HVAC HAND BOOK

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Library of Congress Cataloging-in-Publication Data. Vedavarz, Ali. HVAC: handbook of heating ventilation and air conditioning / Ali Vedavarz, Sunil Kumar, . From to , Wang worked in the field of air conditioning and ity standard, the second edition of Handbook of Air Conditioning and. In the almost sixty years since the publication of the first edition of HVAC Engineer's Handbook, it has become widely known as a highly useful and definitive.


Hvac Hand Book

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Ventilating, and Air Conditioning. Systems. U.S. Department of the Interior. Bureau of Reclamation. Technical Service Center. Mechanical. HVAC Systems Design Handbook, Fifth Edition, features new information on energy conservation and computer usage for design and control, as well as the. the objective usually is to provide an HVAC system which will control. Source: HVAC Systems Design Handbook. Downloaded from Digital Engineering Library .

Chapter 37, Solar Energy Equipment, has new data on worldwide solar technology use, plus an expanded section on photovoltaic equipment. Chapter 38, Compressors, has revisions on general theory; screw and scroll compressors; and bearings, including oil-free technologies. Chapter 44, Centrifugal Pumps, has new content on vertical, inline, split-coupled pumps; hydronic system pump selection; and differential pressure control.

Chapter 45, Motors, Motor Controls, and Variable-Frequency Drives, has updates on standards, bearing currents, and permanent-magnet motors. Chapter 47, Valves, has new content on control valve sizing; electronic actuators; and ball, butterfly, flow-limiting, and pressure-independent control valves. Chapter 51, Thermal Storage, has new content on grid reliability, renewable power integration, heat storage, emergency cooling, water treatment, and commissioning.

For the complete tables refer to the Handbook. Many load calculation programs exist, with varying degrees of complexity and accuracy. Design Procedures: Part 1 26 Chapter Three require large computer systems.

There are several important things to consider when a computer is used: The program to be used must be credible and well documented. Any automated procedure should be capable of being supported in a legal review or challenge. The input must be carefully checked for accuracy.

This is not a simple task since the complete input can be voluminous and complex. In fact, it often takes at least as long to properly input and check the data as it does to manually calculate the loads.

The output must be checked for reasonableness. This is seldom true in HVAC work. Different load calculation programs may yield different results for the same input data. In part, this is due to the way the programs handle solar effect and building dynamics. When using a new program, the designer is advised to manually spot-check the results. There are also many computer programs for estimating energy consumption. Many include subroutines for calculating heating and cooling loads.

Computer calculation has one great advantage over manual calculation.

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The computer can calculate the loads at 12 or more different hours from one set of input data. This is extremely valuable in organizing zones, determining maximum overall loads, and selecting equipment. In this book, we describe manual calculations so that the reader can develop a personal understanding of the principles of HVAC load calculation and will be better able to evaluate the input and output of computer analysis.

These are typically square feet per ton for cooling, Btu per square foot for heating, and cubic feet per minute per square foot for air-handling equipment.

The values used will vary with climate and application and are always tempered by experience. Part 1 Design Procedures: Part 1 27 computational errors. As an example only, the cooling load values in Table 3.

Energy conserving practice in envelope construction, in lighting design, and in system design has resulted in decreased loads in many cases. But increased use of personal computers and other appliances has the opposite effect of increasing the air conditioning requirements.

These data should be listed on standard forms, such as those shown in Figs. Part 1 28 Chapter Three Figure 3. Load factors described below must be determined for all these times. In addition to assumed maximum loads, all zones must be calculated for one building peak time, usually 3 p. Part 1 29 Figure 3. There are some local and regional conditions the designer should be aware of, in setting up calculations.

A similar effect can occur in snow country. Part 1 30 Chapter Three Figure 3. Part 1 31 heat gains. Longitude places the job in a time zone, which may have an almost 1-h plus or minus effect on correlation between local and solar time. Many people will try to operate the systems at lower or higher temperatures than design, and this will be possible most of the time.

Most HVAC systems tend and need to be oversized for various reasons, some of which will be pointed out later. Table 3. For comfort cooling, use of the 2. Note that the maximum wet-bulb wb temperature seldom occurs at the same time as the design dry-bulb db temperature. For sites not listed, data may be obtained by interpolation, but this should be done only by an experienced meteorologist.

The design temperature and humidity conditions should be plotted on a psychrometric chart. Then the relative humidity RH and enthalpy h can be read as well as the indoor wet-bulb temperature. Heat exchanger coil capacities are reduced. Evaporative condenser and cooling tower capacities are slightly—but not entirely—proportionately reduced. The psychrometric chart changes are described in Chap. The air factor—sometimes called the air-transfer factor—is also affected by elevation because it Downloaded from Digital Engineering Library McGraw-Hill www.

TABLE 3. The air factor AF at altitude is obtained by multiplying the sea level air factor 1. Heat gain from people is a function of the level of activity see Table 3. Part 1 35 Heat gain from lights is a function of wattage, at a rate of 3. With ceiling mounted lights recessed some of the heat may go to the ceiling plenum without being a cooling load in the room. The typical allowance for task lighting and appliances is 1. Kitchen appliances, cookers, stoves, ovens, etc.

Transformers mounted indoors must also be acknowledged and accommodated. The transformer vaults or room normally require power exhaust for heat removal. Heat gains from manufacturing processes must be estimated from the energy input to the process.

This is a result of heat storage in the building and furnishings—anything that has mass. The effect is shown in Fig. The longer the heat gain persists, the more nearly the instantaneous cooling load will approach the actual cooling load.

Cooling load factors CLFs for various elements of heat gain are shown in Tables 3. The load factor criteria pages should include schedules of use and occupancy, together with cooling load factors to be applied.

Subsequent editions provide more extensive data. Part 1 37 Figure 3. The units of the U factor are Btu per hour per square foot of area per degree Fahrenheit of temperature difference from inside to outside air. For calculating the cooling load due to heat gain by conduction through opaque walls and the roof, the sol-air temperature concept may be used. Both direct and diffuse solar radiation have a heating effect on the exterior surface of the wall.

The surface temperature will usually be greater than the outside air temperature, which then has a cooling effect. When the exterior surface temperature is greater than the internal temperature of the wall, heat transfer into the wall will take place.

Some of this heat will be stored, increasing the internal temperature of the wall. Some heat will be transferred by conduction to the cooler interior surface and then to the room, as Downloaded from Digital Engineering Library McGraw-Hill www.

The process is dynamic because the exterior surface temperature is constantly changing as the angle of the sun changes. At certain times of the day and night, some of the stored heat will be transferred back to the exterior surface. Only part of the heat that enters the wall becomes cooling load, and this is delayed by storage effects. The greater the mass of the wall, the greater will be the delay.

The sol-air temperature derives an equivalent outside temperature which is a function of time of day and orientation. This value is then adjusted for the storage effect and the time delay caused by the mass of the wall or roof; the daily temperature range, which has an effect on the storage; the color of the outside surface, which affects the solar heat absorption rate; and the latitude and month.

Tables 3. Low ventilation rate—minimum required to cope with cooling load from lights and occupants in the interior zone. Room air circulation induced by primary air of induction unit or by fan coil unit. Very high: High room air circulation used to minimize temperature gradients in a room. When these data are combined with the inside design temperature, a cooling load temperature difference CLTD is obtained.

When light is transmitted, so is solar energy. Up to the end of World War II, fenestrations almost always used clear glass with outside shading by awnings or overhangs and inside shading by roller shades, venetian blinds or draperies. With increased use of air conditioning it was realized that solar heat gains through this type of fenestration were as much as 25 to 30 percent of the total Downloaded from Digital Engineering Library McGraw-Hill www.

Part 1 43 Figure 3. Reducing the amount of glass has a claustrophobic effect on people, so much of the effort centered on reducing the transmission through the glazing material. The mechanism of solar transmission through glazing is shown in Fig. Some radiation is absorbed, heating the glazing material and escaping as convective or radiant heat.

Some radiation passes through the glazing after which it is absorbed by materials in the room, causing a heating effect and thus a cooling load after some time delay. If exterior shading is used, only the diffuse solar component is effective.

As indicated, there is also conduction through the glazing due to the temperature difference between inside and outside.

AHH Handbook: HVAC Applications 2015

At certain times of the year, conduction may represent a heat loss. Partial shading of a pane creates thermal stress along the shadow line. To compute corresponding temperatures for the other locations, select a suitable design temperature from column 6, Table 3. For each hour, take the percentage of the daily range, indicated in Table 3. Calculate the summer dry-bulb temperature at noon for Denver, Colorado.

From Table 3. Most of these problems have been recognized and solved.

These are reproduced here as Tables 3. Figure 3. Additional information is available from the glazing manufacturers. These data are best obtained by means of a solar calorimeter, in which the glazing-shading combination is measured against unrestricted solar transmission.

The value of the SC must be matched against the need to see through the glazing and the use of daylighting to minimize lighting energy use. A note of caution regarding window glazing is in order here.

HVAC Systems Design Handbook, Fifth Edition

Over time, failure may occur in any of these features, reducing the glazing effectiveness and Downloaded from Digital Engineering Library McGraw-Hill www. For limitations and adjustments see notes in the Handbook table.

LM is the latitude-month correction from Table 3. Direct application of the table without adjustments: Values in the table were calculated using the same conditions for walls as outlined for the roof CLTD table, Table 3. These values may be used for all normal air-conditioning estimates, usually without correction except as noted below when the load is calculated for the hottest weather. For totally shaded walls, use the north orientation values.

Adjustments to table values: The following equation makes adjustments for conditions other than those listed in note 1. Part 1 53 Figure 3. Sealed glass units manufactured at low altitudes and installed at high altitudes may not perform as desired. Care must be taken in specifying and installing these materials. It is helpful for the HVAC designer to acquire a sense of the maximum solar heat gain factors for various orientations, time of day, and season.

A few key numbers from Table 3. Later editions have more extensive data. Some of the methods will allow a portion of the solar energy to be delayed in transfer, and this must be accounted for in the cooling-load factor calculations as discussed under Sec.

The cooling-load factor CLF depends on the building construction and the absence or presence of internal shading, as shown in Tables 3. This factor takes into account the heat storage properties of the building and shading and the resultant time lag. Typical U factors for glazing are shown in Tables 3. The design temperature difference must be corrected as shown in Table 3. Then the cooling load from conduction and convection can be calculated from Downloaded from Digital Engineering Library McGraw-Hill www.

The criteria sheets must include the necessary data for each orientation, glass-shading combination, and time of day to be used in the calculations. The inaccuracy results from three factors which are outside the control of the HVAC designer: No building is tight. Data are for values at noon. Tables in source provide values at other times. Walls are porous. Vertical air movement in multistory buildings takes place through elevator shafts, stairwells, utility chases, ducts, and numerous construction openings.

Wind creates a positive pressure on the windward side of a building and a negative pressure on the leeward side. These pressures vary with changes in wind direction and velocity. The chimney effect in a multistory building or even in a singlestory building is related to variations in the air density due to temperature and height and is aggravated by wind.

The buoyancy of the warm air inside compared to a cold ambient condition outside the building makes the air rise, creating a pressure gradient, as shown in Fig.

Part 1 69 Figure 3. See Chapter 21, Indoor Air Quality. Corner zones should always be calculated separately. East-facing zone loads will normally peak from 10 to 12 a. Southfacing zones are similar but will peak usually from noon to 2 p.

Part 1 70 Chapter Three Figure 3. All zones should be calculated at both zone peak for sizing air-handling equipment and building peak for sizing central equipment. Buildings such as churches and restaurants will usually have peaks at times within an hour or two after maximum occupancy occurs. Judgment and experience must be applied.

There are many such forms. Designers should use whatever form is found most satisfactory, or is required. Zone and room calculations must then be summarized, by grouping rooms and zones in the way in which air-handling systems will be applied.

A typical summary form is shown in Fig. Part 1 71 Figure 3. The psychrometric chart is used because this allows us to make a graphical study of the processes. To simplify the analysis, the effects of heat pickup in return air and the effects of fan work are neglected. For comfort cooling Fig. The room condition will include an h of The design condition of the air supplied to the room is determined in one of two ways: Theoretically, the supply air point may be anywhere on this line.

In practice, there are limitations, as discussed below. A present-day cooling coil, even at four rows deep, will do much better, with a bypass factor as low as 5 percent. From experience this would be expected to result in cold drafts and rapid two-position response Downloaded from Digital Engineering Library McGraw-Hill www. Part 1 75 Figure 3. At part-load conditions, which prevail most of the time, the DX system cycles even more often, because the supply air temperature does not modulate but varies between the design condition when the compressor is running and the return-air condition when the compressor is off.

Better control can be obtained at the expense of a slight increase in room humidity.

There is a limited tendency for the entire process to move upward on the psychrometric chart, with a resulting increase in room humidity. This will be more noticeable if the supply air temperature is reset upward with decreasing air volume, as frequently recommended. This can be unrealistic in terms of the CFM and coil bypass factor, and almost always it will result in poor control and wide temperature swings.

This mistake is even easier to make when you are remodeling and rearranging zones with existing AHUs. Part 1 76 Chapter Three While this discussion concerns comfort cooling, it should be apparent that these phenomena become even more important in systems for process cooling, especially those requiring close control of temperature and humidity.

Then reheat becomes a necessity, but the need for energy conservation requires a careful look at the coil TD. It is relatively easy to control either room temperature or room relative humidity, but to control both at the same time requires a more complex control system and expends more energy. When the cooling CFM is compared to the heat loss, the temperature difference for heating will be found to vary from room to room or from zone to zone.

If the variation is small, this will not be a problem; if large, it may be necessary to provide supplemental heating in some rooms or zones. The tabulated heat loss is a gross heat loss at design conditions, but with no credit for the internal heat gains which occur when the room is occupied.

This discussion helps explain why some building systems have a hard time satisfying the tenants.

Owner pressure, or designer inexperience, often results in large area control zones with loads that vary across the zone due to occupancy, use, or exposure. This guarantees subsequent dissatisfaction, and different personal comfort expectations exacerbates the problem.

The CFM per square foot tabulation in the summary is a very useful check item for both manual and computer calculations. Values below about 0. For VAV a minimum design rate should be about 0. Values above 3. Experience has shown that small adjustments of high rates down to 3. Again, judgment and common sense are needed.

The above comments imply that the system concept must precede the calculations. That is, the types of HVAC systems to be used, zoning, location of equipment, and control strategies must be at least approximated before the summaries are made. This comment extends to the sizing of ductwork. After many years of experience, it seems prudent to err on the side of a little too large, rather than a little too small.

But steady-state conditions do not exist in an air conditioning situation. If the HVAC systems and controls are functioning properly, then the indoor environment will vary only slightly. However, the internal and external loads are constantly changing. The research which led to these factors resulted from the widely recognized condition that older calculation methods invariably led to oversizing of HVAC systems and equipment.

Even so, the factors in the tables are conservative, and some oversizing will normally result. Some codes may allow for automatic adjustment of outside air quantities, based on measurement of indoor air quality.

In addition, many processes require large amounts of exhaust, for which makeup air is required. Part 1 78 Chapter Three ing. This whole matter is further complicated by the knowledge that outdoor air quality may not be acceptable in many indoor environments, so that special treatment to remove contaminants may be necessary see Chaps. For heating, it is the minimum outside air quantity multiplied by the design temperature difference and the proper air factor.

Notice that interior zones, with no heat loss, can make use of outside air for winter cooling if the air-handling system is so designed. This will result in some reduction of the ventilating heating load. Part 1 3. Among these are fan and pump work as well as duct and piping losses. These are discussed in subsequent chapters. Energy is neither created or destroyed.

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If energy moves into or out of the building, it must be accounted for and managed. The principal source of information on this subject is the ASHRAE Handbook Fundamentals, to which the interested reader is referred for a more detailed discussion. All load calculations, whether manual or computerized, should be carefully checked for consistency and reasonableness.

This requires the application of judgment, common sense, and experience. Part 2 General Concepts for Equipment Selection 4.

Requirements of comfort or process. These requirements include temperature, always; humidity, ventilation, and pressurization, sometimes; and zoning for better control, if needed. In theory at least, the comfort requirement should have a high priority.

This is happening less often as building occupants become more sophisticated in their expectations. Most often, different parts of the process have different temperature, humidity, pressure, and cleanliness requirements; the most extreme of these can penalize the entire HVAC system.

Energy conservation. This is usually a code requirement and not optional. State and local building codes almost invariably include requirements constraining the use of new, nonrenewable energy. Nonrenewable refers primarily to fossil-fuel sources.

Renewable sources include solar power, wind, water, geothermal, waste processing, heat reclaim, and the like. The strictest codes prohibit any form of reheat except from reclaimed or renewable sources unless humidity control is essential.

This restriction eliminates such popular systems as terminal reheat, two-deck multizone, multizone, and constant volume dual-duct systems, although the two-fan dual-duct system is still possible and the three-duct multizone system is acceptable see Chap.

Most HVAC systems for process environments have opportunities for heat reclaim and other ingenious ways of conserving energy.

Off-peak thermal storage systems are becoming popular for energy cost savings, although these systems may actually consume more energy than conventional systems. First cost and life-cycle cost. A common method of comparing the life-cycle costs of two or more systems is to convert all costs to present-worth values. Part 2 Design Procedures: Very often, someone in authority lays down guidelines which must be followed by the designer.

This is particularly true for institutional owners and major retailers.

Space limitations. Sometimes in existing buildings it is necessary to take additional space to provide a suitable HVAC system.

For example, in adding air conditioning to a school, it is often necessary to convert a classroom to an equipment room. Rooftop systems are another alternative where space is limited, if the building structure will support such systems. There are ways of providing a functional HVAC system in very little space, such as individual room units and rooftop units, but these systems often have a high life-cycle cost.

This criterion includes equipment quality the mean time between failures is commonly used ; ease of maintenance are high-maintenance items readily accessible in the unit? Is there adequate space around it for removing and replacing items? Rooftop units may be readily accessible if an inside stair and a roof penthouse exist; but if an outside ladder must be climbed, the adjective readily must be deleted.

Many equipment rooms are easy to get to but are too small for adequate access or maintenance.

This criterion is critical in the lifecycle cost analysis and in the long-term satisfaction of the building owner and occupants. Central plant versus distributed systems. Central plants may include only a chilled water source, both heating and chilled water, an intermediate temperature water supply for individual room heat pumps, or even a large, central air-handling system.

Many buildings have no central plant. The disadvantages include the cost of pumping and piping or, for the central AHU, longer duct systems and more fan horsepower. There is no simple answer to this choice. Each building must be evaluated separately. Simplicity and controllability. Although listed last, this is the most important criterion in terms of how the system will really work.

There is an accepted truism that operators will soon reduce the HVAC system and controls to their level of understanding. This is not to criticize the operator, who may have had little or no instruction about the system. It is simply a fact of life. The designer who wants or needs to use a complex system must provide for adequate training—and retraining—for operators. The best rule is: Never add an unnecessary complication to the system or its controls.

Most units also include a mixing chamber with outside and return air connections with dampers. Many systems for rooftop mounting are self-contained, with capacities as great as tons or more of cooling and a comparable amount of heating. Some room units for wall or window installation have capacities as small as 0. The two sections are connected by piping.

Cooling coils may use chilled water, brine, or refrigerant direct expansion. Heat reclaim systems of various types are employed. Thus, the designer has a wide range of equipment to choose from.

Although generalizations are dangerous, some general rules may be applied, but the designer must also develop, through experience, an understanding of the best and worst choices. There are some criteria which are useful: In general, packaged equipment is designed to be as small as possible for a given capacity. This may create problems of access for maintenance. Also the supplier should show that capacity ratings were determined for the package as assembled and not just for the separate components.

See particularly the discussion on the effects of geometry on fan performance in Chap. In hotel guest rooms, motels and apartments, individual room units should be used to give occupants maximum control of their environment. Where many people share the same space, central systems are preferable, with controls which cannot be reset by occupants. Noise is a factor in almost any HVAC installation, yet noise is often neglected in equipment selection and installation.

Modern systems are more compact than the old cast-iron radiators and depend more on natural convection than on radiation. Rating methods are standardized by the Hydronics Institute. Hot-water piping or electric resistance heating tape is used. Radiant cooling by means of wall or ceiling panels may also be used. Self-contained package AHUs typically use direct-expansion cooling with reciprocating or rotary compressors. Other AHUs may use direct expansion, chilled water, or brine cooling, with the cooling medium provided by a separate, centralized, refrigeration system see Chap.

Evaporative cooling is used primarily in climates with low design ambient wet-bulb temperatures, although it may be used in almost any climate to achieve some cooling. Centrifugal and screw-type compressors and absorption refrigeration are used almost entirely in large central-station water or brine chillers. Absorption refrigeration may be uneconomical unless there is an adequate source of waste heat or solar energy.

Often both are selected at the same time. The use of individual room units does not preclude the use of central-station chillers; this combination may be preferable in many situations.

For off-peak cooling with storage, a central chilling plant is an essential item. Fuels include coal, oil, natural and manufactured gas, and peat. Waste products such as refuse-derived fuel RDF and sawdust are also being used in limited ways. Electricity for resistance heating is not a fuel in the combustion sense but is a heat source. Heat reclaim takes many forms, some of which are discussed in Chap. For other systems, some kind of central plant equipment is needed.

Large central plants for high-pressure steam or hightemperature hot water, may present safety problems, are regulated by codes and require special expertise on the part of the designer, contractor, and operator. New buildings connected to existing central plants will require the use of heat exchangers, secondary pumping or condensate return pumping, and an understanding of limitations imposed by the existing plant, such as limitations on the pressure and temperature of returned water or condensate.

For a detailed discussion of psychrometrics, see Chap. Consider a single-zone, draw-through air-handling system, as in Fig. Cooling is provided by the cooling coil through the use of chilled water as in this example Figure 4. Part 2 88 Chapter Four or by direct refrigerant expansion. On the psychrometric chart Fig. The return air to the system will usually be at a higher temperature than the space due to heat gains in returnair plenums.

This does not hold for direct-return units. This heat gain can be estimated or calculated from the geometry of the building, the wattage of recessed lighting, etc. A straight line between this point and the outside air state point represents the mixing process. The mixed-air state point lies on this line at a distance from the return-air point equal to the design minimum percentage of outside air—for this example, 20 percent.

Then the mixed-air condition is The design condition of the supply air is calculated as described in Chap. Because this is a draw-through system, there is some heating effect due to fan work.

For preliminary purposes, a temperature rise of 0. Then the air must leave the coil at The resulting point has an h value of The intersection with the saturation curve is called the apparatus dew Figure 4.

Heating will be provided as required to maintain the space conditions some design heating temperature difference will be calculated. If space humidity is uncontrolled, the cycle will automatically fall into a position such that the humidity ratio difference between supply air and space will be the same as that for cooling.

This will typically result in a lower space humidity in winter. Most designers use the psychrometric chart only for the design cooling cycle, or for both heating and cooling if humidity control is provided. It is sometimes useful to look at intermediate conditions such Figure 4. Part 2 90 Chapter Four as in Fig. The inside humidity will depend on the outside humidity, as discussed before.

No mechanical cooling and little or no heating should be needed. Other intermediate conditions can be examined in similar ways. See Fig. But changes in density related to altitude or related to heating or cooling processes may compound all other effects and should not be taken lightly. Inversely, atmospheric pressure increases for those elevations below sea level or some other reference point.

Altitude effects are often ignored below Figure 4. Part 2 91 Figure 4. Most of the basic effects of altitude density variation can be predicted. Air volume must be increased to transport the same amount of cooling or heating capacity for a given air temperature differential.

For the same amount of energy transport, fan speeds must increase, but fan horsepower stays about the same. Airside heat transfer in coils is reduced for a given coil face area, while heat transfer loss can be offset with higher face velocities, but then moisture carryover from a cooling coil may be a problem.

Evaporative media and equipment performance is less affected by altitude and air density change. As air pressure goes down, the water vapor holding capacity of the air increases. The inverse can be observed as water drains from the receiver tank of an air compressor.

There is an often overlooked effect of altitude on steam system performance. Gauge pressures seem to be the same at altitude as at sea level, but the absolute pressure is reduced, which determines the acDownloaded from Digital Engineering Library McGraw-Hill www. Part 2 92 Chapter Four tual steam temperature. Steam-driven absorption chillers may be altitude-sensitive, as may some other heat-exchange devices. Equipment for these applications must be specially designed with fan bearings and other materials suitable for the design temperatures.

Motor horsepower must be adjusted to the nonstandard density. The web seminar or webinar focused…. While every project is unique, here are some guidelines: Gas boilers are the default heat source in the UK and can easily be used in a Passivhaus. Combi boilers provide hot water on demand, and as such have high kW output so tend to…. One of Swegon Air Academy favourite lecturer Peter Simmonds has published a very good guidebook about high-rise buildings and their systems.

And recetly I got remided how good this book is when I saw an…. A new book:For rectangular and oval ducts, an equivalent diameter must be determined from a formula based on the hydraulic diameter of the duct.

Part 4 Fluid-Handling Systems 6. Typically, the air system is designed to control temperature always and humidity sometimes. In terms of perceived comfort, a little higher relative humidity can offset a little lower ambient temperature.

Some codes may allow for automatic adjustment of outside air quantities, based on measurement of indoor air quality. This approach is also less expensive for the designer and avoids discussion and possible argument with the client, who is more often concerned with cost than with innovation.

For comfort cooling, use of the 2. It is easy, under these conditions, to do things as they have always been done and to avoid innovation.

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