After the architecture has established the baseline cooling loads, the HVAC designer can consider some suggestions for keeping heat out of the building through clever engineering.
Engineers have sometimes been described as those who “... can do for a dollar what any fool can do for ten.” Without comment on the related problem of underpriced engineering services, that thrifty bias guides the suggestions on the HVAC side. They are arranged in order, beginning with the lowest-cost, highest value ways to keep heat out of a building.
Seal up all air-side joints and connections
When the connections in air systems are not air tight, the suction and positive pressure generated by the fans is transferred to the building cavities, and then to the exterior walls. Since walls and wall joints are seldom air tight, outdoor air is pulled and pushed through the joints by the pressures created by the HVAC fans. In light commercial buildings, the amount of HVAC-driven, outdoor air exchange is really astonishing.
Figure 9.13 showed the difference in outdoor air exchanges rates in 70 buildings with the systems on and off.8 When the systems are turned on, the air exchange rate skyrockets. This is because in the past, most HVAC designers and most HVAC installers have not understood how important it is to seal up all the duct connections.
The lowest-cost, highest value way to keep extra heat out of the building through HVAC design is to simply specify that all duct joints, and most especially all duct connections to any box containing a fan, must be sealed, using mastic.
All connections need to be sealed, including the connections at VAV boxes, filter boxes, cooling and heating coil housings, PTAC cabinets and all grills, registers and diffusers. That seal-with-mastic specification also includes all joints and connections in exhaust air ducting, such as that from bathrooms, showers or kitchens.
This suggestion should please the most thrifty owners and HVAC designers. According to sheet metal contractors, sealing up the con-the vast increase in outdoor air exchange which comes from leaking
duct work, and especially from leaky return air plenums.8
It’s basically impractical to seal up supply and return air plenums as tightly as metal duct work to avoid this huge increase in cooling and dehumidification loads. Just as soon as the plenum is sealed up—along comes the telephone service guy, or the cable guy, or the plumber, or all three, and then wham—another set of air leaks in that plenum wall. In a recent survey of seven high-budget Federal build-ings with underfloor supply air distribution plenums, the U.S. General Services Administration measured air leakage rates of 40 to 100%
of the total design air flow—after all the air sealing was complete.9 In other words, those systems had to somehow come up with 40 to 100% extra supply air to meet the design HVAC loads.
Certainly, the HVAC designer would prefer to avoid this lost capac-ity and increased load. But if the owner and architectural designer do not allow enough space between floors for supply and return duct work to be fit between the structure (and around plumbing, wiring, fire protection and the attendant support brackets), the designer will be forced into using return air plenums. He or she might then specify in a stern, no-nonsense voice: “All plenums shall be sealed up air tight using spray-applied fire sealant after all carpentry, electrical work, communications cabling, security wiring and plumbing is complete...”
And that specification may even get into the General Contractor’s scope of work. But do you really think that air sealing will actually happen?
We don’t, either. The evidence gathered from field investigations of building-related problems supports that skeptical view.
So that’s why the owner and architectural designer should provide the HVAC designer with enough space between floors, and enough money for air-tight duct connections. That way, the heat and humidity loads in your building will be much less than in typical buildings. Your building will be more comfortable, it will cost less to operate, and it will have a reduced mold risk compared to typical air conditioned buildings in hot and humid climates. Now, some suggestions for the HVAC design.
This is not to suggest that hundreds of thousands of buildings with return and supply air plenums are not “operating successfully”
all over the world. But, success is a relative concept. The air that leaks out of and into those plenums is a huge energy waster and a mold risk. Adding “high-efficiency” cooling equipment to a building with leaky supply and return air plenums is basically like putting lipstick on a pig.
nections with mastic will probably add only 3 to 5% to the cost of the duct installation. And tight connections save a great deal of fan energy, as well as reducing the amount of heat and humidity which the HVAC fans would otherwise pull into the building. The energy losses of leaking duct connections account for about 30 to 40% of the total annual cost of operating the HVAC system.10 Not only that, but those leaking connections are often a major reason for mold problems in buildings in hot and humid climates, as explained in detail in Chapter 5.
So if a hyper-thrifty owner or architect is concerned with the cost of tightly-sealed air connections, they can consider this question: Is there any less-costly way to reduce HVAC-driven mold risk while saving 30 to 40% of annual HVAC operating costs? Sealing connections is very cost-effective, and increasingly, it is required by energy codes.
Don’t use building cavities to carry supply or return air Another description for a ceiling return plenum or for an underfloor supply air plenum is “a very leaky air duct.”8,11 To keep extra heat and humidity out of the building, don’t use building cavities as supply or return air plenums.
If the owner and architectural designer have not provided the money and the space needed for hard-connected, tightly-sealed ducts, it might be in everybody’s best interest for the HVAC designer to point out that the building will pull in more heat and humidity than neces-sary, and that the risk of mold will also be higher than necessary.
If that conversation does not obtain the space and the budget for sealed duct work, then the prudent HVAC designer will make sure the building is equipped with more-than-normal dehumidification capacity, especially for operation during the part-sensible-load hours when humidity is at its peak. It might also be useful to note for the record the concerns about lost AC capacity and increased mold, and to provide the architectural designer with a specification to seal all joints and penetrations of the plenums with fire-rated sealant so that they are air-tight. (One can hope... but see Figures 9.14 and 9.15.)
Fig. 9.14 Air-tight plenums?... not for long
Underfloor supply air plenums and above-ceiling return air plenums are very difficult to seal air-tight, especially over time. Instead of relying on leaky building cavities, use air-tight, sealed ducts and mastic-sealed duct connections.
Fig. 9.15
Air-tight plenums - Attractive in theory, difficult in practice With so many different trades working in building cavities, it is really difficult to ensure that all gaps, holes and joints are sealed air tight. These photos show examples of underfloor supply air plenums that (theoretically) had been
“sealed up, air-tight.”
mates, less ventilation humidity also means less mold risk for the building. So demand-controlled ventilation deserves a careful look from the HVAC designer, especially for public buildings like schools and courthouses. These have highly-variable occupancy, and in the case of schools, long periods of little or no occupancy when the AC system must still operate. Constant-volume ventilation makes no sense for such buildings.
Don’t let air economizers fill the building with humid air In hot and humid climates, using outdoor air for “free cooling” usually results in a higher, not a lower load for the building.
In hot and humid climates, the outdoor air is nearly always more humid than what you’ll want indoors. So for most of the hours in a year, even when the temperature outdoors is below the indoor temperature, the ventilation air will still need to be dried.
Figure 9.17 shows the ventilation load indices (VLI) for several different U.S. locations.12 The VLI is the sum of the energy needed to bring one cfm [or one l/s] from the outdoor air conditions down to neutral indoor air conditions, over all 8760 hours in the year. The VLI has two components—the sensible load and the latent load imposed by that one cfm of ventilation air. Both of these annual loads are expressed in ton-hours per cfm per year. [kW per l/s per year].
Note how the ventilation air’s latent load—its humidity—is far greater on an annual basis than its sensible cooling load. That’s a reminder that an air-side economizer is not usually economical in Install demand-controlled ventilation
Outdoor air is hot and humid, and it costs a lot of money to clean it and dry it out. Don’t bring it in until you need it. And when you have to bring in ventilation air, don’t bring in any more air than you really need for the number of people actually occupying the building.
Easy to say—but difficult and expensive to do. But demand-controlled ventilation is especially worth doing in hot and humid climates because the cost of adequate dehumidification is so high for so many hours per year. In moderate climates, a constant volume of ventilation air is not quite as expensive, because often that air is reducing rather than increasing the AC load. Not so in hot and humid climates. There is nearly always a dehumidification load associated with ventilation air, for nearly all the hours each year.
Note the graphic in Figure 9.16, which shows the hourly dew points for a typical year in Tampa, FL. Note that even during “winter”
months, the outdoor dew point is far above the indoor dew point.
There are several ways to avoid excess ventilation without the risks of inadequate ventilation. More detailed suggestions for different ways to modulate ventilation air in response to vaiable occupancy are described in Chapter 3 (Managing Ventilation Air) and in Chapter 15 (Designing Ventilation Air Systems).
The main point is that demand-controlled ventilation is more cost-effective in hot and humid climates than in moderate climates because the loads are higher. And especially in hot and humid cli-Fig. 9.16
Reduce ventilation air flow when rooms are unoccupied
Outdoor air is hot and humid, all year long. It’s a very large load. So, reduce the ventilation air whenever the building is not fully occupied.
Use exhaust air to precool and predry ventilation air
When the building exhausts large amounts of cool and dry air, it makes sense to use that air to pre-dry and to precool the incoming ventilation air, using an enthalpy heat exchanger. These devices greatly reduce the loads on buildings in hot and humid climates.
In many cases, adding an enthalpy heat exchanger actually reduces the net installed cost of the cooling and dehumidification systems. Plus, the operating cost of the ventilation air is much lower for the entire life of the system. Enthalpy heat exchangers in hot and humid climates has been called “the closest thing to a free lunch in HVAC engineering.”
To take advantage of these big benefits without the downside, just keep in mind three cautions. First, recognize that the effectiveness of ventilation air pretreatment depends on the volume, the temperature and the dryness of the exhaust air. So try to collect as much clean exhaust air as possible and bring it back to the heat exchanger before it leaves the building. Second, an enthalpy heat exchanger cannot dry the incoming ventilation air unless the exhaust air is also dry. In other words, the system still needs effective indoor dehumidification even when the outdoor temperature is low, when the cooling system alone may not be operating long enough to dry effectively.
Third, keep in mind that the heat exchanger presents a signifi-cant resistance to air flow, on both the exhaust and ventilation air streams. For many hours each year, even in hot and humid climates, the outdoor temperature will be low enough that one does not want to heat that incoming air with the warmer exhaust. During those most hot and humid climates. It costs a great deal to remove that
latent load, even when the indoor sensible load is reduced by the economizer air.
If your site-specific analysis shows that an outdoor air econo-mizer cycle will indeed reduce the total annual loads, it’s important that the economizer be controlled not only by the outdoor dry bulb temperature, but also by its dew point. If the outdoor dew point is above the target indoor dew point (usually 55°F [12.8°C]), then the economizer should not flood the building with humid outdoor air, even if the outdoor air’s dry bulb temperature appears attractive.
When the indoor dew point is too high, the occupants crank down the thermostat setting in a desperate search for better comfort, leading to high energy costs even when outdoor temperatures are moderate.
So avoid the use of air-side economizers, unless these are controlled by dew point, as well as by dry bulb temperatures.
Fig. 9.17
Control air-side economizers based on both dew point and dry bulb temperatures
In ventilation air, the annual dehumidification load is far larger than the sensible cooling load. Therefore, any air-side economizer must be controlled by the outdoor air dew point in addition to the outdoor air temperature.
variables of human thermal comfort in more detail. But for purposes of this chapter, it’s enough to note that when the indoor dew point is kept low (below 55°F [12.8°C]), even people accustomed to North American air conditioning levels are often willing to let the thermostat set point rise to 79°F [26°C] before comfort complaints are registered.14,15
So to reduce the amount of heat that gets into the building, keep the dew point low and then let the dry bulb temperature float upwards, until it’s just below the temperature at which occupants notice the temperature. As explained in chapter 2, each building will be different, and each group of occupants will respond differently to temperature and dew point levels. But as a starting point, try keeping the dew point below 55°F [12.8°C] and setting the dry bulb temperature at 79°F [26°C]. Field-measured data suggests that these levels can save about 15% of annual cooling costs.14,15
References
1. Most architectural designers will recognize these quotes, which describe the design aesthetic of many famous modernist architects active during the early and middle of the 20th century. Notable for elegantly simple buildings, moderist design has been a powerful inspiration for the architectural assumptions and therefore the design preferences of owners. Less helpfully however, these build-ings were often a thermal disgrace, reflecting the astonishingly low energy costs in the US during that short period in history.
Many were built with uninsulated, highly conductive steel frames, infilled with huge sheets of glass. HVAC designers could wish that currently famous architectural designers would follow the guideline that “less is more” with respect to glass. Creative designs which use very little glass would provide future generations of designers and owners with a more sustainable visual inspiration than the current wasteful fashion preference for “all glass, all the time.”
hours (sometimes thousands of hours each year depending on the climate) it makes sense to bypass air around the heat exchanger.
Such a bypass avoids the expense of the fan horsepower needed to push the air through the heat exchanger.
Finally, remember that if the design follows ASHRAE Standard 90.1-2004, it’s a mandatory requirement to recover energy from someplace in the building when an individual fan system has a supply air flow over 5,000 cfm, and when the outdoor air portion of that total flow is more than 70%. So when the architectural design allows the exhaust and ventilation air streams to come close together, an exhaust air heat exchanger is one good way to meet this requirement of ASHRAE standard 90.1.
Keep the indoor dew point low, allowing warmer indoor temperatures
When it’s hot outdoors, the colder the air is kept inside the building, the greater is the heat flow through its windows, walls and roof. So to reduce the amount of incoming heat, allow the indoor air tem-perature to rise higher.
In some parts of the world, energy use laws prohibit cooling the indoor air to the levels which are quite common in North American buildings. In Japan for example, office buildings in Tokyo are sel-dom cooled below 81°F [27°C], because lower temperatures are considered quite wasteful of energy.12 In contrast, air conditioned buildings in North America are routinely cooled to 75°F [23.9°C].
Indeed, strange as it may seem to those who live in other parts of the world, U.S. buildings are often chilled down to 72°F [22.2°C] and sometimes even lower.
One of the many reasons for such deep cooling is the occupants’
desperate attempts to achieve comfort when indoor humidity is too high. If the only control you have is the thermostat, then dropping the temperature set point will be the quickest way to improve comfort when the dew point is too high. Chapter 2 explains the interacting
10. Delp, William; Woody, Nance; Matson, E.; Tschudy, Eric; Modera, Mark & Diamond, Richard. Field Investigation of Duct System Performance in California Light Commercial Buildings. 1998.
Report LBNL #40102, Building Technologies Program, Lawrence Berkeley National Laboratory, Berkeley, CA
11. Henderson, Hugh; Cummings, James; Zhang, Jian Sun; Brennan, Terry. Mitigating The Impacts of Uncontrolled Air Flow on Indoor Environmental Quality and Energy Demand in Non-Residential Buildings. 2007. Final Report - NYSERDA Project # 6770. New York State Energy Research & Development Authority, 17 Columbia Circle, Albany, NY 12203-6399
12. Harriman, Lewis G. Kosar, Douglas and Plager, Dean. 1997.
Dehumidification and Cooling Loads from Ventilation Air.
ASHRAE Journal, November, 1997 pp.37-45. ASHRAE, Atlanta, GA. www.ashrae.org
13. Moffett, Sebastian. “Japan Sweats it Out as it Wages War on Air Conditioning.” Wall Street Journal, Sept. 11th, 2007.
14. Spears, John; Judge, James. “Gas-Fired Desiccant System for Retail Super Center” 1997. ASHRAE Journal, October 1997 pp.65-69.
15. Fischer, John; Bayer, Charlene. “Failing Grade for Many Schools - Report Card on Humidity Control” ASHRAE Journal, May 2003.
pp.30-37.
2. Gronbeck, Christopher Window Heat Gain Calculator 2007.
http://www.susdesign.com/windowheatgain/
3. Turner, Cathy and Frankel, Mark. 2008. Energy Performance of LEED© for New Construction Buildings. - March 4th, 2008.
3. Turner, Cathy and Frankel, Mark. 2008. Energy Performance of LEED© for New Construction Buildings. - March 4th, 2008.