What is the ideal size heater for a 20,000 gallon pool? 100,000 - 200,000 BTU 25% 300,000 - 400,000 BTU 75% Get Free Quotes Today! Question: What is the ideal size heater for a 20,000 gallon pool? Top Answer (75% of 4 votes): 300,000 - 400,000 BTU. Answer: 300,000 - 400,000 BTU Explanation: This is the needed btu for a pool that size Precision Pools and Construction WV Answer: 300,000 - 400,000 BTU Explanation: Remember to consider the use of a spa as well when purchasing a kid. NorCal Pool Builders Get Free Quotes Today! Find Local Pros Free Pool Heater Installation Cost Estimates Related Questions How Much Does Pool Heater Installation Cost? How Much Does a Gas Pool Heater Cost? How Long Does a Pool Heat Pump Typically Last?Top answer: Five to ten years How Long Does a Gas Pool Heater Typically Last?Top answer: 5-10 years Can you add a heater to an existing pool?Top answer: In most cases