Thermal mass, building shell air volume, and heat losses over time

In summary, Dan has created a spreadsheet calculator that analyzes temperature changes over time in a building, taking into account factors such as building fabric and ventilation heat losses. He has derived the available heat energy at a given starting temperature and calculated the losses over one hour, deducting them from the total energy. He then uses a simple ratio to calculate the change in internal temperature, but has realized that this may not be accurate due to the thermal time constant of the thermal mass. He is unsure of how to incorporate this into the calculations and is seeking help.
  • #1
dansphere
3
0
Hello,

I'm looking for some feedback on and an analysis of a spreadsheet calculator I've made.

It looks at temperature changes over time, given starting volumes of a building shell and thermal mass. The calc takes into account building fabric and ventilation heat losses.

First I've derived the available heat energy at a given starting temperature and used the specific heat capacities of the air and thermal mass to calculate the total available heat energy at 20 degrees Centigrade relative to absolute zero. Does this make sense?

Then I've calculated the losses over one hour and deducted that from the total energy, and done this recursively for several hours.

The change in internal temperature I'm deriving from a simple ratio.. ((heat energy lost/total heat energy) * original temperature in Kelvin). Although what I've realized is that the energy lost is only translatable to a temperature change via the specific heat capacities of the air and thermal mass, is this correct? Or can it be done via a ratio?

Attached is the spreadsheet I've made.
 

Attachments

  • heat loss from building shell and thermal mass.xlsx.zip
    63.5 KB · Views: 201
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  • #2
I've realized that the calculator is going to be very inaccurate due to the Thermal Time Constant of the thermal mass..
I'm not sure how to include the thermal time constant into the calculations (the calculations of which I'm thinking would best be done through some sort of exponential function.. eek I don't have a clue) Any help on this would be greatly appreciated :)

Dan
 

1. What is thermal mass and how does it affect heat losses in a building?

Thermal mass refers to the ability of a material to absorb and store heat. In buildings, materials such as concrete, brick, and stone have high thermal mass, while materials like wood and insulation have low thermal mass. When these materials are exposed to heat, they absorb and store it, which can help regulate the temperature inside a building. This can reduce heat losses over time as the stored heat is slowly released, making the building more energy efficient.

2. How does the building shell air volume impact heat losses?

The building shell air volume refers to the total amount of air contained within a building's walls, roof, and floor. The larger the air volume, the more heat can be stored and transferred. This means that a building with a larger air volume will generally have lower heat losses as the air can act as a buffer to regulate temperature fluctuations. However, if the building is not properly insulated, a larger air volume can also lead to higher heat losses.

3. What are some ways to maximize thermal mass and reduce heat losses in a building?

One way to maximize thermal mass and reduce heat losses in a building is to use materials with high thermal mass, such as concrete or brick, in the building's construction. Additionally, incorporating passive solar design principles, such as strategically placing windows and using materials with high thermal mass in areas that receive direct sunlight, can also help increase thermal mass and reduce heat losses. Proper insulation and air sealing are also essential in minimizing heat losses.

4. How do heat losses change over time in a building?

Heat losses in a building can change over time due to a variety of factors. These may include changes in outdoor temperature, changes in the building's occupancy and usage, and changes in the building's insulation and air sealing. Additionally, as a building ages, the materials used in its construction may also experience wear and tear, which can affect the building's thermal performance and lead to changes in heat losses over time.

5. How can thermal mass and building shell air volume be measured and monitored in a building?

Thermal mass and building shell air volume can be measured and monitored using various techniques, such as energy audits, thermal imaging, and data logging. These methods can help identify areas of high and low thermal mass, as well as areas where heat losses may be occurring. Regular monitoring and maintenance can help ensure that a building's thermal performance remains optimal and can help identify any potential issues that may arise over time.

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