BUILDING AMERICASYSTEMS ENGINEERING APPROACH TO DEVELOPMENT
OF ADVANCED RESIDENTIAL BUILDINGS
10. Case Study Material
SUBCONTRACT ADC-1-30469-00
MIDWEST RESEARCH INSTITUTE,NATIONAL RENEWABLE ENERGY LABORATORY DIVISION,
1617 COLE BOULEVARD,GOLDEN, CO80401-3393
CONSORTIUM LEADER:
BUILDING SCIENCE CORPORATION
70 MAIN STREET, WESTFORD, MA(978) 589-5100 / (978) 589-5103 FAX
CONTACT: BETSY PETTIT, AIA
CONSORTIUM MEMBERS:
PULTE HOME CORPORATION
ARTISTIC HOMES
CENTEX HOMES
DEL WEBB
ENGLE HOMES
PRAIRIE HOLDIINGS LTD.DEC INTERNATIONAL, INC.
ENERGY EFFICIENT BUILDING ASSOCIATION, INC.THE DOW CHEMICAL COMPANY
15 NOVEMBER 2002
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 2
Conditioning Air in the Humid South:Creating Comfort and Controlling Cost*
BBBBaaaacccckkkkggggrrrroooouuuunnnnddddHouston, Texas. It’s quite likely humid…orhot…or both. You are a builder/buyer ofhigh performance homes (http://www.buildingscience.com/buildingamerica/targets.htm), and naturally want todeliver/buy a comfortable healthy home.So, just how do you most efficiently andcost-effectively cool and dehumidify insideair while maintaining sufficientintroduction of outside air for ventilation?And perhaps most importantly, how do youcondition for humidity when the seasoncalls for neither heating nor cooling? Theseare two knotty, nasty questions with abunch of potential answers, none of whichhave ever been systematically explored orcompared….until now.
One of the great things about the BuildingAmerica program is hooking up withbuilders who, while they may not be astickled with knotty nasty questions asbuilding researchers, are willing and able towork with building scientists and see if areasonable answer can create more valuefor them and their customers. Such abuilder is Pulte Homes of Houston, Texas(http://www.pulte.com/market.asp?pro=0&ck=58|106). Pulte Homes arranged fortwenty of their sold and occupied newhomes in the greater Houston area to be ina year-long study of six different HVAC-integrated dehumidification systems.
The homes were monitored hourly forindoor temperature and relative humidity(five locations), outdoor temperature andrelative humidity, and energy consumptionbroken down by source (cooling, airhandler unit, fan cycler, dehumidifier,ventilating fan). In this way, both thecomfort conditions provided and energyconsumed by each approach over time
could be quantified and compared. Here ishow each home was set up:
• Control Homes Three of the Pultehomes monitored are pre-BuildingAmerica—they are not highperformance homes in terms of theirbuilding envelopes, their air tightness,and ventilation (they lacked mechanicalcentral ventilation). These homesrepresent standard construction practiceand performance in the Houston area.
• Building America High PerformanceHomes Three homes were built to theBSC Building America performancetargets (see: http://www.buildingscience.com/buildingamerica/targets.htm). Thesehomes had no special dehumidificationcapability, just what the standardcooling system can provide (see theHVAC glossary and discussion ofsensible heat ratio that follows). Theresults from these high performancehomes provide a baseline in terms ofcomfort conditions and energyconsumption.
• Stand-alone conventionaldehumidifiers Two homes have aconventional dehumidifier installed in alouvered closet within the living spaceand two have the units installed in theconditioned attic. The “dry” airsupplied by these units is distributedthroughout each home by the central-fan-integrated supply ventilationsystem (http://www.buildingscience.com/resources/presentations/practical_approaches_residential_ventilation.pdf), a system that is standardin BSC Building America homes.
• Continuous filtration/ventilationThree homes have the DEC FilterVent (http://www.thermastor.com/
* This resource is based on the technical report of the study written by Armin Rudd – “Building America final Technical Paper: Advanced System PerformanceDehumidification Project, October 31, 2002.”
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 3
ventilation/ventilationcentral.html)and ducted dehumidifier located in theconditioned attic. These two unitscombine to provide air filtration, freshair ventilation, and dehumidification.
• Continuous filtration/ventilation/dehumidification Three homes havethe DEC Ultra-Aire APD (AirPurification Dehumidifier) (http://www.thermastor.com/dehumid/dehumidresid.html), a single unit thatprovides air filtration, fresh airventilation, and dehumidification.
• Ventilation with energy recoveryThree homes have Venmar ERVs(http://www.efi.org/products/ventilat/venterv.html) (Energy RecoveryVentilator) located in the attic. ERVsrecover energy associated with both theheat and the moisture content of theexhausted indoor and supplied outdoorair. The Venmar system uses adessicant wheel to accomplish thetransfer and recovery.
• High-efficiency, two-stagecompressor and ECM AHU Onehome has a high efficiency Carriercooling system with two-stagecompressor ( http://www.residential.carrier.com/products/airconditioners/set_aircond.htm) and an ECM(electronically-commutated motor) airhandler. The two-stage compressorprovides a lower capacity run settingfor more sustained operation and hencegreater dehumidification. And thevariable speed capability of the ECMair handler permits lower fan speedsettings that increase the contact time ofindoor air at the coils, also increasingdehumidification.
All of the test systems provide some levelof ventilation, cooling, and dehumid-ification. They differ in:• the way in which they dehumidify (as
part of cooling, as part of ventilation, or
as a part of an individual conditioningactivity);
• what air they dehumidify (circulatinginterior air, incoming fresh air, orboth);
• when they dehumidify (at what stage inthe conditioning sequence);
• under what range of conditions theycan dehumidify (only during cooling,only during ventilation, any time humidconditions exist), and finally;
• the level of air filtration they provide.
As much as possible, there was more thanone home with each system to reduce theeffect of occupants (their number, theirthermostat setpoints, and their activitiesthat generate moisture such as cooking,cleaning, bathing). And the homes areconcentrated in just a few Pultedevelopments to reduce the impact ofmicro-environmental differences.
This study set out to answer the followingquestions:• Which system(s) was the least
expensive? We report the builder’sinstalled cost.
• Which system(s) was the leastexpensive to run (operating cost)? Wereport both the total HVAC systemdaily energy cost as well as the cost ofjust the dehumidifying component, ifavailable or applicable.
• Which system(s) did the best overalljob in providing humidity control? Wereport the % of hours each system keptthe house average relative humiditybelow 60%.
• What are other specific advantages/disadvantages of each system? Wereport builder and homeownercomments made to researchers over thecourse of the project, in a written shortquestionnaire, and in a face-to-face exitinterview with the homeowner.
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 4
BBBBaaaassssiiiiccccssssHumidity 101Humidity is the water vapor in the air.There are two expressions of this vaporcontent—absolute and relative humidity.Absolute humidity refers to the totalamount of water vapor in the air. Relativehumidity refers to the amount of watervapor in air relative to the total amount ofwater vapor the air can actually hold. Aircan hold different amounts of water vaporat different temperatures—warmer air canhold more than colder air. That’s whywater can and often does condense on acold glass—the air just around the glassgets cold enough that it can’t hold all of itswater vapor and some condenses on theoutside surface of the glass.
So, if you raise the temperature of the air,it’s relative humidity goes down; cool theair down and the relative humidity goes up.Cool the air down enough so that therelative humidity becomes 100% and youhave reached that air’s dewpoint, or thepoint at which that air just can’t hold thatamount of moisture anymore. Raising orlowering the temperature of a given volumeof air changes the relative humidity but itdoes not change the absolute humidity. Thedewpoint temperature of air is often used todescribe or quantify absolute huimidity.
Relative humidity is critical to theperformance of buildings for two reasons.One, all living things sense and respond tohumidity based on the concentration ofwater in air--relative humidity--more thanthe absolute amount of water in the air--absolute humidity, within the temperaturerange of indoor environments. Two, in asurprising twist of science, materialsadsorb and desorb water based on relative,not absolute humidity.
H V A C G L O S S A R Y Air Conditioner A mechanical unit that uses a refrigerant(selected for performance attributes including boiling point,chemical stability, heat exchange capacity) under pressure incoiled tubes to pick up interior heat and release that heat to theexterior. An air conditioner consists of a compressor in linewith the tubes filled with refrigerant, a fan to move outside airpast outside coils (which shed heat), and an air handler tomove interior air past the interior coils (which pick up heat).As air cools on the interior coils, water vapor condenses anddrips off of these coils—this is why all air conditioners, yourrefrigerator included, have a condensate line or pan to collectand handle this liquid.
The EPA Energy Star program includes central (and room) airconditioners —http://yosemite1.epa.gov/estar/consumers.nsf/content/cac.htm. Qualifying units have a SEER of 12 or greater.
Dehumidifier There are three ways to wring (or condense)water vapor out of air—cool the air (thereby decreasing itsability to hold moisture); increase the pressure (“squeeze theair to reduce its ability to hold moisture); and pass the air overa dessicant (a substance that picks up moisture, like the silicabeads that are packed with electronic equipment). Aconventional room dehumidifier is like an air conditioner withall the parts of the system inside the conditioned space. As airis cooled, moisture in the form of either water droplets (or ice)builds up on the refrigerant coils and is collected in a pan formanual removal or piped to a drain. The key to efficientoperation of a dehumidifier is to continually move “new” airpast the coils to avoid frosting the coils or the energy penaltyassociated with defrosting the coils.
The EPA Energy Star program includes dehumidifiers—http://yosemite.epa.gov/estar/consumers.nsf/content/dehumid.htm. Qualifying dehumidifiers are evaluated based on
MainReturnDuct
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Manual damper (toadjust flow rate)
Central Return Box
Return grille
Gypsum ceiling
20 CFM ofconditionedair to circulatein attic
5" mediafilter
Wallcap
Supply air
20 CFM
Conditioned Attic
Living Space
Motorized damper(to control open time)
Normal stand-alonedehumidifier in hallcloset w/ louvered doornear central return
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 5
Why—believe it or not--you careabout humidityIt affects your comfort If air “feels” too dry(generally at relative humidity below about20% for most people), throats and eyes canget scratchy, skin can get cracked or flaky.If air “feels” too moist (generally at relativehumidity above about 70% for mostpeople), skin to skin contact can feelclammy and make you feel either “colder”or “warmer” depending on other factorssuch as the air temperature, the season, andclothing. The scientific understanding ofthermal comfort and the contributionrelative humidity makes to thermal comfortis laid out in ASHRAE Standard 55 andASHRAE’s 2001 Fundamentals, Chapter 8.Perhaps most importantly for this study,Chapter 8 states:
“…The upper and lower humiditylevels of the comfort zones are lessprecise….”
and:“…The upper humidity limits ofASHRAE Standard 55 were developedtheoretically from limited data….”
We will come back to this later afterpresenting the results of the study.
It affects the “comfort” of your buildingLet’s face it—if you ever have high relativehumidity and temperatures inside yourhome compared to the outside (or viceversa!), then you have just set up thepotential for a little dewpoint experimentsomewhere INSIDE your walls and/or roof.You just don’t want water vapor coolingdown and condensing inside your buildingassembly—the results are never good. Ifyou live in a cold climate you have thismoisture drive from the inside to theoutside in your whole house all winter. Ifyou live in a hot climate, you have thismoisture drive from the outside to the
their capacity (liters of water removal per day) and energyfactor (liters of water removed per kilowatt-hour) understandard test conditions.
Latent load This is the portion of cooling energy necessary toremove water vapor content of indoor air as the ability of theindoor air to hold moisture drops with lower air temperatures.
Sensible load This is the portion of cooling energy necessarystrictly to lower the indoor air temperature.
Sensible Heat Ratio SHR is the portion of total coolingcapacity that a specific HVAC unit delivers for sensible heatload removal. (Conversely the Latent Heat Ratio is 1 – SHR,or the portion of total cooling capacity that delivers latent heatload removal.) Several factors in the design of air conditionersaffect the unit’s SHR and performance tables containing thisrating are available to HVAC contractors. For hot humidclimates, the SHR should be no greater than .75.
SEER The Seasonal Energy Efficiency Rating is astandardized test for air conditioners and heat pumps. In thetest, the unit is operated under three different sets of indoorand outdoor conditions. It represents the total seasonal coolingoutput in Btu divided by the total seasonal electric input inwatt-hours. The minimum SEER by federal law is 10, whileunits are available with a SEER as high as 18. Most unitsinstalled today are between 10 and 13.
Manual S The Air Conditioning Contractors ofAmerica (ACCA) has standards for correctlysizing HVAC equipment (Manual J), for designingduct systems (Manual D) and for selecting airconditioning equipment to meet the design
loads—Manual S. Proper use of Manual S ensures that boththe sensible capacity and the latent capacity of your systemwill be adequate to meet the cooling load.
inside whenever you run your airconditioner.
What to do about humidity insideyour homeLet’s start with the easiest season—winter.We tend not to worry about high humidityas much during the winter because theoutside air is dryer than the inside air.Localized areas of highhumidity—bathrooms, kitchens, andlaundry rooms—are easy to control withexhaust fans and occupants know they havea problem that must be addressed whencondensation appears on the inside ofwindows in these rooms. And don’t
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 6
forget—moist interior air that is exhaustedis ultimately replaced with dry outside air.
It’s a different story in the summer—theoutside air often holds a lot of moisture andour air conditioners cool this air when webring it inside. Remember, inside air comesfrom outside—always. Cooling this airincreases its relative humidity.
But can’t an air conditioner removehumidity as well as heat? The answer isyes, to some degree. As interior air movespast the cold coils in your air handler, theair gives up heat (this is called sensibleheat removal) and moisture in the aircondenses out on the cold coils (this iscalled latent heat removal). There is even ameasure of how much of each type of heatremoval any air conditioner accomplishescalled the sensible heat ratio or SHR. AnSHR of .80 (a pretty typical SHR forresidential AC units) means that 80% of theAC unit’s heat removal capacity is sensibleheat removal and 20% is latent heatremoval.
Why your AC system may not provideenough humidity control• Climate and the shoulder seasons
You may live in a climate where youneed humidity control when you don’tneed cooling or when you don’t want topay to run the AC system just to giveyou a comfort bump (What everyoneelse calls fall and spring, buildingscience folks demote to “shoulderseasons” because, alas, there may belittle to no mechanical equipmentoperation called for!).
• Internal moisture generation Whileyour interior humidity levels areprincipally dependent on the moisturecontent of outside air, a significantdriver may be moisture generated by
occupants. Cooking, house cleaningwith water, showering/bathing, andclothes laundering/drying all dump lotsof moisture into the air; some homesneed humidity control year round justto handle internal moisture generation.
• Spot ventilation Externally-ventedexhaust fans in bathrooms, kitchens andlaundry rooms can go a long waytowards controlling for internalmoisture generation, but the switch hasto go on! Many people will use thesespot ventilation fans for odor control,but do not consistently use them formoisture control. Combine lack ofexhaust fan use with lots of internalmoisture generation, and you are aprime candidate for humidity controlthat goes way beyond your AC system.
The switch has to go off too, though.Remember that any air exhausted isreplaced with outside air. In hot humidclimates, extended or continuous use ofexhaust fans just means thatunconditioned outside air is beingpulled into replace conditioned insideair.
• High performance homes It’s ironicbut it’s a fact: high performance homeswith thermally efficient envelopesreduce the total sensible heat gaincompared to the latent heat gaincausing your AC to run less and givingit less of a chance to handle the latentload. High performance homes alsoreduce how much your AC runsbecause they extend the shoulderseasons, parts of the spring and fallwhen the higher performance meansless heat gain and less need for AC.
But can’t I just switch to an AC unitwith a lower SHR to reflect my high
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 7
• AC unit High performance homes inthe hot humid south need an AC unitwith an SHR of about .5 to deliveradequate humidity control as part oftheir operation, and there simply aren’tany residential units with SHRs thislow on the market. So, we just don’t
have the equipment to make the AC theanswer to controlling relative humidityin many homes.
Unfortunately, this is quite a list andchances are, more than one applies to youand your home. That’s the bad news. Thegood news is that the rest of this paper isabout systems that provide interiorhumidity control.
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 8
Dehumidifier portion $ 40 1Humidity Control Performance4: 93% 3
System Description The stand-alone dehumidifier system includes the installation of anoff-the-shelf dehumidifier in an interior closet with a louvered door, along with central-fan-integrated supply ventilation. Outside air is intermittently drawn in by normalthermostat-driven operation of the central cooling and heating system or by activation ofthe central air handler blower via the AirCycler™ control. The stand-alone dehumidifierhas a built-in dehumidistat that will energize the dehumidifier whenever the humiditylevel rises above the setpoint (nominally 50% relative humidity). Intermittent operationof the central air handler fan moves the dry air throughout the house. In this way, yeararound humidity control is achieved along with controlled mechanical ventilation forimproved indoor air quality.
1 The ranking is a simple way of comparing the relative performance of each system in the study. Bear inmind that these results are not normalized so the differences in house size, number of stories, number ofoccupants, thermostat setpoints, etc. have not been accounted for in this ranking. In other words, take eachranking with a grain of salt.2 The labor portion of the installed cost for each system was about the same. The difference in installed costis almost entirely determined by the cost of the equipment.3 This assumes an electric utility average rate of $0.11/kWh, a number derived from calculations used in theEnvironments for Living™ energy guarantee in Houston.4 This percentage is the amount of total time that the average interior relative humidity remained below thethreshold of 60%. The selection of 60% for the threshold in terms of comfort, health, and impact onbuilding durability is certainly an arguable one and is discussed in detail in a later section of this paper.
Coo
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MainReturnDuct
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Manual damper (toadjust flow rate)
Central Return Box
Return grille
Gypsum ceiling
20 CFM ofconditionedair to circulatein attic
5" mediafilter
Wallcap
Supply air
20 CFM
Conditioned Attic
Living Space
Motorized damper(to control open time)
Normal stand-alonedehumidifier in hallcloset w/ louvered doornear central return
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 9
System Advantages The off-the-shelf Energy Star dehumidifier in the hall closet is asimple, relatively inexpensive system for homes that have central-fan-integrated supplyventilation. It also provided good humidity control for quite reasonable operating cost.
System Disadvantages There is the loss of closet space and the noise of operation withinthe living space. There is the uncertainty of service life for light-duty dehumidifiers whenused for conditioning the volume of an entire house.
Homeowner Comments: The two homeowners with this system reported high levels ofsatisfaction with the system.
Researcher Comments/Observations: This system performed well even in a home with5 occupants, two of whom were home all day, and with almost no use of spot exhaustventilation fans in the kitchen, baths, and laundry rooms.
Building Science Corporation November 2002
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 10
Dehumidifier portion $ 482 6Humidity Control Performance: 99% 1
System Description The stand-alone dehumidifier in the conditioned attic systemincludes the installation of an off-the-shelf dehumidifier in the conditioned attic, alongwith central-fan-integrated supply ventilation. Outside air is intermittently drawn in bynormal thermostat-driven operation of the central cooling and heating system and byactivation of the central air handler blower via the AirCycler™ control. The stand-alonedehumidifier has a built-in dehumidistat that will energize the dehumidifier whenever thehumidity level rises above the setpoint (nominally 50% relative humidity). A smallreturn duct near the dehumidifier, and intermittent operation of the central air handler fan,moves the dry air throughout the house. In this way, year around humidity control isachieved along with controlled mechanical ventilation for improved indoor air quality.
System Advantages The off-the-shelf Energy Star dehumidifier in the conditioned atticis a simple, relatively inexpensive system for homes that have central-fan-integratedsupply ventilation. It also provided good humidity control for quite reasonable operatingcost. This system’s remote location in the conditioned attic also alleviated the problem ofoperating noise.
System Disadvantages As both the annual operating cost and humidity controlperformance numbers suggest, off-the-shelf dehumidifiers were over-dehumidifying inthe conditioned attic much of the time (see Researcher Comments for more detail). Thereis also the uncertainty of service life for light-duty dehumidifiers when used forconditioning the volume of an entire house.
Coo
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Coi
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Hea
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Coi
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MainReturnDuct
Gab
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nd W
all
Manual damper (toadjust flow rate)
Central Return Box
Return grille
Gypsum ceiling
20 CFM ofconditionedair to circulatein attic
5" mediafilter
Wallcap
Supply air
20 CFM
Conditioned Attic
Living Space
Motorized damper(to control open time)
Stand-alone dehumidifiernear 20 CFM attic returnair duct to pull dry airinto house
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 11
Homeowner Comments: The two homeowners with this system reported high levels ofsatisfaction with the system.
Researcher Comments/Observations: This system performed well even in a home with5 occupants (three of whom were home all day), with the homeowner reporting use of thekitchen and bath spot ventilation exhaust fans only about 50% of the time, and no use ofthe laundry room exhaust fan. But note that since this home and the representative one forthe dehumidifier in the hall closet were identical down to the model type, the largedifference in dehumidifier operating cost is likely due to over-dehumidification by thesystem with the dehumidifier in the attic. And the difference in total HVAC systemoperating cost may in large part be due to occupant behavior, such as thermostat setpoint.
Off-the-shelf dehumidifiers have a rather primitive humidity-sensing device that isapparently sensitive to both air moisture content and air temperature. This can result inthe dehumidifier running more than it should at a given setting when the dehumidifier isoperating in a space such as the conditioned attic where air temperatures ranged 10 ormore degrees higher than the main house. A solution to this performance problem wasdeveloped near the end of this study: “hardwire” the humidifier to a run-position but wirethe unit to a remote humidistat/control located in the conditioned space. This would addan estimated $40 to $50 to the system, but make its performance comparable to that ofthe off-the-shelf dehumidifier located in the hall closet.
System Description The Filter-Vent ventilation/dehumidification system includes theinstallation of a blower and filter system that is ducted to a stand-alone dehumidifierlocated inside a sheetmetal cabinet. The sheetmetal cabinet is ducted to the main supplyduct of the central air distribution system. These components are located in theconditioned attic. The Filter-Vent blower will operate continuously, drawing in about 40cfm of outside air, and mixing it with about 120 cfm of recirculated inside air. Themixed air is filtered and pushed through the dehumidifier cabinet where moisture isremoved as necessary according to the dehumidistat setting on the dehumidifier. Theresultant air is delivered to the main supply duct of the central air distribution system. Inthis way, year around humidity control is achieved along with controlled mechanicalventilation for improved indoor air quality.
System Advantages The Filter-Vent with ducted dehumidifier demonstrated excellentperformance in terms of humidity control and uniformity of conditions throughout thehouse, including the conditioned attic. It also has the added benefit of superior whole-house air filtration—the standard filter for the FV is a MERV-8, 2-inch pleated, but theunit accepts up to a total of 6 inches of filters, including activated carbon.
System Disadvantages This system has an installed cost more than double that of thestand-alone dehumidifiers. It also exhibited relatively high operating costs, in partbecause of the additional fan motor for the filtration/ventilation component and in part
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 13
because the system is continuously drawing in outside air and the associated added latentheat load.
Homeowner Comments: The one homeowner with this system who completed thewritten questionnaire gave the system a moderate satisfaction rating but noted that, as anasthmatic, she thought the system was far more effective in dealing with her symptomsathan the HVAC system in their previous residence. During the homeowner interviews,none of the homeowners expressed performance concerns with their systems, but one didexpress concern about the operating cost.
Researcher Comments/Observations: As with the stand-alone dehumidifiers located inthe conditioned attic, the ducted dehumidifiers in the Filter-Vent system had long runtimes, high operating costs, and tended to over-dehumidify. Since this situation can mostlikely be attributed to the dehumidifier's sensing device and its sensitivity to both relativehumidity and air temperature, the performance of this system would likely improvesignificantly if the ducted dehumidifier were controlled by a remote humidistat located inthe living space.
Ventilation portion $ 120 n/aHumidity Control Performance: 80% 6
System description The Energy Recovery Ventilation (ERV) system includes adesiccant wheel energy exchanger installed in the conditioned attic. The ERV bloweroperates continuously, drawing in about 40 cfm of outside air, and exhausting about 40cfm of inside air. Heat and moisture are exchanged between the incoming outside air andthe outgoing inside air, such that much of the heat and moisture stays on the side that itcame from. In this way, during the cooling season, the introduction of heat and moisturefrom ventilation air is lessened. This system will not dehumidify house air, but willlessen the need for dehumidification. During the heating season, the introduction of coldand dryness from ventilation air is lessened. The house exhaust air stream exits throughthe roof, and the tempered ventilation air is supplied to the main supply air duct of thecentral air distribution system.
System Advantages The ERV system has one primary advantage—operating cost. Itsenergy recovery reduces the load on the air conditioning system (and furnace, for thatmatter) and reduces latent load that incoming ventilation air could introduce to theconditioned space. In homes with low internal moisture generation, it’s lack ofdehumidification capacity for re-circulated air may not be critical to overall homeperformance.
System Disadvantages This system has an installed cost almost six times greater thanthat of the stand-alone dehumidifiers. Its humidity control performance is entirelydependent on occupant numbers and behavior in terms of internal moisture generationand use of spot ventilation.
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 15
Homeowner Comments: Two out of the three homeowners gave their overallHVAC/dehumidification system the highest satisfaction rating possible, while admittingthat they could not separately assess the value of their ventilation/dehumidificationcomponents.
Researcher Comments/Observations: It’s really interesting to look at a relativehumidity distribution for one of the homes with the ERV system and try and reconcilethat with the lack of comfort complaints from homeowners. Perhaps this is due tovariation in individual tolerances for humidity comfort conditions and/or personalschedules that mean a home is unoccupied during the periods of highest interior relativehumidity (For comparison sake, take a look at the RH distribution bar graph for one ofthe Ultra-Aire systems on page 17).
Room Relative Humidity19926 Ashland, 31-Jul-01 to 31-Jul-02
System Description The Ultra-Aire system includes a high-efficiency dehumidifierinstalled in the conditioned attic. The Ultra-Aire blower operates continuously on lowspeed, drawing in about 40 cfm of outside air, mixing it with about 120 cfm ofrecirculated house air, filtering that air, and supplying it to the main supply air duct of thecentral air distribution system. A dehumidistat located in the conditioned space willactivate the dehumidifier compressor if the humidity level rises above the setpoint(nominally 50% relative humidity). In this way, year around humidity control is achievedat high-efficiency along with controlled mechanical ventilation for improved indoor airquality.
System Advantages The Ultra-Aire system delivers near-perfect humidity control andamong-the-best total HVAC operating cost. The Ultra-Aire high efficiency filtrationsystem also delivers well above average overall air quality as well.
System Disadvantages This system has an installed cost nearly five times greater thanthat of the stand-alone dehumidifiers.
Homeowner Comments: All three homeowners with this system gave high marks foroverall satisfaction. The sole occupant of the representative house spoke highly of thissystem’s performance in light of her allergies, asthma, and sensitivity to some chemicals.
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 17
Researcher Comments/Observations: The only two homes reporting 100% use of bothbath and laundry spot ventilation exhaust fans and 80% use of kitchen fans were Ultra-Aire homes. This certainly contributed to the overall humidity control of the systems,although other systems probably needed this sort of diligence a lot more than the Ultra-Aire! The bar graph below certainly attests to the stellar moisture control of this system.
Room Relative Humidity19923 Ashland, 18-Oct-01 to 02-Aug-02
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Per manufacturer recommendations, both the Ultra-Aire and the Filter-Vent systems wereducted to the supply plenum (see diagrams). This configuration could result in reverse airflow across the air handler when the air handlers were not running but the 160 cfm,continuously-operating Filter-Vent and Ultra-Aire systems were. This could mean the re-evaporation of condensed water off of wet cooling coils, defeating at least a part of thelatent heat removal of the AC system. We did not observe this phenomenon in thisstudy's Filter-Vent and Ultra-Aire systems most likely because of the resistance imposedby the 5-inch pleated air filters in the systems. An alternative duct configuration thatalleviates the potential for this problem, however, is described in the summary section atthe end of this paper.
Dehumidification portion $ 120 3Humidity Control Performance: 80% 5
System description The enhanced dehumidification with cooling and central-fan-integrated supply ventilation system includes theinstallation of a Carrier cooling system with a 2-stagecompressor, an ECM indoor fan motor, and aThermidistat controller. The 2-stage compressor allowsbetter matching of the load to the cooling systemcapacity to avoid poor humidity control inherent withshort-cycling of over-sized systems. The electronicallycommutated motor (ECM) allows lowering the flow rateof air over the cooling coil for enhanced moisture
removal. The Thermidistat control is the smart thermostat that coordinates the 2-stagecompressor and ECM fan features to achieve enhanced humidity control, especially atpartial-load conditions.
System advantages The overall HVAC system is state-of-the-art ultra high efficiency.The total HVAC operating costs reflect the energy-saving capacity of the two-stagecompressor and ECM fan unit.
System disadvantages This system provided relatively poor dehumidification and comeswith among the heftiest cost premiums of all the systems.
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 19
Homeowner Comments: The single homeowner with this system gave it relatively lowvalue, stating that they could not really tell much of a change in comfort levels after thechange-out of the compressor and AHU fan motor.
Researcher Comments/Observations: This system did not provide the level ofdehumidification that was expected. It is possible that the humidity control performancecould be improved by:• lowering the fan speed during first-stage cooling (to keep the evaporator coil colder)
and• having the fan shut off immediately at the end of cooling calls (preventing re-
evaporation of condensed water on the evaporator coil).
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 20
SSSSuuuummmmmmmmiiiinnnngggg UUUUppppWe field-tested six different systems thatprovide supplemental dehumidificationbeyond what standard air conditionersprovide. The systems varied widely interms of their installed cost, operating cost,and moisture control performance. As in allstudies, we got some answers to thequestions we asked as well as some newquestions.
• The reduced sensible heat gainresulting from the superior thermalenvelope of Building America highperformance homes also reduces theopportunity for standard, evenproperly-sized AC systems to removelatent heat gain; that is, to dehumidify.
• Of the six humidity control systemstested in this field study, the systemsthat dehumidified recirculated air as afunction independent of the coolingsystem performed significantly betterthan the others.
• The stand-alone dehumidifier systemsdelivered good moisture controlperformance at a reasonable installedcost and reasonable operating cost. Thestand-alone dehumidifier systemrepresents the best overall strategy forproduction builders who choose to offersupplemental dehumidification in theirnew homes. But note the following:
The moisture control performanceof the stand-alone dehumidifiersystem is dependent on effectiveand regular mixing of all of theindoor air. A fan cycler that alsoincludes central-fan-integratedsupply ventilation handles both theintroduction of fresh outside air aswell as this necessary mixing.
If the preferred location of thestand-alone dehumidifier is theconditioned attic, then its
performance in terms of moisturecontrol and operating cost isdependent on remote location of anindependent humidistat.
Further, its performance can beimproved by using the ductconfiguration shown below. Thisessentially ties its function to theliving space while maintaining itslocation in the attic for soundattenuation and space saving.
Coo
ling
Coi
l
Hea
ting
Coi
lMain
ReturnDuct
Gab
le E
nd W
all
Manual damper (toadjust flow rate)
Central Return Box
Return grille
Gypsum ceiling
20 CFM ofconditionedair to circulatein attic
5" mediafilter
Wallcap
Supplyair
20 CFM
Conditioned Attic
Living Space
Motorized damper(to control open time)
Outside Air Duct -6" insulated flex-duct
Remote dehumidistat on wallinside house (disconnect�dehumidistat on unit)
Conditioning Air in the Humid South: Creating Comfort and Controlling Cost 21
• The Ultra-Aire system providedexcellent moisture control at areasonable operating cost with theadded benefit of high performance airfiltration. Hence, its relatively highinstalled cost may be acceptable toindividuals with particular indoor airquality sensitivities.
• The ducting configuration shown beloweliminates the potential for reverse airflow in both the Filter-Vent and Ultra-Aire systems.
• Because spot ventilation fans were usedso infrequently in most of the testhomes, the impact of their consistentuse in kitchens, bathrooms, and laundryrooms on overall humidity control isuncertain.
• The following factors affect the needfor supplemental dehumidification inBuilding America homes in hot humidclimates:
the relative humidity at which anindividual experiences and/orexpresses thermal discomfort
the number of occupants in a home
the use of spot ventilation fans inkitchens, baths, and laundry rooms
the local outdoor dewpointtemperatures throughout the year.
The bad news is that there is only one ofthese—dewpoint temperatures—thatbuilders can get hard-and-fast informationon. And there is only one other—use ofspot ventilation fans—over which builderscan exert any degree of control (by hard-wiring exhaust fans to light switches oreducating homeowners about theimportance of using exhaust fans formoisture control).
The good news is that this study identifieda good supplemental dehumidificationsystem that has both a reasonable first andreasonable operating cost, and that can bereadily installed either during constructionor after occupancy. The prudent thing todo? Install the supplementaldehumidification in hot humid climates andsell the greater comfort and improvedindoor environmental quality as part of ahigh performance home package.
Conditioning Air in the Humid South—Creating Comfort and Controlling Cost
About this Report
This report was prepared for the US Department of Energy’s Building America
Program.
Direct all correspondence to: Building Science Corporation, 30 Forest Street,Somerville, MA 02143.
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