Outdoor SPA Heat Loss
By definition ALL SPAS NEED HEATING.
If you have an in-ground concrete spa then the heat losses are identical to a pool (see pool heat loss) .
If you own a spa with a pre-fabricated acrylic , fiberglass or vacuum-formed plastic shell either above or in ground, it will be very well insulated and usually have a built in hard cover - all of which eliminates most heat loss. Since the hard cover is usually opaque it also eliminates most re-radiation heat loss.
Once heated to temperature these spas will only lose heat rapidly when UNCOVERED just like a pool does - (via evaporation, convection and radiation heat loss) unless air jets are on.