This article was originally published in the November/December 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online November/December 1996
Heat Pumps and Manufactured Homes:
In my job as the factory-built home specialist for North Carolina's Alternative Energy Corporation (AEC), I have been crawling under, in, and around manufactured homes for the last dozen years, looking for causes of, and solutions to, building energy problems. I have seen lots of things. I even have a series of wildlife photographs taken from the special environment found only underneath these homes. Perhaps, I have speculated, it's this wildlife that makes it so hard for contractors to deliver quality work on manufactured homes. There is, indeed, nothing quite like lying on your back amidst rural North Carolina's abundant flora and fauna, with 10 tons of home jacked up above you. Contractors watching for snakes and other common crawly creatures may be tempted to cut corners. Recently, however, one animal that I had seen only rarely before around manufactured homes has begun to appear in abundance-the Heat Pump.
Heat pumps are ideal for manufactured homes in the Southeast. Since little natural gas is available in rural areas, almost all new manufactured homes leave the factory with an electric furnace. When the home is purchased, a central air conditioner is often included as part of the package. The air conditioner is actually installed after the home is set up on site. Because of the Southeast's low heating loads, it often makes sense for a home buyer to get a heat pump, rather than central air conditioning, when buying a manufactured home. When a heat pump is installed, the existing furnace is left in place. Its air handler is used for the heat pump and its coils are used for backup heat. The higher cost of the heat pump is easily recovered in the first year and a half by savings from reduced electric resistance heat. According to North Carolina electric utilities, between 1994 and 1995, sales of manufactured homes with heat pump package deals went up by 250%.
Multi-section homes have a marriage wall between the sections. This wall is often poorly sealed, so it can fill with outside air, making it respond to outdoor temperature even more than insulated exterior walls. A thermostat on a leaky marriage wall won't respond to the inside temperature, and will likely be operated as an on/off switch. This is an annoyance to the homeowner, and is bad for the heat pump-a heat pump thermostat that senses too much heating load will turn on wasteful backup resistance heating elements.
Heat Pump Sizing Improperly sized air conditioning equipment causes many problems (see Sizing Air Conditioners: If Bigger Is Not Better, What Is? HE Sept/Oct '96, p. 13). The most notable of these is that oversized units cause shorter run times, which result in poor efficiency, and, in humid climates, poor dehumidification. Individual heat loss and gain calculations were not carried out for each house in this sample. However, all the homes were similarly insulated as required by the HUD Code (see Manufactured Housing-an Evolutionary Home), were located within 60 miles of Raleigh, North Carolina, and had window areas equal to 8%-11% of floor areas. The Air Conditioning Contractors' Association Manual J calculation for similar homes with various orientations and worst-case window orientations specifies 1 ton of cooling for every 600 to 800 ft2 of floor area. Only one of the field study heat pumps fell within this size range; the rest were oversized by 18%-60%.
|Table 1. Field Performance of Five Heat Pumps in North Carolina Manufactured Homes.|
|Test Site||Home A||Home B||Home C||Home D||Home E||Target|
|Floor area (ft2)||1,266||956||1,677||1,460||2,083|
|Outdoor unit rated capacity||3 tons||3 tons||3.5 tons||3 tons||3.5 tons|
|Ratio of floor area to equipment size (ft2/ton)||420||320||480||490||600||600-800|
|Air flow to the coil (CFM/ton)||380||290||400||230||200||400|
|Duct leakage to exterior (CFM at 25 Pa)||270||150||140||130||320||5% of floor area|
|Capacity in field as % of rated capacity||103%||78%||60%||69%||78%||100%|
|Note: All homes were less than one year old, and were built to the October 1994 HUD Code.|
Manufactured Housing-an Evolutionary Home
In the 1950s, house trailers became 10 ft wide and it took special trucks to move them from place to place. They came to be called mobile homes. Many manufacturers were competing in this industry, and quality was sometimes compromised. Some states felt that if mobile homes were going to be used as permanent housing, they should be regulated with fire and safety standards. Several states began specifying their own standards for mobile homes.
This presented a particular problem for the mobile home industry: homes built to the standards of one state would often not meet the codes in another state. What is the point of being a mobile home if you can't go anywhere? Also, one of the benefits of mass production would be lost if the factory had to produce each home to satisfy a different local code.
In 1976, the U.S. Department of Housing and Urban Development (HUD) began administering standards for manufactured homes. The Manufactured Housing Construction and Safety Standards Act (often referred to as the HUD Code) set what seemed, for the time, reasonable but aggressive standards for these homes. In turn, the mobile home industry got an exemption from state code jurisdiction. These standards changed the industry. In fact, they were so challenging that the number of mobile homes sold dropped over 40% between 1974 and 1976.
The industry has since tried to differentiate homes built to the HUD Code from the older pre-HUD Code homes by officially calling new units manufactured homes. This change was also intended to move away from the implications of transiency inherent in the term mobile homes. And, in fact, more of the homes were being set up on permanent foundations and were no longer really mobile. The term mobile home, however, has not obliged by going away. It is still commonly used both within and outside of the industry. Since the 1970s, the trend in the manufactured housing industry is towards multisectional, custom-ordered homes with removable axles. However, there are still many single-section homes being built, and some of these are lower end homes that remind us of the links with trailers and mobile homes of the past.
The HUD Code was updated in October 1994. This long-overdue change significantly increased, among other things, the thermal insulation and wind resistance requirements. This time, the accompanying increase in costs did not scare away buyers. In 1995, 340,000 HUD Code homes were sold, double the 1991 sales, and the most sales since the original HUD Code was enacted.
This picture was taken in a two-month-old manufactured home, looking through the crossover boot towards the furnace. Although the home is new, there is already a major failure in the duct where the finger joints have separated from the main supply plenum. Because this is the point of highest pressure in the system, an air leak here is quite severe.
Like most manufactured homes that are being equipped with heat pumps, the units had factory-equipped electric furnaces with four-speed blowers. The typical four-speed blower provides 910, 1,130, 1,260, and 1,400 CFM standard air flow at 0.2 inches of water column static pressure. Measured static pressure was generally close to 0.2 inches, and never exceeded 0.26. Thus, proper air-flow capacity for these 3- to 3.5-ton heat pumps was available. However, in most cases, the blower speed was not changed when the systems were converted from electric furnaces to heat pumps.Indoor Thermostat Placement To save money at the factory, a thermostat control cable with only two wires is installed with the electric furnace, but a heat pump thermostat often requires an eight-wire control cable. If a heat pump installer is required to run new thermostat wiring, to avoid crawling under the house (remember the creepy crawlers) she or he will frequently put the thermostat in the most convenient spot to wire, not in a logical spot for heating and cooling purposes.
One test home's new heat pump thermostat was installed in the kitchen adjacent to the air handler closet, directly over a supply grille. Of course, it was easily installed from inside the house; but every time the equipment operated, the supply air blew directly on the thermostat, quickly shutting the system off. This system was destined to an entire lifetime (albeit a short one) of start-and-stop operation.
The polyethylene air barrier under this mobile home had large holes torn in it during a telephone wire installation. Running the wires required a space of about 14 inch, but messy work resulted in 18-inch-square holes, a serious breach of the air barrier. This wire runs between two sections of a multi-unit home, disrupting air sealing in both parts of the house.
Condensate All of the heat pumps examined had prefabricated condensate line traps. However, in order to route the condensate from the trap to the outside, the pipe connected to the trap was sometimes angled downward. Traps that are not level will leak outside air into the equipment, and will often cause water in the drip pan to overflow. Some new equipment has an internal condensate trap that should prevent shoddy installations in the field.
Leaky Ducts Several Home Energy articles have challenged the practice of installing high-efficiency air conditioning equipment into site-built homes when the duct system performs so poorly that the effective efficiency of the equipment is seriously reduced. No one has checked whether this principle holds true in the new manufactured home designs. However, another recent study of manufactured homes showed air distribution systems were responsible for losing 40% of HVAC systems' heating efficiency and 18% of their cooling efficiency.
A supply leak fraction of 14% was found in the North Carolina homes. This means that 14% of the air coming out of the air handler was delivered somewhere other than to the supply registers. Because of the floor design, some of this conditioned air entered the living space through holes in the floor, and additional energy was recovered by conditioning the floor.
Besides ubiquitous random holes in the ducts throughout the belly of the home, duct leaks also result from poor crossover connections (see Duct Improvement in the Northwest, HE Jan/Feb '96, p. 27). The catastrophic failure where the crossover duct has fallen away from the in-floor plenum is usually discovered by the observant homeowner, who notices the lack of climate control on one side of the home or receives a shocking utility bill. Less obvious leaks, where only a portion of the crossover air is directed into the crawlspace, may go unnoticed for years, until someone crawling under the home sees spiderwebs waving in the leaking air, or until the ductwork is tested.
In practice, however, many breaches in the air barrier are common, and these defeat the design scheme. The setup crew can meticulously repair the inevitable holes torn in transit and setup. Still, other contractors-including HVAC installers-anxious to get out from beneath the home have been known to cut large holes in the air barrier to install a single wire. In another AEC study, we found that the bellies of manufactured homes had an average 29 Pa pressurization with respect to the inside during 50 Pa depressurization tests. Patching holes can help the air barrier work.
However, patching is not all that's needed. In one home where our crews repaired every visible hole in the belly (including a 2 ft x 4 ft hole made by the cable TV installer), the floor cavity was still communicating with the outside. A pressure probe placed in the floor cavity during a 50 Pa depressurization test showed an average belly pressure of 14 Pa with respect to the inside. No one has conducted a complete test to determine how air gets around the air barrier, but one explanation is that it leaks through gaps between the staples that hold the perimeter of the air barrier to the home. In fact, the whole design of this external air barrier is so prone to problems that in some areas, such as the Pacific Northwest, weatherizers and manufacturers alike have accepted that this sheet of fabric is a road barrier, not an air barrier.
Heat Pumps-What Should You Look for?
But They Can Work Fine! Heat pump systems that work properly can provide affordable space conditioning to manufactured homes. In a demonstration project conducted by AEC several years ago, five occupied manufactured homes conditioned with heat pumps were monitored for energy use. In these homes, many potential problems, such as duct leakage, proper equipment sizing, and holes in the air barrier, were repaired. The homes had remarkably little energy use for climate control. The annual consumption by the heat pumps in these new homes was less than 5,000 kWh. At 8¢ per kWh, this would cost the homeowner $33 per month.
As manufactured housing competes for a bigger chunk of the new housing market, the general quality of the product seems to be on the rise. More manufacturers are paying attention to understanding and reducing performance problems such as duct leakage. Several states are also increasing the role that their local code inspectors play in manufactured home setups. The incidence of inferior home installations should decrease as code officials become trained to enforce minimum home setup standards.
Manufacturers typically do a better job of quality control than retailers and installers. The more work that is done at the factory, the more reliable the quality tends to be. Some manufacturers offer a heat pump ready option which ensures that extras like the eight-wire thermostat control cable and a multispeed air handler motor are part of the home specifications. Other manufacturers have begun to offer a heat pump that can be installed in the factory. As more heat pumps are installed in manufactured homes from the start, many installation problems should become a thing of the past.
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