• Gas heaters (natural gas or propane) heat water quickly and are ideal for colder climates or spas that need on-demand heat.
• Electric heaters are typically used for smaller spas and cost more to operate but are easy to install.
• Heat pumps are energy-efficient and ideal for warmer climates, pulling heat from the air to warm the water.
  

Each has different pros, cons, and operating costs, so your choice should depend on your pool size, climate, and usage needs.

To size your heater correctly, you'll need to know your pool or spa's water volume (in gallons) and desired temperature rise. Heaters are rated in BTUs (British Thermal Units)—the higher the BTU, the faster it heats.

As a general rule:

• For pools: 250,000–400,000 BTUs
• For spas: 100,000–250,000 BTUs

It depends on the heater type, BTU rating, and starting water temperature. On average:

• A spa can heat up in 30–60 minutes
• A pool may take 8–24 hours

Gas heaters work fastest, while heat pumps are slower but more efficient long-term. Using a can help retain heat and speed up the process.

• Cleaning out debris and checking vents for blockages
• Inspecting for corrosion, especially on gas heaters
• Flushing the heat exchanger annually
• Ensuring proper water chemistry to prevent scale buildup

Regular upkeep not only extends the life of your heater but also helps maintain performance and reduce energy costs.