What Is Heat Load Calculations ? Basic Design Fundamentals For Heat Load Calculations ?(Also with Excel Sheet)

 

The heat load calculations is one of the most important part of the HVAC designer or HVAC consultant.HVAC design impacts a home’s construction costs, comfort, air quality, durability, and energy efficiency, and load calculations will dictate the size of the HVAC system needed in a home.

And when your Figuring a house’s load it actually depends on a number of variables, and a building or room gains heat from many sources. Inside occupants, computers, copiers, machinery, and lighting all produce heat. Warm air from outside enters through open doors and windows, or as ‘leakage’ though the structure. However the biggest source of heat is solar radiation from the sun, beating down on the roof and walls, and pouring through the windows, heating internal surfaces.

The sum of all these heat sources is know as the heat gain (or heat load) of the building, and is expressed either in BTU (British Thermal Units) or Kw (Kilowatts).

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Basic Design Fundamentals For Heat Load Calculations :-

Orientation :-

• East 
• West 
• North
• South
• North East 
• North West
• South East 
• South West.

Inputs required :-
Indoor Conditions.
Outdoor Conditions.

Partition :-
It divides the heating/Cooling space from non- heating/cooling space.

• BOB - Bottom of Beam.
• BOS - Bottom of Slab.
• BOR - Bottom of Roof.
• BOFe - Bottom of False Ceiling.
• Open to sky 
• Air well 
• Shaft 
• Service duct – all the above are same with different names. 

Types of heat sources :-

What are the types of heat sources into the conditioned space? 

Out door and Indoor

Out door:-

• Fresh Air 
• Wall 
• Roof 
• Windows 
• Doors

Indoor :- 

• Electrical Appliances – Light etc., 
• Yellow Bulb – Consume energy in the following ways 
• 10% Light, 10% Conduction & Convection, 80% Radiation 
• Tube Light or Florescent Bulbs – 
• 25% Light, 25% Conduction and Convection and 50% Radiation 
• Equipments Inside – Xerox, Printers, Computers, Fax machines, etc 

Human (And his activity) :-

Q = AxTDxU

 Where 

Q = Heat
A = Area in Sqft
TD = Temperature Difference
U = Transmission of coefficient
U is inversely proportional to R (Resistance)

  

Walls Construction :-

• Light 40 - 60 lbs/sqft > 4” - 6” thick 
• Medium 80 -100 lbs/sqft > 8” - 10” thick 
• Heavy 120 lbs/sqft > 12” thick.

Roof :-

• Shaded Area 
• Water Stagnate 
• Spray Pound 
• Exposed to sun

People :-

• Room temperature 
• Activity 

Watt/Sqft :-

• Residence – 1.25 watts
• Office - 2.0 watts
• Show room – 3.0 watts
• Operation theater – 5.0 watts 
• Watt = 3.41 Btu/h

Heat load calculations :-

There are several different methods of calculating the heat load for a given area:

Quick calculation for offices :-

For offices with average insulation and lighting, 2/3 occupants and 3/4 personal computers and a photocopier, the following calculations will suffice:

Heat load (BTU) = Length (ft.) x Width (ft.) x Height (ft.) x 4
Heat load (BTU) = Length (m) x Width (m) x Height (m) x 141

For every additional occupant add 500 BTU.
If there are any additional significant sources of heat, for instance floor to ceiling south facing windows, or equipment that produces lots of heat, the above method will underestimate the heat load. In which case the following method should be used instead.

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