 °C) = the boiling point of water, water at any temperature below 212°F (100°C) is sub-cooled.

If we drop the water temperature to 200 °F (93.3 °C) then the water has been subcooled by 12 degrees. (212 - 200 = 12)

So in fact on 20 July 2017 the Venice lagoon water was subcooled by (212 - 77) = 135°F.

The concept of subcooling applies only to liquids and solids whose state is remaining un-changed. When we cool a gas sufficiently, it changes to a liquid.

Definition of Superheat or the latent heat of vaporization, condensation, solidifcation

This definition has its own page - please see DEFINITION of SUPERHEAT

Definition of Sensible Heat

Sensible heat is defined as the amount of heat that we can sense or feel or measure.

If we stick a thermometer into the Venice lagoon (shown earlier) we are measuring the water temperature or its amount of sensible heat. (It was 77°F (25 °C) when we checked).

It's hot and muggy in the air Venice in the summer too - not just the lagoon, so some of the more posh apartments and hotels provide air conditioning.

When an air conditioner system is working, the larger diameter tubing on the low-side of the system (the suction line) combined with the effects of the refrigerant metering device (cap tube or thermostatic expansion valve) results in a reduced pressure on the low side (compared with high side pressure).

The reduced pressure causes vaporization of the liquid refrigerant inside the cooling coil, which in turn means that SENSIBLE HEAT [separate definition in a companion article] is absorbed by the cooling coil.

When the same air conditioner system is working, the smaller diameter tubing on the high side reduces available volume so that (along with the effect of the compressor itself) we increase the pressure and temperature of the refrigerant so that sensible heat can be transferred to ambient outdoor air.

A sensible heat system is one that uses a cooling media that does not change state - for example water to air.

See AIR VELOCITY RATES TYPICAL FPM for an example of the use of sensible heat.

Definition of Sensible Cooling

Sensible cooling is the removal from air of heat that is felt by people.

See also LATENT HEAT & LATENT COOLING - the energy associated with removal of moisture from air.

Definition of Specific Heat

Specific heat is defined as the amount of heat required to raise the temperature of a given substance by one unit of temperature (in our examples by one °F.) Specific heat is also defined as the amount of heat (in calories) to increase the temperature of one gram of a substance by one deg C (Celsius).

The specific heat of water is defined as a constant and = 1

The specific heat of ice is is 5

A definition of one BTU is the energy required to raise one pound of water by one degree F (sensible heat).

Definition of Therms, Convert Therms to BTUs

Therms are used as a measure of natural gas for pricing or on some gas meters.

1 Therm = 100,000 BTUs

1 Therm = 0.10 MMBTUs

100 cubic feet of natural gas = 1.037 Therms or 103,700 BTUs

Details are at GAS BTUH, CUBIC FEET & ENERGY

In which direction does heat flow

Heat energy always flows from the warmer substance to the cooler substance, down to -460 °F where all molecular movement stops.

A neat fact is that the heat flows more rapidly (efficiently) between two substances when there is a greater temperature difference between them. That's why the thermal conductivity of finned copper tubing heating baseboard is exponentially greater at higher degrees of heating water temperature, and that's why we like to run our heating boiler at a higher rather than a lower upper limit temperature.

An example of the application of the importance of heat flow direction is given

at CLOTHES DRYER TEMPERATURE MEASUREMENTS

where we measured roof surface and roof cavity temperatures to determine when heat from the roof might explain elevated temperatures at a clothes dryer vent that passes through a hot roof cavity.

When the roof temperature and roof cavity temperature are above the otherwise-natural temperatures in the clothes dryer exhaust vent system then the roof adds heat to the dryer vent and we measured temperatures at the dryer vent wall outlet as high as 143 °F (62 °C).

But when the roof temperature and roof cavity temperature were lower than the otherwise-natural temperatures in the clothes dryer vent system, then the roof system actually removes some heat from the dryer vent system and we would expect to measure exhaust temperatures lower at the vent exit than in the clothes dryer itself.

That's likely to happen in winter when the roof surface is cold.

Definition of Joule - What is a Joule?

Note: Outside of the U.S. and some other places, BTUs is being replaced with the SI unit of energy, the Joule. (J). The English have beaten out the Scots by James Prescott Joule who defined this value. since there are 3600 seconds in an hour) the following formulas equating Watts, Joules, and Newton meters can be written:

1 Watt second (Ws) = 1 joule (J) = 1 newton meter 1 Watt hour (Wh) = 3600 Joules

1 kilowatt hour = 3.6 x 10⁶ Joules, since there are 1000 watts in a kilowatt.

We can think of an air conditioner's "efficiency" as expressed either in the total operating cost for a season of use, or you may prefer to just express the air conditioner's efficiency as its operating cost to run the system for one hour.

The equation shown at page top is designed to reduce all of the parameters describing air conditioning efficiency to a single efficiency number, SEER. SEER numbers are useful when we're comparing one air conditioner with another.

But suppose we want to know the actual air conditioning cost per season, or air conditioning cost per operating hour to operate our air conditioner?

To translate our air conditioners SEER rating into actual air conditioning operating costs we need to know the amount of electricity used per hour.

How Much Electricity Does An Air Conditioner Use Per Hour?

How much electricity our air conditioner uses per hour is easy to calculate. Let's assume that the data tag on our air conditioner says that the unit is a 5000 BTUh device with a SEER rating of 10.

This means our A/C unit will produce 5000 BTUs of cooling in an hour of running. Since SEER=10 means that 10 BTUs used per Wh, then

5000 BTUh / 10 SEER = 500 Watts per hour that our A/C unit will use.

How Much Electricity Does An Air Conditioner Use in one Cooling Season?

A common example we use (because the math is easy) is to assume we have 125 days of cooling season during which we run the air conditioner for eight hours per day.

8 x 125 = 1000 hours of cooling operation over a season

500 Wh (watts used per hour) x 1000 (hours per season) = 500,000 Wh per season

So we are using 500,000 Watt Hours of energy (electricity) per cooling season. We divide this by 1000 to convert to Kilowatts since that's how our electrical bill will express our electricity usage.

500,000 Wh / 1000 = 500 kWh or kilowatt hours per season of use
That's how much electricity we're using over the cooling season.

What is the Definition of High Side and Low Side in Air Condition & Refrigeration Systems?

Definition of Low Side in an Air Conditioning System refers to the components on the low-temperature and low-pressure side of the compressor unit.

In an air conditioner, the low side includes the suction or intake side of the compressor unit, suction piping connected to the evaporator coil, the evaporator or cooling coil, and the output-end of the metering device or TEV.

Definition of High Side in an Air Conditioning System refers to the components on the high-temperature (above ambient air temperature) and high pressure side of the compressor unit.

In an air conditioner in cooling mode these include the output or high pressure side of the compressor unit, the high pressure gas refrigerant line connected to the condensing coil, the condensing coil itself, and the inlet side of the metering device located near the evaporator coil.

These parts are named and illustrated at AIR CONDITIONING & HEAT PUMP SYSTEMS

and at COMPRESSOR PRESSURE READINGS we discuss air conditioner system high side and low side further.

At OPERATING COST we determine the actual dollar cost of running an air conditioner either by the hour of by the season of use. It's easy to get from that data to actual air conditioning operating costs in dollars.

Other Heating & Air Conditioning System Performance Measurements & Standards

 

Definition of AFUE

The AFUE (Annual Fuel Utilization Efficiency) rating number for heating equipment is a measure of heating system efficiency. AFUE tells you, for each dollar you spend on energy for heating by gas, oil, or another fuel, just how much of your dollar shows up inside the occupied space of your building as heat.

See AFUE DEFINITION, RATINGS for more details and for current required AFUE percentages for U.S. & Canadian heating equipment.

Definition of Heating Seasonal Performance Factor - HSPF

HSPF is a heating performance measurement used to evaluate the efficiency of heat pumps when in heating mode. Higher HSPF numbers mean greater heating efficiency. Currently heat pumps in the U.S. should have an HSPF of 6.8 or higher.

Also see ENERGY STAR PROGRAM and for tips on how to cut air conditioning or heat pump operating costs, see AIR CONDITIONING HEAT PUMP SAVINGS.

Definition of Watt & Watt Hour: What is a Watt Hour or Wh?

What's a WattHour? Watt hours (Wh), sometimes written W.h, can measure either electrical energy produced, say by a power station, or Watts can measure the amount of electrical energy consumed (say at a light bulb or an air conditioner in our home).

For air conditioners, the A/C units' total Wh is the energy used in running the air conditioning system for an hour.

See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.

How do we calculate watts, volts, and amperage for an electrical device like an air conditioner?

Watts (W) as used in a simplified manner here and by electricians, is a measure of electrical power and is expressed by any of the formulas shown below. [All forms of power are measured in units of Watts, W, but this unit is generally reserved for real power (see definitions further below.]

DC circuits: W = V x I (this is a simplified formula and is technically exactly correct for DC circuits. For AC circuits,

• V*I=VA  not watts.   In an AC circuit, things are more complicated. An electrical load in an AC circuit will typically use both real power - P - and reactive power - Q - (definitions below).

AC circuits: Watts   W=V*I*PF  where PF = power factor

See DEFINITIONS of ELECTRICAL TERMS for details about volts, watts, amps, and power factor.

Also see AMPS & VOLTS DETERMINATION "How to estimate the electrical service ampacity and voltage entering a building".

Reader Daniel Mann adds that "Watts is correctly shown as Watts-Voltage times Current times power factor.  Since power factor varies all over the place,..." [W = V x I] "perpetuates misinformation". We include additional more technical explanation of power factor, real power, apparent power, complex power, and reactive power as we elaborate

at DEFINITIONS of ELECTRICAL TERMS.

Lots of electrical appliances include a label providing the appliance's wattage, and in the case of heating and air conditioning equipment, lots of other details are provided too.

See A/C DATA TAGS for details.

Because no amount of insulation can keep a drafty building warm, also review ENERGY SAVINGS PRIORITIES.

BTUs & BTUH

See DEFINITION of BTUs and DEFINITION of BTUH

Translate BTUs to Tons of Air Conditioning or Cooling Capacity?

This discussion has been moved to CONVERT BTUs to TONS of COOLING CAPACITY

Tons of Cooling Capacity

We have moved this discussion to TONS of COOLING CAPACITY

"One ton" of cooling capacity, historically, referred to the cooling capacity of a ton of ice.

U.S. Climate Zones

U.S. Heating Climate zones and Cooling Climate Zones are now found at CLIMATE ZONES for the U.S

Typical Heating BTUh Requirements Per Square Foot

Typical BTUs per square foot requirements to heat a building now found at HEATING CLIMATE ZONE BTU REQUIREMENTS

Also see GAS BTUH, CUBIC FEET & ENERGY

Typical Cooling BTUh Requirements per Square Foot

Typical BTUs per square foot requirements to cool a building now found

at COOLING CLIMATE ZONE BTU REQUIREMENTS

...

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