ASHRAE Journal - April 2010 - 66

Dual-Capacity Heat Pumps
By Steve Kavanaugh, Ph.D., Fellow ASHRAE

A

rchitect Jaquelin Camp was invoiced $17,250 in 2008 for a circulator pump module and a 5 ton (18 kW), dual-capacity water-to-air heat pump with a rated EER of 25.6 (Btu/W · h). The ground loop and duct costs were an additional $10,500 and $6,000, respectively. Although she has been pleased with the reduction in heating costs in her home compared to the replaced oil-fired furnace, she wondered if she may have paid too much when she more clearly understood the method used to attain the high rating of the heat pump. This column explains the rating system and the actual performance when efficiency and capacity are corrected to the full-load and part-load conditions that are experienced in the field. This will enable building owners, such as Camp, to determine if the added cost and complexity of dual-capacity heat pumps are warranted. The emphasis is in the cooling mode where the discrepancies between the rated and actual values are more pronounced.
ISO Standard 13256-1

boiler. Groundwater heat pump (GWHP) ELTs are based on groundwater being pumped directly to the units and are appropriate for residential applications in moderate climates. The full-load and part-load ELTs for ground loop heat pumps (GLHPs) are appropriate for cold-climate residential applications but not optimal for most commercial systems, and moderate- or warm-climate residential systems.2 The power used to compute EER does not include the fan power required to overcome duct losses or high performance filters. The standard allows the fan power to be reduced from the value measured during testing to a value for zero external static pressure by assuming the fan is 30% efficient. The heat that is generated by this fan power is added back to the cooling capacity. The assumption is that the designer is aware of this procedure and has access to the necessary tools to make the corrections to actual capacity and efficiency once the pressure losses of the air-distribution system and filter are known.
Efficiency Comparisons

Table 1 summarizes the air and water temperatures used to generate the rated performance of water-to-air heat pumps.1 Reported values are total cooling (TC in Btu/h), energy efficiency ratio (EER in Btu/W · h), heating capacity (HC in Btu/h) and coefficient of performance (COP in WThermal/WElect). Four sets of rating conditions are used to represent approximations of conditions occurring for various applications as shown in Table 1. Water loop heat pump (WLHP) entering liquid temperatures (ELTs) assume the units are connected to a cooling tower and

Table 2 compares the rated EERs and COPs of a 4 ton (14 kW) dual-capacity heat pump with a similar size single-capacity heat pump from the same product line.3 Note the single-capacity heat pump EERs are 8% to 13% higher when compared under full-load conditions. The dualcapacity machines have a high EER at part-load conditions, and this is the primary reason dual-capacity machines are claimed to be more energy efficient. However, the standard permits the fan to be operated at full-load flow rate, which increases cooling capacity and total power. The significant reduction in capacity and efficiency is not readily apparent since much of

Dual-Capacity Versus Single-Capacity Heat Pumps
Disadvantages
o o o o Higher electrical demand and lower efficiency at full-load Lower or equal efficiency at part-load Higher cost Added mechanical complexity likely to increase service requirements o Added mechanical complexity likely to decrease equipment life o Added electronic complexity likely to increase service requirements o Added electronic complexity likely to increase required technician sophistication o Predecessor technologies abandoned (reversing compressor, inverter drives) o Current technology of choice only reduces capacity by 20% to 25%

Advantages
o Suited to applications with heating load-to-cooling load mismatches so that high-speed capacity can reduce need for less efficient auxiliary heating o Modest reduction (20% to 25%) in on-off cycles o Higher rated efficiency provides marketing advantage (for vendor/manufacturer)

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ASHRAE Journal - April 2010

Table of Contents for the Digital Edition of ASHRAE Journal - April 2010

ASHRAE Journal - April 2010
Contents
Commentary
Industry News
Letters
Meetings and Shows
Feature Articles
The Science of Evaporation is Key to Defense in Murder Trial
Selecting DOAS Equipment with Reserve Capacity
Technology Award Case Studies: Greening Hospitals
Technology Award Case Studies: Sustainable Remedy for Hospital
Building Sciences
Emerging Technologies
Technical Topics: Selecting Efficient Fans
Technical Topics: Dual-Capacity Heat Pumps
IAQ Applications
International Column
Classified Advertising
Advertisers Index
ASHRAE Journal - April 2010 - Intro
ASHRAE Journal - April 2010 - ASHRAE Journal - April 2010
ASHRAE Journal - April 2010 - Cover2
ASHRAE Journal - April 2010 - 1
ASHRAE Journal - April 2010 - 2
ASHRAE Journal - April 2010 - Contents
ASHRAE Journal - April 2010 - Commentary
ASHRAE Journal - April 2010 - 5
ASHRAE Journal - April 2010 - Industry News
ASHRAE Journal - April 2010 - 7
ASHRAE Journal - April 2010 - 8
ASHRAE Journal - April 2010 - 9
ASHRAE Journal - April 2010 - 10
ASHRAE Journal - April 2010 - 11
ASHRAE Journal - April 2010 - 12
ASHRAE Journal - April 2010 - 13
ASHRAE Journal - April 2010 - 14
ASHRAE Journal - April 2010 - 15
ASHRAE Journal - April 2010 - 16
ASHRAE Journal - April 2010 - 17
ASHRAE Journal - April 2010 - Letters
ASHRAE Journal - April 2010 - 19
ASHRAE Journal - April 2010 - Meetings and Shows
ASHRAE Journal - April 2010 - 21
ASHRAE Journal - April 2010 - The Science of Evaporation is Key to Defense in Murder Trial
ASHRAE Journal - April 2010 - 23
ASHRAE Journal - April 2010 - 24
ASHRAE Journal - April 2010 - 25
ASHRAE Journal - April 2010 - 26
ASHRAE Journal - April 2010 - 27
ASHRAE Journal - April 2010 - 28
ASHRAE Journal - April 2010 - 29
ASHRAE Journal - April 2010 - Selecting DOAS Equipment with Reserve Capacity
ASHRAE Journal - April 2010 - 31
ASHRAE Journal - April 2010 - 32
ASHRAE Journal - April 2010 - BRC1
ASHRAE Journal - April 2010 - BRC2
ASHRAE Journal - April 2010 - 33
ASHRAE Journal - April 2010 - 34
ASHRAE Journal - April 2010 - 35
ASHRAE Journal - April 2010 - 36
ASHRAE Journal - April 2010 - 37
ASHRAE Journal - April 2010 - 38
ASHRAE Journal - April 2010 - 39
ASHRAE Journal - April 2010 - 40
ASHRAE Journal - April 2010 - 41
ASHRAE Journal - April 2010 - Technology Award Case Studies: Greening Hospitals
ASHRAE Journal - April 2010 - 43
ASHRAE Journal - April 2010 - 44
ASHRAE Journal - April 2010 - 45
ASHRAE Journal - April 2010 - 46
ASHRAE Journal - April 2010 - 47
ASHRAE Journal - April 2010 - 48
ASHRAE Journal - April 2010 - 49
ASHRAE Journal - April 2010 - Technology Award Case Studies: Sustainable Remedy for Hospital
ASHRAE Journal - April 2010 - 51
ASHRAE Journal - April 2010 - 52
ASHRAE Journal - April 2010 - 53
ASHRAE Journal - April 2010 - Building Sciences
ASHRAE Journal - April 2010 - 55
ASHRAE Journal - April 2010 - 56
ASHRAE Journal - April 2010 - AP1
ASHRAE Journal - April 2010 - AP2
ASHRAE Journal - April 2010 - AP3
ASHRAE Journal - April 2010 - AP4
ASHRAE Journal - April 2010 - 57
ASHRAE Journal - April 2010 - 58
ASHRAE Journal - April 2010 - 59
ASHRAE Journal - April 2010 - Emerging Technologies
ASHRAE Journal - April 2010 - 61
ASHRAE Journal - April 2010 - 62
ASHRAE Journal - April 2010 - 63
ASHRAE Journal - April 2010 - Technical Topics: Selecting Efficient Fans
ASHRAE Journal - April 2010 - 65
ASHRAE Journal - April 2010 - Technical Topics: Dual-Capacity Heat Pumps
ASHRAE Journal - April 2010 - 67
ASHRAE Journal - April 2010 - 68
ASHRAE Journal - April 2010 - 69
ASHRAE Journal - April 2010 - IAQ Applications
ASHRAE Journal - April 2010 - 71
ASHRAE Journal - April 2010 - 72
ASHRAE Journal - April 2010 - 73
ASHRAE Journal - April 2010 - International Column
ASHRAE Journal - April 2010 - 75
ASHRAE Journal - April 2010 - 76
ASHRAE Journal - April 2010 - 77
ASHRAE Journal - April 2010 - 78
ASHRAE Journal - April 2010 - 79
ASHRAE Journal - April 2010 - 80
ASHRAE Journal - April 2010 - 81
ASHRAE Journal - April 2010 - 82
ASHRAE Journal - April 2010 - 83
ASHRAE Journal - April 2010 - 84
ASHRAE Journal - April 2010 - 85
ASHRAE Journal - April 2010 - Classified Advertising
ASHRAE Journal - April 2010 - 87
ASHRAE Journal - April 2010 - Advertisers Index
ASHRAE Journal - April 2010 - Cover3
ASHRAE Journal - April 2010 - Cover4
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