HVACR416 - Design Heat Loss / Heat Gain Part 1

Why? The primary function of Air Conditioning is to maintain conditions that are… o Conductive to human comfort o Required by a product or process within a space.

How? To perform this function the equipment installed must be of proper capacity and controlled throughout the year. Proper capacity is determined by actual PEAK load. Partial load conditions are handled by other controls during the year.

Loads It is impossible to measure the actual loads in a peak condition. Loads must be estimated based on heat gains and heat losses. The heat gain and heat loss will not ever equal the exact equipment size as there are other considerations.

System Design Proper system design takes into account: o Building Heat Load o Building Requirements o People Requirements and Comfort o Air Flow o Humidity o Energy Costs

A few items you need to know The amount of heat instantaneously coming into a space. A heat loss is the amount of heat instantaneously leaving a space.

ASHRAE American Society of Heating, Refrigeration and Air Conditioning Engineers. ASHRAE is the body that sets the standards for all heating, ventilation and air conditioning formulas and calculations.

BTU The BTU is the British Thermal Unit. 1 BTU = the amount of heat it takes to raise 1 pound of water one degree. All heating and air conditioning calculations are based on the BTU. 12,000 BTU’s is equal to 1 Ton.

CFM Cubic Feet Per Minute The CFM is the measurement used for air flow. 400 CFM of air is the standard for 1 Ton of Air Conditioning.

Square Foot Used to measure floor and wall space. This is Length x Width If a space is 12’ long and 12’ wide it is 144 sq. ft. A space 12 inches by 12 inches = 1 sq. ft.

Cubic Foot A cubic foot is a measurement of length, width and height. The cubic foot is used for a measurement of VOLUME. For example a room that has a ceiling height of 10 ft and a length of 10 ft and a width of 10 ft has a volume of 1000 cu. ft. (or ft 3 )

Infiltration Heat that moves into and out of a structure through doors, cracks, and holes in a building. Infiltration is uncontrolled air/heat movement. Infiltration gets worse the larger the temperature variance from inside to outside.

Ventilation A planned and controlled movement of air and heat into or out of the building envelope. Ventilation occurs in rest-rooms, through air exchangers, or over grills in kitchens. Some ventilation is mechanical, some is natural.

Natural Ventilation Remember Hot Air Rises Cool Air Falls Heat moves from hot to cold. Air moves from high pressure to low pressure environments. All of these points are used in natural ventilation.

Sensible Heat & Latent Heat Sensible Heat is….. The heat that is measurable. Latent Heat is…… Heat that is not measurable and causes a change in state. o For example water to steam o Ice to water o Water to ice o Vapor to liquid o Liquid to vapor

Types of Heat Conduction: o Heat transfer from one molecule to another within a substance or from one substance to another. Convection: o Heat transfer from one place to another using a fluid. Radiation: o Heat transfer via rays, or infra-red light. The sun is an example of Radiation.

Load Estimating The first step in load estimating is to organize the sources of heat as internal or external loads. This is done through a building survey.

External Loads External loads are loads that have conducted heat, solar heat and outside air load from ventilation or infiltration.

Internal Loads Internal heat may come from people, lights, electric motors, office machines, computers, appliances, kitchen equipment, and processes. Internal heat may also come from the equipment you are using to cool the space.

How external heat affects a space? The heat that flows through a wall or other structure depends on the temperature difference on the two sides. The larger the temperature difference the greater the heat flow. The lower the temperature difference the lower the heat flow.

Heat Flow The amount of heat flow also depends on the area, and the type of construction. The type of construction is known as the “U Factor”. The total heat flow is known as Q and is a measure of BTU/hr or BTU’s per hour.

Heat Flow INSIDE – 70 degrees OUTSIDE – 95 degrees Wall 12ft long and 10ft high Tc = Temperature Cooler Area Tw = Temperature Warmer Area Q = BTU/H of Heat Transfer U = Heat Transfer Factor of the Wall

Heat Transfer The formula for heat transfer is: o Q = U x Area x (Temp W – Temp C) In English this translates to total conducted heat (Q) is found by multiplying the U factor by the Area by the temperature difference across the wall.

Heat Transfer Example For example – a 30 ft long by 10 ft high partition wall has a temperature of 90 degrees on the unconditioned (outside) side and 80 degrees on the conditioned (inside) side. From a set of “U” factor tables, the value of a typical 8 inch masonry partition is.40 BTU / sq. ft. / degree temp difference

Heat Transfer Example The wall has an area of 300 sq. ft. The temperature difference is 10 degrees. The conducted heat is calculated out to.40 x 300 x 10 which equals 1200 btu/hr.

Heat Transfer In addition to heat transfer through walls and partitions heat can enter a building through: o The roof o The walls o The windows Glass is a great conductor of heat.

Heat Transfer Example If the same area of wall in the last example: o 30’ x 10’ = 300 sq. ft. Was single paned glass with a U factor of 1.13 then: Q = 1.13 x 300 x 10 or 3390 BTU/hr.

Heat Transfer One square foot of ordinary window passes as much heat as about 4.5 sq. ft of residential wall, or 4 sq. ft. of residential ceiling, or 3.5 sq. ft. of commercial wall, or 3 sq. ft. of commercial ceiling.