Systems Flashcards
Skin-load dominant building
Buildings that have a lot of surface area compared to interior space-buildings that will be influenced by exterior conditions
Internal load dominated buildings
Minimal surface area compared to volume - generate a lot of heat - influenced by people, lighting and equipment -typically sheds heat year round
Factory, hospital, office buildings
Solar heat gain coefficient ( Shgc)
The fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward
Between 0 and I
O = less solar/radiant heat goes through (higher performance) (low=0.2 →> 0.4)
1 = 1/8” untreated plate glass la lot of heat transfer)
SHG through my window/ SHG through 1/8” glass
Solar insolation
Radiant energy per sf
British thermal units (btus)
The heat required to move 1 pound of water 1º
212°- steam
How many BTUS to go from 212° boiling to 212° steam? 1061 BTUS (changing state liquid → gas)
How many BTus to go from 72° to 212°? 140 BTUS
Do you sweat more when It’s humid or dry?
Dry - because the air evaporates the skin faster so it doesn’t feel sweaty
Humid air can’t hold anymore moisture so it can’t evaporate it (think like the air is a sponge or glass of water)
Air temperature and humidity interaction
Psychrometry-
Relative humidity
% Of moisture in the air at a particular temperature
What spills over from relative humanity to a lower temperature is dew, condensation, clouds - cooler air can’t hold any more moisture
Absolute humidity =humidity ratio
Warm our can hold more moisture than cool air can
When things evaporate, it makes you feel cold
Enthalpy
Total heat in the air
Sensible heat ( air temp) + latent heat ( moisture level)
Wet bulb temp is a good indication of total heat
Low - e (emissivity) glass
Helps prevent radiant heat
I is like no glass at all
O is like there is an opaque object
Low-e glass boasts a low SHGC
Balance point temperature
65°
. Heating or cooling degree days
If it’ cooler outside, we heat, IF its warmer, we cool
Degree days
The number of days per year and the differences in degrees from the baseline (balance point) temperature
Heating degree days (winter calculation)
Helps plan if we need high efficiency windows _ creates a relationship between high efficiency and low energy - more effective
Cold winter = higher HDDs
More than 5500 HDDs is long and cold
Less than 2200 HDD s is mild
Cooling degree days (summer calculation)
A hot or long summer will have more cooling degree days
Conductivity (k)
The rate at which heat passes through a specified material
The measure of efficiency of conduction of a material
If heat moves quickly, conductor
If heat moves slowly, insulator
Resistivity (r)
1/k = r
The rate at which a material resists the transfer of heat
High resistivity doesn’t change much when heat is applied
We generally want high resistivity because we don’t want materials exchanging heat - especially in cold or temperate climates
Conductance (c)
Homogenous materials of any given thickness or for a heterogeneous materials with known thermal properties (cmu block)
Higher number means heat transfers more quickly
C= 1 / R
Inverse of resistance (R)
Resistance (R)
Homogenous materials of any given thickness or for heterogeneous materials with known thermal properties
Higher number means heat moves slower
R =1 / C
Inverse of conductance (C)
heat takes 2x longer to go through thicker material = 1/2 as efficient transfer
U -value
The measure of the overall ability of a series of conductive and connective barriers to transfer heat
U = 1 / RI + R2+ R 3…
Thermal transmittance of an assembly
Conduction
Heat exchange between two surfaces that are in contact ( your elbow and the desk )
Q (BTU / hr )=u (u-value) x A (area )x delta T (temp change)
I F we double the u-value, what happens to the rate of heat exchange across the assembly?
It will double the heat exchange ( Q)
If we double the A (area) what happens to Q?
It will double the heat exchange
If we cut delta T by 50% what happens to the rate of heat exchange through the assembly ?
It reduces heat exchange by 50%
Air change per hour ( ACH )
Intentional ventilation- pull air in through HVAC to keep fresh air (internal load dominated buildings require more)
Unintentional - infiltration- cracks that allow air exchange (typically enough for homes)
0.9 ACH means 90% of air refreshes per hour