Author: Oliver Smith

To achieve the best water heating results for your wood-fired hot tub, you need to educate yourself at least a little on the best firewood for wood-fired hot tubs. This article will help you to dip your toasty toes into the topic so you can make the right choice for maximum hot tub efficiency and enjoyment. 

By comparing different types of wood to burn for heating your hot tub water and learning how to use them properly, you will be able to make an informed decision.

We’ll also discuss some applicable rules and regulations so you understand why you can only buy dry wood with less than 20% moisture content. If you’re a land owner and can produce your own firewood, in this article you will be advised on the reasons why only dry wood is authorised for burning in domestic stoves in the UK.

Let’s get started! 

Which type of firewood heats the water the fastest? 

The fastest heating result can be achieved by burning the types of wood that yield the biggest heat output when dry enough and cut into medium or small pieces. Check out the table below to see the different species of firewood and their heat output grades. This is a good reference to plan your firewood purchase, but, as you will find out later, you must also ensure you have selected the right sized logs and followed the correct procedure of lighting the fire and attending to it to heat the water in a wood-fired hot tub. 

Common UK firewoods and their characteristics

Name of firewood	Type of wood	Grade	Comments
Oak	Hardwood	High	One of the best firewoods that produces high levels of heat and burns slowly.
Ash	Hardwood	High	One of the best firewoods with a low moisture content that burns slowly with high heat output and good flame.
Hazel	Hardwood	High	Can be burnt without splitting.
Birch	Hardwood	Good – high	Burns quickly, so it’s best when mixed with oak or elm. Birch bark can be used as an excellent fire-lighter.
Hawthorn	Hardwood	Good – high	Good heat output and little smoke. Can be more challenging to split.
Beech	Hardwood	Good	Needs long seasoning due to its high water content. Can spark when burning.
Apple	Hardwood	Good	Slow-burning and producing good heat output, occasional sparks and a nice smell.
Sycamore	Hardwood	Good	Burns well, producing a moderate heat output.
Elm	Hardwood	Low – good	Burns slowly, providing a lasting heat. Good to combine with faster-burning wood like birch.
Elder, walnut, willow	Hardwood	Poor – low	Burn quickly and produce low heat output.
Spruce	Softwood	Low	A low-quality firewood, but good to use for kindling when starting the fire.
Pine	Softwood	Low	Splits easily and burns very well, making for good kindling. Leaves sooty deposits.

Which hardwood should I buy? 

From the table above, you can see that oak and ash produce the highest heat output. 

However, they are also the most expensive types of firewood, so it can be more economical to buy a mixture of hardwood logs to save money. For example, at The Log Company, a bulk bag of select ash or birch logs cost £149.00, while the same amount of mixed hardwood is valued at £109.00. 

In any case, the firewood you buy must be ready to burn, meaning it has been dried so that it contains no more than 20% moisture.

Do I need softwood at all? 

Yes, you do. The best firewood for wood-fired hot tubs is not limited to hardwoods only. While hardwoods yield more heat and burn for longer, they are harder to ignite from cold. Softwood (such as pine or fir) burns much more quickly, plus it is more affordable. At The Log Company, you can buy a bulk bag of softwood logs for £89.00. 

If you burn only softwood, the heating time will be longer. More resinous softwood logs will also result in more soot in the heating system, requiring more regular and thorough cleaning. 

You should mix softwood with hardwood logs to complement one another to achieve the best results.

What types of wood are not suitable to burn in a wood-fired hot tub? 

We are aware of at least one case where burning Leylandii (Leyland Cypress), even after having been dried for a year, produced terrible amounts of soot and resin stalactites in the chimney and heater of the wood-fired hot tub, so we strongly recommend avoiding using such firewood in your stove. 

Driftwood is another ‘species’ of firewood that should be avoided. When burning salt-saturated wood, toxic or harmful chemicals are released into the air, so it’s best to just use driftwood for garden décor, if you so desire.

It’s also a bad idea to burn twigs, fallen leaves or any other bits of wood you may find when cleaning up your garden. These types of wood are wet and produce a lot of smoke, so it’s better to compost them.

Never burn construction or furniture wood that is covered with paint, impregnants, varnish, priming or similar toxic or harmful materials. You should dispose of any such wood in accordance with your local council’s regulations. 
To protect the surrounding woodland areas from the spread of invasive pests, try to purchase only locally-produced firewood where possible. Scroll down to “Where do I get my firewood?” to click on a link to find local firewood suppliers.

Why should I only buy “Ready to Burn” certified firewood? 

There has been a legal obligation under the Air Quality Regulations in the UK since May 2021. The legislation focuses on certifying wood fuels sold in volumes under 2m³, with the aim of preventing harmful pollution due to the domestic burning of all solid fuels. Firewood with moisture content of under 20% emits less smoke and fewer particulates than wetter wood. To decrease the amount of smoke and particulate matter emitted by a wood-burning stove chimney, and to speed up the hot tub heating process, it is best to use kiln-dried barkless firewood.

Naturally seasoned firewood still contains a moistre content of 25-30% which does not feel damp to the touch. That’s why certification and labelling demonstrate the dryness of the wood. 

Green firewood is too wet to burn effectively, causing it to make a hissing sound when burning. Burning wet wood is also highly inefficient as it produces less heat than dry logs of the same type of firewood. On a practical note, wet wood also produces more soot and ash, which means you need to clean the heater, chimney and glass door more regularly.

Where do I get my firewood? 

If you are a customer of The Log Company, then look no further! This is one of the trusted suppliers of authorised firewood for burning in a wood-fired hot tub compared to any other brand’s unit. 

You can also visit the Woodsure website to find numerous local firewood suppliers by entering your postcode. Woodsure is a woodfuel quality assurance scheme in the UK. Rest assured that it will only refer you to suppliers who sell firewood with the right level of dryness. 

The rule of thumb is to buy your firewood in larger quantities if you have the storage capacity. This will ensure you receive better pricing, save on transportation costs and use less packing material.

Can I produce my own firewood? 

You most certainly can, as long as you follow the applicable tree felling rules. The firewood you prepare must be seasoned and kept in such a way as to ensure it dries out. This is so it does not produce a lot of smoke or pollution with particulate matter. To ensure proper drying and seasoning of your firewood, we recommend following these  tips on how to dry firewood at home. 

How much firewood do I need to heat the water in my hot tub? 

Some wood-fired hot tub sellers estimate that you are likely to burn 12-25 kg of dry wood to heat the water to the optimal temperature, and those figures are in line with our experience. 

Below is a visual representation of how much firewood you need to heat the water in your hot tub from cold. On average, two nets of logs would be required. Some suppliers also sell firewood in barrow bags, which is more than enough for heating your hot tub once. 

Photo credit: The Log Company

The actual amount of firewood required will depend on the season, wind, type of wood, initial water temperature and your skills at operating the stove. Less firewood is needed if you keep the hot tub covered and maintain a good routine of preparing and burning the fire. 

How do you effectively operate a wood-burning stove? 

Ideally, you will start the fire in your stove by burning softwood kindling. Keeping lighters at the ready will make the process much easier and faster. Stack the kindling in a way that allows for good air circulation, such as a pyramid or Jenga (see below). 

Continue with a mix of medium-sized chunks of hardwood and softwood to generate heat in the most effective manner. The idea is that softwood produces fire more quickly that then heats and ignites the hardwood. At that point, the hardwood takes over to yield much more heating power to transfer into the water. 

Use only hardwood logs at the final stage of heating and to maintain the heat once the optimal temperature has been reached. You may then use larger chunks of wood so that it burns slower and more steadily.

Check out this useful article on how to heat your wood-fired hot tub over on the Gardenvity blog where they take you through the process step-by-step.

How do you reduce the amount of smoke? 

First and foremost, the best firewood for wood-fired hot tubs is dry firewood. Kiln-dried, seasoned firewood generates more heat, and its lower moisture content produces less smoke. 

Even the size of the logs you use matters: medium and small logs burn faster with more heat and less smoke.

Ensure that you remove the bark from the firewood to achieve even better results by further minimising the amount of smoke.

Do not burn paper – it spreads a significant amount of smoke which can also be harmful. 

We are sure you would not be doing this with your wood-fired hot tub, but we’ll repeat it here for the avoidance of doubt – you should never, under any circumstances consider burning plastics, rubber, textiles, old batteries or similar materials in your wood-burning stove.

IMPORTANT: 

• You must never use any fuels other than wood in a wood-fired heater. Coal and charcoal burn at much higher temperatures and will damage the system. 
• Never ignite the fire if the hot tub is not filled with water. 
• Do not drain the water while the heater is hot. If you need to pack up and leave in a hurry, then you must put out the fire completely and empty the hot ash from the heater. Only then can you drain the water, if necessary.

After reaching the end of this article, you are now well-prepared to shop for the best firewood for the wood-burning stove in your hot tub. So, enjoy the process, the crackling sounds and the smell of smoke on your palms before sinking in for a relaxing soak!

Not everything you’re going to need to enjoy hot tub ownership will be covered in the instruction manual, and knowing how to efficiently use your wood-fired hot tub comes with experience. Avoid the trial-and-error approach with our tips and tricks to make your hot tub life easier from day one.

How to get the temperature up while keeping the heating bills down

It sounds simple, but the goal is to keep your burning time to a minimum. What’s less simple is that there are a number of different ways to go about it. 

The two most important factors in cutting the time it takes to heat your water are choosing the right firewood and using an insulated hot tub cover. In the same way that your house is better at retaining heat than a tent or garden shed, an insulated cover will stop your tub’s heat escaping much more effectively than plastic, wood or canvas. Artificial leather thermal covers are also lighter and more manageable than some other options.

In a wood-burning stove, it’s crucial to build the fire up gradually. Different sizes of logs have different jobs to do, so avoid piling up large hardwood logs as soon as the kindling has taken. Use a mix of soft and hardwoods (pine, fir, birch, ash or oak), and burn medium-sized logs first. Once you’ve got a healthy-looked fire, use the larger logs in the last stage of heating the water to generate hot embers that will last longer, needing less attention and leaving you free to enjoy the tub!

Make sure your firewood is as dry as possible (ideally kiln-dried) so it burns more efficiently, generates more heat and produces less smoke. 

If you plan on using the hot tub a few days in a row, or even skip a day in the middle, don’t let it cool right down. It’s much more economical to maintain warmth than it is to start from cold again. If your tub has a hybrid heating system, you could leave the electric heater on to keep the water up to temperature, or if not, feed the fire a few logs at least twice a day. Once you’ve put the work in to get the fire going, it doesn’t need a lot to keep the water toasty. 

As a side note, if you use the massage system to move the water in your hot tub while heating, it’s always better to do that with your hydro jets. Using an air jet system to move the heat around the tub creates bubbles which can actually slow down the heating process. If the water gets too hot, then air jets can help – add some cold water, stir it well, and use the bubbles to help bring it back to bathing temperature. Always check before you get in, and you’re aiming for between 37-39℃.

How to make sure your artificial leather cover lasts

If you don’t take good care of your insulated cover, it can start to gather mould, or even harden and crack from being exposed to the UV rays of direct sunlight. 

Firstly, don’t shut the cover tight straight after draining the tub. Moisture left in the hot tub needs to evaporate, and if you trap it inside, it’ll collect on the underside of the cover. Without air circulation, this condensation will create the perfect conditions for bacteria and mould. So, for a couple of days after draining, it’s best to either leave the cover off, cover it but leave it half-open, or you could put something under the edge of the cover to create a tiny gap for the moisture to escape. After you’ve given it a few days, you can secure the cover tightly.

Secondly, the hot tub cover needs to be cleaned monthly to avoid the buildup of bacteria. While a simple wooden or plastic cover only needs a scrub with some detergent and a good rinse, an insulated faux leather cover requires a little more attention. Clean it regularly using leather or saddle soap, and treat it with a faux leather conditioner with UV protection twice a year to keep it looking good, and make sure it lasts.

How to avoid regularly changing the water

Keep your water clean by preventing debris from building up in the tub. When it’s not being used, keep the tub under cover. 

More importantly, properly check the water on a regular basis (ideally every day). A quick glance under the cover isn’t enough here –  knowing the pH level tells you whether you need to increase or decrease the water alkalinity with the right chemicals. Regular water maintenance ensures that you will need fewer chemicals overall, and don’t have to resort to more drastic treatment actions.

When making these adjustments, be sure to use the right combination and quantities of chlorine, bromine or oxygen-based chemicals and closely follow the manufacturer’s instructions!

If you don’t change the water after using your wood-fired hot tub, you must use the water filter and run it for a few hours (most manufacturers recommend 4–5 hours) daily, even if you’ve not used the hot tub. An automated switch or timer plug socket can come in handy to make sure this step doesn’t get forgotten!

Another additional option that experienced users choose to keep their hot tubs in good shape is  a UV light for the water filtration system. It makes the system more efficient at killing pathogens and also reduces the amount of chemical products needed to keep the water in your hot tub safe and free from bacteria.

What you should know about cleaning the hot tub

Cleaning the hot tub is all about getting into good habits. As long as you’re careful with the following points, you’ll cut down on bigger maintenance jobs and keep your hot tub running smoothly. 

Never put off cleaning the liner after draining your tub, and definitely don’t let it dry out. If you wipe it down immediately after draining the hot tub, you’ll remove any sediment and debris from the liner surface before it dries. If you don’t, they’ll stick, and they’ll take more effort and more cleaning products to get off later.  

Clean your hot tub regularly. Don’t leave it until the water starts to smell! Use the right specialist cleaners to treat the hydro massage system to prevent the tubes from getting clogged, or even blocked, which can damage the tub. 

If you have a hot tub with a light-coloured liner, you should also keep the cleaning schedule more frequent. The mains water in some parts of the country can have a higher level of iron or other metals, which can eventually cause discolouration of the liner turning it yellow or greyish. Don’t compromise on your cleaning and upkeep regime, and your hot tub can look and work like it’s brand new for years to come.

During the cold season

When there’s even the slightest chance of temperatures dropping below zero, you have to be extra cautious about protecting your wood-fired hot tub from the damage that frozen water can cause.  Follow these tips to keep your hot tub safe in the colder months:

• If your tub has an electric or hybrid heater, keep it on to maintain a warm water temperature, or drain it from the tub. 
• When you drain the water from the hot tub, turn on the massage systems for up to thirty seconds to blow out any water residue from the tubes and nozzles. (Check with the manufacturer if they allow this function so as not to accidentally damage the massage system.)
• Leave the drain valve open to save it from cracking should any water collect in it and freeze.
• Make sure there’s no water left inside the water filter or massage system motor. Check your model’s user manuals very carefully to find out if you need to take any specific steps when preparing the whole system for winter.
• Check your manufacturer’s recommendation for the water filter – some companies clearly require you to disconnect the water filter and store it in a warm place through winter.

In the UK, the cold season is often accompanied by strong, blustery winds, so remember to remove the upper section of the chimney to protect it from wind damage, or from becoming detached. 

When using your hot tub in cold weather, be careful not to heat the water to more than 39℃. It might not feel like it, but your body is still very sensitive to high water temperatures, even when the ambient temperature is low. 

If you’re having a winter soak, keep your head covered. Of course, you’ll feel warm when in the tub, but the reality is that your head will be at a much lower temperature than your body. Losing body heat from your head can be bad for your health, especially if you let wet hair stay exposed to cold winds. 

Hopefully, we’ve covered some of the more practical pointers to help you get the most out of your hot tub. We’ll go into more detail on some of these elsewhere on the blog, and will keep adding to our how-to lists to keep you up-to-date with the latest user hacks and improvements for your wood-fired hot tub. You can also find tips related to wood-fired hot tub maintenance works and expected costs in one of our earlier articles. 

Enjoy your toasty soaks!