For pool owners who don’t want to wait and enjoy a heated pool, or need an extra boost of power, 400,000 BTU pool heaters will get the job done in a pinch. These heavy-duty heaters are designed for heating speed and efficiency. And despite the power, a few of the 400,000 BTU heaters at Pool Express are designed to be eco-friendly, for the environmentally conscious pool owner.Swimming pool heaters are rated according to a measurement known as British Thermal Units, or BTU. One BTU is capable of raising the temperature of 1lb of water by 1 degree Fahrenheit. This means the more BTUs a pool heater produces, the faster it will heat the water in your swimming pool. And 400,000 BTUs is more than enough power. Pool heaters with 400,000 BTUs are a safe bet to quickly heat any sized swimming pool. The pool heater should always be somewhat higher than the actual pool surface area, and the desired temperature rise required. But because there’s no such thing as “oversizing” a pool heater, pool owners typically choose a 400K BTU heater for their fast heating speed.