Included with an all-in-one pool/spa is the industry standard 11 KW electric heater for spa only, which works at 55° outside temperature and above. For a more efficient heater, you may upgrade to a larger electric, larger gas, or one that will heat both pool and spa. Gas heaters come in propane or natural gas. Heater cost does not include gas line or hookup from gas pipe to heater location on back wall of house. This requires an additional fee, quoted according to distance. The average cost for heater upgrades is $3500-$4500. The average cost for gas lines starts from $4500. Also available are heater/chiller combos, with an average cost of $7500.