This article was originally published in the September/October 1994 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online September/October 1994
Waterbed Heating: Uncovering Energy Savings in the Bedroom
by Ted Rieger
Ted Rieger is a freelance writer who specializes in energy topics. He lives in Sacramento, California.
Waterbeds can easily put refrigerators to shame in a kWh-guzzling contest and nearly one of every six homes has one. Some utilities are now targeting them in their energy conservation programs.
The crest of the wave of waterbed sales may have passed, but energy consumption by waterbed heaters still accounts for a significant portion of energy costs in some households, in some cases for consumers who can least afford this ongoing cost. Some utilities are discovering that waterbed heating can be cost-effectively targeted for significant energy savings as part of demand-side management and weatherization programs.
Industry statistics show that domestic sales of all types of waterbeds have declined annually from a 1988 peak of 3.8 million wholesale units to 2.5 million wholesale units sold in 1992. During this same period, the number of manufacturers of flotation products sold in the United States declined from 326 to 158. The Waterbed Council, Inc., an industry trade association, says that 18% of all beds sold in the United States today are waterbeds, with soft-sided waterbeds making up nearly 20% of all waterbed sales. Statistics on total market penetration vary by source, with an estimated 15%-20% of all U.S. households having a waterbed.
Improvements and options in waterbeds have reduced energy use since the full flotation models were introduced during the 1960s. Mattresses are available that require less water than before, such as shallow-fill or mid-fill models. The hybrid, or soft-sided beds, which look like innerspring mattresses, are more energy-efficient because of the insulating effect of the soft sides. Reduced-motion or waveless models, which have fiber inserts within the mattress chamber, also help reduce heat loss by reducing water movement.
Many variables affect waterbed energy consumption: the climate where the bed owner lives; the size of the bed (king, queen, or super single); the type of bed (traditional hard frame with full flotation mattress, or soft-sided beds); the type of heater used; add-on energy saving measures (fitted mattress pads, foam pads on top of the mattress, use of quilts and comforters); user behavior (making the bed when not in use); and the user's temperature comfort level.
When it comes to waterbed heaters and thermostats, consumers really have few options to choose from. According to Debb Beatty-Bovett, marketing director for Aqua Queen, a heater manufacturer, only three companies worldwide manufacture waterbed heaters, although their products are sold under a number of brand names. Beatty-Bovett says waterbed heating systems are subject to tight UL guidelines for approval.
Heaters come in three wattages--150, 330, and 380. Manufacturers' guidelines should be followed in matching the heater to the bed based primarily on the fill-depth of the mattress. Some shallow-fill soft-sided beds may not even need a heater. Shallow-fill to mid-fill mattresses (up to 6 inches in depth) may use 150-W heaters. Beds with dual mattresses are available that allow sleeping partners to individually adjust mattress firmness and temperature. These use two, individual 150-W heaters, one for each mattress, with a thermal divider between mattresses.
The heater consists of a heating pad--the circuitry of a heating element encased in vinyl--placed on the bed frame under the water-filled mattress. Beatty-Bovett says efficiency is largely based on how much of the pad the circuitry covers. Heater pads can be all vinyl, or may have a rigid aluminum back which may help reflect heat into the mattress instead of losing it to the bed frame, according to some manufacturers.
The heater has a thermostatic control with a temperature range of 70deg.F-100deg.F. Thermostats do not necessarily correspond to actual mattress water temperature, but temperature gauges can be added for more accurate readings. Bulb and capillary controls are the standard mechanical types of temperature controllers. Manufacturers claim that higher-priced solid-state models are more efficient for control of temperature and energy usage.
User perceptions and comfort levels also affect energy use. A nationwide survey, Waterbeds and the Consumer, conducted in 1984 for the Waterbed Manufacturers Association stated, At least two out of every three waterbed owners feel that because a waterbed is heated, it saves on the cost of heating homes since the thermostat can be turned down. This perception may not always produce energy savings, because of other factors such as the room where the bed is located, the comparative efficiencies between the space-heating and waterbed-heating systems, the type of bed, and the user's comfort level.
Some people use waterbeds for health and comfort reasons associated with heated sleep, in addition to flotation benefits, and for them these features outweigh the energy costs. Users adjust heater thermostats to make the mattress comfortable to skin temperature and their own thermal comfort. Beatty-Bovett recommends that a mattress be heated to a minimum of 76deg.F at all times. The American Sleep Research Institute says thermal comfort for most people is in the range of 84deg.F-95deg.F. In some climates, users may turn down the thermostat setting during summer months, but Beatty-Bovett claims the heater should never be turned off, because it is necessary to ensure minimum temperature for comfort and to prevent mold growth.
Energy Consumption Tests
Metered usage and testing of waterbed energy consumption is not extensive, nor is data from available tests comprehensive or conclusive. A 1982 study by the California Department of Consumer Affairs (see Waterbeds: the Silent Guzzlers, HE Jan/Feb '88, p.29) which is still cited by energy professionals, calculated an average usage of 125 kWh per month (1,500 kWh per year) for its baseline case--a king-size bed (with a quilted mattress pad, two sheets, and a comforter) heated to 90deg.F in a 70deg.F room.
In a study of miscellaneous residential electrical energy use (published in the Proceedings of the 1990 ACEEE Summer Study on Efficiency in Buildings), researchers with Lawrence Berkeley Laboratory concluded that waterbed energy use is highly seasonal, with up to a factor of five difference between the coldest and warmest months. Their literature review found that waterbeds use 500-2,000 kWh per year. In studies by Central Maine Power and Southern California Edison, a waterbed's load shape was found to be unfavorable to most utilities, because peak usage occured in the morning, begining at about 7 a.m., and didn't fall significantly until afternoon (see Figure 1). This load shape suggests owners tend to leave the covers off their waterbed once they get up, and don't make their beds until later in the day.
A 1987 study by an independent lab comparing an Aqua Queen Mark 4 heater with a Thermafoil heater was conducted with two queen-size wood-frame beds placed side-by-side and made up with an insulator pad, a waterbed sheet, and a comforter, with the beds heated to 90deg.F. The beds were in a draft-free room, but no attempt was made to control ambient room temperature. Power consumption was monitored for 15 days in March and April, with daily kWh readings taken at midnight. The daily averages were 2.9 kWh for the Aqua Queen and 3.9 kWh for the Thermafoil. Extrapolating these averages gives respective average monthly consumptions of 87 kWh and 117 kWh, and annual consumptions of 1,044 kWh and 1,404 kWh. Neither of these tests accounted for seasonal variations in room temperature over the course of a year, however, and neither included the effects of people using the bed for several hours a day, nor any other user behavior.
Utilities Target Low-Income Users
Statistical and anecdotal evidence suggests a trend in waterbed purchase and ownership among lower-income people. The initial cost of new waterbeds make them affordable to lower-income people, when compared with innerspring beds of similar size. For these consumers, the benefit of heated sleep is not necessarily the main motivation for purchase.
Some utilities that target low-income households for demand-side management or weatherization programs based on billing data have found even higher percentages of such households with waterbeds. Waterbed Council statistics show that 20% of waterbed purchases are by people with incomes under $20,000. Jeff Newburn, senior marketing planner with Midwest Power in Des Moines, Iowa stated at this year's Affordable Comfort Conference, Studies have shown that one-quarter to one-half of Iowa low-income households have waterbeds. According to Newburn, Waterbeds tend to be less expensive to buy, so they give you a low first cost, but you have an ongoing cost for energy to run the heater.
Newburn says that Midwest Power began installing foam mattress pads this year on waterbeds to insulate and save energy, as part of the utility's direct-install programs for all customers. The foam pads are also being used this year for low-income weatherization programs in Iowa administered by community action agencies.
The foam pads have egg carton contours on one side and are flat on the other, with a base thickness of 1/2 and a 13/4 peak height. They are placed directly on the waterbed mattress under the regular mattress pad and sheets. The pads come in king, queen, and single sizes, and can be purchased at some discount department stores as well as through some energy-efficient product suppliers. Newburn says the utility is assuming an installed cost of $30 per pad, and preliminary indications show payback in as little as 5 months.
Newburn says the pads were voluntarily tested on waterbeds during the '93-'94 winter by 79 utility employees and contractors, including himself. Pre-installation metering of 8 participants carried out for one month during the winter, showed an average waterbed-energy use of 3.5 kWh/day. Post-installation metering is incomplete, but in a survey of 72 participants, 45% said they turned the waterbed heater off for the winter and 55% said they turned the heater thermostat down. Additionally, 76% said the foam pad also made the bed more comfortable to sleep on.
In Ohio, waterbeds were targeted as part of a low-income demand-side management program in 1992 for Columbus Southern Power customers through a contract with the Corporation for Ohio Appalachian Development (COAD). This program included weatherization measures, heating/cooling system tuneups, electric water heater and appliance repairs and measures, lighting measures, and consumer education. The program served 931 households and waterbed mattress foam pads were installed on 279 waterbeds in 223 households. In the early stages of the project, waterbed energy consumption was metered before installing the foam pads. It soon became apparent that pad installation would always be effective, so consumer education was provided and pads were installed by an inspector in every case.
The foam pads were similar in appearance and function to those used in Iowa. COAD determined cost-effectiveness based on a $20 cost per pad with a three-year life span. According to COAD's Bob Pitts, Savings are generated not only because of the insulating value of the foam pad, but also because it allows the mattress thermostat to be turned down as soon as it goes on, without a reduction in the bed's comfort level. Although COAD's metering was not a random or extensive sampling, and is not considered scientific or conclusive by Pitts, pre-pad metering of 55 beds indicated an average annual metered use of 1,636 kWh. Pitts estimated the pads produced an average annual energy savings of 60%, and that the worst-case scenario would yield annual savings of at least 800 kWh.
Duquesne Light Co. in Pittsburgh, Pennsylvania, took a different approach in a 1992 low-income pilot program for non-electric-heating customers selected based on electrical usage. Duquesne offered a complete mattress changeout to a standard foam mattress, eliminating the total waterbed energy load. Of 185 program participants, 78 owned waterbeds (42%), but only 34 agreed to the mattress replacement, which was also tied to a refrigerator replacement and/or lighting efficiency measures. Those who refused the mattress changeout cited health reasons or a preference for the waterbed, and a few said the waterbed was used as a heating device in rooms that were unheated. Of the mattresses replaced, 50% were king size, 35% were queen, and 15% were super singles.
Energy usage data for the program was drawn from customer billing for one year before and one year after installation of measures. Specific data on waterbed energy savings could not be isolated, since other measures occurred with the mattress changeout, but engineering estimates indicated that households with mattress changeouts saved an average of 1,300 kWh annually, from this measure alone. The utility concluded that the program and each category of installed measure was cost-effective, based on a seven-year payback criterion established by the Pennsylvania Public Utility Commission.
Figure 1. Monitored waterbed average energy use profile.
Source: Central Maine Power (CMP) and Southern California Edison (SCE).
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