By Lew Harriman
Key Points
In current HVAC design practice, a computer will be used to help estimate cooling loads. This chapter is not a substitute for computer-powered calculations. Instead, we focus on a few of the designers’
basic assumptions about sensible loads and about the cooling systems which remove them. The designers’ assumptions, sometimes unstated and unrecognized, will always strongly influence the results from computer-assisted calculations, for better or for worse.
Suggestions and cautions for calculating sensible cooling loads in hot and humid climates include:
• First—Glass decisions rule the sensible cooling loads.
Calculate glass loads early, to benefit the owner by mini-mizing sensible cooling loads.
• Next—separate and calculate the dehumidification loads independently from sensible cooling loads, to avoid com-mon errors in HVAC design in humid climates.
• The peak outdoor temperature is not the worst case for the ventilation load. The highest total ventilation load (highest enthalpy) comes at part-load sensible cooling conditions, when the outdoor dew point reaches its peak.
• Peak ventilation loads can be greatly reduced with exhaust air heat exchangers.
• Use ASHRAE research to avoid the common mistake of overestimating “plug loads.”
Quantify glass-related loads early and often, to improve the architectural design
The solar load through glass—combined with its consequences for lighting energy and the heat generation from those lights—dominates the sensible cooling loads. The HVAC designer’s most influential (and very brief) opportunity for improving the building is when he or she informs the owner and architect of the dimension of those loads, along with their implications for energy consumption, thermal comfort and construction cost.
If the owner and architect want a low-energy building, the HVAC designer is in the best position to help them accomplish that goal before the building enclosure design is settled. The owner and architect seldom understand the fatal energy consequences of their glazing decisions. The HVAC designer can help them make more in-formed design decisions. An early cooling load calculation tells them the size of the cooling equipment for different glazing choices.
After the architectural decisions have been made concerning the size, shape, location, exterior shading and number of windows, the look of the building and the space-planning for each floor are basi-cally fixed. These will be very difficult for the owner to change. The marketing for the building strongly depends on its look-and-feel, and so do its regulatory approvals. Also, the architect will not gratefully embrace changes after the look-and-feel of the building has been established and agreed to by the owner and the regulatory authori-ties. The architect begins to lose money quickly when the building’s entire exterior and its interior space planning must be redesigned and re-approved because of different locations, aspect ratios, shading or percentage of glazing on exterior walls.
So the magic moment for improvement is at schematic design—
before design development and long before construction documents.
Early in schematic design of the building’s exterior, the HVAC design-er’s load calculations can make a real improvement. After the glazing decisions are finished (if those glazing decisions were poor ones) all the HVAC designer can do are the usual things—moan and groan,
Glass is an exceptionally bad insulator1, and also, even modern glass still lets in far too much radiant heat.
Excellent modern glass might now have a solar heat gain coef-ficient as low as 0.35. But that must be compared to an insulated wall at 0.0 solar heat gain. Zero percent of the solar heat is a lot less than 35% of the solar heat. Therefore, the more glass on the building, the more money and the more energy it will take to cool it. In other words, lots of glass surface on the building is bad, from a cooling load perspective.
The HVAC designer can make this fact clear to the owner and architectural designer through early load estimates using different percentages of glazing on each exterior wall.
After the amount of glass has been reduced to its minimum, the next issue is the thermal quality of the glass, and the amount of shad-ing which should be above that glass.
A first step is glass which has a low solar heat gain coefficient (SHGC). Then add shading to reduce the SHGC still further, as illus-trated in figure 12.2. The problem with specifying an extremely low SHGC for the glass (and leaving off the shading) is that such glass will be so dark that it may not be pleasant for occupants to look though. Or it may be so reflective that nearby drivers can occasion-ally be blinded by the reflected glare outdoors. So selecting glass with a moderately-low SHGC combined with a horizontal window overhang of more than one meter often provides a more congenial way to reach a low net SHGC.
Reduce the glazing towards 20% of the wall surface
The ASHRAE Advanced Energy Guides3,4,5,6 provide benchmark target values for glazing as a percent of the wall, and for the net solar heat gain coefficient of those windows, including shading. In hot and humid climates, the guides call for a target of 20 to 30% glazing on any wall. Also, the solar heat gain coefficient of those windows should be less than 0.31.
enhancing his reputation for being negative and uncreative, and then try to make the best of a bad situation, using a budget which won’t be adequate for all the equipment and controls needed to provide comfort, much less the mechanical space needed to service them.
That all-too-common situation can be avoided when the HVAC designer makes the decision to participate (positively, creatively and firmly) in the architectural design decisions at an early stage. At that point, quantification of the glazing loads can make the difference between a low energy building and a wasteful one.
Rough out the loads quickly to start the conversation along pro-ductive lines, informing and guiding the glazing decisions. Then take more time later, during the design development stage, to make the detailed load calculations based on actual details and specifications.
It’s difficult and uncomfortable for technical professionals to do this.
No engineer is comfortable guessing about the thermal characteristics of walls and windows before the architect has decided what they are, exactly, and defined them in detail. But that’s the point. At this stage, the architectural design is more flexible.
Early glazing decisions that make big differences in cooling loads
Some glazing decisions make a bigger difference than others. Here are several which come early in the schematic design of the building’s exterior. These usually have the greatest influence on the sensible cooling loads, for better or for worse.
A lot of glass is bad
Thermally, the best building is one with little glass instead of a lot.
And the reduced amount of glass must exclude most of the solar heat gain.
There have been major improvements in glass technology in recent years. Architectural designers have been especially impressed by these improvements. Many have designed buildings which are basically large glass boxes, under the apparent misimpression that lots of glass has somehow become a low-energy technology. It isn’t.
Fig. 12.2 SHGC and its effect on annual cooling load2
The lower the solar heat gain coefficient, the lower the annual cooling load. The HVAC designer can help improve the architectural design by quantifying this difference at an early stage, before the exterior fenestration has been “set in stone.”
With poor glazing (glass with a high SHGC) the radiant heat from hot window surfaces overheats the nearby occupants. In response, they turn down the thermostat. At lower temperatures, the AC system uses much more energy to cool the building. At the same time, the temperature further away from those windows (in the core of the building) often becomes far too cold for comfort, sometimes even triggering the need for supplemental heat. The people near the win-dows are being slowly broiled while the people in the core are being flash-frozen. Nobody is comfortable, and they all blame the HVAC system, even though the high solar heat gain coefficient of large, low-budget windows is often the cause of the problem.
To capture and quantify the equipment and energy cost reduc-tion that comes from installing fewer and better windows, the HVAC designer can use the computer to quickly run the loads at 71°F vs.
78°F thermostat set points. [At 21.7°C vs. 25.5°C] The reduced en-ergy and smaller equipment for the warmer set point will add to the arguments in favor of better windows, and smaller ones.
Understanding modern glass
If the HVAC designer wants to make a big improvement in the sustainability of the building, he or she would be wise to become intimately familiar with modern glass and window technology. With that understanding, the HVAC designer can become a more useful resource (and a more persuasive advocate) during those critically important early conversations with architects and owners.
Reference 7 provides an excellent starting point for this educa-tion. It is a brief, engagingly-written and well-illustrated description of current window terminology, technology and issues. After reading that introduction, the recommendations in the ASHRAE Advanced Energy Guides will be easier to understand for those who may not have extensive experience with recent advances in window technology.
Then, references 8 and 9 go beyond basics to more detailed information, to help the design team implement the specifics of the low-energy glazing recommendations of ASHRAE Std. 90.1-2007.
In cool or mixed climates, guidance to architectural designers is a bit different. In climates far from the equator, glass that lets in some solar heat is sometimes not all bad, because it can, depending on how it is arranged, reduce the need for winter heating. But in hot and humid climates, the basic guideline is simple: less glass is better. Then make sure the glass itself has a low SHGC and shade it if necessary, for a combined total SHGC of less than 0.31. The lower the better.
Using calculations to show the hidden energy benefit of better glass Another important-but-seldom-recognized benefit of glass with a low solar heat gain is better thermal comfort near windows without the need to drop the thermostat setting. This is a big benefit that seldom shows up in typical single-point load calculations, because it involves human response, over time, to radiant heat from solar-heated window surfaces.
A single-point load calculation usually assumes that the build-ing thermostat is set somewhere near 78°F [26°C]. But those loads would increase substantially if the thermostat setting were pushed down to 71°F [21.7°C]. Such thermostat twiddling often happens in the real world, especially when the glazing has a high solar heat gain coefficient. Here’s why.
Fig. 12.3
ASHRAE Advanced Energy Guides Target values for all components of the exterior enclosure are clearly outlined in the ASHRAE Advanced Energy Guides, which are available—at no cost—for downloading from the ASHRAE website.
As of the publication date of this book, Advanced Energy Guides are available for schools, offices, warehouses and small business hotels.3,4,5,6
horizontal, so they will be effective in daylighting. But when view windows are also necessary on the western wall these should:
• Cover the least-possible area of the wall
• Have glass with a very low SHGC
• Be shaded with vertical louvers
During peak cooling load hours, (afternoon and early evening) the sun is closer to the horizon as it beats on the western wall. So a shading device made with vertical blades, set at an angle and extending down the full face of the window will be helpful in limiting the solar heat gain during peak cooling load hours.
Daylighting can reduce peak cooling loads
When the building is equipped with well-designed daylighting, its cooling loads can be significantly reduced. Daylighting windows are Western glass is the worst
When calculating the loads from windows, look very carefully at the load from windows on the western walls. The west face of the build-ing is the worst place to locate windows, from a thermal perspective.
There, the cooling loads come at the worst-possible time—after the building has been heated up all day long by the sun and by the oc-cupants’ activities. Figure 12.4 shows the load from both south and west-facing windows. Note that during the hottest months of the year, the load through the west-facing windows is 2.7 times larger than the load through the south-facing windows.
This fact will be most apparent when the designer looks at the peak hour loads, rather than the total load for the entire day or year.
Over 24 hours, the cooling load from western windows is the same as the load from eastern windows. But the eastern windows generate that load during the early morning, when the internal loads are very small or non-existent. So the cooling system can handle those loads using very little of its capacity. Later in the day, when all the sensible loads are peaking at the same time, the cooling system will struggle.
(The usual occupant complaints about temperatures are: too cold in the morning, too hot in the afternoon—never just right.)
So, the computerized calculations can be very helpful in under-standing how important the western windows are to the peak load.
Calculating different percentages of glazing, and calculating the effect of different shading geometry will be especially helpful to the owner and architect as they plan the look and feel of the building, and the uses of the spaces which would be affected by western windows.
Thermally, the western side of the building would be a good location for storage rooms, mechanical rooms and similar uses which don’t benefit from windows. When windows are necessary on the western wall, it’s best to make them high on the wall, small and
Fig. 12.4 Western walls are the worst place to locate windows That’s because during the hottest months of the year, and at the hottest time of the day (the afternoon), they allow more than 2.7 times more heat into the building than windows located on the south wall.2
Glazing substitutions can ruin HVAC designs and occupant comfort Running multiple cooling load calculations can help the HVAC de-signer head-off last-minute substitutions. Glass, especially good glass, is expensive. And it’s often easy for the owner to substitute lower-cost glass without major architectural design changes when the construc-tion bids come in over budget.
However, for buildings with a great deal of window area, glass substitutions could be catastrophic for the HVAC budget and system design. The sensible cooling loads could be far in excess of what the HVAC designer expected when the original calculations were run.
With multiple computer load calculation runs at an early stage, the HVAC designer will have a faster and more informed response (with more useful suggestions) if the client needs to substitute lower-cost glazing. Figure 12.2 showed a small example of the difference that good glass makes in reducing the cooling load.
Separate and then calculate the dehumidification loads In hot and humid climates, the dehumidification (D/H) loads are large and nearly continuous. D/H loads also peak at different times than the sensible cooling loads. That’s why, especially in hot and humid climates, it is wise to calculate dehumidification loads separately from sensible loads. Also, it’s best to calculate the lbs or kg which must be removed per hour—rather than thinking about Btu’s per hour or kWh or the sensible heat ratio. Tracking the D/H loads in pounds or kg per hour allows the designer to deal with the key variable directly.
Estimating the dehumidification loads can be done quickly, using hand calculations guided by Chapter 11 of this book. Then, after the three major dehumidification loads of people, ventilation air and infiltration are clearly identified and quantified, the designer can return to the computer to calculate the sensible cooling loads.
Costly experiences with mold, wasted mechanical system budgets, occupant complaints of swampy, cold buildings and outlandish costs for energy all underline the importance of quantifying the D/H and sensible loads separately.
small, narrow, horizontal and set near the ceilings. (See figure 12.5 for an example.) These can have a higher solar heat gain coefficient, in order to transmit the maximum amount of visible light. But the view windows, usually much larger and set below the daylighting windows, will have a very low SHGC to limit heat gain.
Even more significantly, with effective daylighting the electric power needed for lights in the perimeter zones is greatly reduced during peak cooling load hours. And when that lighting power is reduced, so too is the cooling load generated by lighting.
It is still true that full lighting power will be installed in the build-ing, for use at night. But that cooling load comes long after the peak cooling load hours, at a time when the cooling system has a great deal of unused capacity.
Since daylighting is becoming more popular with owners and architects, it’s useful for the HVAC designer to clearly understand its thermal importance, and to design the HVAC equipment accordingly.
It would not be wise to assume, as many have in the past, that the daylighting will be ineffective at reducing the cooling loads. Done right, daylighting greatly reduces the loads in the perimeter zones.
The cooling equipment should be reduced accordingly, to avoid the overchilled occupants, high indoor humidity and needless reheating of the supply air that comes from an oversized cooling system.
Fig. 12.5 Daylighting design Daylighting reduces both the solar heat gain and the heat from the interior lights it displaces. These reductions are especially welcome during the afternoon, when sensible heat loads are peaking.
ing” load. With that weight of water in the mind’s eye, the water’s volume every hour also becomes part of the designer’s thinking.
(See figure 12.8) More attention will likely be given to components like drain pans which really drain, and condensate piping which has functioning traps rather than ineffective bends in undersized flexible tubing. The weight and volume of the hourly D/H load also reminds the designer that all that water has to go someplace. So it’s less likely that the designer will forget to specify how the condensate drain should get to the storm or sanitary sewer instead of spilling out of the drain pan and onto the floor.
Using the D/H load from computerized load calculations
All computerized load calculations will calculate the latent load (the dehumidification load ) along with the sensible cooling loads. In some programs, that latent load is not reported separately from the
All computerized load calculations will calculate the latent load (the dehumidification load ) along with the sensible cooling loads. In some programs, that latent load is not reported separately from the