HVACR416 - Design Heat Loss / Heat Gain Part 2. External Loads The greatest external load is the sun. The suns heat can get into a building in one of.

HVACR416 - Design

Heat Loss / Heat Gain Part 2

External Loads

• The greatest external load is the sun.

• The suns heat can get into a building in one of two ways.

o Through glass

o Through walls and the roof.

External Loads

• Solar heat through glass is absorbed instantaneously in the room. This is in addition to the conducted heat passed by the glass.

• Solar heat heats up the walls and the roof and then heat is conducted into the room.

External Loads

• Depending on the type of construction there is usually a time lag of from 2 to 10 hours before this heat reaches the room.

• This means that the heat may be pouring into a space after the sun has gone down.

External Load

• The exact amount of heat gain from either source depends upon the area, the direction faced by each wall.

• It also depends on the shading and the type and color of surface exposed to the sun.

External Load

• The external load also depends on the position of the sun. This position is determined by:

o The season of the year

o The time of the day

o The latitude where the building is located.

Solar Effects

North

South

West East

BTU/hr

6am 12 noon 6pm

Glass Facing East

Solar Effects

• Glass on the east side of the house faces the sun as it is rising.

• It would be expected that the sun rising in the east causes the solar heat to be very high at about 8am and then drops off at noon through the afternoon.

Solar Effects

North

South

West East

BTU/hr

6am 12 noon 6pm

Glass Facing South

Solar Effects

• Glass facing south shows the greatest load at noon and is lower before and after noon.

Solar Effects

North

South

West East

BTU/hr

6am 12 noon 6pm

Glass Facing West

Solar Effects

• Glass on the west side of the house faces the sun as it is setting.

• It would be expected that the sun setting in the west causes the solar heat to be very high at between 12pm and then drops off as the sun sets.

• The sun reaches its peak at about 4pm.

Solar Effects

• North facing glass and glass shaded all day gets very little solar gain, only what is reflected by small dust particles in the air.

• This is a very small amount compared to the direct solar gain.

Solar Gain

• Understanding these load curves can often help you in making better decisions on the equipment and building construction.

• Sometimes in new construction facing a window another direction or providing shading can reduce the size and the cost of cooling.

Solar Gain

• Additionally there are times, for example at a car dealership, where the building owner may be willing to have slightly higher costs in the morning in exchange for more comfort in the afternoon.

• The sun can also help provide heat in the winter months.

Ways to help solar gain

• If complete outside shading of glass is impossible there are a few other methods to reduce solar gain.

o An outside awning will turn away 75% of the solar heat.

o Inside roller shades will reduce solar heat by 35%.

Ways to help solar gain

• Solar heat can also be reduced by using glass other than ordinary window glass.

o Heat absorbing glass (high E glass) will reduce by 25%

o Double pane windows – 10-20%

o Stained glass – 30-65% depending on colors used.

Windows – Final Note

• It is important to remember the effects of solar gains on windows as they are the largest portion of heat gain in a building.

• Whenever possible it is important for the mechanical design team to be involved as early as possible in the construction cycle.

Walls and Roofs

• The sun heats up the outside surface and then this heat gradually works its way into the building.

• Since the outside surface temperature due to the sun is higher that the air temperature, not all of the heat goes inside, but some goes into the outside air.

Walls and roofs

• The effect of the type of construction and other factors on this solar gain can be looked at by studying several examples.

Wood Frame Roof

BTU/hr

6am 12 noon 6pm 8pm

Wood Frame Roof

• The wood frame roof is a light wood construction which peaks at about 2pm in the afternoon with a heat transfer rate of 18 btu’s/hr.

4” Concrete Roof

BTU/hr

6am 12 noon 6pm 8pm

4” Concrete Roof

• The 4” concrete roof peaks at about 4pm with 19 btu’s/hr.

• Still better, but not great.

6” Concrete Roof

BTU/hr

6am 12 noon 6pm 8pm

6” Concrete Roof

• The 6” concrete roof peaks between 4 and 6pm with 15 btu’s/hr.

• Notice the building is not moving, but it is the amount of time it takes for the sun to heat up the roof and the heat to work its way into the building.

Roofs

• There is very little difference in the amount of heat flow through un-insulated roofs except for peak time.

• The heavier the roof it peaks later and has a smaller amount of heat transfer.

Roofs

• Adding insulation will reduce heat transfer by 50% with one inch.

• Two inches reduces heat transfer by around 2/3.

• Three inches reduces heat transfer by about 3/4.

Roofs

• Heat transfer can also be reduced by spraying water on a roof or by covering the roof in water.

• A completely shaded roof results in a 75% reduction from the peak sunlit roof load.

Walls

• The effect of sun beating down on the walls are very similar to the roof.

• However another factor is involved, the direction the wall is facing.

• The roof and walls will both cool off at night.

Walls

• The time the heat starts getting inside depends on when the sun starts hitting the wall.

• The east wall heats up first, then the south, then the west, and then the north. Just like in the windows.

Walls

• More heat will get through the west wall than any of the others.

• This is due to the outside temperature being at its highest at the same time the sun is beating down on it.

• Heavy commercial west wall construction may also allow the most heat in between 8pm and 10pm at night.

• The effect of solar heat through walls is small compared to that of the roof and glass.

External Loads

• The other source of external loads is that of infiltration or ventilation.

• Remember ventilation is controllable.

• Infiltration is uncontrolled without building modifications or changes of pressure.

Outside Air

• Outside air is brought into a building by an outside air duct or by depending on infiltration around doors and windows.

• Whichever way this air gets in it must be cooled and de-humidified in the summer to bring air to the room conditions.

Outside Air Ventilation

• Bringing in outside air for ventilation is one of the true functions of air conditioning.

• Outside air can dilute a number of pollutants in the inside air and make a sick building healthy.

• Outside air can also be used to change the pressure of a building.