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ENERGY FACT SHEETHeat Pumps for Residential

Heating and CoolingSome Questions and Answers

Larry W. TurnerExtension Agricultural Engincer

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WHAT IS A HEAT PI,IMP?

A heat pump is an electrically-ponered refrigeration-cycle device which is capable ofproviding both heating andcooling to a residence. In the sumrrcr, it operates like an airconditioner by "moving" heat from inside the house to theoutside. In the winter, the refrigerant flow is reversed,"moving" heat into the house by further chilling the al-ready cold outside air and transferring the heat gained intlrc process to the inside.

HOW DOES A HEAT PI,'MP WORK?

A heat pump works according to the same principlesas refrigerators or air conditioners. In a refrigerator, forexample, heat is rejected from inside the refrigBrator to thesurrounding air. On older models you can actually see thecondenser coil on the outside. It is hot wtren the unit isrunning because heat from inside is being rejected throughthis condenser coil.

The basic components of any such system are acompressor, evaporator coils, condenser coils, expansionvalve or mOtering device, and a reversing valve. Figure Ishows how the components work together in the coolingcycle of a heat pump. Warm indoor air passes over theindoor evaporator coils and gives up heat to the refrigerantcausing the refrigerant to change from a liquid to a coolvapor, or "evaporate". The compressor then pumps thevapor, increasing its pressure to make a hot, high-pressurevapor. When this hot vapor passes through the outdoor con-denser coil, heat is given off to the air and the vaporcondenses to form a liquid. This hot high pressure liquidpasses through an expansion valve, which reduces thepressure, cooling the liquid. The cool liquid refrigerant thenpasses back through the evaporator and returns to a vaporfornl continuing the cycle. Thus, heat has been movedfrom inside the house to outside, cooling the inside space.The enerry required to operate the cycle is the electricalenergy which powers the compressor and fans.

When a heat pump is used for heating a home, therefrigerant flow is reversed. The indoor coil now becomesthe condenser and the outdoor coil becomes the evaporator(Figure 2). In this mode heat is picked up from the outdoorair by the refrigerant and is moved to the space to beheated. Even though the outdoor air may be OoF or lower,

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there is still some heat available. Since the refrigerantvaporizes at a very low temperature, some of this heat fromthe outdoor air can be added to the refrigerant. Thecompressor pressurizes the cool vapor, which warms it to ausable temperature for space heating. Return air is warmedas it passes over the inside condenser coil, and the refriger-ant continues through the cycle.

HOW IS HEAT PT,JMP CAPACITY RATED?

Heat pumps are rated according to their cooling andheating capacities in BTU's per hour. The capacities for

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heating and cooling are different. They also vary with out-door temperature. Heating capacities are rated at 47oFoutdoor or condenser air temperature for a standard ratingand also at l7"F outdoor air temperature for an applicationrating. Figure 3 shows the variation of capacity of a typicalheat pump with outdoor temperature. As the outdoortemperature decreases, the heating capacity decreases. Thisis due to the fact that there is less heat available in the airwhich causes the evaporator refrigerant pressure to drop,decreasing the gas density of the refrigerant. Since therefrigprant is less dense, fewer pounds are circulatedthrough the system and less heat can be moved. Efficiencydecreases also with decreasing outdoor temperature, al-though it does not drop as rapidly as capacity. This isbecause at the lower temperatures less work is required to

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move the refrigerant through the system, since it is lessdense. The S-shaped curve indicates capacity. The straightline represents the change in heat loss of a typical housewith temperature. The balance point is where the heatpump capacity is equal to the house heat loss. At outsidetemperatures below the balance point additional heat mustbe provided beyond that available from the heat pump ifthe house is to be kept at the desired temperature.

Cooling capacity is also variable with outdoortemperature, although not as severely, as shown in Figure 4.The variation is less because of the smaller temperaturerange over which the heat pump operates in cooling. Heatpump cooling capacity is rated at 95oF outdoor airtemperature for the condenser coil.

Cooling capacity may also be expressed in tons.Before the era of mechanical refrigeration, ice was used as acooling medium. A ton of ice melting in 24 hours willabsorb heat at the rate of 12,000 BTU/hr. Thus, a heatpump which can remove heat at the rate of 12,000 BTU/hris referred to as having a capacity of one ton.

HOW IS THE EFFICIENCY OF AIIEAT PT.JMP EVALUATED?

There are several methods of measuring heat pumpefficiency. One is the Energy Efficiency Ratio, or EER. TheEER is commonly used as a measure of the performance ofair conditioners. Since a heat pump acts as an air condi-tioner when on the cooling cycle, it may also be rated usingthe EER. The EER is simply the number of useful BTU's ofcooling capacity for every watt of electrical energy inputrequired to drive the compressor and fans. For example, if aheat pump is operating on the cooling cycle with a capacityof two tons of refrigeration (24,000 BTU/hr) and is con-suming 3,120 watts per hour, including power for fans, the

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The higher the EER, the more efficient is the unit.The measure of heat pump efficiency in the heating

cycle which has been used for nxlny years is the coefficientof performance, COP. This rating is similar to the EER,except that the comparison is entirely based on BTtPs. Todetermine the COP of a heat pump, the following formulacan be used:

.^D : BTU's useful enerry pumpedL v r _ @

To convert the energy input to the compressor and fans toBTLfs, we must multiply the wattage by 3.412 BTU's/watt.Thus:

COP =BTU/hr capacity

Unit wattage x3.412 BTUiwattFor example, if a heat pump operating at a 40o outdoortemperature has a heating capacity of 25,000 BTU/hr (as inFigure 3) and is consuming 3,660 watts per hour, the COPwould be:

25,000 BTU/hr = 2.03,660 watts/hr x3.412 BTU/watt

When electric resistance heat is used, it generates3.413 BTtIs of heat for each watt. Therefore, the COP of aresistance heater is one (1.0) and can never be any higher.

Electric resistance heat is produced by the resistanceof a coil or wire to the flow of electricity. For example, theheat produced by electric baseboard heaters, the coils onyour electric range, or by your toaster is resistance heat. Inan electric furnace using resistance heat, air is blown acrossthe coils to warm the air. If the COP of the heat pump inthis example is compared with that of resistance heat, it isapparent that the heat pump is more efficient. Under theoperating conditions evaluated, the heat pump produces 2.0times as much heat energy as an electric resistance heaterof the same wattage.

Although the heat pump performance looks attractiveat 40oF, the COP will drop as outdoor temperature drops.However, it will not fall off as rapidly as the capacity curveof Figure 3, since power input also decreases. Figure 5 indi-cates the variation of COP with outdoor temperature. Notethat at some point the heat pump is no more efficient thanan electric resistance heater in producing useful heat,although this point is near the extreme on the scale ofwinter temperature.

WHAT IS TTIE SEASONAL PERFORMANCE FACTOR?

The seasonal performance factor (SPF) of a heatpump is an expression of efficiency of the unit similar tothe COP. The difference is that the SPF is calculated overthe total heating season and includes the portion of theheat supplied through backup electric resistance heat. Thishas the effect of lowering the SPF from what it would bewith a heat pump alone. The SPF varies from slightly above1.0 (near electric resistance heat) up to typically not morethan 2.0, depending upon the installation and maintenanceof the unit. Although difficult to estimate, the average SPFof a well installed system is probably in the range of 1.7 -2.0. An SPF of 1.7 would provide 70% more BTU's of heatoutput for the electrical energy input than resistance heatalone. This would be a savings of 40Vo of the total energyused. Of course, the SPF will vary with the severity of thewinter, insulation level, quality of construction, type ofhouse and the quality of installation.

DOES A HEAT PT.]MP FI.]NCTION AS EFFECTIVELYIN THE NORTH AS IN THE SOUIH?

No. The more severe the winters. the lower the SPFwill be. This is due to the fact that the COP of a heat pumpdecreases as outdoor air temperature decreases, and the

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This resembles the cooling curve of an equally sized air conditioner.

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supplementary resistance heat must be used a largerpercentage of time.

WIIY DO I NEED ELECTRIC RESISTANCEHEAT WITHA HEAT PUMP?

Heat pumps are usually sized for the air conditioningload, since it is generally smaller than the heating load. Thereason for sizing the unit based on the smaller load is thatthe moisture levels in the home can be better controlled inthe summer if the heat pump operates over longer periodsto cool the home rather than providing relatively shortblasts of cold air. Since the unit is sized for the smallerload, some form of supplementary heat must be providedto boost the heating capacity for winter operation;although often a majority of heating hours fall in the rangewhere the heat pump alone can meet the heating require-ments.

The capacity of a given heat pump will drop withoutdoor temperature, as shown in Figure 3. If the heat loadcurve of the house is superimposed on the heat pumpcapacity curve as in Figure 3, it can be noted that at somepoint the curves intersect. At this point the house heat lossis equal to the heat pump capacity. This is known as thebalance point. Below this point supplementary heat mustbe provided, usually in the form ofelectric resistance heat.

Recommended practice is to add the supplementaryresistance heat in two or three stages or steps as outdoortemperature drops below the balance point. For example, ifthe balance point is at 32oF, the system might be designedso that the first stage (one third of total) would activateslightly below the balance point, the second stage in 22oF,

and the third at 12"F. This would maintain the environ-ment in the house until outside temperature was down to adesigr temperature of 2oF. The first stage is generally con-trolled by the indoor thermostat. The other two stages arecontrolled by thermostats which sense outdoor temper-ature.

Although the above practice is strongly recom-mended, often the method of installation is to tie all stagestogether to the indoor thermostat so that the entire bank ofresistance heat is activated if the heat pump cannot warmthe home. This will not allow the overall system to operateas efficiently since the total resistance heater bank is calledfor more frequently. By following the recommended prac-tice of staging the resistance heat in two or three stages,which depend on outdoor thermostats as well as the indoorthermostat, less cycling should be observed, an increasedcomfort level should be maintained. and the cost of opera-tion will be lowered.

CAT{ OPERATING A HEAT PI.IMP ON TIIEHEATING CYCLE DURING PERIODS OFWARM WEATHER DAMAGE TTIE I.JNIT?

Yes! A heat pump has a maximum outdoor tempera-ture above which it should never be operated on the heatingcycle. Typically, the range is 7Oo-75"F. The manufacturer'sliterature should be checked to find the exact temperature.High outdoor temperature results in high evaporator pres-sure which increases gas density and flow rate. This causesthe compressor to work harder and it may overload whenoperated above the specified temperature.

WHY DOES TIIE AIR FROM A HEAT PI.]MPSYSTEM FEEL COOLER THAN TTIATMY OLD FI,'RNACE PROVIDED?

A conventional oil or gas furnace, which burns fuel toproduce heat, raises the temperature of the air culculatedover the heat exchanger to a reldtively higher level thandoes a heat pump system. The conventional fumace typi-cally delivers l40oF air when the thermostat calls for heat,compared to 90o-100"F air from the condenser on the heatpump systern Since the l40oF air is obviously hot andbrings the house temperature up rapidly, the furnace mayrun for a very short period of time, depending upon theheat need. The heat puhp, in contrast, increasing thetemperature of the recirculated air in the house only20o-30oF, tends to feel cooler as the unit sraduallv addsheat to the home.

IS CHECKING AIR FILTERS IMPORTANT?

Yes! Airflow in the heating cycle of a heat pump iscritical to maintenance free operation of the unit. If airflow is reduced due to dirty filters, worn fan motor belts,air returns covered or blocked, or other causes, problemswill develop. Reduced air flow causes the condenser coil(the inside coil on the heating cycle) to overheat. Thiscauses the compression ratio to increase and can causeproblems in the compressor and also can tend to breakdown the lubricating oil. The potential compressor prob-lems include a higher amperage draw and thus a higherelectric bill and also overheating of the motor windings,which can destroy the compressor if left for long periods.The problems will not be readily apparent, either, due tothe fact that warmer than normal air will be circulated tothe space, rather than cooler air, in this situation. To avoidthese problems filters should be checked and cleaned orreplaced often.

CAN DUCT WORK BE A PROBLEM INMAINTAIMNG AIRFLOW?

Yes. The ducts must be sized so that proper airquantities can be maintained. If existing duct work is toosmall, it must be replaced to allow for the airflow needed.Long lengths of duct work not contained within the heatedspace can be a big source ofheat loss. These should be vrellinsulated to reduce that heat loss. Performance can suffergeatly if ducts in an attic, crawl space or unheated base-ment are not properly insulated. In addition, the duct workshould be as air-tight as possible. Any air leakage canincrease heat loss and adversely affect heat pump perform-ance.

HOW MUCH AIRFLOW SHOULD BE PROVIDED?

The correct amount of airflow is the amount whichwill heat or cool the space adequately without being draftyand keep the equipment operating within safe reliabilitylimits. The fan should be sized to provide air-flow in therange of 400450 CFM/ton. Drafts may be noticeable above450 CFM/ton. kss than 350 CFM/ton could cause com-pressor problems in the heating cycle and coil freeze-up inthe cooling cycle.

Sometimes drafts are created even with proper airflow due to improper register type or location. If theprimary function of the heat pump is heating, high walllocations for air returns should be avoided if possible.Drafts can be created and the heat pump will not operatequite as efficiently as when it draws cooler air along thefloor.

WHY DOES MY OUTSIDE I.JNIT SEEM TO*STEAM'OR "SMOKE''?

During the heating season when outdoor tempera-tures are below 45oF, frost or ice may periodically build upon the outdoor coil. When frost accumulates in the coil, theamount of air passing oyer the coil is reduced and thefinned surface is insulated by the frost, cutting down onheat transfer. This reduces efficiency. When the efficiencyis affected enough to reduce capacity appreciably, the frostmust be removed. This is done automatically by reversingthe flow of the refrigerant and temporarily operating in thecooling cycle. During this cycle, operation of the reversingvalve can be heard, the outdoor fan will stop, the compres-sor will continue to run and water may drain away from thebase pan of the outside unit. After the hot refrigerantwhich is circulated through the outside coil has melted thefrost, the unit will automatically resume a normal heatingfunction. When the outdoor fan restarts, a puff of steammay be blown from the outdoor unit. This is due to waterleft on the outdoor coil by the melting frost.

Since cool air will be blown into the room during thedefrost cycle, one stage of the electric heater is usuallyenergized to maintain the air temperature.

WHAT IS THE EMERGENCY IIEAT SWITCHON THE THERMOSTAT?

If a heat pump fails when the outdoor temperature isabove that required to energize the backup resistanceheaters, heat will not be supplied to the conditioned spaceuntil the thermostats close. In this situation, a manualswitch will bring on the electric strip heaters and locla outcompressor operation. The system should be operated inthis mode only until the heat pump unit can be serviced.

HOT{ IMPORTANT IS TTIE LOCATION ANDINSTALLATION OF THE OUTDOOR TJNIT?

Several factors will influence the placement of theoutdoor unit. The unit should be sheltered from prevailingwinds. Air recirculation should be avoided. This will reduceefficiency and could cause defrost problems. A commoncause is an overhead obstruction ofair flow such as locatingthe unit under a porch. The air blowing out the top of theunit tends to deflect off the ceiling and is drawn backthrough the fan.

The unit should be placed so that water can drainfrom the coil on a defrost cycle. It is possible that ice couldbuild up on the unit so that a normal defrost cycle wouldbe ineffective. To allow for proper air flow, drifting snowshould also be prevented if possible. The unit should bechecked periodically during the winter to make sure there isno ice or snow buildup restricting air flow.

Noise could be a problem if the unit is located near abedroom. Also, it should be close enough to the house tominimize the length of connections between the outdoorand indoor units.

WHAT ABOUT MGHTTIME SETBACKOF TTIE THERMOSTAT?

It is not recommended for heat pump systems. Someenerry can be saved through night setback (about 8-l0Vo)nmost heating systems. In the case of a heat pump, however,a problem arises when the thermostat is turned back up intlrc mornings. The thermostat will sense that the tempera-ture is not rising rapidly enough. This will then activate theelectric resistance heat and therefore reduce the heatingefficiency, negating the positive effects of reducing thethermostat setting.

CAT{ ZONE HEATING BE INCORPORATEDINTO A HEAT PT,JMP SYSTEM?

It is possible, but only if separate units are used foreach zone. Since heat pumps are so dependent on air flow,the registers in each room cannot be shut offindividually orperformance will be affected. The use of tuo heat pumpunits can allow for individual control ,in two zones, butmight involve additional initial expense. Howver, in sometwo-story or rambling type houses, the reduction in theduct work required will balance the extra expense.

DO ALL HEAT PI.JMPS USE OUTSIDE AIRAS TIIE IIEAT SOURCE?

No. Water is also used, although for residentialsystems air is much more predominant. Wells and other

such sources can sometimes be used, although large suppliesof water are necessary. Earth-buried coils have also beenused but not extremely effectively.

HOW OFTEN SHOILD THE REFRJGERANTCHARGE BE CHECKED?

The refrigerant charge should probably be checkedonce a year by a competent serviceman, preferably prior tothe cooling season. A routine check of filters, belts, etc.should also be done prior to the heating season. In general,maintenance of a heat pump is more critical than it is forother systems. A well maintained unit will give betterperformance in terms of a higher COP and by saving moneyin operating costs.

INW DO I TELL IF MY I.]NIT IS NOT WORKING?

Obvious situations are those where safety pressureswitches or circuit breakers have tripped due to high pres-sures in the system and/or compressor overload. This mayoccur when air flow is reduced due to dirty air filters orblocked air registers. It also could be the result of atemporary overload. If the unit repeatedly trips safetydevices, then a service repairman should be called.

Some thermostats include a light to indicate whenresistance heat is being used. This is a separate indicatorfrom the emergency heat light. If the unit operates onresistance heat for long periods of time on fairly warmdays, there may be a problem with the heat pump.

A less obvious problem is when the supply air temper-atwe is either too high or too low. As discussed previously,the supply air temperature will be lower than that of aconventional furnace. Still, it should be at least 1OoF abovethat of the space to be heated. The normal temperaturerange across the indoor coil is 15o-20"F. This might bechecked by using an indoor-outdoor thermometer with theoutdoor bulb placed in the supply register while the indoorthermometer is used to measure room temperature. Thetemperature should be monitored when the resistanceheaters are not on and for a period of time to insure thatthe defrost cycle is not in progress. The problem ofsupplyair temperatures which are too high is more difficult todetect unless the results become apparent in terms of veryhigh temperatures or the tripping of safety switches.

Another test for proper functioning of the heat pumpis to simply feel the large line coming into the inside unit.This line should be too hot to hold on to. If it is cool, waitat least l0 minutes and check it again as it will be coolduring the defrost cycle.

WIIAT FACTORS SHOT'LD BE CONSIDERED INCOMPARING A IIEAT PI'MP TO OTHERIIEATING SYSTEMS?

The cost of fuel is one major consideration. A heatpump falls somewhere in the midrangS of fuel costs. At thepresent time, natural gas would be cheapest, then fuel oil,heat pump, P g"r, and finally electric resistance heat.Where electricity is the only fuel available, a heat pump hasa definite advantage over resistance heat. The initial equip-ment gpnerally costs more tlran a conventional heatingsystem, but the heat pump also provides air conditioning in

the same unit. Zone heating and night setback of thethermostat are not as practical with a heat pump.

Installation and maintenance are important in theperformance of a heat pump system. If qualified individualsare not available to install and service the unit, there maybe problems later.

A well installed and maintained heat pump systemwill be efficient and provide a clean, flameless heat.Particularly where electricity is the only option arailableand/or air conditioning is desired, a heat pump appears tobe a wise choice for heating.

Thecoiegeo|AgticUl|urisanEqualopportunityorganizationwithrspecttoedrindividualsandins|ilutionsthat|unctionwithoutrega|dtorce'coloLna|ionaloigin1964,Titlelxo|theEduca|ione|Amendments,seclion504o|lheRehabil itationActandothe|rla|edmalte.sshouldbedirctedtoEqUalopduKenlucky, Room S-105, Agticulturcl Science Building-North, Lexinglon, Kentucky 40546.

Seryice, University of Kenlucky College of Agriculture. Lexington. and Kentucky Slale University. Franklort.

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