Reducing Home Thermal Heat Loss

Well the first thing you need to consider regarding heat loss is

Every building loses heat (it also gains heat but we'll get to Heat Gain later). Thermal heat loss occurs mainly via Conduction and Air infiltration.

The percentage of thermal heat loss from a building is also dependent on the 'form' (the shape) of the building. The more aspects the more thermal heat loss.

If you are thinking about building a new house then it is important to know that simplicity in 'form' will save you heaps of cash in energy running cost.

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Well it's like this. The more surface area your building has the more heat it will lose.

Example

If you had two buildings with the same area in floor space, one is a two storey design with only four external walls. The other a single storey with eight external walls. Even though the two storey building may have a little more wall area the single storey is more complex in design having more surface area as well as corners means it losses heat at a greater rate and therefore is more energy demanding.

Remember this, architects, designers and builders generally deal with thermal heat loss when the design is complete. You can always get better doors and windows, thicker insulation to cope with a complex design, but it cost more. A simple form means reduction in both build and running cost.

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Yes you can. If you wanted a precise reading of thermal heat loss then you would need to have a lot of information based upon temperature differences internally and externally (Heating Degree days). The 'Thermal Transmittance Coefficient' or "U" Value and the 'Thermal Resistance Coefficient' or "R" Value of all the material that you would be using in your build and furnishings, and a raft of other factors, which is quite a lot.

So to have an approximate idea you can use a rule of thumb method (the value this produces is overestimated). Calculate the room volume in cubic meters, then divide by 20 to get the heat required in kilowatts. So a room that measures 10 meters in length, 5 meters in width, and 2 meters high will require approximately 5 kilowatts of heat. ((10x5x2)/20) = 5 kilowatts.

Note: The temperature inside of an unheated building is usually around 2.78^oC higher than the outside air temperature, so when the outside air temperature is at 15.6^oC the inside temperature will be 18.3^oC and this is close to the average internal design temperature. The 'Degree Day' is the difference between 15.6^oC and the daily 'mean' outside temperature when it is below 15.6^oC.

However, if you prefer to be warmer use 18 instead of 15.6^oC as the base temperature.

For example, if the mean outside temperature on a particular day was 8^oC then the number of 'degree Days for that day would be; 15.6 - 8 = 7.6, or 18 - 8 = 10 (Degree Days).

Celsius to Fahrenheit conversions
Enter Celsius
Value in Fahrenheit
provided by metric conversions

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Yes.The thermal heat loss calculations for underfloor heating is slightly different in that there is a 1^oC reduction in internal air temperature to reflect the added value of radiant heat, however no reduction in temperature is made in bathroom areas.

Adjustments are also made to the volume of the room, e.g, the calculated height of the room for heating is around 2 meters with underfloor heating systems to reflect our actual living space and not the ceiling height (why spend money heating the ceiling a space we do not use).

Underfloor heating creates weak convection currents this enables lower air change rates to be used in the thermal heat loss calculations.

Underfloor heating does not produce the same convective gradients of heat associated with natural and forced air convection systems as it warms you from feet up, where you like it the most. Who wants a hot head?

If you would like a heat loss calculator software from 'Slant/Fin' to work out your own thermal heat loss - Click here to check for availability.

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All buildings need to be ventilated to remove moisture. However, air changes means thermal heat loss. Cold air is sucked into buildings by the warmer air inside. This process reduces the internal temperature.

So to raise the temperature of 1 cubic meter of air through 1^oC in 1 second will require approximately 1340 watts. If you use this air multiplying factor 0.33 you can work out the approximate ventilation heat loss yourself.

The "U" Value make it possible to calculate the amount of heat which can be conducted through the various layers of materials in the building structure. It takes into consideration the thickness of structure and the resistance to heat flow, caused by the film of air on the surface of the structure.

The "U" Value is the amount of heat which will flow in 1 second through a particular piece of structure for every 1K (Kelvin degree) temperature difference. "U" = W/m²K

The amount of heat loss through a section of the structure such as a wall is obtained from;

area m² x temp rise ^oC x "U" value W/m²K = Heat Loss(W) watts

Example

A room has an external wall 5m x 3m. it has a 280mm brick wall with an unventilated cavity, and it faces east. The "U" value is 1.8W/m²K. the temperature difference is from -1^oC outside to 18^oC inside. 18 - (-1) = 19. Area of wall is 5m x 3m = 15m²

15 x 19 x 1.8 = 513 watts

This is a simplified example of heat loss by conduction through a wall as it does not take into account the heat losses from the other building fabric such as:

• Floors
• Ceilings and Roofs
• Windows and Doors

Nor what type of floor finish, whether tiled or carpeted.

Well as I mentioned above there are a raft of factors and data that you would need to know to preform a precise thermal heat loss calculation. However, the rule of thumb method - volume divided by 20 will give you an approximate guide and that it is 'only a guide' and not an exact calculation of heat loss.