BTU Requirements

The first and most critical step in the designing of a geothermal heating and cooling system is to determine the BTU requirements of the building.

BTUs are a measure of how much heat is required to heat a home to a specified design temperature. This number tells the designer the size of the system they must design and build to heat and cool a specific building.

When sizing a building for geothermal, the BTU requirements are expressed in tons. One ton equals 12,000 BTU’s. So a building requiring 48,000 BTU’s would require a 4 ton loop field to supply the needed BTU’s.

Retrofit Buildings

It is difficult to determine the BTU requirement of an existing building because  most of the information needed is buried under the finished walls and ceilings.SAM_0607 Therefore it takes a great deal of detective work and experience to accurately determine the BTU requirements of a home or commercial building.

The BTU requirement in retrofit homes vary greatly depending on numerous factors including the amount of insulation, types and amount of windows, direction the house is facing, the number of external walls, the amount of air penetration into the house, and the outside environment around the house including shade and exposure to wind. In short, the “tighter” the house is, the less Btu’s that are required to heat it. The “looser” the house is, the more Btu’s required to heat it.

Rule of Thumb

As a rule of thumb, a building will require between 12 to 20 Btu’s per square foot. The amount varies by floor – basements the least BTU’s and upper stories the most. But this rule of thumb is almost always wrong because every home performs differently in terms of heat loss and gain. And sizing a geothermal system using these numbers is a recipe for disaster.

Computer Modeling & Measuring

Most reputable designers will model the home or buildings existing equipment using loop field design software. This software generates an analysis of the performance of the existing equipment to determine fuel and energy usage. Think of this like the factory rated gas mileage on a new car.

You then measure the Btu usage of a home by calculating the homes actual fuel usage over a period of three years. This is the equivalent of taking that used car out for a spin and seeing how much gas it actually uses.

The designer can now compare to the actual energy used (measuring) over the last three years, with the projected usage (modeling) of the computer software. The computer software allows the Btu load to be adjusted up and down until the measurements and the models are delivering the same fuel consumption.

The measuring and modeling process utilizes science and computer analysis to consistently develop high performing and quality geoexchange systems. But even this process requires a designer to draw on their experience to interpret and adjust the data for each situation.

Eyeballing

An experienced installer can also identify issues simply by examining the building and applying their past experience. For example, the following issues were taken into accountCUSTOMER HOME CROPPED when designing a system for this house:

  • Additional wall space – The unique design of this home increases the amount of total square footage of external walls. External walls are a major source of heat loss/gain. Therefore this house will require more Btu’s that a traditional square box shaped home with the same square footage.
  • Exposure – The home faces south west into a large open field. This will dramatically increase the heat gain of the house during the day time hours when the sun is pounding down on the building.
  • Windows – The large number of windows on the SW side will increase heat loss and gain even more. This will results in a significant temperature swing as this portion of the house “cooks” in the sun during the day and gets cold at night.

This home will require significantly more Btu’s than a traditional two story home. And its peak loads will be much greater due to the large amount of heat loss and gains from the windows and walls

If the system is designed to meet the the peak demands of the SW corner of the home it will be significantly larger than what is required for the rest of the home. Therefore, the design had to make some trade offs in performance in order to design a system that will work for the entire building.

New Construction

Determining BTU requirements in new construction is accomplished using a process call a Manual J Calculation. The Manual J determines the amount of heat the house loses in winter and gains in summer.New Home Cropped

The procedure can be done room-by-room to determine how much conditioned air, in cubic feet per minute (CFM), each room needs for both heating and cooling. It factors in all the surfaces of the building envelope, with their areas and insulation levels. Each wall is given its proper orientation, as well as the windows and doors attached to them. Additional important data is the location and tightness of the duct system, the infiltration rate of the house, the internal loads (appliances and people), and area where the house is located.