Thermal and HVAC Flashcards
Cooling/Heating Degree Days
The number of degrees that a day’s average temperature is above/below 65 degrees F, which is the temperature above/below which buildings need to be cooled/heated
Enthalpy
Total amount of sensible and latent heat in the air-moisture mixture. Use enthalpy line on psychometric chart to determine the amount of heat to be removed or added from conditioned air for thermal comfort.
Describe psychometric chart
Dry bulb - bottom Wet bulb - left Relative humidity - curves Enthalpy - far left Comfort zone in middle left Hot Arid - bottom right Hot Humid - top right
Conductance
(C) The rate at which heat passes through a heterogeneous material or material of any given size. If heat moves quickly, it is an conductor. If it moves slowly, it is an insulator. ex. CMU block (composite of air and concrete)
Resistance
(R) Inverse of conductance. 1/C = R
High resistance means it is a good insulator.
R = r x d (depth of material)
aka R Value of a material
U-Value
Thermal transmittance of a material. U = 1 / (R1 + R2 + R3 +R4…)
Conduction
Heat Exchange between two objects which are in contact (ex. elbow on a table: heat transfer is a function of U value of shirt sleeve, area of elbow and temperature difference between elbow and table)
Q (in BTU, rate of heat exhange per hour) = U value x Area x Temp difference
Outdoor Design Temperature
Worst case scenario temperature to design HVAC system around
R-Value
Resistance: # of hours needed for 1 BTU to pass through a material of a given thickness when the temperature differential is 1 degree F.
Air changes per hour
Number of times the air enters and exits a room from the HVAC system in one hour.
AC/Hr = (CFM x 60min) / Volume of room
4 ways the body can lose heat
convection, radiation, evaporation and conduction (must lose heat one way or the other ex. little evaporation happens at low temps, little convection or radiation happens at high temps)
Effective Temperature
(ET) combines the effects of air temperature, humidity and air movement (air movement increases evaporation and heat loss through convection)
Relative Humidity
Ratio of the percentage of moisture in the air to the maximum amount of moisture that the air can hold given a certain temperature without condensing. Cold air can hold less moisture than warm air so it has a higher RH. Comfortable range: 30-65%. Tolerable range: 20-70%.
See Amber video Enclosure 12.3
Emissivity
The measure of an object’s ability to absorb and then radiate heat. Shiny materials have low emissivity. High = better ability to radiate heat. White or light colored roofs have high emissivity,
Mean Radiant Temperature
(MRT) Weighted average of the various surface temperatures in a room and the angle of exposure of the occupant to these surfaces, as well as any sunlight present.
Mean Radiant Temperature
(MRT) Weighted average of the various surface temperatures in a room and the angle of exposure of the occupant to these surfaces, as well as any sunlight present. Important for thermal comfort in winter - rooms with cold surfaces will feel colder even if air temp is comfortable. Warming the surfaces and providing radiant heating panels will help.
Operative Temperature
Average of air temperature of a space and MRT of the space. Measured with black-globe thermometer which is a thermometer in a black globe. Used by athletes and the military to determine appropriate temperatures for exercising.
Operative Temperature
Average of air temperature of a space and MRT of the space. Measured with black-globe thermometer which is a thermometer in a black globe. Used by athletes and the military to determine appropriate temperatures for exercising.
Natural ventilation requirements
4% of floor area being ventilated
Mechanical ventilation requirements
Supply must be approximately equal to return (exceptions: hospitals and stair towers need positive pressure). IMC: Amount of outdoor air brought in depends on use, occupancy classification and occupant density (not the same occupancy classifications as IBC)
Thermal gradiant
Shows variance in temperature through a cross-section of a construction assembly. Helps determine where to plae vapor barrier to ensure it is on the warm side of the assembly (depending on the climate).
Infiltration equation
qv = V(1.08) delta T
1.08 accounts for the amount of heat that air at a certain density can hold
V= volumetric rate flow (air changes per hour)
T = temperature differential
qv = heat loss thru infiltration
Skin-load dominated building vs. internal load dominated building
Skin-loaded: Large surface area relative to volume. Thermal response heavily influenced by conditions outside. (ex. small house / hot arid climate uses thermal mass and shading to control temp / salt box house maximizes southern exposure in cold climates)
Internally-loaded: Large surface area relative to volume. Thermal response NOT heavily influenced by conditions outside. ex. skyscrapers, hospitals, mid-scale office buildings, theaters (when full), factories (equipment). Think of buildings with a lot of people, lighting and equipment relative to the surface area of the building. Only need to heat perimeter in coldest months.
SHGC
Solar heat gain coefficient. Low = Good (less heat transmitted) Range = 0-1
High: 0.7-0.9
Low: 0.2-0.3
For a skin-load dominated building you want at HIGH SHGC to warm the building in the winter.
Solar heat gain coefficient(SHGC) is the fraction ofsolarradiation admitted through a window, door, or skylight – either transmitted directly and/or absorbed, and subsequently released asheatinside a home. The lower theSHGC, the lesssolar heatit transmits and the greater its shading ability.
Solar Insolation
Radiant energy per SF of the sun
Solar Insolation
Radiant energy per SF of the sun
Albedo
Measure of a surface’s reflectivity of solar radiation.
0 = absorbs all radiation (black)
1 = reflects all solar radiation (white)
Where does vapor barrier get located
On warm side of insulation in cold climates.
Optimum angle of solar panels
Year round: same as latitude of site
Summer: latitude - 15 degrees
Winter latitude + 15 degrees
Why is temperature range in a desert so much greater than in a humid climate
Water vapor in air reflects heat during day and prevents it from leaving at night.
R value of air space
No shiny material: 1
One side shiny material: 3
Two side shiny material: 10
Sensible vs latent heat
Both removed with mechanical air conditioning
Centralized vs decentralized mechanical cores
Centralized: More energy efficient, easier to service, better control of air quality, quieter.
Decentralized: Economical to install, short distribution runs and allow each zone to have individual temperature control.
Factors to consider when selecting an HVAC System
- Performance
- Efficiency
- Initial and life cycle costs
- Access required for maintenance
- Construction requirements for enclosure
- Structure requirements for weight of equipment
- Degree of visibility
- Centralized or decentralized core
All air systems
Central fans circulate conditioned air to and from the spaces through long runs of ductwork. Great for controlling air quality, fresh air intake, filtration, humidification and temperature. Can use economizer function when it’s cool outside. Maintenance is concentrated on one area of the building with all pipes coming together in an unoccupied part of the building.
Air and water systems
Air is ducted to each space. Heated and chilled water are also piped to each space where they are used to modify the temperature of the circulated air to meet the local demands. This system circulates less air which makes them more compact and easier to fit in a building. More control in individual spaces but more maintenance.
All water systems
No circulated air. Most compact system. More control in individual spaces but more maintenance.
Central, all air, single duct VAV and variations
Most versatile and wisely used for large buildings. Great temperature control and moderate cost. Cannot heat one space and cool another at same time.
Variations:
- Use induction or hydronic convectors at perimeter when buildings have a lot of exterior glazing.
- With reheat system for better control of individual rooms
- With induction system. Small, high velocity ducts to move air but not control temperature well. Used in spaces with limited space and not a high demand for hearing/cooling.
- Dual duct system. Carries heat and cool to all parts of building side by side. Air is mixed for optimum control. Not energy efficient, expensive and space consuming.
Central, all air, single duct CAV and variations
One master thermostat controls temperature for whole building. Good for spaces with large open areas, few windows and uniform loads (lobbies, department stores, theaters, auditoriums and exhibition halls). Simple, easy to maintain. No individual zone control.
Variations:
1. Furnace
2. Reheat system: wasteful to cool air and reheat. Used in renovations of rooms that need precise control such as labs or OR’s.
3. Multizone system (sep ductwork runs to each zone from fan room)
Demand controlled ventilation (DCV) technology and best types of spaces to use it
Uses carbon dioxide sensor to increase or decrease ventilation of a space according to occupancy. Best used in spaces where the number of occupants varies greatly during open hours such as a bowling alley. Not good for spaces that must be constantly exhausted such as a locker room, pet store or dry cleaner.
Appropriate spacing for ceiling diffusers
Space them approximately the same distance apart as the room is high.
% of construction budget spent on HVAC: Restaurant Hospital Climate controlled warehouse Parking garage
Restaurant: 40
Hospital: 20
Climate controlled warehouse: 10
Parking garage: 0
Fan powered VAV System and where to use it
In a multi-zone building, you may need to heat the perimeter while you cool the core. The VAV with terminal reheat can accomplish this, but by adding a fan, you short-circuit the system and draw room-air back into the VAV with a fan to reheat the already warmed air. Fresh air is also still being supplied and some air is stil being returned to the fan room.
Kitchen exhaust
Room air is exhausted out and must be made up with intake air. Self-balancing vents bring in their own makeup air. CFM for hoods is found in charts that vary with size of range, BTU of range, size of kitchen.
HVAC dampers and types
Parallel blade: best for all open or all closed (sim to gate valve)
Opposed blade: best for subtly moderating (sim to globe valve)
HVAC filter types
Charcoal: Removes odors and chemicals from air. Resistance to air flow so they must be run at low air velocities. Must be replaced regularly.
Electrostatic: 2 charged plates attract dust. Expensive to install but don’t need to be replaced. Must be cleaned regularly.
Fiberous: Remove dust, dirt, mold. Must be replaced regularly.
Air film
Accounts for surface which is not perfectly smooth. Accounted for in U Value of a wall assembly.
Interior: R 0.70
Outside: R 0.10
Single duct VAV systems are more energy efficient than constant air volume systems because…
VAV’s use variable pitch blades or variable speed fans that allow air volume to be modulated from zero to the required demand
Vapor control barrier classes
Measured in perms Class I: <0.1 perm = very little moisture passes (plastics, glass, metals), polyethylene sheet, foil facing Class II: 0.1-1.0 perm = plywood, some latex paints, kraft paper, rigid foam insulation, closed cell spray foam Class III: 1-10 perms = gyp wall with latex paint No Class (vapor permeable): fibrous insulation, cellulose, house wrap, Ideal is to have class I surrounded by Class III, surrounded by vapor permeable to allow some vapor in and give it a place to escape (bell curve)
Ways vapor gets into buildings
- Vapor diffusion through materials
2. With air via infiltration through enclosure weak points
Which vapor barriers to use in different climates?
Climate 1,2,3,4a or 4b: no vapor barrier
Climate 4c, 5, 6,7 8:Use Class I or Class II
Works in all climates:
ventilated cladding
air gap
thermal control (rigid on its own, or mineral wool with air/vapor barrier behind it)
sheathing
structure (insulation not required for thermal)
interior finish
ERV
Energy Recovery Ventilator. Uses a heat exchanger (wheel or static plate) to transfer heat between indoors and outdoors and both supply and return. An energy transfer wheel transfers fresh air but not moisture in the summer, and fresh air and moisture in the winter. Moisture transfers from high humidity to low. A residential HRV (Heat Recovery Ventilator) does not control humidity, just fresh air.
Types of mechanical systems that conserve energy
Economizer cycle Boil fuel economizer Heat pipes Runaround coils Energy transfer wheels Ground source heat pump Dual condenser chillers
Minimum distance between fresh air and exhaust air
10’
