Calculating Radiated Heat Loss in Dwellings, Ovens and Other Structures - A Quick and Dirty Method
Bill Venn Hot Flash Thermography
ABSTRACT
The need for a quick method to calculate Infrared Heat Loss has been underemphasized in this field. Too many times, the effort has been to calculate actual temperatures at the area where hot or cold impingements occur. After this is done, the thermographer will make calculations for convectional or conduction heat loss while less emphasis is on the actual Apparent radiated heat losses. This paper will present arguments for the need for these calculations to be included and will provide a formula for the construction of templates to easily quantify these losses in the Reporter 7 program.
INTRODUCTION
The first and most important condition for this calculation is the ‘apparent’ temperature of the energy lost calculation. Many different building materials are used in the construction of structures and normally these materials are medium to highly emissive. However for the purposes of this paper the apparent temperature is only needed for the calculation. Therefore, the Emissivity on the camera or thermograph will always be set at
1.00 E. The actual temperature is not valid.
Often thermographers will use an air temperature reading to make calculations for radiated heat loss. However air is only able to transmit infrared. This tool allows the thermographer to use the nearest reference temperature of an object in infrared to enable the radiated power to be calculated. Thermometers are very unreliable for measuring temperature as they are not calibrated to the same accuracy as the camera. As well, radiated temperatures are much different than air temperature and it is radiated temperatures which are of interest.
It is argued that the building must be at stasis when the thermograph is taken. The validity of the calculation will be compromised in the event that the building is artificially pressurized to make the thermograph. The building will already have a natural pressure gradient which the thermographer can use. This is shown in Figure 1.
Wind Direction
Area of Maximum Outdoor Air Pressure
Area of Lowest Indoor Air Pressure
Area of gradual increased Air Pressure
Area Of Maximum Indoor Air Pressure
Area of Lowest Outdoor Air Pressure
Figure 1. Pressure gradient in a building
The difference in pressure for the interior of the building will only be measurable in tenths of an inch (28 inches water column equals one pound per square inch).
Field of View is an important part in the use of these templates. The thermograph must show the warmest and coolest non-energy generating terrestrial reference points (using the sky as a reference invalidates the calculation as would a furnace or air conditioning A-coil).
CALCULATIONS AND TEMPLATES
The templates have been built to allow for different building calculations. Both of these templates use the Stefan-Boltzmann formula to calculate radiated power.
W=σT4 (1)
Where:
W = power lost {watts per square meter} σ= Stefan-Boltzmann constant = .000000057 Watts/K4m2 T= Temperature in Kelvin
Since heat travels from warm to cold, an equation can be made to calculate power lost.
Wpower lost = σ(T4warm - T4cool) (2)
Since the calculation is in Kelvin where absolute zero equals – 273.15 degrees Celsius: W power lost = σ [ (273 + twarm)4 – (273 + tcool)4 ] (3) Where: t = temperature in Celsius Two templates were then built to allow the quick generation of a report.
EXTERIOR WALL CALCULATED RADIATED HEAT LOSS
This is an estimate of heat loss from radiation on an exterior wall. This calculation will be affected by the size of warm area and cold spot, convective cooling (wind) on the surface being measured, changing temperature from day to day as well as other factors. The actual heat loss could be SUBSTANTIALLY higher.
Figure 2. Exterior wall calculated radiated heat loss in watts per square meter lost to radiation is in the box marked Fo1.
Object Parameters Value Emissivity 1.00 Cold Spot Temperature 7.4 °C Warm Area Average 27.0 °C Temperature Fo1 109.5
This template is used to calculate warm walls. These are generally the lateral or leeward walls of buildings in winter conditions. It can also be used as the template for interior walls on the windward side of a building in summer with air conditioning. The general application of this template is appropriate. There should be a minimum of 10 degrees Celsius temperature differential between the interior and the exterior of the subject building.
This template is used for interior walls of the windward side of a building in winter or an exterior wall of an air conditioned building.
INTERIOR WALL CALCULATED RADIATED HEAT LOSS
This is an estimate of heat loss from radiation on an exterior wall. This calculation will be affected by the size of cold area and warm spot, convective cooling (wind), changing temperature from day to day as well as other factors. The actual heat loss could be SUBSTANTIALLY higher.
70.0 °C
60
40
20
3.9
Object Parameter Value
Emissivity 1.00
Label Value
Warm Spot 59.2 °C
Cold Area: Average 7.4 °C
Fo1 342.0
CASE STUDY 1
The owner of this dwelling had renovated to change a seasonal residence into a year round home. A bathroom was added to the south facing portion of the structure where there was a covered porch. Insulation was installed in the new walls but the porch roof had been covered over and had been mistakenly omitted when the insulation had been in installed. The owner could not determine why the bathroom was cold in a south wind. The wind was from the north and this is the lee side.
Exterior Wall Calculated Radiated Heat Loss
This is an estimate of heat loss from radiation on an exterior wall. This calculation will be affected by the size of warm area and cold spot, convective cooling (wind) on the surface being measured, changing temperature from day to day as well as other factors. The actual heat loss could be SUBSTANTIALLY higher.
3.4 °C
2
0
-2
-2.9
Object Parameters Value
Emissivity 1.00
Cold Spot Temperature -2.7 °C
Warm Area Average 0.4 °C
Temperature
Fo1 14.3
The area box highlights the area of escaping heat and has heated the exterior wall. This has caused infrared radiation which can be used to calculate radiated heat loss. The area measures at 2 square meters so the loss of heat at this point is 2*14.3 = 28.6 watts. The cold spot has been placed on the coldest point in the thermograph which is vegetation in front of the home. Note the temperature of the cold spot. The thermometer read -1 degree C.
CASE STUDY 2
The owner of this home has an indoor swimming pool directly behind the wall section shown. The wall is cinderblock and has no insulation on the interior surface; R value for the cinder block wall is 1.1. The owner has turned the pool heater off in the winter due to the high costs of heating the pool. However there is no insulated wall between the pool room and the rest of the house. Heat from the occupied part of the house heats the pool room. The pool room temperature was 2.5 degrees Celsius. There was a stiff north breeze which cooled the room down. The thermograph was taken from the south side.
Exterior Wall Calculated Radiated Heat Loss
This is an estimate of heat loss from radiation on an exterior wall. This calculation will be affected by the size of warm area and cold spot, convective cooling (wind) on the surface being measured, changing temperature from day to day as well as other factors. The actual heat loss could be SUBSTANTIALLY higher.
Figure 5. The calculated wattage in watts per square meter lost to radiation is in the box marked Fo1.
Object Parameters Value Emissivity 1.00 Cold Spot Temperature -18.7 °C Warm Area Average -11.7 °C Temperature Fo1 27.4
The measured warm area of this thermograph is 22 square meters. Therefore the total heat lost at the temperature of the day (indicated by the cold spot) is 22*27.4 = 602.8 watts.
CASE STUDY 3
In this case study the cool wall template is used. The room in this thermograph had repairs made to electrical wiring in the ceiling and to make the repairs, the fiberglass batt insulation was removed to facilitate the repair. The insulation was replaced over the electrical wiring, leaving an airspace at the ceiling which compromised the building envelope and allowed the impingement of cold air. This thermograph was taken inside the house.
Interior Wall Calculated Radiated Heat Loss
This is an estimate of heat loss from radiation on an exterior wall. This calculation will be affected by the size of cold area and warm spot, convective cooling (wind), changing temperature from day to day as well as other factors. The actual heat loss could be SUBSTANTIALLY higher.
Figure 6. The calculated wattage in watts per square meter lost to radiation is in the box marked Fo1.
Object Parameters Value Emissivity 1.00 Warm Spot Temperature 17.7 °C Cold Area Average 14.4 °C Temperature Fo1 17.9
This area is about 2 square meters so the radiated heat loss is 36 watts.
CONCLUSION
The use of these templates will allow the thermographer to quickly report to a client, the power lost by impingement on the building envelope. The accuracy of the calculation will be greatly enhanced by the use of the camera to read both radiated temperatures needed to calculate radiated heat lost. The thermographer can generate a Rapid Report to facilitate the findings.
REFERENCES
- FLIR Systems Thermacam Reporter 7 (2004)
- Oak Ridge National Laboratories web site
- Hi-Mark Industrial Industrial Training Manual (2007)
ACKNOWLEDGEMENTS
Thanks to Rob Milner at FLIR Canada for helping to make the template load into Rapid Report. The group at Hi-Mark is an excellent resource for many home heating and building dynamic information. The Oak Ridge facility has an excellent web site which covers many studies it has done at the behest of the United States Government in building and window insulation in conduction, convection and radiation.
ABOUT THE AUTHOR
Bill Venn is a retired industrial maintenance electrician from General Motors. The GM facility at Oshawa produces one million cars and pick-up trucks per year and is the largest facility with the highest quality rating of any manufacturer in North America. His work history includes almost every department in the giant Oshawa facility including stamping, plastics injection, the many paint shops, welding, sub and final assemblies and all the automated systems used in the manufacturing process. He was one of the many thermographers at the facility during his last three years of employment and has amassed hundreds of hours of time behind a thermal camera in an industrial environment in electrical, mechanical systems as well as natural gas and steam heating applications. As part of his energy management duties for the energy management initiative at the truck plant paint shop, he authored a thermal study rejecting the use of a new composite polymer insulation which was to be applied to the exterior stainless steel housing of paint ovens. The insulation was an almost perfect radiator. Upon retirement he started his company Hot Flash Thermography. Bill is an Electrician, Industrial Maintenance Technician (natural gas) and a Level 1 Thermographer.