Hot tub heaters and heat pumps

A hot tub heater is an electric element, much like the one in a kettle, sealed in a steel or titanium tube in the plumbing. The pump pushes water through the tube, the element warms it, and the water flows back to the seats. Almost all the electricity becomes heat: California's Energy Commission puts spa resistance heaters at about 98% efficient or better. So heater size decides how fast the water warms, and the insulation and cover decide how much heat you have to replace.

The arithmetic is simple. One kilowatt-hour is 3,412 Btu, and one Btu raises a pound of water by 1°F. A gallon of water weighs about 8.3 pounds, so a 400-gallon tub holds about 3,300 pounds of it. A 4 kW heater adds about 13,648 Btu an hour, or about 4.1°F an hour with the cover on and nothing lost. Real heat-up is slower, because the tub keeps losing heat while it warms.

Much smaller. Jacuzzi says 240V tubs carry 4 to 5.5 kW heaters and 120V plug-in tubs about 1 to 1.5 kW, that a 120V tub takes about 8 to 12 hours to reach temperature, and that it may struggle to hold temperature with the jets running in very cold weather. Cal Spas' 128-gallon Aloha heats with 1 kW on 120V, which works out to roughly 3°F an hour. See plug-and-play hot tubs for the 120V models and hot tub electrical requirements for what a 240V circuit involves.

No. A 5.5 kW heater replaces the same lost heat as a 4 kW heater, just in less time, so it uses about the same energy over a month. What does differ is the repair bill once the warranty ends. Hot Spring's own parts store lists its in-house No-Fault elements at $509 to $621, while a compatible heater made by HydroQuip sells for $299.95. Ask whether the heater is a standard part a local technician can buy.

A heat pump does not make heat from electricity. It pulls warmth out of the surrounding air and moves it into the water, the way a heat pump water heater does; the Department of Energy says those can be two to three times as efficient as resistance water heaters. Few hot tub makers offer one. Jacuzzi lists its J404L, J408 and J5 models as heat-pump ready, and Wellis sells an optional 3 kW heat pump built into a step for its Life tubs. If a dealer offers one, ask what it costs installed and how it performs on your coldest nights.