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!
