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Survey of the Residential Building for Heat Load Calculations

written by: Haresh Khemani • edited by: Lamar Stonecypher • updated: 10/17/2009

Once you have understood the various sources of heat in the residential building, we have to now carry out the survey of the building for heat load calculations.

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    Survey of the Building or Room

    Once you have understood the various sources of heat in the residential building, we have to now carry out the survey of the building for heat load calculations. The survey of the building involves measuring various dimensions of the building, orientation of the building, and applying various factors to calculate the heat load calculations. With the help of survey one can also decide the best air conditioning system suitable for the building and also the methods to install it.

    For the survey of the building the accompanying heat load calculation form has to be used. To carry out the heat load calculations you have to determine all the parameters mentioned in the form, fill them in the form and carry out various calculations, which will eventually help you find the total heat load of a single room. Let us see the various parameters of the building to be surveyed as mentioned in the heat load calculations form. The discussions that follow will explain you all the parts of the form, the survey to be carried out and how to fill the form. For each of the rooms separate heat load calculations form has to be filled. For your convenience the heat load calculations has been attached below, please refer it for all further reading and calculations.

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    Heat Load Calculations Form

    Heat Load Calculation Form
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    Topmost Portion of the Heat Load Calculations Form

    In topmost part of the heat load calculations form you have to fill the basic details like name of the customer, their address, the person who buys the air conditioning system, the person who will install the air conditioning system, estimate number, heat load estimated by, and date. After carrying our heat load calculations you can also fill the details of proposed equipment manufacturer, model, and size of the machine. After the survey of the building you will also be able to fill the details of the direction of the house or room, total floor area for which the heat load calculations are done, and the inside volume of the space that has been surveyed.

    After these basic details, you have to fill the design conditions for the room. These are very important and will have a major impact on the total tonnage of the air conditioning system required for the room. In the design conditions space you must note down the outside and desired inside dry bulb and wet bulb temperatures. The difference between the two, as mentioned in the form, has also to be noted.

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    Orientation of the Room and Various Dimensions

    Now you can come to the lower part of the heat load calculations form and start the survey of the room. For this you should have with you two instruments: the magnetic compass and the measuring tape. The magnetic compass will help you determine the orientation of the room including the directions of the all the walls and the windows. The measuring tape will help you determine all the dimensions of the all the walls, partitions, windows, floor, roof, ceiling etc. If you are ready with these instruments let us carry out the following measurements:

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    In topmost part of the heat load calculations form you have to fill the basic details like name of the customer, their address, the person who buys the air conditioning system, the person who will install the air conditioning system, estimate number, heat load estimated by, and date. After carrying our heat load calculations you can also fill the details of proposed equipment manufacturer, model, and size of the machine. After the survey of the building you will also be able to fill the details of the direction of the house or room, total floor area for which the heat load calculations are done, and the inside volume of the space that has been surveyed.
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    1) Solar Heat Gained by the Windows

    Please refer the heat load calculations form above.

    Firstly, find the direction of all the windows using magnetic compass. In the accompanying heat load calculations form, you will notice six directions of the windows: northeast, east, southeast etc. Separate north direction has not been mentioned since it is assumed that heat absorbed by the windows in north direction is very small. For all the windows in all the directions measure the dimension of windows and fill the details in appropriate column. For instance, if there is a window of 24 sq ft in north east direction, fill the details for North East in the “Item" column the area of 24 in the “Area" column. Similarly, fill the areas for all the windows located in all the directions.

    Next, select the proper factor for each of the windows from “Factor" column and round the appropriate factor. For example, for the northeast direction window if there is external awning round off the factor 20 associated with outside awning. Reduce this factor by appropriate margin for glass block windows, storm windows or windows with double glass and find corrected factor. In a similar manner, round off the associated factors for all the windows in all the directions and apply correction factor wherever necessary.

    Next, multiply the area of each window with the corrected factor associated with it. These details are filled in the “Area x Factor" column. The maximum value of this column has to be filled in the last column of BTU/HR that indicates the solar heat gained by the windows.

    Next, you have to find out the solar heat gained by the windows due to designed dry bulb temperature difference between outside and inside. For this firstly, find the total area of all the windows and fill them in the area column. Find out if the windows are of single glass or double glass and fill the details in the appropriate areas column. Now, round off the proper factor for the designed dry bulb temperature difference. For example, if the desired dry bulb temperature difference between the atmosphere and the room is 17F the associated factor would be 22 for single glass window and 10 for double glass window. Multiply the total area of the window by this rounded factor and fill the result in the last column that indicates the total BTU/HR gained by the windows.

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    Solar Heat Gained by the Walls

    To find the heat gained by the wall find the total area of all the walls of the room (excluding partitions). Find out if walls are non-insulated or insulated and the thickness of insulation if any. Accordingly, fill the details in the areas column for the appropriate type of wall. Now round off the associated factor with it for the designed dry bulb temperature difference. Multiply the total area of the wall with this factor and fill the result in the last column of BTU/HR that indicates the total heat gained by the walls per hour.

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    To the subtotal of heat gained by the room add additional 30% of the subtotal to account for the latent heat inside the room. This will give the total heat load inside the room. The air conditioner of suitable tonnage that can remove all the total heat gained by the room should be selected. The suitable manufacturer and best possible model of the air conditioner can also be recommended to the customer. This ends the total process of heat load calculations using the ready-made form.
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    Heat Gained by the Partitions

    In a similar manner, find the total area of the partitions, round off the factor associated with it, multiply the two and fill the result in the last column to find the total BTU absorbed by the partition per hour. Only the partitions that are connected to the non-air conditioned rooms are to be considered.

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    Solar Heat Gained by the Roof

    To find the total heat gained by the roof, find out the total area of the roof and check if it is insulated or non-insulated. Find the thickness of insulation if it is insulated. Now round off the associated factor. Multiply the total area with this factor and fill the result in the last column to find the total BTU of heat gained by roof per hour.

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    Heat Gained by the Ceiling and Floor

    By the same procedure as explained for the roof, find the heat gained by the ceiling and the floor.

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    Heat Gained by the Room Air from the Outside Air or Infiltrated Air

    The total outside air or infiltrated air that enters the room has been linked with the total floor area of the room. Find the total floor area of the room and multiply it with the associated factor that gives the total BTU gained by room from the outside air.

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    Heat Gained by Room Air from the People

    To take into account the total heat gained by the room air form the people inside the room, find the average number of people that will stay in the room most of the time. Multiply it by 200 as mentioned in the form and put the result in the last column for total heat gained from the people per hour.

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    Subtotal of Heat Gained by the Room Air

    The subtotal gives the total heat gained by the room air from the windows, walls, partitions, roofs, ceiling, floor, outside air and people inside the room.

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    Total Heat Load Inside the Room and Total Tonnage of AC Required

    To the subtotal of heat gained by the room add additional 30% of the subtotal to account for the latent heat inside the room. This will give the total heat load inside the room. The air conditioner of suitable tonnage that can remove all the total heat gained by the room should be selected. The suitable manufacturer and best possible model of the air conditioner can also be recommended to the customer. This ends the total process of heat load calculations using the ready-made form.

    In the next article we have shown the sample heat load calculations using the above form.

How to Do Residential Heat Load Calculations

This is the series of articles that explains how to perform heat load calculations for a residential building or other smaller buildings in easy manner by using the ready-made form.
  1. Residential Heat Load Estimate Form: Heat Load Calculations Made Easy
  2. Survey of the Residential Building for Heat Load Calculations
  3. Example of Residential Heat Load Estimate/ Calculations

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