Presentation on theme: "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."— Presentation transcript:
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.
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 suns 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 materials 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.
Emissivity or Emittance Material SurfaceEmittance Asphalt0.90 - 0.98 Aluminum foil0.03 – 0.05 Brick0.93 Fiberglass0.80 – 0.90+ Glass0.95 Steel0.12 Wood0.90
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 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 The best windows have air leakage rating between 0.1 and 0.6 cfm/ft. Varies from 0 to 1.0 The higher the # the more light is transmitted.
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. –XventXvent
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.
Ventilation Heat Recovery System - uses fans to maintain a low-velocity flow of fresh outdoor air into the building (incoming air stream) while exhausting out an equal amount of stale indoor air (exhaust air stream). Fresh air is supplied to all levels of the building while stale air is removed from areas with high levels of pollutants and moisture.
Ventilation Heat Recovery System Air Exchange - Expels stale, polluted indoor air and gaseous pollutants and continually exchanges them with a continuous flow of fresh, revitalized outdoor air to improve Indoor Air Quality.
Ventilation Heat Recovery System Excess Humidity Control - Helps prevent uncontrolled excess humidity by expelling excess humidity from the air, thereby reducing the risk of window condensation, mildew and mold, which prevents structural damage and deterioration to your home.
Ventilation Heat Recovery System Heat Recovery Core - As warm air is expelled from your house, it warms the incoming cold, fresh air before its circulated throughout your home. The result is a constant supply of fresh air, no unpleasant drafts and greater home comfort.
Sized to ventilate the entire house at a minimum of.35 air changes per hour. Minimum CFM requirement can be calculated as follows Determine square footage and multiply times ceiling height. Divide by 60 minutes Multiply times.35 (minimum air changes)
HRV Calculation Example Determine square footage and multiply times ceiling height. Divide by 60 minutes Multiply times.35 (minimum air changes)
HRV Calculate the minimum CFM for a home with 2000SF main level, 1000SF second level and 750 SF finished basement Note: Main and second level have 9 foot ceilings and basement has 8 foot ceiling.
Solution 3000 SF x 9 = 27000 750 x 8 = 6000 Total 33000 33000/60 = 550.35 x 550 = 192.5 CFM
Radiant Floor Heat Three types Radiant Air Floors Electric Radiant Floors Hot Water (Hydronic)
Radiant Floor Heat Types of installation Wet Installations Large thermal mass of a concrete slab floor lightweight concrete over a wooden subfloor Dry Installations Where the installer "sandwiches" the radiant floor tubing between two layers of plywood or attaches the tubing under the finished floor or subfloor.
Radiant Floor Heat Air Heated Radiant Floors Not recommended for residential applications Electric Radiant Floors -