Outdoor Design Flashcards
Describe operational peak conditions.
Heating and cooling equipment is sized to meet energy load requirements for a certain number of hours during the year. This is because they only occur for a few hours every year.
How should heating and cooling equipment be sized?
To optimize comfort, costs, operational effectivenes, and efficiency.
What is the cost improperly sized heating and cooling equipment?
- higher cost and opperates less efficiently, whereas undersized equipment does not meet the required comfort level for the homes.
what is a 99 percent design temperature for a given location?
- the outdoor temperature is above the design temperature for 99 percent of all the hours in the year.
- this means that the heating system inside a building or home does nto meet the thermostat set-point temperature for a few hours.
- therefore, a home’s thermal inertia helps absorb the effects of many short peak weather conditions.
How can you achieve higher efficiency ?
Systems with modulating or multi-stage capacities should be selected to meet partial heating loads since equipment rarely runs at full capacity.
How many hours in a year on average would the outdoor temperature colder that the design temperature for 99 percent winter design?
88 hours ( 1 percent of the year).
Describe an example of maximum heating efficiency.
When equipment is operating at full load, and therefore higher effiency and lower losses.
eg. a conventional non-condensing boiler has an efficiency of 82 percent at 90 percent load and only 46 percent at 10 percent load.
What is winter outdoor design temperature?
The temperature below which heating equipment cannot meet indoor temperature set-points.
Define heating load.
Heating load is the power (kW or Btu/h) required to maintain indoor temperature set-points associated with given outdoor conditions.
It’s also used to describe heat loss as sum of:
- heat transmitted through walls, ceilings, floors, glass etc; and
- the heat required to temper outdoor air entering a structure throug infiltration and ventilation.
Important: a calculated heat load does not need to include the impact of a structure’s thermal mass
During coldest hours of the night, peak heat loss is considered equivalent to peak heat load.
what is transmission heat loss?
- conducton through the envelope is calculated by adding up the transmission heat losses through the various envelop components (walls, floor, roof, etc). Each componentets heat loss is estimated by using the heat transfer coefficient U [W/m2degreeC or BTU/hft2degrees Fahrenheit] and area (m2 or ft2)
equation: q=U x A z (T in - T out)
what is air infiltration?
discontinuity in the air barrier system ie. poor installation, through cracks around doors, windows, lighting and lecrical fixtures, joints between walls and the floor, and to a lesser extent through building materials.
what effects amount of infiltration?
deoends on total area, types of cracks and pressure difference on each side of cracks, wich in turn depends on wind speeds, indoor-outdoor air pressure differeces and outdoor temperature.
remember: lower outdoor temperatures cause higher infiltration due to stack effects.
How is infiltration rate calculated?
Q = (ACH) V / C subset t
Q - infiltration rate, cfm or m3/s
ACH = number of air changes per hour
V = gross space volume, ft3 or m3
Csubset t - constant; 60 imperial units and 3600 SI units
Define sensible heat loss.
the heat required to bring outdoor air to indoor ambient conditions.
How is sensible heat loss calculated?
q subset s = Q/Vo (Cp)(T indoor - T outdoor)
Q = infiltration rate, cfm or m3/s subsets = sensible heat loss, Btu/h or W Cp = specific heat of air at room temperature and atmosphere is (SI) 1.01 (kJ/kg K) or 0.24 Btu/(ibmdegrees F) Vo = specific volume
Define cooling load.
Cooling load is the power [kW or Btu/h) required to maintain the indoor temperature set-points associated with given outdoor conditions.
the rate at which energy needs to be removed from a space to maintain the temperature and humidity within an acceptable range.
Why is cooling used?
- Mitigate heat gains transmitted through surfaces and solar heat gains.
- Cool and dry outdoor air entering a home (infiltration and ventilation)
What is the difference between latent and sensible heat?
Sensible heat: amount of energy needed to change temperature, independent of a phase change (such as, gas to liquid). It’s what registers on your thermostat. It reflects a temperature change.
Latent Heat: is the heat that results from an increase or decrease in the amount of moisture held by the air. Specifically, it’s the amount of energy needed to cause a phase change (liquid to gas; gas to liquid).
Remember that humidity itself isn’t latent heat, but it contains latent heat.
***the main culprit is air infiltration or latent heat
Two components of heat gain?
Sensible and latent heat
Cooling equipment provides?
dehumidification by condensing water vapour out of the air.
Cooling load versus heat gains?
CL differs from instantaneous or transient heat gains because of transient effects of the time delay caused by convection effects, radiative effects and heat storage in the furnishings and building.
THis thermal lag defines the relatinship between the heat gain and cooling load.
Because there are limited hours at maximum heat gain during the day time, even if the thermal mass is lower, it is reasonable to assume that the peak cooling load is smaller than the peak heat gain.
What do cooling load claclulations need to take into account?
- solar heat gains via fenestrations
- Heat transfer through the building envelope
- Heat and moisture from outdoor air by means of natural infiltration and mechanical ventilation.
- heat generated by appliances, equipment and lighting
- Heat and moisture generated by occupants and activities.
Designs to reduce the building loads?
- Proper orientation and positioning of windows
- exterior shading and permanent structure of proper overhang
- Increased reflectivity of roof/outdoor surfaces
- Increased air tightness
- Proper ventilation rate
- use efficient heat recovery ventilators and energy ventilators
- proper attic venting
- Highly efficient building envelopes
Define power.
Power is the rate at which energy is produced, consumed or transmitted.
