Prospective and current owners of spas or hot tubs need to know information about energy-efficiency features so they can make informed decisions when they are ready to buy or upgrade. Many people base their purchasing choice on the lowest retail price. Often, this has an immediate—and negative—effect on their monthly electric bill. Fortunately, there are features available that will reduce energy use and lower operating costs.
How spas and hot tubs use energy
To understand how to reduce operating costs, start by learning how spas and hot tubs use energy. They typically use electric energy to heat and continuously circulate the water, and a very small amount of electricity for lighting. Tubs are covered and unused more than 95 percent of the time, yet this is when they use 75 percent of their energy. Thus, energy conservation starts at this “steady-state” mode and at reducing heat losses from the cover, floor and walls. While most of the energy used in spas goes into the heater, the energy for pumping is also significant. When the cover is removed and the tub is in use, heat losses increase six-fold or more. If the jets are activated and a couple of people get into the tub, evaporative heat losses from the surface increase even more. Pumps for the high-speed jets use many times the energy of circulating pumps. An activated air pump or bubbler increases heat loss beyond the capacity of most heaters, and water temperature begins to drop
When considering the energy efficiency of a spa or hot tub, the key elements to look at are the cover, tub wall and floor insulation and pump system efficiency.
Cover – The insulation value of the cover and the tightness of its seal to the tub are the most important construction details in terms of overall energy use. Designing a cover that is well insulated, provides a good air seal, and is light enough for a single person to handle is a real challenge.
The warm, humid air trapped between the cover and the water surface is rich in energy – a small air leak in the seal increases evaporation from the water surface, bypassing the cover’s insulation and increasing heat loss from the
tub. The cover insulation should be good-quality, closed cell foam that will not absorb water. It should be supported adequately so it does not sag in the middle, and one person should be able to remove it alone. Some insulation absorbs water as the cell structure breaks down from chemicals used in the tub. If the manufacturer offers a “premium” or “upgraded” cover with a longer useful life and higher R-value, it is usually a very good investment.
If you currently own a tub and notice that the cover is becoming heavier from water saturation, replace it – waterlogged insulation loses effectiveness and wastes costly energy
Tub walls – Spa and hot tub walls are not always insulated to optimum levels, so they can be a significant source of energy loss.
Thermal insulation can perform two different functions:
Reducing heat losses and, if it is rigid foam insulation, physically supporting the tub.
As insulation thickness increases, the benefit from each additional unit of insulation decreases relative to its cost. These diminishing returns make the optimum insulation thickness somewhere around six inches, depending on the average ambient temperatures and energy costs.
Some spas and hot tubs use a two-inch layer of rigid foam insulation and fill the rest of the cavity with fiberglass insulation; this saves a few dollars—until it gets wet. Then the insulation value drops almost to zero, significantly increasing in your electrical bill.
Pumps – The circulator pump(s) move water through a filter and heater continuously during steady-state operation. Some tubs have a two-speed motor, using the same pump for low-speed circulation in steady-state mode and for high-speed operation when the jets are on. These pumps are not usually very efficient in any mode, but particularly in steady state because the motor is very lightly loaded and running at low efficiency most of the time. Since these are air-cooled motors, getting rid of the waste heat from the motor in the summer is a problem too.
Some manufacturers use separate pumps for circulation and jets. While initial costs are slightly higher, this helps optimize the circulation pump and can yield good savings during steady-state operation.
A few studies have looked at the potential for reducing spa energy use. Improving spa covers is estimated to save 10 percent, or about 250 kWh annually – the same amount of energy as improving tub insulation.
More efficient pumping systems save even more – 15 percent, or about 375 kWh/yr.
Improved controls can save an estimated 5 percent, or 125 kWh/year.
If you are paying .085/kWh for electricity, you can save 15 GBP/year with spa covers, another 15 GBP/year with tub insulation, 24 GBP/year with improved pumping systems and about 8 GBP/year with improved controls.
Other than replacing the cover, these efficiency improvements will only be available at the time of purchase – so make sure you take advantage of them. More efficient pumping systems would include a separate pump for continuous circulation. Improved controls would incorporate the ability to set back tank temperature and minimize other electrical uses.
But this still an estimate. There are several other factors that will affect this price, including:
- The size of your tub — more water requires more heating
- Your tub's thermostat setting
- Outdoor air temperature and wind speeds
- The quality and age of your water heater
There are also multiple features that factor into the cost of maintaining your hot water temperature when the tub is not in use:
- The quality of the tub's insulation
- The quality and fit of the tub's cover
- Whether you use a thermal blanket for additional insulation
- Whether you use a heater timer to emphasize heating during off-peak hours when electricity is cheaper