J a n u a r y 2 0 0 4 ASHRAE Jou rna l ’ s O f f i c i a l P roduc t & Show Gu ide S157
By Glenn Friedman, P.E., Member ASHRAE
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About the Author
Glenn Friedman, P.E., is a principal at TaylorEngineering in Alameda, Calif. He is the chair ofASHRAE Technical Committee 9.5, Residentialand Small Business Applications, and the programchair of ASHRAE TC 4.1, Load Calculations.
hen an HVAC system works, building occupants don’t even knowit’s there—no news is good news. The bad news is, in a recent
survey done by the International Facility Management Association,1 toocold and too hot ranked first and second of top 10 office complaints.
Hot and cold problems can be causedby many factors: too little capacity, toomuch capacity, lack of zoning, too littlecontrol, varying space usage, varying so-lar load, changes in occupancy, changesto the space envelope, drafts and diffuserperformance, or too little ventilation.
The following is a guide for engineers,contractors, building managers and own-ers confronted with hot/cold problems.The article covers the basics of hot/coldcomplaint diagnosis and then describesadditional technical steps for diagnos-ing complaints.
ComplaintsComplaints usually come from build-
ing occupants who may not be able todistinguish between drafts and being toocold or between stuffiness and being toohot. If you hear, “The room is stuffy,” thetrue problem could be temperature, solarload, radiant heat from windows andwalls or lack of ventilation.
Start the hot/cold complaint review byevaluating an occupant’s complaint. Thenreview the HVAC equipment performancefor proper operation followed by a sys-tem design analysis.
It is worth validating the hot/cold com-plaint to evaluate whether the complaintis an HVAC issues or a management is-sues. Chapter 8, Page 8.17 of the 2001ASHRAE Handbook—Fundamentalspoints out that, for typical comfort con-ditions, 5% of the people will be dissat-isfied. Research reported in ASHRAETransactions finds that even with per-fect temperature control complaints oc-cur.2 If individual occupant control is notjustified, do not expect every occupantto be satisfied with the same conditions.
Did you ever sit by a campfire andnotice how comfortable you wereeven though the temperatures werehot on your clothing facing the fireand cold on your clothing away fromthe fire? At this same time the out-side air temperature might have been50°F (10°C) or colder. The tempera-ture you perceived was radically dif-ferent than the actual air temperature.
The body’s perceived temperatureis dramatically affected by direct ra-diation such as a campfire. The sameradiation affect holds true sitting indirect sunlight, such as in a car. Carair conditioning allows the occu-pants to vary temperatures and cre-ate drafts for occupant comfort.Standard building design is for asteady temperature and draft freeenvironment. For building comfort,keep sunlight off occupants.
StratificationResearch
Researchers are studying the ef-fects of stratification on human com-fort. Though the 2°F (1.1°C) floor todesktop test for stratification in theoccupied zone for a conventionalcommercial space with ceiling sup-ply diffusers is achievable for a welldesigned space, this low level of strati-fication is not required for comfort.However, when the stratification isgreater than this, it is likely the diffus-ers are “dumping,” or not mixing prop-erly at the ceiling. For underfloor airdistribution stratification of 5.4°F (3°C)within the lower 6 ft (1.8 m) of a spaceis not uncommon and consideredcomfortable. Therefore, it would behard to condemn a quality installationsimply because there is stratification.Since there is ongoing research in thisarea and little documentation to ref-erence, this guide is purposefullyvague about stratification.
ValidationStart by getting a detailed explana-
tion of the hot/cold complaint. Then,visit the space. Observe and measure theconditions using the Validation sectionof the checklist. The following tests ref-erence common industry practice designparameters. If specific building guide-lines are available, they should super-sede the industry common practicedesign parameters.
If all answers to the seven Validationquestions are “no,” the complaint is amanagement issue rather than an HVACsystem issue per normal building guide-line performance expectations.
If the answer to Questions 1, 2, 3, or 4is “yes,” proceed to Load Calculations.
If the answer to Question 5 is “yes,”remedy the situation with internal or ex-ternal shading. If the occupant wishesto experience direct sunlight, then ex-plain that normal comfort space tempera-tures will not satisfy an occupant indirect sunlight.
If the surfaces are hotter or colder thanthe space temperature, they can producea radiant effect for nearby occupants. Ifthe answer to Question 6 is “yes” and theradiant temperature of surfaces is thecause of the hot/cold complaint, reducethe radiant effect. Options may includemoving the occupant further from theradiating surface, installing shades orcurtains on glazing, retrofitting windowswith thermally insulated glazing, add-ing insulation and furring the walls.
If the answer to Question 7 is “yes,”this is not a true hot/cold complaint.Address the lack of outside air ventila-tion by adjusting the outdoor air venti-lation control.
HVAC Equipment Operation ReviewOperate the HVAC systems through
their sequences that relate to the hot/coldcomplaint; measure the system perfor-mance including fan operation, heatingand cooling cycles and compare the read-ings to the design system specifications
and the equipment manufacturers’ speci-fications. This work is potentially dan-gerous and is best performed by skilledtechnicians experienced in the testingand commissioning of HVAC systems.Any equipment or system operation thatdoes not perform to design specificationor manufacturer’s operating specifica-tions should be analyzed and repaired.
If the system is equipped with a build-ing automation system, trend logs canbe recorded to gather much of the sys-tem performance. Trend logs should bereviewed for proper operation and per-formance anomalies. Anomalies shouldbe analyzed and corrected.
Load CalculationsLoad calculations are usually the next
item checked after verifying the equip-ment is working properly. Load calcu-lations model the building envelope andinternal heat gains to size HVAC equip-ment and distribution systems. Blockloads are load calculations for an entirebuilding or major equipment area. Spaceloads are load calculations for indi-vidual spaces.
The Load Calculations section of thechecklist compares the load to the in-stalled equipment capacity. If the an-swer to either Block Load Question 1 or2 is “no,” the building requires addi-tional capacity.
If the answer to either Space LoadQuestion 1 or 2 is “no,” the space requiresmore capacity.
After the capacity issues are addressed,this is often the point when the basic hot/cold complaint evaluation concludes andmany hot/cold complaints are not re-solved. However, there is more to hot/cold complaints than insufficient capac-ity. Now let’s investigate zoning, control,drafts and humidity.
ZoningThe ASHRAE Terminology of Heating,
Ventilating, Air Conditioning, and Re-frigeration, defines zoning as:
Zoning1. Division of a building orgroup of buildings into sepa-rately controlled spaces (zones),where different conditions canbe maintained simultaneously.
2. Practice of dividing a build-ing into smaller sections forcontrol of heating and cooling.Each section is selected so thatone thermostat can be used todetermine its requirements.
To properly zone spaces, the seconddefinition must hold true “…that onethermostat can be used to determine itsrequirements.” If one thermostat does notaccurately represent all the areas of thezone, expect hot/cold complaints.
Traditionally, zoning is done by ex-perienced engineers as an art, not a sci-ence. It would be wonderful if loadcalculations provided the science ofevaluating whether space temperatureswould deviate from the thermostat spacetemperature within a given zone. Unfor-tunately, as stated in the 2001 ASHRAEHandbook—Fundamentals, Chapter 29,most load calculation procedures makeassumptions such as:
“The most fundamental as-sumption is that the air in thethermal zone can be modeledas well mixed, meaning it has auniform temperature through-out the zone.”
Thus, zoning requirements are not cal-culated by normal load calculation pro-cedures. However, we can use conventionalload techniques to evaluate some zoningproblems. For zones to work with single-point thermostatic control, the propor-tional capacity requirement must notchange significantly for a variety of in-ternal and external load variations. Thisapproach is discussed in a subsequent sec-tion of this guide.
Rather than doing a significantly morecomplex analysis, investigate the zon-ing conflicts using the Zoning Conflicts(Simple) section of the checklist.
If the answer to any of the questions in
CostOf ComplaintsThe savings of not zoning is far ex-
ceeded by the cost of retrofitting addi-tional zones. But the cost of retrofittingcan be further eclipsed by the cost oflost opportunity for a project once it islabeled an HVAC deficient building.
We received a hot/cold complaintfrom a housing development. Not onlywas the developer having trouble sell-ing its homes, but also the owners ofsome of the homes that had been soldwere threatening legal action. Our in-vestigation showed the developerhad saved money on their smallertwo-story homes by installing a singlecentral HVAC system. The solutioninvolved retrofitting a costly variablevolume variable temperaturechangeover system to create sepa-rate zones for each level in the homes.
GamblingWith ComfortShortly after it opened we ana-
lyzed the main gambling room of acasino for hot/cold complaints. Theroom was served by a single zoneconstant volume unit with 60% out-side air. The airflow was high, morethan 7 cfm/ft2 (35.6 L/s per m2). Oncold days the supply temperaturewould vary from 50°F (10°C) duringthe no-heating call to a supply air tem-perature of 85°F (29°C) on a call forheating. We watched as occupantsput their jackets on or take their jack-ets off every time the heating cycledoff or on, respectively. Though the oc-cupants were too intent on gamblingto complain, we saw there was a prob-lem. We found that at 7 cfm/ft2 (35.6L/s per m2) airflow the air tempera-ture changed more quickly than thespace temperature sensors could re-act. To fix this we changed the controlto a cascading loop that controlledsupply air temperature reset from theroom sensor. This control graduallychanged the supply air temperatureat a rate that the space temperaturesensors could react to and solved theproblem.
the Zoning Conflicts (Simple) section is“yes,” consider additional zoning.
ControlBuilding controls are intended to main-
tain steady space temperatures. Controlloops that respond too fast or too slowcan result in hot/cold complaints. A con-trol loop response time is too fast whenit responds faster than the control sensorresponds. A control loop is too slow whenit responds slower than the control sen-sor responds.
If the control loop response is too fast,the space temperature can overcorrect andresult in wide temperature fluctuations.This situation is worst when there is ex-cess capacity. Two examples of this includeoversized packaged units without modu-lating control and variable-volume, vari-able-temperature, changeover systemswith oversized space ducts.
The first example, oversized packagedunits without modulating control, dealswith variations in load by cycling heat-ing or cooling stages. If the staged ca-pacity of the packaged equipmentserving a space is oversized for the de-mand and the control loop response timeis too fast, the space will cycle stages ofheating or cooling and condition thespace faster than the control sensor canprovide feedback resulting in over con-ditioning. The zone will cycle from heat-ing to cooling to compensate resultingin hot/cold complaints.
The second example, variable-volume,variable-temperature, changeover systems,typically uses packaged equipment to pro-vide heating and cooling to multiple zonesby both modulating dampers and chang-ing from heating to cooling to satisfy mul-tiple simultaneous demands. In avariable-volume, variable-temperature,changeover system with oversized spaceducts, when there is a minor demand forcooling in one zone while the other zonesare satisfied or in heating, at some pointthe system will change over to a demandfor cooling. This demand for cooling willclose all the other zone dampers but thesingle zone damper demanding cooling,which results in significant cold airflow to
J a n u a r y 2 0 0 4 ASHRAE Jou rna l ’ s O f f i c i a l P roduc t & Show Gu ide S161
the zone demanding cooling. The zone temperature cools morerapidly than the zone temperature sensor and the space overcools.The analogous problem can happen in the heating cycle.
If the control loop response is too slow, the space tempera-ture does not respond to changes in the load. For example, ifthe sun goes behind a cloud reducing the solar heat load theslow control loop response time causes the zone to overcoolbefore responding.
Confirming temperature fluctuations may require more fre-quent and sensitive monitoring than typical trend logs usingthe direct digital control (DDC) system room sensors. If theDDC system room sensors are responding too fast or too slow,a trend log may not indicate the temperature fluctuations feltby the occupants. Also a long trend log interval may be longenough to effectively even out the temperature fluctuations. Afive-minute trend log interval may appear fine, even thoughpeople in the space may have hot/cold complaints.
A good tool for checkingtemperature fluctuation is afast acting and sensitive datalogger to log the space tem-perature at one-minute inter-vals or less. Verify that thespace temperature at the loca-tion of the hot/cold complaintsdoes not vary more than 5°F(2.8°C) in less than 10 minutes.Raise the space temperaturesetpoint by 4°F (2.2°C) and re-view the response. Are thereunusual fluctuations in tem-perature? After the space hasstabilized at the new setpoint, reset the space temperaturesetpoint back to the original setting and review the response.If the space temperature response is too slow, the space tem-perature may not recover fast enough.
If the space temperature does not control properly, revise thecontrols design, which may require revising the system type.
DraftsDrafts can be the result of poor system performance or grille
selection. The initial observation walk through can reveal manyof the potential problems without requiring detailed measure-ments or expensive tests.
Primary air is the air coming from the supply duct. The pri-mary air is mixed with the room air as it is introduced to thespace through the supply grille. Occupants should not be sub-ject to contact with primary air before it is well mixed.
Examine the grilles and observe their direction airflow pat-tern based on the grille configuration. See if you experiencedrafts. Use the Draft Problems section of the checklist.
If any of the draft problem answers in the Draft Problemssection is “yes,” resolve the problems using ADPI criteria (see
the Technical Draft Analysis section later).If diffusers are not throwing air far enough (with enough
velocity), which can happen at part load in VAV systems, coldair may drop to the floor without mixing. The result is exces-sive table top to floor stratification. A side effect of this is thatthermostat response can be extremely slow since the space isnot well mixed. Selecting diffusers that have longer throw willoften change the thermostat response significantly, resultingin better response to changing loads and system fluctuations.
When heating from the ceiling, the ASHRAE Handbook—Fundamentals, Chapter 32, recommends that the primary airtemperature not be more than 15°F (8.3°C) above room tem-perature to avoid stratification and ventilation short-circuit-ing. BSR/ASHRAE Addendum n to ANSI/ASHRAE Standard62-2001, Ventilation for Acceptable Indoor Air Quality (for-merly Addendum n to ANSI/ASHRAE Standard 62-1999), nowrequires an increase in ventilation rates if discharge tempera-
tures exceed 15°F (8.3°C)above room temperatures.
Continuous drafts usually re-sult from the supply jet of a dif-fuser projecting into theoccupied space. Change the dif-fuser locations, settings or selec-tions to correct these situations.
HumidityThe upcoming revision to
ANSI/ASHRAE Standard 55-1992/55-2000R, Public ReviewDraft, Thermal EnvironmentalConditions for Human Occu-
pancy, defines an upper humidity ratio of 0.012. This is 65% RHat 75°F (24°C) or a 62.2°F (16.8°C) dew point. There is no lowerlimit humidity level defined in the updated standard. Measurethe humidity level using a wet bulb thermometer and confirmthe humidity ratio does not exceed 84 grains of moisture or0.012 lbs of moisture per pound (12 g/kg) of dry air during acomplaint period. If this level is exceeded, calculate the dehu-midification load for the wet-bulb design condition. Do not usethe mean coincident wet-bulb design condition to calculate thehumidity load. If the existing system has enough capacity forthe cooling and dehumidification load, resolve the humidityproblem by adding humidity control to the existing HVAC sys-tem. If necessary include reheat as part of the humidity control.If there is not enough capacity in the existing HVAC system,design a new system to supplement the existing HVAC systemor replace the existing HVAC system.
Technical Zoning AnalysisDesign conditions are for design heating and cooling days.
These conditions are normal peak weather and normal peakoperating conditions. Run the load calculations for a variety
Alternatives to ReduceHot/Cold Complaints
ANSI/ASHRAE Standard 55-1992/55-2000R, public re-view draft, Thermal Environmental Conditions for HumanOccupancy, states that building occupants like direct con-trol over their environment. Under the same environmentalconditions when people have direct control, more peopleare satisfied. Building owners are finding positive resultsfrom providing building occupants individual direct con-trol features such as access to thermostats, operable win-dows and underfloor air distribution with adjustable grilles.
S162 J a n u a r y 2 0 0 4
of annual design conditions and see if the proportional capac-ity requirements between different spaces in the zone stay fairlyconstant. Let’s look at the simple example in Figure 1. Theexample building is a single-story, stand-alone, three-roomoffice building with single pane glazing. Table 1 is the cool-ing airflow requirements per space at different months’ out-door design temperatures.
If the airflow is designed at the peak load conditions forSeptember, the system would be reasonably conditioned inApril. However, in February the office and the conference spaceswould be overconditioned. This indicates that this is not ap-propriate zoning; more zoning is required.
Most buildings do not warrant the thoroughness of an expo-nentially more complicated building simulation to analyze allthe different conditions during the year such as sunny days andrainy days, summer and winter conditions, and varying internalloads like conference rooms with and without people and lights.However, for a useful and more rigorous zoning analysis thanthe Zoning Conflicts (Simple), run monthly load calculationsfor the Zoning Conflicts (Advanced) as described in Table 1.
If the answer any of the questions in Zoning Conflicts (Ad-vanced) section is “yes,” additional zoning is required.
Technical Draft AnalysisThe following discussion of drafts is more involved and
technical than most hot/cold complaint investigations re-quire. Observation, standing in the space, feeling for draftsand simple temperature measurements at the floor, tabletopand 6 ft (1.8 m) above the floor will provide most of the usefuldiagnostic information.
The 2001 ASHRAE Handbook—Fundamentals, Chapter 32,describes the Air Diffusion Performance Index (ADPI) for cool-ing conditions. ADPI takes into account the cooling effect ofair temperature and air motion on comfort. ADPI can be calcu-lated using the procedures in Fundamentals.
Fundamentals states a comfort criterion using the ADPI ef-fective draft temperature equation:
“A high percentage of people are comfortable in sed-entary (office) occupations where the effective drafttemperature θ …is between –3 and +2°F and the airvelocity is less than 70 fpm.”
θ is defined as:
( ) ( )30 07.0 −−−=θ xVtt cx
whereθ = effective draft temperature, °Ftx = local airstream dry-bulb temperature, °Ftc = average (control) room dry-bulb temperature, °FVx = local airstream centerline velocity, fpm
ADPI is the comfort criteria expressed as a percentage. Thus,an occupied space with an ADPI rating of 80 translates to a pre-diction that 80% of the occupants are satisfied in the occupiedspace. More importantly, when the ADPI is 80% or higher, effec-tive mixing in the space is assured and stratification is unlikely.
Since the procedures above are technical, and generally notperformed, a practical approach is to determine ADPI from grillemanufacturers’ data. For each occupant location, use the grillemanufacturers’ data to determine the ADPI at 6 ft (1.8 m) abovethe floor. The ADPI should be equal to or greater than 80%.
Measuring the air velocity is more complicated and notusually done. For field analysis the ANSI/ASHRAE Standard113-1990, Method of Testing for Room Air Diffusion, Appen-dix A states:
For summer (cooling) conditions the ADPI shall beequal to or greater than 80% and the test zone aver-age air velocity (V,) shall not exceed 50 fpm (0.25 m/s).
[Author’s note: 50 fpm (0.25 m/s) maximum draft require-ment indicates excellent performance. After all, walking pastanother person in a room creates a 75 fpm draft. (0.38 m/s)]
For winter (heating) conditions the ADPI shall beequal to or greater than 80%, the test zone averageair velocity (V,) shall not exceed 30 fpm (0.15 m/s)and the maximum vertical air temperature difference(At,) shall not exceed 5°F (2.8°C).
Airflow velocity measurements for drafts require special-ized equipment. Confirming proper grille selection is moreimportant and practical indicator than testing air velocity. Ifairflow testing is available, the measured velocity in the occu-pied area in all directions should be equal to or less than 30fpm (0.15 m/s) for heating and 50 fpm (0.25 m/s) for cooling.Anemometers measure air velocity in a single direction. Anomnidirectional anemometer is required to check air move-ments in all directions, but it is a delicate instrument.
Table 1: Cooling airflow requirements per space for Figure 1.
Reception
Office Conference Room
Figure 1: Simple example building for zoning analysis.
T
J a n u a r y 2 0 0 4 ASHRAE Jou rna l ’ s O f f i c i a l P roduc t & Show Gu ide S163
If these conditions are not met, resolve the draft problems bymodifying the grilles using ADPI criteria.
ConclusionThe majority of hot/cold complaints can be diagnosed by
observation, load calculations and recording thermometers.The steps to investigating a hot/cold complaint are:1. Define and validate the complaint;2. Check the HVAC system equipment operation;3. Calculate the building a space loads and verify that there
is sufficient capacity;4. Review zoning conflicts;5. Test the zone for good and stable temperature control;6. Review draft problems;7. Measure the humidity level to verify it is below the Stan-
dard 55 upper dew-point limit of 62.2°F (16.8°C);(Sometimes more rigorous and costly measures are neces-
sary as explained in Items 8 and 9.)8. Compare the load variation characteristics of the differ-
ent spaces in each zone; and9. Analyze the ADPI or measure the omni-directional drafts.
AcknowledgmentsThe author would like to acknowledge the following people for their
advice and contributions to this article: Allan Daly, P.E., MemberASHRAE, principal, Taylor Engineering; Charlie Huizenga, researchspecialist, Center for the Built Environment, University of California,Berkeley; Cliff Federspiel, Ph.D., Associate Member ASHRAE, re-search specialist, Center for the Built Environment, University of Cali-fornia, Berkeley; Dan Int-Hout, Member ASHRAE, chief engineer,Krueger; Fred Bauman, P.E., Member ASHRAE, research specialist,Center for the Built Environment, University of California, Berkeley;Jeff Stein, P.E., Member ASHRAE, senior engineer, Taylor Engineer-ing; Jerry Hurwitz, president, J&J Air Conditioning; Tom Webster,P.E., Member ASHRAE, research specialist, Center for the Built Envi-ronment, University of California, Berkeley; and Tracy Cornish, P.E.,Associate Member ASHRAE, senior engineer, Taylor Engineering.
References1. International Facility Management Association. 2003. Press re-
lease, “IFMA survey ranks top 10 office complaints…and some thatscore high on the laugh meter.” www.ifma.org.
2. Martin, R.M., C.C. Federspiel, and D.M. Auslander. 2002. “Re-sponding to thermal sensation complaints in buildings.” ASHRAE Trans-actions 108(1).
3. ANSI/ASHRAE Standard 113-1990, Method of Testing for RoomAir Diffusion Section 8.
4. ANSI/ASHRAE Standard 55-1992/55-2000R, Public Review Draft,Thermal Environmental Conditions for Human Occupancy, Section 5.2.4.3.
5. ANSI/ASHRAE Standard 62-2001, Ventilation for AcceptableIndoor Air Quality Appendix C.
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