DUCT EFFICIENCY AND HEAT PUMP PERFORMANCE

Paul Francisco

David Baylon

Ecotope, Inc.

House Assumptions

• House over crawl space– 1350 square feet– 3.5 ton heat pump or electric furnace– 10% leakage on both supply and return sides

• House with half basement– Approx. 2200 square feet– 3.5 ton heat pump or electric furnace– 5% supply leakage, no return leakage

Equipment Assumptions

• 3.5 ton heat pump is rated at HSPF of 8.2, SEER of 14

• For heating, resistance added to meet load, compressor used as much as possible

• Electric resistance furnace sized to meet load

• Some prototypes use different heat pumps, both bigger and smaller

Effect of Ducts

• Duct losses add to the load that must be met by the heat pump or furnace– supply leakage and conduction result in

capacity that does not make it to the house– return leakage and conduction result in changed

entering conditions at the equipment

Effect of Ducts

• Ducts in buffer spaces see different ambient conditions than the house– in heating season, crawl space often warmer

than outside but colder than house– in cooling season, crawl space often cooler than

house or outdoors– in cooling season, attic often hotter than house

or outdoors

Savings from added insulation - Portland electric furnace

Heating - 1350 ft^2 house - EFAPre-1980 1980-1993 Post-1993

Uninsulated ducts -31.2% -33.8% -33.8%Add R-4 Supply 6.0% 6.4% 6.4%Add R-11 Supply 10.3% 11.0% 11.0%Add R-4 Return 0.5% 0.5% 0.5%Add R-11 Return 0.8% 0.9% 0.9%

R-4 to R-11 Supply 4.6% 5.0% 5.0%R-4 to R-11 Return 0.3% 0.4% 0.4%

Heating - half-basement house - EFAPre-1980 1980-1993 Post-1993

Uninsulated ducts -12.6% -13.8% -13.8%Add R-4 Supply 3.2% 3.4% 3.4%Add R-11 Supply 5.5% 6.0% 6.0%Add R-4 Return 0.0% 0.0% 0.0%Add R-11 Return 0.0% 0.0% 0.0%

R-4 to R-11 Supply 2.4% 2.6% 2.6%R-4 to R-11 Return 0.0% 0.0% 0.0%

Duct Loss for Electric Furnace

Location

Sea

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Sea

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Loss

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10

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Uninsulated DuctsWith R-11 added to Supply

Crawl Space House Half Basement House

Duct Loss for Cooling

Location

Sea

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Por

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Sea

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Loss

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Uninsulated DuctsWith R-11 added to Supply

Crawl Space House Half Basement House

Duct Loss for Heating(Heat Pumps)

Location

Sea

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Por

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Sea

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20

40

60

80

100

120

Uninsulated DuctsWith R-11 added to Supply

Crawl Space House Half Basement House

Summary of Duct Effects

• Duct insulation can account for up to about 1/3 of duct losses at these levels of leakage

• Return insulation has little impact• Ducts have little impact in prototype with half basement• Duct efficiency changes between heating and cooling, with

cooling being higher• Largest impact of duct loss is for heating with heat pump due

to greater use of resistance at warmer temperatures - this effect very large in colder climates (more than doubles energy use in Missoula if ducts are uninsulated)

• Duct efficiency and percentage savings not heavily dependent on house vintage

Heat Pump Performance

• Heating performance dependent on outdoor temperature.– Heating capacity of compressor reduces 30% between

47oF and 30oF– Air delivery temperature is reduced by 10oF to less than

85oF resulting in severe comfort problems– Electric resistance is brought on to increase heating

capacity and delivery air temperature– To control these effects the manufactures recommend a

“Low Ambient Cutout” control set to about 30oF that transfers turns off the compressor and uses the elements only.

Seattle

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Portland

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Boise

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Spokane

Missoula

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Heat Pump Performance Adjustments

• Heat pump set-up determines the overall performance– Improper charge and/or air flow results in a 5-12%

reduction in COP – Defrost control reduces performance by 2% for demand

defrost and 10-12% for timed defrost– Crankcase heater can be similar to defrost in cold

climates– QC protocol necessary to insure that proper installation

and specification are met

Partload adjustments by Climate zone: Heating

PTCS Installation SpecsHSPF Climate adjustment

ARI Rating 6.8 7.1 8 8.12 8.3

Portland 7.1 7.4 8.3 8.4 8.6 1.04Seattle 7.1 7.5 8.4 8.5 8.7 1.05Boise 6.3 6.5 7.4 7.5 7.6 0.92Spokane 6.2 6.5 7.3 7.4 7.6 0.91Missoula 6.1 6.3 7.1 7.2 7.4 0.89

No PTCS Installation Specs: Charge & FlowDF&charge adjustment (Proctor, 91)

Portland 6.7 7.0 7.9 8.0 8.2 0.95Seattle 6.8 7.1 8.0 8.1 8.3 0.95Boise 5.9 6.2 7.0 7.1 7.3 0.95Spokane 5.9 6.1 6.9 7.0 7.2 0.95Missoula 5.7 6.0 6.8 6.9 7.0 0.95

No PTCS Installation Specs: Control with Low Ambient CutoutControl adjustment

Portland 6.1 6.4 7.2 7.3 7.4 0.906Seattle 6.3 6.6 7.4 7.5 7.7 0.928Boise 4.3 4.5 5.1 5.2 5.3 0.728Spokane 4.2 4.4 5.0 5.0 5.2 0.719Missoula 4.0 4.2 4.7 4.8 4.9 0.695

Performance Adjustments

Heat Pump Cooling Performance

• Cooling performance rating uses SEER to indicate cooling energy requirements

• Actual cooling seasonal COP depends on ambient humidity and temperature in the climate

• Effective average for seasonal cooling performance about 65% of the SEER rating– A SEER of 14.0 is 9.1– Duct performance can reduce this efficiency by 10-30%– Not very dependent on Northwest climate

Seasonal COP & DuctsCooling - Spokane

Seasonal COP & DuctsCooling - Portland

Duct Efficiency Impact on Heat Pump Performance

• Duct losses can reduce heat pump performance to less than half of rated COP

• Much more severe in cold climates

• Overall performance depends on quality duct installation as much as quality heat pump installation

Seasonal COP & DuctsHeating - Spokane

Seasonal COP & DuctsHeating - Portland

Summary• Overall system efficiency strongly interactive between ducts and

conditioning system• Significant duct losses can largely eliminate potential savings from

heat pumps• Overall system efficiency requires quality control of both heat pump

installation and duct installation• Heat pump controls crucial to overall performance, especially in cold

climates• HSPF and SEER rating numbers are poor approximations of actual

performance, even with ideal installation• Percentage improvement from lower to higher SEER may be

appropriate