How fast will a 400000 BTU pool heater heat a pool?

A 400,000 BTU pool heater can heat a pool relatively quickly depending on the size of the pool. The heating time will vary based on factors like the starting water temperature, desired heating temperature, ambient air temperature, and wind speed over the pool. But generally, a 400,000 BTU heater provides a high heating output that can raise pool water temperature at a fast rate.

Quick Answer

A 400,000 BTU pool heater can typically heat a standard 20,000 gallon in-ground pool by about 1-2 degrees Fahrenheit per hour. So to heat the pool from 70°F to 80°F would take about 5-10 hours. The heating time may be longer if the starting water temperature is very cold or if the target temperature is especially warm. Windy conditions will also slow down heating.

Heating Time Estimates

Here are some estimates for how long it could take a 400,000 BTU pool heater to heat different size pools under typical conditions:

Pool Size Heating Time from 70°F to 80°F
10,000 gallons 2-4 hours
15,000 gallons 3-6 hours
20,000 gallons (standard in-ground pool) 5-10 hours
25,000 gallons 6-12 hours
30,000 gallons 8-15 hours

As shown, a typical 20,000 gallon pool would take about 5-10 hours to heat 10 degrees with a 400,000 BTU heater, assuming normal weather conditions. Smaller pools may heat up in just a few hours, while larger pools could take up to 15 hours. These are general estimates only.

Factors Affecting Heating Time

Several key factors influence how quickly a 400,000 BTU pool heater can warm the pool water:

Pool Size

Larger pools will naturally take longer to heat than smaller pools because there is more water volume to warm up. Doubling the amount of water would roughly double the heating time under similar conditions.

Starting Water Temperature

The cooler the starting water temperature is, the longer it takes the heater to raise it to the desired temperature. For example, heating from 50°F to 80°F will take about twice as long as heating from 70°F to 80°F.

Target Heating Temperature

The higher the target temperature, the longer the heating duration. Heating to 90°F could take 50% longer than heating to 80°F depending on conditions.

Air Temperature

Colder air temperatures will make the water lose heat faster, slowing down the heating process. Heating times will be faster on warmer days.

Wind Speed

High winds increase evaporative and convective heat loss from the pool surface, countering the heat added by the pool heater. Low wind speeds allow for faster heating.

Pool Shape

Freeform and irregular shaped pools provide greater surface area for heat loss compared to rectangular or oval pools, potentially increasing heating duration.

Covered vs Uncovered

Using a pool cover prevents surface heat loss, allowing much faster heating compared to an uncovered pool exposed to wind and cold air.

Filtration System

An inefficient filter system that does not properly circulate all the pool water past the heater will extend heating times. A high-performance pump and filter ensure even heating.

Geographic Location

The general climate of the region can impact heating times. Pools in cooler northern areas may heat more slowly than those in warmer southern regions.

Heater Sizing Guidelines

As a general rule, correctly sizing the pool heater for your pool size is important for efficient heating performance. Here are some common heater sizing guidelines:

  • 100,000 BTU heater for pools up to 15,000 gallons
  • 200,000 BTU heater for pools 15,000 to 25,000 gallons
  • 300,000 BTU heater for pools 25,000 to 35,000 gallons
  • 400,000 BTU heater for pools 35,000 to 45,000 gallons

Oversizing the heater capacity for your pool allows you to heat the water faster. But very oversized heaters run the risk of frequent cycling on and off, reducing efficiency.

Tips for Faster Heating

Here are some tips to help speed up heating time when using a 400,000 BTU pool heater:

  • Use a pool cover when heating to retain heat.
  • Run the filter pump continuously during heating for better circulation.
  • Heat during warmer daytime temperatures instead of overnight.
  • Select a higher thermostat temperature to maximize heater output.
  • Ensure good water flow through the heater for best efficiency.
  • Maintain proper water chemistry to prevent scale buildup in heater.
  • Shield pool from wind using landscaping or wind breaks.
  • Preheat pool in advance of a planned event.

Comparing Natural Gas and Propane Heaters

400,000 BTU pool heaters are commonly available in both natural gas and propane fuel models. The heating capacity is similar for both fuel types. But there are some performance differences to note:

Factor Natural Gas Heater Propane Heater
Heat Output Slightly higher BTU output Slightly lower BTU output
Heating Time 10-20% faster heating Slightly slower heating
Fuel Cost Lower fuel cost Higher fuel cost per BTU
Convenience Requires fixed gas line Portable propane tanks

So natural gas pool heaters will generally heat a bit faster and cost slightly less to operate than propane models. But propane heaters don’t require installing gas lines and offer more flexibility in placement.

Electric vs Gas Pool Heaters

Electric and gas heaters both have pros and cons:

Factor Electric Pool Heater Gas Pool Heater
Installation Typically easier, plug-in Gas line must be installed
Heating Speed Slower heat times Faster heat up rate
Climate Suitability Best in warmer climates Better in colder regions
Operating Cost Higher electrical costs Less expensive gas fuel
Upfront Cost Lower purchase price Higher initial cost

In general, gas pool heaters are better suited to heating pools quickly and efficiently in colder climates. Electric models work well for maintaining temperature in warmer regions but are not ideal for rapid heating in cold weather.

Heat Pump Pool Heaters

Heat pumps provide an energy efficient alternative to gas and electric resistance heaters. Their performance attributes include:

  • Very energy efficient – Can have over 5 times higher efficiency than gas heaters
  • Slow steady heating – Not designed for rapid temperature increases
  • Low operating costs – Uses less electricity than an electric heater
  • Quiet operation – No noisy combustion
  • Long life expectancy – Last over twice as long as gas heaters

The downside of heat pumps is a high upfront cost. But they can pay back long-term with lower utility bills for pool heating compared to gas heaters. Heat pumps are better for maintained temperatures rather than quickly heating a cold pool.

Solar Pool Heaters

Solar pool heating systems use the sun’s thermal energy to heat pool water as it passes through solar collectors. Benefits of solar heating include:

  • Free heating using solar energy
  • Lower utility costs than gas or electric heaters
  • Clean, renewable energy source
  • Very long life expectancy
  • Low maintenance requirements

Drawbacks of solar heaters are that they require sunny weather to function, have a high upfront cost, and do not heat pools as quickly as fossil fuel heaters. But they can provide substantial utility savings in the long run.

Combination Heating Systems

The fastest and most flexible pool heating is achieved by using a combination of systems:

  • Gas heater – Provides fast heating for initial pool warm up or to quickly heat in cool weather.
  • Heat pump – Maintains set temperature efficiently during normal operation.
  • Solar – Offsets some daytime heating needs for free.

By combining the strengths of different technologies, a hybrid heating approach provides speed, efficiency, and economy for heating any pool.


A 400,000 BTU gas pool heater can raise pool temperature at around 1-2°F per hour for a standard 20,000 gallon in-ground pool. Smaller pools may heat up in a few hours, while large commercial pools may require up to 15 hours to reach desired temperature. Heating times are impacted by the starting water temperature, target temperature, weather conditions, and other factors. Following sizing guidelines and tips like using a pool cover, running the filter continuously, and shielding from wind can help accelerate the heating process. While gas heaters provide the fastest heating, combining technologies like solar and heat pumps can offer greater efficiency and economy for pool heating needs.

More Questions?

Here are answers to some other common questions about pool heaters:

How long should I run my pool heater each day?

You should size your heater to run between 6-8 hours per day on average to maintain a desired temperature. Run times depend on factors like the season, climate, and pool use. In summer, the heater may only run a few hours. In winter, it may run 8-12 hours on colder days.

What temperature should I set my pool heater to?

A good target temperature for most pools is around 78-82°F during main swimming season. You can set the heater between 75-85°F depending on personal preference and how much the temperature fluctuates.

How much does it cost to run a pool heater?

Gas heaters cost between $0.40-$0.80 per hour to operate for around 200,000-400,000 BTU models. Electric and heat pump heaters average $0.10-$0.30 per hour. Solar heating has minimal costs after the initial equipment purchase.

Should I run my pool heater overnight?

Running a pool heater overnight in cooler weather can help maintain the water temperature. But this is generally less efficient than running it during daytime. Using a pool cover at night also helps retain heat.

How long do pool heaters last?

With proper maintenance, gas pool heaters typically last 5-10 years. Electric heaters last 10-15 years. And heat pumps can last 15-20 years or longer. High-end models tend to exceed these estimated lifespans.

How do I calculate necessary pool heater size?

A good guideline is 100,000 BTUs for every 10,000 gallons of pool water plus 15,000 BTUs if the temperature drops below 70°F. So a 20,000 gallon pool in cool climate would need 200,000 BTUs. An online BTU calculator can also help size your heater.

Should I get a solar cover for my heated pool?

Yes, using a solar pool cover can raise heat retention by 50-70% compared to an uncovered pool. This allows your heater to operate much less to maintain desired temperature.

Is a bigger pool heater better?

Oversizing your pool heater too much can reduce efficiency and increase operating costs. Stick close to general sizing guidelines for optimal performance and value. A heater too large for the pool size may cycle on and off frequently.

Where should I install my pool heater?

Locate gas/propane pool heaters at least 5 feet from the pool itself according to manufacturer specifications for safety. Position away from excessive wind or rain exposure. Allow adequate access for maintenance and winterization.

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