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August 17th, 2010 at 8:08 pm

Spa and Hot Tub Thermal Efficiency

in: spaeval
About Thermal Efficiency..
A lot can be said these days regarding the ability of a spa to retain heat.  However, in the portable spa market, these days, it is virtually a non-issue. 

From Sof-Tub to even the inexpensive Jacuzzi brand spas at Home Depot… there’s hardly a portable spa manufactured out there that won’t retain heat extremely well.  So, in most cases these days, it’s definitely nothing to lose sleep over.


There are three different ways that heat is transferred…
  • By Conduction.  One surface touching another.
  • Convection, the natural flow of heat through air movement, and the rising effects of it.
  • Radiation, can be easily understood as being the radiation as felt by the sun, or from a stove burner.
The most commonly used insulation material is an expanding poly-foam, which is usually yellow to tan in color, and is used by almost every spa manufacturer.  It primarily stops the flow of heat by the prevention of convection.  Without the foam, the heat would transfer from the inside of the shell, to the outside (by conduction), and radiate outward, and be carried away by air convection.  Note that all three factors come into play here….  but by spraying the foam in the cabinet, the chance for convection is nil.
Some spa manufacturers utilize full foam insulation, that is, foam that fills the spa skirt 100 percent.  This is one of the most effective means of controlling temperature loss, and has been in use for many years by most nationwide manufacturers.
Many other spas, (and if you shop around, you’ll probably find that most of them are), are constructed with a thick fiberglass shell, with an acrylic type of sheet on top of the shell.  The thicknesses of these shells will vary anywhere from 1/8″ to as much as 3/4″ in some areas.  These shells are generally more expensive to manufacture, and are usually made in a facility that is dedicated to this purpose…. then shipped to the spa builder/manufacturer for drilling, plumbing, skirting, etc. 

Usually what you will find with a spa built in this fashion, is that all exposed surfaces of the spa shell, plumbing, jets, and manifolds, will be sprayed with foam, in some areas thicker than others.  Additionally, the foam is used to keep these components “in place”, preventing the movement of the plumbed parts, and subsequent leak problems from occuring.
Recently, I have begun to see spas built with a new type of insulation scheme, which is rather remarkable.  It uses a laminated metallic foil/air bubble seal material on the inside of the skirt, and at the same time, uses poly-foam on the spa shell and plumbing.  The effectiveness of this is insulation is extremely good…. as the metallic foil contains the radiation of heat from the spa.  Actually, the foil material reflects the infra-red component of heat back into the spa itself, literally creating its own convection oven effect inside the skirt.  The result?  Extremely effective recycling of heat from components such as the pump motor and spa control system, which is normally lost through equipment ventilation. 
What should you buy then?

Like I’ve said in the buyers guides, buy what you like!  All well made spas will exhibit superb thermal efficiency characteristics… 

Personally, I prefer the feel of the thin shell material used by Dimension One and others, but I also like the ease of service of a full thickness fiberglass shell with limited foam… and the superb thermal characteristics of the reflective foil material on a spa skirt…  HA!  It’s a tough choice!  But ALWAYS remember:

Buy what you like folks!  They’re ALL pretty good!
 
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