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3.1 PRINCIPLES OF SPACE CONDITIONING - Coggle Diagram
3.1 PRINCIPLES OF SPACE CONDITIONING
1) HEATING PRINCIPLES
FACTORS AFFECTING COMFORT
Air temperature
Usually varies between the floor and the ceiling, especially near windows.
The optimum temperature required for comfort depends on
Air movement
The amount of thermal radiation to or from room surfaces( ESPECIALLY WINDOWS)
And the level of physical activity and the kind of clothing worn by occupants
Mean Radiant Temperature
Mean radiant tempertature (MRT) is an average temperature of surfaces surrounding a person, and is one of the most important factors affecting comfort.
Is a measure of the radiant heat exchange between a person and his or her surroundings, independent of air temperature.
Relative Humidity
Is the amount of moisture in the air relative to the amount of water the air can hold at that temperature.
Cold air hold less water vapour than warm air.
Space cooling and dehumdification both reduce relative humidity.
When RH reaches 100%, the air is saturated.
Any additional water vapour will condense in the form of water/frost/ or ice depending on the temperature of the condensing surface
If the humidity falls TOO LOW, the air will draw moisture out of the furniture, carpets,walls and ceilings.
If the humidity becomes TOO HIGH
Occupants may complain of DRY THROATS ans STATIC ELECTRICITY
MOULD AND MILDEW CAN FLOURISH
Air movement
Generates CONVECTION and evaporative heat loss and cooling sensation on exposed skin.
The design of the space conditioning system should manage air delivery velocities and consider the LOCATION of the AIR DIFFUSERS.
Effective Temperature
Is an index that represents
the combination of Temperature, Humidity, and Air Velocity at which people are most comfortable.
2) HEATING DESIGNS & COOLING LOADS
Outdoor design temperature is based on long-term local climate data that has been established for every community and listed in NBC. The outdoor design temperature typically used is the lowest temperature that may only be exceeded for 2.5% of the house in January.
Heat loss and heat gain calculations are done for each component of the house: Walls, Ceilings, Windows, Doors and Floors.
There has been a tendency for heating systems to be OVERSIZED. The result is Furnace Cycling that results in short periods of operation that don't let furnaces reach their efficient operating mode. In the case of cooling, equipment over-sizing results in high humidity levels due to short cycling, which can adversely affect occupant comfort.
A typical practice is to select a space heating system based on the design load plus 40% and then choose the next highest capacity in a supplier catalogue.
SOLUTIONS TO FURNACE OVERSIZING:
Use a smaller furnace consistent with lower heating requirements
Use modulating furnace which combines a modulating gas vavle and variable speed fan. The furnace gas vavle operates between 40% and 100% of total capacity.
use 2-stage furnace, which, while running is the first stage, uses about 68% of its capacity and is sufficient to warm a house on milder winter days.
The most important factor for thermal comfort is the construction of the envelope - the degree of thermal insulation, the selection of windows and doors, and the control of air leakage.
Heat loss calculations take into account the fact that
HEAT is lost mainly by the BUILDING ENVELOPE and AIR LEAKAGE
VENTILATION PRINCIPLES
Ventilation involves 4 main principles:
Indoor-outdoor exchange: the process of removing stale indoor air and replacing it with fresh air from outside.
Treatment: Air filtering, heating or cooling.
Distribution: Air delivery to each room
Circulation: Air mixing within each room
Backdrafting is a dangerous condition that occurs when a house is depressurized, resulting in combustion gases being drawn back into living space rather than being expelled via chimney and flues.
It can be avoided by:
Using applianmces that are less likely to backdraft such as direct- or power-vented furnaces and hot water tanks.
Provide make-up air for appliances that EXHAUST AIR (example: bathroom fan, range hood, clothes dryer, central vacuum, open full burn fireplaces.)
ENERGY SOURCE SELECTION
Combustion Air
When house envelopes were less airtight than they are today, air from combustion and dilution came from random crack and openings in the house envelope, which wasted energy. The modern well-sealed envelope requires a dedicated source for combustion air and dilution air.
Most new furnaces and boilers are sealed-combustion, direct-venting appliances to eliminate combustion spillage. Dedicated combustion air supply is usually not required if an appliance is power-vented.
Make-up air
Is required in house where exhaust appliances induce a negative pressure that might lead to the backdrafting and spillage of combustion products.