DUCT EFFICIENCY AND HEAT PUMP PERFORMANCE

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DUCT EFFICIENCY AND HEAT PUMP PERFORMANCE

• Paul Francisco
• David Baylon
• Ecotope, Inc.

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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

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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

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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

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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

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Savings from added insulation - Portland electric
furnace
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Duct Loss for Electric Furnace
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Duct Loss for Cooling
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Duct Loss for Heating(Heat Pumps)
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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

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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.

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Seattle
13
Portland
14
Boise
15
Spokane
16
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

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Performance Adjustments
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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

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Seasonal COP DuctsCooling - Spokane
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Seasonal COP DuctsCooling - Portland
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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

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Seasonal COP DuctsHeating - Spokane
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Seasonal COP DuctsHeating - Portland
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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