HVAC improvements in manufactured housing crawlspace-assisted heat pumps.
Over 6500 energy-efficient manufactured homes were built and sited in the Pacific Northwest in 2005, with roughly 60% built to energy-efficiency program specifications (NEEM 2004). Roughly 30% of these homes have air-source split-system heat pumps.(1) Pacific Northwest electric utility incentive programs encouraged the use of high-efficiency heat pumps in energy-efficient manufactured homes instead of electric forced air furnaces to reduce annual energy use and winter peak heating loads. One challenge for these utility programs is to maximize heat pump performance by ensuring adequate system airflow and refrigerant charge and by minimizing defrost and excessive resistance heat energy use (Lubliner et al. 2005). The factory installation of crawlspace air-source heat pumps in energy-efficient manufactured housing may help overcome some of the commissioning and first-cost challenges associated with site-installed heat pumps.
In the late 1970s and early 1980s the U.S. Department of Energy (DOE) funded research on crawlspace-assisted heat pump (CAHP) technologies. Product development in the early 1980s resulted in a commercially available technology targeted for factory-built housing with ventilated crawlspaces (EPRI 1982). Over the years a number of manufacturers have sold the technology under a specific brand name, but it is no longer available on the market.(2) The single packaged unit CAHP technology was expected to be installed and commissioned at the factory at a lower cost than typical after-market heat pump split systems installed by HVAC contractors.
In 2002, two energy-efficient research homes were built to U.S. Department of Housing and Urban Development (HUD) code, sited near Lewiston, ID, and were occupied and monitored. One of these homes was built to ENERGY STAR[R] (ESTAR) specifications typical of new energy-efficient HUDcode Pacific Northwest homes, while the other, called the zero energy manufactured home (ZEMH), was built as a research and demonstration home to help DOE evaluate higher levels of energy efficiency and renewable energy systems. Both homes have vented concrete foundation crawlspaces and insulated floors. The ZEMH has R-19 insulation on the above-grade crawlspace walls, while the ESTAR home has none. The ZEMH is not commercially available but represents the nation's most efficient HUD-code home. The researchers conducted detailed monitoring and field testing of the HVAC systems on these homes, and have included CAHP heat pump performance evaluations in this paper. Information on data instrumentation and field testing results is provided in a previous research paper (Lubliner et al. 2004).
The researchers also evaluated maintenance and operation information from a trailer park rental community of 49 single-section energy-efficient manufactured homes where CAHP units were installed in the 1990s, other Pacific Northwest installations, and the experiences of a HUD-code home manufacturer who installed over 1000 CAHP units in the US Southeast between 2002 and 2004.(3) The CAHPs are rated with a heating seasonal performance factor (HSPF) of 6.8, and a Seasonal Energy Efficiency Ratio (SEER) of 10. The heating capacity is 25,000 British thermal units per hour (Btuh) at 47[degrees]F and 15,600 at 17[degrees]F (ARI 2005).
The CAHP uses typical indoor blowers, indoor coils, compressor, bi-flow thermal expansion valve (TXV), strip heating, and controls. Unlike split system heat pumps, the crawlspace air-source heat pump's outside coil is located in the back of the indoor unit, and utilizes a centrifugal blower with forward-curved blades attached directly to the motor shaft of a 1/3 horsepower (HP) permanent split capacitor (PSC) motor. This blower draws roughly 800-1000 cfm of air from outside through a vented crawlspace, over the coil, and exhausts it above the roof (i.e., single pass). The indoor coil is located in the front of the unit. Air is drawn by a typical indoor 1/3 HP blower fan motor from a central return grill over the indoor coil and distributed to a trunk ductwork in the "belly" below the floor and above the floor insulation and bottom board. It is common in multiple section homes to connect trunk ductwork using a 10-12 in. diameter crossover insulated flex duct system located in the crawlspace. The indoor and outdoor coils are isolated from each other within the heat pump, as shown in Figure 1.
[FIGURE 1 OMITTED]
The heat pump defrost controls come factory set to operate after 60 min of compressor runtime in heating mode, but have 30-and 90-min field adjustable settings.
Previous research has shown that improvement potential in heat pump performance during the winter can be realized by employing this crawlspace preheating concept using single pass and recycled systems. These studies suggest savings can be improved by air-sealing and insulating the ductwork and the floor and ensuring proper heat pump sizing (Ternes 1982; EPRI 1982). This research also suggested that adding R-5 insulation to the above-grade crawlspace walls only improves performance slightly. The ZEMH used an R-33 foam insulation system in the floor, which resulted in significantly less floor penetration, ductwork air leakage, and conduction losses to the crawlspace. The reduced heat loss through the ZEMH floor insulation system reduces the temperature of the crawlspace and available "waste" heat to the CAHP. The ZEMH R-19 perimeter insulation was installed to reduce the above-grade foundation wall heat loss.
The flip-flop tests compare the two winters of typical heat pump performance (flip) with two multiweek periods of electric resistance heating only (flop). Figure 2 provides the flipflop test monitoring for the CAHP units in the ENERGY STAR and ZEMH homes. Average daily space heating energy use in kWh is plotted against average daily indoor to outdoor temperature differences that are greater than 20[degrees]F. The flipflop ratio is useful information for Pacific Northwest utility heat pump rebate programs, since it compares electric furnaces to heat pumps, while accounting for performance factors, such as duct leakage, conductive losses, equipment cycling, defrost and strip heat, thermal regain, etc. It should be noted that high duct losses are known to cause greater efficiency problems for heat pumps than forced air furnaces, as shown in ANSI/ASHRAE Standard 152-2004, Method of Test for Determining the Design and Seasonal Efficiencies of Residential Thermal Distribution Systems (ASHRAE 2004). The measured duct leakage rates in the ZEMH and ENERGY STAR homes were 37 CFM25 and 150 CFM25, respectively, using Standard 152. Using the regression fits, the flip-flop ratios appear to be 2.2-3.9 at outdoor temperatures of 20[degrees]F- 50[degrees]F, respectively, in the ZEMH case, and 1.8-3.3 in the ESTAR case. These ratios are still statistically significant even with the poor R-squared fit.
[FIGURE 2 OMITTED]
Figure 3 shows the measured compressor and fan space heat, electric resistance strip heat, interior temperature, and supply air temperature in the ZEMH on Dec. 16, 2005. As is common with many heat pumps, defrost is activated every 60 min of compressor operation for 3.5 min of reverse compressor operation, provided the coil temperature is no greater than 32[degrees]F. During this reverse operation, strip heat (10 kW) is activated to counteract potential cold air from the air-conditioning mode during that time. The heat pump's 60-min factory-set defrost was changed to 90 min in the ZEMH, resulting in some energy savings. Note the very regular amount of compressor operation throughout the day to maintain an interior temperature average of 69[degrees]F (only varied from 68.4[degrees]F to 69.5[degrees]F). The outdoor temperature was also very steady during this day, from a low of 21.9[degrees]F to a high of 24.6[degrees]F. Unlike some other colder days, this day was ideal in that the compressor was able to meet load at all hours without auxiliary. Total fan and compressor energy use for the entire day was 32.3 kWh. The strip heat used during the nine defrost cycles was 1.65 kWh/day during the 30-min daytime defrost periods. The compressor energy use (operating in cooling mode) during all defrost cycles was 0.89 kWh. The average compressor energy use per defrost cycle was 0.1 kWh. The average strip heat energy use was 1.65 kWh per day, or 0.183 kWh per cycle. The total consumption per cycle was 0.283 kWh, or a consumption of 2.55 kWh. The compressor energy use with defrost was 32.3 kWh, so the total without defrost operation was 32.30-2.55 kWh = 29.75 kWh. Thus, the basic increase in heating energy use due to defrost operation was 2.55/29.75, or 8.6% on that day.
[FIGURE 3 OMITTED]
As can be seen in Figure 4, which shows a close-up of the fourth defrost cycle, the strip heat was able to slightly more than offset the compressor energy losses from reverse operation. Thus, the average increase in space heating due to defrost operation in this case was, as evaluated, 9%. This is well within the range of other estimates of defrost energy, which vary from 4%-20% (Parken et al. 1977; Rettberg 1980; Baxter and Moyers 1985), but average about 8%. The defrost cycle starts in the middle of the second compressor cycle in Figure 4, where there is a drop of roughly 200-400 W in compressor energy use when operating in cooling mode versus heating mode. The insignificant change in performance before and after the defrost cycle, shown in Figures 3 and 4, suggests little need for defrost during this period.
[FIGURE 4 OMITTED]
An estimate of annual defrost energy use for the ZEMH home was determined by taking the measured 0.283 kWh/cycle and multiplying it by the number of defrost cycles per year. Defrost cycles per year was determined by dividing the total annual winter compressor runtime by 30-, 60-or 90-min defrost runtime control setting. Using this procedure, the overall annual energy penalty of defrost was determined assuming 30-, 60-, and 90-minute runtime control settings. The maximum potential estimated energy savings from field adjusting the defrost control from 60 to 90 minutes of compressor runtime is 65 kWh/year. This assumes the coil temperature is below 32[degrees]F every time the compressor runtime reaches the setting. Before benefits from the improved defrost timer setting can be generalized, issues of compressor performance degradation resulting from longer runtimes and issues associated with higher winter crawlspace relative humidity, need further investigation.
A simple and inexpensive way to increase efficiency of heat pumps used in homes with vented crawlspaces is to use the soil beneath the crawlspace as a heat source or heat sink. Previous modeling simulations and field monitoring research suggested a crawlspace heat pump in northern regions of the United States could significantly reduce heating season purchased energy compared to a conventionally installed heat pump.
The heat transfer in the crawlspace is a complex dynamic system of heat flux and regain to the home and CAHP that includes: conduction heat exchange associated with the ground, floor, and crossover ductwork and plumbing. There is also airflow to and from the crawlspace due to duct and floor envelope leakage and CAHP operation. Investigations were made to evaluate the impact of the crawlspace on the CAHP performance. Figure 5 shows the average monthly ambient temperature, slab temperatures, and the crawlspace temperatures for the ZEMH and ENERGY STAR (base) homes. The crawlspace air temperatures appear to be as much as 5[degrees]F warmer than the ambient temperature in the winter and 5[degrees]F cooler than the ambient temperature in the summer. Figures 3 and 4 show the crawlspace temperature slightly drops each time the heat pump operates, and rises again during the heat pump off-cycles.
[FIGURE 5 OMITTED]
Previous research suggests the crawlspace air heating results primarily due to the heat transferred from the earth as opposed to additional heat loss from the house. The total heat transfer to the crawlspace air is comprised of the sum of three heat transfer components: the heat transfer between the ambient and the crawlspace, the heat transfer between the indoors and the crawlspace, and the heat transfer between the ground and the crawlspace. The goal of the ESTAR and ZEMH improved floor insulation and air-sealing and the ZEMH perimeter insulation is to increase the percentage of heat transfer between the crawlspace air and the ground, while limiting heat transfer from the house to crawlspace. The ground temperature is usually warmer than ambient air in the winter and colder in the summer, so greater thermal contact between the ground and the crawlspace air should cause higher operating efficiencies for the heat pump.
Seasonal ground heat flux combined with higher crawlspace ventilation rates due to CAHP results in greater crawlspace ground temperature reductions over the heating season, with the CAHP expected to operate more efficiently in the fall than the spring heating months. To get a better understanding of these seasonal impacts, the researchers evaluated data from July 2003 to June 2005. Figure 5 shows the average monthly ambient, crawlspace, and crawlspace slab surface ground temperatures for both the ZEMH and ESTAR (base) homes. The ZEMH and ESTAR (base) ground temperature appears to track well except for the peak summer and winter of 2005 where the ZEMH is slightly warmer. Both the ZEMH and ESTAR home crawlspace temperatures are warmer than ambient air in winter and cooler in summer. It also appears that the ZEMH crawlspace temperature is slightly warmer in the winter and summer. Previous research shows the ESTAR occupant used less air conditioning than the ZEMH occupant, which may explain why the ZEMH crawlspace is warmer in the summer.
While it is beyond the scope of this paper to clearly separate, identify, and quantify the dynamic and interactive impacts on crawlspace temperatures, the monitoring results suggest considerable buffering impacts that can result in improved heating and cooling performance by the CAHP when compared to a typical split system heat pump that is exposed to outside ambient temperatures.
Thousands of CAHP systems have been manufactured and installed in HUD-code housing throughout the United States. Since 1996, a number of heat pump manufacturers have marketed CAHP systems under a specific brand name. Currently, there is no company marketing or providing warranty and/or technical support. Two reasons the product is no longer sold are: (1) low demand, and the need to redevelop a CAHP that meets current SEER, and (2) HSPF requirements (ARI 2005). (4)
Improvement to the SEER/HSPF in CAHP units presents design challenges associated with outdoor and indoor coils since both are located inside the indoor unit. It should be noted that ARI Standard 210/240 (ARI 2005) testing is based on lab test conditions that cannot recognize the thermal efficiency crawlspace temperature buffering benefits associated with the CAHP.
Typical split system air-source heat pumps installed in new HUD-code manufactured homes are sold as part of the home package and installed on site by an HVAC contractor; and in some cases the homebuyer contracts directly with the HVAC contractor. A typical 2-3 ton split system heat pump costs the homebuyers around $3500.(5) In the case of one brand name heat pump, the manufacturer purchased directly from the heat pump manufacturers and installed the unit at the factory. The typical cost to the home manufacturer for this 2-3 ton heat pump was $1600-$1700, which included roof and floor adapter kits. This left a significant potential margin for the manufacturer and retailer to cover installation costs and markups. The combined manufacturer's and retail typical 220% markup and (Conner 1992) would amount to a final cost to the homebuyer comparable to an HVAC contractor installed split system. This market structure eliminates the HVAC contractor and provides additional benefits to the manufacturer.
HVAC Market Structure
In spite of this margin, HUD-code manufacturers have been reluctant to sell CAHP technology largely because it links them closer to heat pump service and warranty issues. Discussions with the HUD-code manufacturer indicated concerns about factory heat pump installations related to qualified service technicians, warranty responsibility, and homeowner complaints about "cold blow."(6) For homes built and sold to retailers as stock homes, there was also concern that a homeowner may want the home, but not the heat pump, which means a change out in the field and limits the market to customers that are purchasing presold homes (i.e., homes built after the customer orders them). Both heat pump manufacturers and home manufacturers have expressed concerns that the direct marketing approach alienates the HVAC service contractors network that they rely on to field install and service systems.(7) This current HVAC market distribution system is believed to be another major reason for the lack of CAHP market acceptance in HUD-code housing.
Maintenance and Operational Observations
Results from an informal telephone survey conducted to evaluate CAHP installations in the Pacific Northwest are presented in Table 1. (8) In general, very few equipment problems were encountered. All units are still operating and customers generally are satisfied with performance. Owners of the oldest project, Vincent Village, in operation for more than ten years, have been very satisfied with the performance of the units in their 49 rental single-section manufactured homes. One homeowner's (NBI) HVAC service contractors had to special order an indoor blower relay board, which took a few weeks to be installed. There are currently only two parts distributors. This contractor was concerned that while the replacement board was an easy repair, future cleaning of the face side of the coils would require removal of the unit. The HUD-code manufacturer who installed over 1000 units has been satisfied with the low level of service issues, once the in-plant personnel became familiar with the installation process. The ESTAR home had a plugged condensate drain that overflowed at the vent tee, resulting in saturating the crossover duct and floor insulation. As with any heat pump system, the proper condensate drain installation and maintenance are critical.
Table 1. Anecdotal Feedback on maintenance and Operation Issues Case Year Number of Location Repairs Sections ZEMH 2001 Double Lapwai, ID None ESTAR 2001 Double Lapwai, ID Pipe froze near vent, condensate leak WSU 1999 Triple Olympia, WA None ODOE 2003 Double Salem, OR None Vincent 1996 Single Kennewick, WA None, condensate plug (one home) BPS 2000 Double Cle Elum, WA None, motor replaced (one home) NBI 2003 Double Mosier, OR Indoor blower relay failed in 2006 Clayton 2002 Single Southeast USA Limited, related to plant training
Observations from homeowners and manufacturers suggest that additional indoor noise associated with the outdoor blower and especially the defrost cycle can present a challenge. In the Washington State University (WSU) House the unit was located in a hallway adjacent to the bedrooms, causing occupant complaints during the nighttime defrost cycles. The ZEMH, ESTAR, and ODOE home floorplan located the unit in a utility room at the opposite side of the home from the bedrooms and no significant occupant noise complaints were observed. Noise was also reported as a greater concern in the smaller single-section homes.(9)
OBSERVATIONS AND RECOMMENDATIONS
Flip-flop tests and estimates of in-situ heating COP performance suggest 200%-260% improved performance over an electric furnace in the 30[degrees]F to 50[degrees]F outdoor temperature range.
Changing the ZEMH defrost cycle from a 60-to 90-minute runtime is estimated to have saved a maximum 65 kWh/year, assuming the setting could be changed without impacting system efficiency due to ice on the coils. Defrost systems need to be improved to accommodate a variety of climate, heat pump balance points, and crawlspace temperatures and relative humidity conditions.
CAHP buffering impacts are considerable and result in improved heating and cooling performance when compared to a typical split system heat pump, by in effect moving the heat pump to a warmer climate in winter and cooler climate in summer. This crawlspace buffering results in higher steadystate COP, larger capacity, and lower heat pump balance point, which decreases supplemental resistance heat and defrost frequencies. Energy-efficient floor and duct insulation and air sealing systems help CAHP systems maximize crawlspace-ground-to-crawlspace-air heat transfer, while minimizing floor and duct heat loss and gain.
The seasonal crawlspace temperature changes may result in better CAHP performance earlier in the heating and cooling season. For instance, by using the regression equation for the ZEMH from Figure 2 and assuming a 5[degrees]F winter crawlspace buffer from Figure 5, the crawlspace buffering effect appears to provide a 5 kWh per day energy savings. However, additional field and modeling research is needed to better quantify the seasonal impacts on crawlspace buffer temperatures and the complex dynamic thermal interactions impacting crawlspace temperatures. Future research should also focus on moisture issues in cooling mode.
Given the close proximity of both coils and indoor location, further evaluation of cycling impacts is suggested.
Anecdotal evidence from owners and manufacturers indicates no significant equipment problems in CAHP systems; however, longer-term evaluation is suggested.
Servicing and warranty concerns, current market structure, and new equipment standards are all barriers that have limited adoption of the CAHP technology.
With this equipment, noise does remain an issue compared to outside units. Operational noise was particularly noticeable at the beginning of defrost cycles when the reversing valve position changed. However, locating the CAHP away from bedrooms, and the use of improved sound attenuation of the CAHP and mechanical room and improved defrost controls may improve occupant acceptance, especially during defrost cycles.
Evaluation of brushless permanent magnet motors in outdoor and indoor blowers may help to achieve improved HSFP/SEER ratings, and address noise issues.
The authors would like to acknowledge the Bonneville Power Administration, Benton Co. PUD, Florida Solar Energy Center, Idaho Department of Water Resources, Northwest Energy Efficient Manufactured (NEEM) Program, DOE Building America Industrialized Housing Program, Washington State University Extension Energy Program, Clayton Homes, Valley Manufactured Housing, Fleetwood Homes, Brady Peeks, Ken Eklund, Margaret Thomas, and Andy Gordon. This work is sponsored in large part by the DOE Building Technology's Building America Industrialized Housing Program under Cooperative Agreement DE-FC36-99GO10478.
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Larry J. Hughes, President, Alpha Engineering, Inc., Bear, DE: Crawlspaces are typically damp with mold spore propagation. Does the crawlspace benefit or suffer from outside ambient air venting through the space?
Michael Lubliner: In the Pacific Northwest, code requires venting of crawlspaces through the use of holes in the foundation walls. Venting is intended to continually remove moisture from the crawlspace and allow it to recover from any significant moisture-related incidents. The authors believe an exchange with more outside air, through use of the crawlspace-assisted air-source heat pump, will only increase the potential benefits of the standard passive venting system and, therefore, have a positive impact on the condition of the crawlspace. In hot, humid climates, the issue of venting crawlspaces presents greater challenges, although no anecdotal reports of crawlspace moisture problems were reported in CAHP homes in this study.
(1.) Baylon, David. Ecotope, Inc. Aug. 2005.
(2.) Starr, George. Friedrich Air Conditioning Company. Telephone conversations, March 16, 2006.
(3.) Author Michael Lubliner has personal experience living in a residence with a crawlspace-assisted heat pump (from 1999-2006). In March 2006, he discussed CAHP maintenance and operation issues in telephone conversations with representatives from Vincent Village, Cle Elum Fish Facility, Clayton Homes, and another homeowner.
(4.) Phone conversations with Mark Ezzo and Craig Haynes at Clayton Homes, March 17, 2006.
(5.) Phone conversations with Tisha Busey at Valley Manufactured Housing, March 17, 2006.
(7.) Phone conversations with Mark Ezzo and Craig Haynes at Clayton Homes, March 2006.
(8.) Author Michael Lubliner has personal experience living in a residence with a crawlspace-assisted heat pump (from 1999-2006). In March 2006, he interviewed representatives from Vincent Village, Cle Elum Fish Facility, and Clayton Homes, plus another homeowner.
(9.) Phone conversations with Mark Ezzo and Craig Haynes at Clayton Homes, and Tisha Busey at Valley Manufactured Housing, March 2006.
Adam Hadley, PE
Associate Member ASHRAE
Michael Lubliner is a building science specialist at Washington State University Extension Energy Program, Olympia, WA. Adam Hadley is a mechanical engineer at the Bonneville Power Administration, Portland, OR. Danny Parker is a senior researcher at the Florida Solar Energy Center, Cocoa, FL.
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|Title Annotation:||heating, ventilation, and air conditioning|
|Author:||Lubliner, Michael; Hadley, Adam; Parker, Danny|
|Article Type:||Technical report|
|Date:||Jul 1, 2007|
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