Hot Tubs and Energy Use

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Hot tubs have become much more energy efficient in recent years, thanks largely to California Energy Commission requirements that took effect in 2009. Yet even with these standards, a hot tub still costs about a dollar a day to operate. Even in standby mode, a tub will use 63 cents' worth of energy just to keep the water warm. Editors at Spasearch magazine say it's not worth turning off the tub's heater tub to save energy between uses; it actually uses more power to bring the water back up to temperature the next time you hop in. Most of the energy a hot tub uses goes into heating the water, so anything that helps keep the water warm will also cut operating costs.

According to Spasearch's editors, insulation is the most important factor in preventing heat loss. There are three main types of insulation for hot tubs.

  • Basic insulation is a thin layer of polyurethane foam applied to the underside of the hot tub shell. Most of the space under the tub is empty air, which makes it easier to access the plumbing. Basic insulation is inexpensive but won't do much to control heating costs, especially in standby mode.
  • Layered or blanket insulation combines basic insulation with an additional layer of foam on the inside of the cabinet. The best blanket insulation has multiple layers of high R-value material on all four walls. If the spa has removable walls, the insulation can be moved aside for maintenance.
  • Full-foam insulation is a thick layer of solid foam that completely fills the space under the tub. More than 70 percent of above-ground hot tubs on the market use this type of insulation. In addition to slowing heat loss, it provides soundproofing benefits, and helps support the plumbing and the tub itself. Editors at Spasearch recommend looking for closed-cell foam, which will insulate better because it can't absorb water.

Of course, insulating the sides of the tub can help only so much if the water itself is exposed to air, which makes an insulated cover essential. A typical cover has a core of polystyrene foam wrapped in polyethylene plastic. The foam varies in density from 1 to 2 pounds per cubic foot. A cover of high-density foam will weigh more, but will also be stronger and insulate better. Ideally, the plastic outer coat should be thick and well-sealed so moisture can't penetrate the foam.

Most covers fold in half for storage with a gap between the two halves, but a sealing gasket can improve the insulating power by as much as 5 percent. The best covers also have a generous "skirt" that completely covers the edges of the spa. If your hot tub is outdoors, the cover should have tie-downs made of nylon webbing sewn into it at several spots to keep it in place. If you have children, look for a locking cover that can only be opened with a key that you can keep out of their reach.

Other features of a hot tub can help keep the heat in. For instance, the pump generates a significant amount of heat during use, so some models recycle this heat into the tub. That can be used to either heat the tub's pipes or produce a stream of warm air directed into the water. Hot tubs with an economy mode automatically lower the water temperature when the tub isn't in use without shutting off the heater completely. As the editors at Spasearch magazine note, it's important to make sure your tub's heater puts most of the heat it produces into the water.

While a hot tub's heater is the biggest energy hog, other parts use power, as well. Spasearch's editors say you can save energy by avoiding oversized pumps, which don't actually improve performance. Instead, opt for either one high-powered pump or multiple pumps that use less power. Other energy-saving features include LED lighting and an automatic shutoff, which can extend the life of your hot tub.