1. Load Estimating Fundamentals

2. Definition
Proper design of space heating , air conditioning, or refrigeration systems, and other industrial applications require a knowledge of thermal insulation and the thermal behavior of building structures

3. Basic Concepts
Target : proper sizing of the HVAC equipment and evaluation of energy consumption

4. Typical AHU components: 1 - Supply duct 2 - Fan compartment 3 - Flexible connection 4 - Heating and/or cooling coil 5 - Filter compartment 6 - Return and fresh air duct

11. Load Estimating Fundamentals
100% outside air means no recirculation of return air through the (AHU)
Conventional all-air-handling systems for commercial and institutional buildings have approximately 10-40% outside air
An air – handling unit that provides 100% outside air is typically called makeup air unit (MAU)
To size properly the heating and cooling load , the gain and loss energy within the conditioned space must be considered

12. Gain and Loss Energy Within
the Conditioned Space
Ventilation: intentional introduction of air from outside to inside 9natural or forced)
Infiltration: uncontrolled flow of outdoor air into building through cracks and other unintentional openings
Heat Transfer gain or loss by conduction and convection through walls , roofs windows and floors
Solar gain through windows
Internal heat gain from people , equipment and light

13. The U Value
The U-value indicates the rate at which heat flows through a specific material or a building section
The smaller the U-value , the better the insulating value of the material or group of materials making up the wall, ceiling or floor

14. The U-value is the reciprocal of the thermal resistance:

15.  The U is computed by adding up all of the R-values , including those of inside and outside air films, the air gap, and all building materials

16. Overall U Value The overall wall U-value , Uwall is : Where :
Rcwl=L/k; K m2/W

17. The total heat transfer is :
U: W/K m2

18. Heating Load Calculation
Compute for worst case scenario
Pre-dawn at outdoor design dry bulb temperature
Do not include:
Radiation from sun
Heat gain from people , lights and equipment
Infiltration in non-residential buildings
Ventilation in residential buildings

19. General Procedure
To calculate a design heating load , prepare the following information about building design and weather data at design condition.
Select outdoor design weather conditions (temperature , wind direction and wind speed)
Select the indoor air temperature to be maintained in each space and humidity level of the return air , if a humidifier is to be installed, during design weather conditions

20. Estimate the temperatures in adjacent unheated spaces
Select or compute the heat transfer coefficient for outside wall and glass; for inside walls, non-basement floors, and ceiling , if these are next to unheated spaces, and for the roof if it is next to heated space

21. Determine the net area of outside wall, glass and roof next to heated spaces , as well as any cold walls , floors or ceiling next to unheated spaces
Compute the heat transmission losses for each king of wall , glass , floor, ceiling and roof in the building by multiplying the overall heat transfer coefficient in each case by the area of the surface and the temperature difference

22. Compute heat losses from grade-level slab floors using the heat loss rate per unit length of exposed perimeter
Compute the energy associated with infiltration of cold air around outside doors, windows, porous building materials and other openings

23. 9. Finally, sum the coincidental transmission losses or heat transmitted through the confining walls , floor , ceiling, glass and other surfaces, and the energy associated with cold air entering by infiltration and /or the ventilation air , to obtain the total heating load

24. Example Problem
Calculate the heating load of the building

26. An office in Mumbai, is of  size 20 ft x 40 ft, with a ceiling height of 12 ft. A false ceiling is proposed at 10 ft height. One 40 ft long wall  faces SouthWest, and one 20 ft long wall faces SouthEast. (Both are exposed to sun. Walls which are not, are considered partitions). The other two walls are partitions with non-airconditioned offices next to them. (20ft x 12ft)+(40ft x 12ft )=720 sqft) (Toilet, staircase and passage walls are also partitions). There are 2 windows on the SouthWest  wall, each of size 4 ft x 6 ft. The roof is exposed to the sun or floor above is non air-conditioned. (20ft x 40ft = 800 sqft) The floor below is non- airconditioned. (20ft x 40ft = 800 sqft). Lighting load is taken as 1.5 watts per sqft. There are 12 persons to be seated in the office. There are 12 PCs / terminals, one for each person. There is 1 server. There is a 5 KVA UPS for emergency. All the above values have been plugged into the example heat load for your convenience. Just click the solve button at the lower end of the Example Heat Load page, and read the results. The percentage distribution of the load is also calculated, to enable you to determine where energy could best be saved.

27. Windows & Heat Loss
Windows are very different from insulation in walls and ceilings
Windows let the light in and allow people to see out, and they interact with their environment in ways that insulation does not.
They react to outside air temperatures, sunlight, and wind, as well as indoor air temperatures and occupant use.
Windows are strongly affected by solar radiation and the airflow around them. R-value does not accurately reflect this interaction
Therefore, the energy efficiency of windows is measured in terms of thermal transmission, or U-factor.
U-factor measures the rate of heat transfer through a product.
Therefore, the lower the U-factor, the lower the amount of heat loss, and the better a product is at insulating a building

28. What’s the Difference between U-factor and R-value?
The biggest difference between U-factor and R-value is that U-factor measures the rate of heat transfer (or loss) while R-value measures the resistance to heat loss.
R-value is a measure of conductivity.

29. U-Factor
U-Factor measures how well a product prevents heat from escaping a home or building.
U-Factor ratings generally fall between 1.13 and The lower the U-Factor, the better a product is at keeping heat in.
U-Factor is particularly important during the winter heating season.
1 h·ft²·°F/Btu = K·m²/W, or 1 K·m²/W = h·ft²·°F/Btu

30. Solar Heat Gain Coefficient (SHGC)
Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat from the sun. SHGC is expressed as a number between 0 and 1.
The lower the SHGC, the better a product is at blocking unwanted heat gain. Blocking solar heat gain is particularly important during the summer cooling season.

31. Visible Transmittance
(VT) measures how much light comes through a product.
VT is expressed as a number between 0 and 1.
The higher the VT, the higher the potential for day lighting.

32. Air Leakage (AL)
Air Leakage (AL) measures how much outside air comes into a home or building through a product.
AL rates typically fall in a range between 0.1 and 0.3 (US UNITS).
The lower the AL, the better a product is at keeping air out.
AL is an optional rating, and manufacturers can choose not to include it on their labels.

33. Condensation Resistance (CR)
Condensation Resistance (CR) measures how well a product resists the formation of condensation.
CR is expressed as a number between 1 and 100.
The higher the number, the better a product is able to resist condensation.
CR is an optional rating, and manufacturers can choose not to include it on their NFRC labels.

34. Example How to calculate the heat loss for a structure
To calculate the size of heater's) required to heat a structure, we need to know:
The temperature to be maintained within the structure.
The lowest ambient (outside temperature) which can be expected for the area.
The direct heat loss from the overall surface area of the structure.
Heat loss through natural or mechanical ventilation.
The difference between the ambient and internal temperatures gives the temperature lift required.
Temperature lift = Internal temp. - ambient temp.
The heat loss for a structure is calculated by taking each surface in turn, calculating its overall area and multiplying by its thermal transmittance co-efficient or ‘U - value’

35. 'U' values for commonly used building materials
'U' values for commonly used building materials. Normal exposure conditions. (W/m2deg.C).
Basic materials
Glass
5.7
Corrugated Asbestos
Sheet Asbestos
6.5
Wood 25mm
5.0
Brick (unplastered) 228mm
2.6
Concrete 100mm solid
4.0
Earth floor
1.9
Concrete floor
0.7
Windows
Single glazed wood
4.3
Single glazed metal
5.6
Double glazed wood
2.5
Double glazed metal
3.2
Composite materials
Insulation block 140mm rendered both sides
1.1
Corrugated double cladding with 25mm glassfibre over polythene vapour barrier
1.4
70mm extruded polystyrene  bonded to 6mm fibre cement each side
0.43
40mm glassfibre over polyethylene vapour barrier and 3mm fibre cement
0.5
60mm glassfibre over polyethylene vapour barrier and 3mm fibre cement
0.4
140mm thick concrete block rendered outside, lined inside with timber battens, 40mm extruded polystyrene and flat fibre cement sheet
0.44
Plywood exterior cladding, 40mm extruded polystyrene, fibre lining
0.46

36. Heat loss through surface = width(m) x length(m) x U value
Note the area of windows and doors should be calculated and deducted from the area of the surface they are in, and their heat loss should be calculated separately.
The total surface heat loss for the structure is the sum of all surface heat losses.
Total surface heat loss = loss for walls + loss for roof +   loss for floor + loss for windows + loss for doors
An allowance should be made for heat loss through ventilation, which also includes leaks of air through badly fitting doors, windows, damage to the structures surface etc
Putting a value on this can be very difficult, figures range from 20% to 66% depending on the type and condition of the structure.
Total heat loss = total surface heat loss x allowance for heat loss through ventilation
Finally to calculate the heater size needed, the total heat loss is multiplied by the temperature lift.
Heater size required = total heat loss x temperature lift

37. Example heat loss calculation (see diagram)
Temperature lift = 20 - (-5) = 25 deg. C Area of roof = 2 x 5.09 x 30 = m2 Area of walls = (2x2x30) + 10(2+3) = 180 m2 Area of floor = 30 x 10 = 300 m2 Heat loss through roof = x 5.7 = 1740 W/oC Heat loss through walls = 180 x 2.6 = 468 W/oC
Heat loss through floor = 300 x 0.7 = 210 W/oC Total surface heat loss = 2418 W/oC Assuming 20% for heat loss through ventilation Total heat loss = 1.2 x 2418 = 2901 W/oC Heater size required = total heat loss x temperature lift                = 2901 x 25 = W In this example kW Activair Ace electric fan heaters , kW Activair portable heaters or 4- 21kW Activair wall mounted electric heaters are needed. Calculating heater running costs per hour

38. Calculating the running cost per hour for an electric heater is straightforward. Electricity is sold by the unit (kWh), multiply this by the heater size in kW.
Running cost per hour = heater size (kW) x unit cost of electricity (kWh)
Estimating annual heater running costs
Annual running costs will be greatly affected by physical and geographical location and prevailing weather conditions from year to year.

39. Estimated annual energy requirement (kWh)= total heat loss x x figure from degree table/
Finally to calculate the annual running cost multiply the estimated annual energy requirement by the unit cost of your electricity.
Annual running cost = annual energy requirement x unit cost of electricity

40. HEAT LOSS BY INFILTRATION
On an average, a reasonably tight house will have one air change per hour.
A radiant floor heating system reduces drafts and the factor 0.7 air/hour may be used.
Heat loss by infiltration:
Imperial: ft3 x Btu/ft3 x (tindoor - toutdoor) oF x air change / h = Btu/h
Metric: m3 x 0.35 W/m3 x (tindoor - toutdoor) oC x air change / h = Watts

41. Design Heating Load
The amount of heated air, or heating capacity, to be supplied by a heating system
It is usually the maximum amount to be delivered based on a specified number of heating degree days or design outside temperature.