HIGH-PERFORMANCE HVAC
FOCUS ON O
GLAZING
ne morning I was oddly excited because when I looked at my weather station out on the deck the temperature was -21ºC and that happened to be 3ºC below the “design day” temperature listed in the National Building Code for the city I live in. As a geeky HVAC enthusiast, I imagined I might be getting some calls that day from builders or homeowners who were uncomfortably cold or simply had concerns that the “furnace was running all the time.”
I prepared myself by putting on an extra sweater and by noon I was in the great room of a beautiful model home of a large builder, helping a small group of their sales team learn about the technical features of the homes. Within a short period of time, I was uncomfortably warm. The great room featured a nine-ft. ceiling with beautiful, large garden door assemblies that happened to face due south. On this cold sunny day the great room was quickly overheating due specifically to the radiant gains through that expanse of glass. While I appreciated the fact these windows were able to contribute valuable energy in the winter, I did the math as to what that heat gain would do to cooling loads.
DOING THE MATH The math starts with the amazing power of our sun. The power per unit area of electromagnetic radiation received from the sun is called the solar irradiance. On a clear day, at sea level, the maximum normal surface irradiance is approximately 1000 W/m2 (317 Btus/hr/ft2); think of a one kW baseboard heater on every square meter of your house or yard. The impact of that power source varies depending on at least the elevation, angle, shading, and the colour and material type of the surface it illuminates. The power of the sun that illuminates the window glass is commonly referred to in codes and standards as the incident solar radiation or solar incidence. It again is expressed in watts per square metre and it varies as a function of the orientation of the window, any shading of the window and the slope of the window. This radiant gain is separate from the conduction heat flow through the glass at any given time. The conductive heat flow is primarily a function of the difference in air temperature between outdoors and indoors. In doing the math for that large great room window for the worst-case summer condition, we have to consider both the conductive and radiant flow.
Gord Cooke
70
M e c h a n i c a l
Glazing gurus tell me that comfort complaints due to glazing tend be more prevalent in rooms where the window area in the room is greater than 15 per cent of the floor area of that room. This may be a quick test for contractors to use in determining the cause and solution of comfort challenges.
The formulas for heat flow are well described in the CAN/CSA F280-12 Determining the Required Capacity of Residential Space Heating and Cooling Appliances Standard. The conductive flow is given by the common heat loss/heat gain formula; Heat flow equals the area of the window times the temperature difference across the window divided by the insulation value of the window, usually expressed as the RSI value (R-value if Imperial values are used).
HF = Area x Temperature difference/RSI OR Area x Temperature difference x U-Factor The radiant heat flow equals the area of the window times the incident solar radiation times the solar heat gain coefficient of the window.
HF = Area x solar radiation x solar heat gain coefficient (SHGC) The CSA F280-12 Standard provides a table of incident solar radiation values to be used across Canada. It should be of interest that the values are effectively the same across Canada except for a small latitude correction for south, southeast and southwest facing windows. If windows are well shaded with overhangs, trees or adjacent buildings, the solar radiation for north can be used in any orientation.
Gord is a professional engineer who has spent 35 years helping builders and HVAC contractors implement innovative technologies into high-performance homes. He has particular expertise in IAQ and airflow management in houses, and can be contacted at gordc@buildingknowledge.ca.
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