·         On Avg. you need about 100 Btu for every 10 sq. ft of surface area for every 1 degree F. of temp. rise.

·         L x W = sq. ft.

·         sq. ft/10= a

·         Answer  x 100Btu = Btu needed for each degree of heat rise desired.

·         Answer  x your desired heat rise in degrees F= Btu needed per hour to hold temp.

·         answer X 24 hrs/day = Btu total

·         Btu Total/ hrs of circulation per day = size of heater


Ž      answer X .75% efficient = actual out put


Ž      Or cubic ft x 7.5 (gallons per cu ft) x 8 lbs. per gal x heat rise desired


Heater size / 15,000= # ton heat pump



q       Spa


·         Sq. ft. x  avg. depth = cubic ft.

·         Cu. Ft. x 7.5(gal in each cu ft.) x 8 (lbs. Per gal.)= POUNDS OF WATER.


·         POUNDS  x avg. 25° heat rise = Btu total



q       Cost of operation.


Knowing one Btu is the amount of heat needed to raise the temperature one pound of water one degree Fahrenheit .  A Therm  (unit of measurement on gas bill) = 100,000 Btu/hour of heat.


·         Heater Btu.  Divided by 100,000 Btu per hr. = Therm per hr

·         Therm per hr x  hrs per day of use = Therm per day to run heater.

·         Therms per day  x  cost per Therm = cost per day.



·         250,000 Btu heater / 100K = 2.5 Therm per hr

·         2.5 x 8 hrs per day = 20 Therm per day

·         20  x  50 cents = $10.00 per day.  Wouldn’t that be nice?