Do Wood-Burning Pool Heaters Actually Work? (It Depends!)

Swimming pools are great for recreational activities for friends and family year in and year out. Although pools are more popular and practical to use in the summer months when it’s hot, some of us still want to have fun in them even in mild weather.

This is possible of course with pool heaters. Thanks to necessity, many kinds of pool heaters have been developed through the years. 

There are modern pool heaters that use solar, gas, or electricity –and, then there’s the old-fashioned way of burning wood.

While wood burning heaters have been around for ages –especially as a way of heating our homes– you may wonder “Do wood burning pool heaters work?”

In this article we’ll assess whether wood burning pool heaters work, and some important considerations before pursuing this option.

Can You Heat a Pool with Wood?

Technically, yes –just like wood can be used to heat the air in a house, it can also be used to heat water in a swimming pool. This doesn’t mean having a fire pit built under your pool, or manually filling your pool with hot water boiled through a firewood stove.

Wood-burning pool heaters, commercial or not, do exist. For some, this is a cheaper and more efficient way to heat a pool as long as there’s plenty of wood (more on that later).

It may not be as convenient or as popular as modern pool heaters that use gas or electricity, but wood-burning pool heaters may be an option under the right circumstances. 

This is especially true for places with a lot of available firewood. Wood-burning pool heaters can actually be an economical option compared to paying a premium utility bill.

How Does a Wood-Burning Pool Heater Work?

There are several types of wood-burning pool heaters, including stoves and boilers.  In some instances, people can even use existing wood-burners that serve their house to also serve their pool!

Design characteristics of the different types of wood-burners can vary, but they all operate under similar heat transfer principles.

Wood-burning heaters use a chamber, usually made of stainless steel or aluminum, to burn the firewood.

Water from the pool is piped to the wood-burner where it moves through tubes inside a heat exchanger, which transfers the energy from the burning wood into the water.  The heated pool water is then returned back to the pool.

Some types of wood-burning pool heaters operate with natural thermosiphon circulation without need for a pump.  Other wood heaters require the use of forced circulation, such as through the pool pump.

These wood-burning systems can be left completely manual, where it will require someone to monitor the burner temperature and adjust the flow rate of water.  Or, they can have automated controls added that will make adjustments based on the pool temperature.

How Fast Does a Wood-Burning Pool Heater Heat a Pool?

There are several factors that affect how fast the water in your swimming pool will heat up.  The type and size of the wood-burner will be a key factor in how fast you can heat the pool.

The rate of heating also depends on several other factors:

• Volume of the pool: A larger pool will take more energy to heat than a small pool. Depending on the size of the wood burner, this will likely mean it will take longer to heat a larger pool. 
• Temperature of the pool: A colder pool will absorb more heat than a warmer pool.  This means you’ll see a quicker increase of temperature when heating a cooler pool, as it will be more efficient.  
• Type of wood: Different species of wood provide more thermal energy (BTUs) than others.  Many hardwoods will provide more heat than softwoods.  

Commercial wood-burning pool heaters have a specific power rating and efficiency rating.  Large heaters can provide 300,000 BTU or more of heat to your pool.  The larger the heater, the faster your pool will heat.

A 200,000 BTU wood-burning heater can raise the temperature of 20,000 gallons of water by 1.2°F per hour.  This provides up to a 10 degree increase in just over 8 hours of running the wood burner. 

One caveat with these calculations is that they assume 100% efficiency, which is near impossible.  So, instead of taking 8 hours to heat the pool in the previous example, it may take 10 hours depending on the actual efficiency.  

As you’re heating the water, you may also be battling heat loss at the pool due to evaporation.  Especially during cold weather, you may be trying to fight a near impossible battle where the rate of cooling exceeds the rate of heating.  Using a solar blanket can help retain that heat, though.

How Much Wood Do You Need to Heat a Pool?

While the time it takes to heat the pool with a wood-burner is one consideration, it may be even more important to know how much wood you’ll need.  You’ll need to ensure you have plenty of wood ready to feed the fire.

Here are some basics to determine how much wood you need to heat a pool.

It requires 1 BTU (British Thermal Unit) to heat 1 pound of water by 1°F.  The BTUs will come from the energy released by burning wood.

While different wood species can provide varying amounts of energy or heat, a common rule of thumb is that one pound of dry wood will provide 8,600 BTUs.  

To determine how many pounds of wood you need to heat a pool by 1°F, you need to know the weight of water in your pool.  

Each gallon of water weighs approximately 8.3 pounds.  Based on this information, an average 20,000 gallon pool has 166,000 lb of water.

Since each 1 lb of wood provides 8,600 BTU, you’ll need about 20 lb of wood to heat a 20,000 gallon pool by 1°F.  A simple calculation means you’ll need at least 200 lb of wood to increase the pool temperature by 10°F, assuming no heat loss.

For reference, a cord of firewood weighs about 2,500 pounds.  This cord of wood can provide up to 125°F worth of heating for a 20,000 gallon pool.

The length of time that the cord of wood lasts will depend on the size of your wood-burner and the duration that you run it.   

How Much Does It Cost to Operate a Wood-Burning Pool Heater?

If you’re fortunate to have a plethora of firewood on your property, then operating a wood-burning pool heater will be virtually free.  It’ll just be a trade-off of your time and effort to prepare the wood.

However, if you do not have wood available and need to purchase it, the wood-burning heater could become quite costly to heat your pool.  On average a cord of wood costs $300 and contains about 2,500 pounds of wood.  

To heat a 20,000 gallon pool by 1°F, you’ll need to burn at least 20 pounds of wood.  This equates to a cost of $2.40 per degree that you’re heating the pool.  

By comparison, here’s how much it’d cost to heat the same 20,000 gallon pool by 1°F using other traditional heaters: 

• Natural Gas Heater: $1.66 (assumes $1.00 per therm)
• Electric Heat Pump: $1.25 (assumes $0.13/kWh)

As you can see, the price of wood will heavily determine whether a wood-burning pool heater is actually cheaper to run than a gas or electric heater.

Pros of Wood-Burning Pool Heaters

Wood is a natural resource that we can use not just to heat our homes but also our swimming pools. Here are two reasons why a wood-burning pool heater may be a worthwhile option:

Low Operating Cost

Wood-burning pool heaters will be significantly cheaper to operate than gas or electric pool heaters if you have an abundance of firewood available.  Instead of paying a high utility bill for gas or electricity, you can simply walk to your backyard to grab some wood.

If wood is scarce, it may be in your best interest to keep an eye out for neighbors that are looking to get rid of some downed trees! 

If you do not have free wood available, then the low operating cost is pretty much nullified.  

Potential Dual Use

If you have an existing wood burning stove or boiler that you use for your home, there’s potential you may be able to retrofit it to also heat your pool.  Even some outdoor grills, fire pits or pizza ovens could be used to heat your pool.

Just remember, the design of your wood burner will limit exactly how much heating capability can be provided to your pool.  However, if you’re already relying on wood for heat around the house, you may as well give some benefit to your pool as well!

Cons of Wood-Burning Pool Heaters

While there are some obvious benefits of using a wood-burning pool heater, it may not always be the best option.  Here are some downfalls to keep in mind:

Requires Free or Cheap Wood

While wood-burners can operate with low cost if you have an abundance of free wood available, the cost to heat your pool can quickly escalate if you need to purchase wood.

As mentioned previously, it could cost $2.40 or more for each degree that you heat a standard 20,000 gallon pool.  For comparison, a smaller 5,000 gallon pool may only cost $0.60 in wood to raise the temperature by a degree.

Requires Attention During Use

Unlike traditional pool heaters where you can simply flip a switch to turn it on, a wood-burner will require routine attention during use.  If you want to keep the pool heating you’ll need to feed or stoke the fire routinely.

The frequency that you need to add wood will depend on the size of the burn chamber, the type of wood, and how hot the burn chamber is.

If you install an automated temperature control system this can help ensure your wood-burner maintains a set temperature –but, you’ll still need to add wood routinely.

May Not Overcome Cold Temperatures

While it takes 1 BTU to heat 1 pound of water by 1 degree, evaporation due to cold weather can quickly remove that heat from the pool.  This is because each pound of 80°F water lost to evaporation results in a loss of 1,048 BTUs!

So, if you’re running the wood-burner to heat a pool in cold weather, it’s very possible it could become a losing battle depending on if you’re able to exceed the rate of cooling.

In such situations, it’s important to use a solar blanket to prevent as much evaporation as possible.

Other Types of Pool Heaters

There are other types of pool heaters that many people use today, especially those who have limited access to firewood.

Electric Heat Pump – This type of heater uses the heat from the air to heat the water in the swimming pool. It’s efficient and requires less electricity than a traditional electric pool heater.  However, it doesn’t work as well in cooler weather since there’s not as much heat to draw from the air.

Gas Heater – This type is effective if you use your swimming pool often and year-round. However, the cost of maintaining it will depend on gas prices in your area. This type of heater needs a gas line and typically needs professional installation.

Solar Heater – This type of heater uses solar panels instead of fuel or electricity to transfer heat. For this heater to work, sunlight is required and heating is more gradual.  You can even make your own solar heater for relatively cheap.

Regardless of what type of heater you use, it’s highly recommended to use a solar cover.  This can prevent up to 85% of evaporation, which will conserve the heat that you’ve been adding to the pool.

Final Takeaway

Under the right circumstances, a wood-burning pool heater can be a great alternative to other traditional pool heaters. This is particularly true if you have tons (literally) of wood available for free or cheap.

For average size pools, you may need to burn through hundreds of pounds of wood to see a noticeable difference in water temperature.

If you don’t have lots of firewood available, a wood-burning heater could actually be more expensive to run than gas or electric heaters.

Whatever heating method you use, ensure a solar cover is part of your plan!

Happy (Warm) Swimming!