HVAC HEATING COOLING VENTILATION
Human Comfort Zone As humans we try to maintain a body temperature of 98.6° F • Three Mechanisms • Heat generated within the body • Heat gained from surroundings • Heat lost to surroundings
Human Comfort Zone We shiver to generate heat
HumanComfort Zone We sweat to Give off heat
Human Comfort Zone We get goose bumps
Human Comfort Zone Blood Flow • Decreases to hands and feet in winter • Increase in summer to encourage heat loss
Thermal Neutrality To be comfortable humans must loose heat at the same rate as it is produced or gained.
Factors Affecting Human Comfort • Air temperature • Air Speed • Humidity • Mean radiant temperature Each has a direct influence on heat loss or gain to the human body
Factors Affecting Human Comfort • Air Temperature - This affects temperature differences between the body and the surroundings, consequently affecting the rate of heat loss or gain by convection.
Factors Affecting Human Comfort Air Speed - This affects the rate at which the body loses heat by convection. • An air temperature of 35°F and a wind speed of 20 miles/hour combine to give a wind chill temperature of 11.2°F. • Air speed is also very important during summer when the body is trying to lose heat to maintain comfort.
Factors Affecting Human Comfort Humidity - Affects the rate at which the body loses heat by evaporation. During hot weather, high humidity increases discomfort by making it more difficult to evaporate perspiration into the air.
Mean Radiant Temperature • Mean Radiant Temperature' (MRT). This is defined as the temperature of a sphere at the point in question which would exchange no net radiation with the environment.
Factors Affecting Human Comfort Mean Radiant Temperature (MRT) - MRT is the average surface temperature of the surroundings with which the body can exchange heat by radiant transfer. Radiant heat transfer to and from the body is quite apparent when sitting near a fireplace (high MRT) or large cold window area (low MRT).
Mean Radiant Temperature In general for every 1 degree F that the MRT drops, the air temperature must be raised about 1.4 degrees F to achieve comfort conditions. How can you raise the MRT? • Close blinds and curtains • Solar Film on windows • Seal heat leaks
Comfort • Comfort is achieved by either increasing the ambient temperature or by raising the mean radiant temperature of an environment. • A higher radiant temperature means that people become comfortable with a lower ambient temperature and the reverse is also true.
Example 1 • Dry Bulb 73° • Relative Humidity 50%
Example 2 • Dry Bulb Temp. 78° • Relative Humidity 70%
Example 2 • Dry Bulb Temp. 78° • Relative Humidity 70% • Requires a wind speed of 250 FPM (250*60)/5280 MPH = 2.84
Example 3 • Dry Bulb Temp. = 50°F • Relative Humidity 55%
Example 3 • Dry Bulb Temp. = 50°F • Relative Humidity 55% BTU/Hour = 250
Definitions • Conduction A method by which heat is transferred from a warmer substance to a cooler substance by molecular collisions. Direct contact. • Convection A method by which heat is transferred by currents in a liquid or gas. • Radiation A method by which heat can be transferred through objects and empty space. Electromagnetic.
Conduction Examples • Liquid - Liquid - Pouring cold cream into coffee • Liquid - Gas - Ocean and Atmosphere • Gas - Gas – Cold and warm weather systems mixing • Solid - Solid – Touch a hot pot on a stove
Conduction Rate Factors • Contact Area • Type of Material Cast Iron vs Stainless Steel • Temperature Difference • Distance heat must travel
Convection Examples • In a closed room cool air will settle to the bottom while warm air will rise • Bowl of soup – Hot liquid in the center moves to the cooler outside where it drops and is reheated at the center and the cycle continues. • Warm air rising through a heat register
Radiation Examples • The sun’s heat • A bonfire • Warm soil on a cool night
Radiation Rate Factors • Surface area • Type of material • Temperature difference
More Radiation Terms • Reflectance (or reflectivity) refers to the fraction of incoming radiant energy that is reflected from the surface. Reflectivity and emissivity are related and a low emittance is indicative of a highly reflective surface. • For example, aluminum with an emittance of 0.03 has a reflectance of 0.97.
More Radiation Terms • Emittance (or emissivity), refers to the ability of a material’s surface to give off radiant energy. All materials have emissivities ranging from zero to one. The lower the emittance of a material, the lower the heat radiated from its surface.
R-Value • R-Value is the measure of resistance to heat flow through the defined material. The higher the R-Value the less heat will transfer through the wall, making the system more energy efficient. • U-Value –is the reciprocal of the R-Value (1/R) and is a measure of the rate of heat loss
WINDOWS - 4 Ways to Evaluate • U-FACTOR • Solar Heat Gain Coefficient • Visible Transmittance • Air Leakage
U-FACTOR U-FACTOR The rate of heat loss is indicated in terms of the U-Factor of a window assembly. The insulating value is indicated by the R-Value which is the inverse of the U-Value. The lower the U-Value the greater a windows resistance to heat flow and the better the insulating value.
Solar Heat Gain COEFFICIENT The SHGC is the fraction of incident solar radiation admitted through a window. SHGC is expressed as a number between 0 and 1. The lower a windows solar heat gain coefficient, the less solar heat it transmits.
VISIBLE TRANSMITTANCE The visible transmittance is an optical property that indicates the amount of visible light transmitted. Theoretical values vary between 0 and 1, but most values are between 0.3 and 0.8
Air Leakage Heat loss and gain occur by infiltration through cracks in the window assembly. Air leakage is expressed in cubic feet of air passing through a square foot of window area. .3 is recommended for Oregon
Low-E Windows • Glass is coated with silver or tin oxide which allows visible light to pass through but reflects infrared heat radiation back into the room. • Reduces heat loss • Allows visible light to pass through but reflects infrared heat radiation away from the room • Reduces heat gain
High number for cold climate. Low number for warm climates The lower the number the better the insulating value Varies from 0 to 1.0 The higher the # the more light is transmitted. The best windows have air leakage rating between 0.1 and 0.6 cfm/ft.
Double-Glazed with High-Solar-Gain Low-E Glass, Argon/Krypton Gas
Triple-Glazed** with Moderate-Solar-Gain Low-E Glass, Argon/Krypton Gas
Ventilation • Multi Point Fan Systems • One fan located in the attic • Connects to baths and kitchen • Timed to run at high speed during high use times such as morning (showers, bacon ) and evening. • Xvent
Heat Recovery Ventilation How it works • In the heating season the core transfers heat from the outgoing, stale household air to preheat the incoming, fresh air. • Cross-current sections, ensure the two air streams are always kept separate preventing the incoming fresh air from being contaminated by the outgoing stale air.
Heat Recovery Ventilation • During the air-conditioning season, the HRV reverses this process, removing some of the heat from the incoming air and transferring it to the outgoing air.