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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%.
The Manufactured Homes StudyIn 1995, as part of a four-state study on manufactured homes, I studied seven new multisection manufactured homes that had been equipped with heat pumps. Carolina Power and Light Company conducted an instantaneous-performance test on these heat pumps. The AEC had previously done a similar study for site-built homes (see "Air Conditioner Efficiency in the Real World," HE May/June '92, p. 32), which demonstrated that the actual performance of air conditioners and heat pumps was as low as 26% of the performance rated by the Air Conditioning and Refrigeration Institute (ARI). The inefficiencies were primarily due to installation and service problems. We found similar problems in manufactured homes.Actual and Rated CapacityHeat pump capacity is rated by ARI at standard outdoor temperatures of 47oF for heating and 95oF for cooling. In the field, these temperatures are not often conveniently available. However, performance data under local conditions can be adjusted by formulas to compare to ARI ratings, with a margin of error of 10%. Of the equipment that could be field-tested for performance, only one unit out of five was satisfactory. The others had significant performance degradation, due to either poor air flow across the coil, improper charge, or some combination of both. Adjusting for the conditions at testing, the average measured heating or cooling capacity of installed units was only 78% of their rated capacities (see Table 1).Heat Pump SizingImproperly 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%.
Air Flow to the CoilProper air flow across the indoor coil is critical for good heat pump performance. The standard air-flow requirement for the indoor coil is 400 cubic feet per minute (CFM) per ton of cooling. In the study houses, air flow was calculated using the temperature difference measured across the coil and the amount of heat added by the strip heaters. Only three of the five systems were operating within the acceptable 350-450 CFM/ton range. The one system with properly sized equipment had air flow that was 43% too low.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 PlacementTo 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.
Outdoor ThermostatA desirable control for heat pumps is an outdoor thermostat that prevents the operation of the resistance heat when the heat pump can handle the load. These "lockout" outdoor thermostats need to be set at an appropriate temperature, above which the resistance heat won't be used. This study found one lockout set at 55oF, when it should have been set closer to the balance point of 35oF-40oF.CondensateAll 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 DuctsSeveral 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.
Pressure Boundary BreachesSouthern manufacturers assume that these homes have a warm floor design, meaning that the thermal boundary is on the outermost layer of the floor construction. The bottom side of the floor is wrapped in a blanket of insulation and protected from the outside with a continuous woven polyethylene road barrier. This barrier is also intended to serve as the pressure boundary. Leaky ducts have not been considered very seriously, because theoretically they are within the home's thermal shell, in the area known as the belly.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.
Pressure ProblemsDuct leakage always causes unwanted induced pressure differentials when the air handler is operating. Manufactured homes typically have no return ducts, so leaks usually induce negative pressures, which drag in outside air and increase heating and cooling loads. Problems in homes with leaky ducts can be masked by oversizing the cooling equipment. Recall the problems with oversized equipment. The prevalent duct problems have convinced some retailers for life that their rule of thumb for equipment sizing is better than using any load calculation procedure.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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