How to Heat Up a Wood‑Fired Hot Tub and Maintain the Perfect 37–38°C Temperature

Heating a wood‑fired hot tub is simple once you understand the process. Whether you own an acrylic, fibreglass or Ofuro‑style tub, the goal is the same: reach a comfortable 37–38°C and keep it stable throughout your soak. This guide explains how to heat your tub efficiently, how long it takes, and how to maintain the ideal temperature.

1. Fill the Hot Tub to the Correct Level

Before lighting the fire, make sure the water level is above the top of the internal stove or above the inlet/outlet pipes on an external stove. Running the heater without enough water can cause serious damage.

2. Use Dry, Seasoned Hardwood

The best fuel for fast, efficient heating is kiln‑dried hardwood. It burns hotter, lasts longer and produces less smoke. Avoid softwood, wet logs or treated timber.

3. Start with a Small Fire

Begin with kindling and small logs to establish a strong flame. Once the fire is burning well, add medium‑sized logs. A steady fire heats the water faster than overloading the stove early on.

4. Typical Heating Time

Most wood‑fired hot tubs heat from cold in 1.5 to 3 hours, depending on:

• Outside temperature
• Water starting temperature
• Size of the tub
• Type of stove (internal heats faster)
• Quality of firewood

5. Stir the Water for an Accurate Temperature Reading

Hot water rises to the top. Use a paddle or long stick to gently stir the water before checking the temperature. This gives a true reading and prevents overheating.

6. Maintaining 37–38°C

Once the tub reaches your ideal temperature, add one or two small logs every 20–30 minutes. This keeps the fire alive without overheating the water.

7. Avoid Overheating

If the temperature rises above 40°C, stop adding wood and remove the lid. Stir the water to release heat. You can also add a small amount of cold water if needed.

8. Safety Tips

• Never leave the stove unattended while heating.
• Keep children away from the stove and flue.
• Do not operate the stove without water.
• Use the flue guard at all times.

9. After You Finish

Let the fire burn out naturally. Do not throw water on the stove. Once cooled, remove ash and ensure the tub is drained if freezing temperatures are expected.

Conclusion

Heating a wood‑fired hot tub is easy once you understand the basics. With good firewood, proper water levels and steady heat control, you can enjoy a perfectly warm 37–38°C soak every time. For more guides, tips and product information, explore the RG Hot Tubs blog.

Frequently Asked Questions

How long does it take to heat a wood‑fired hot tub?

Most wood‑fired hot tubs heat from cold in 1.5 to 3 hours depending on outside temperature, tub size, and firewood quality.

What is the ideal temperature for a wood‑fired hot tub?

The perfect soaking temperature is 37–38°C. This is warm, comfortable and safe for most users.

What type of wood heats a hot tub the fastest?

Dry, kiln dried or seasoned hardwood such as ash, oak or birch burns hotter and heats the tub faster than softwood.

How do I keep the temperature stable?

Add one or two small logs every 20–30 minutes once the tub reaches 37–38°C. This maintains steady heat without overheating.

What should I do if the water gets too hot?

Stop adding wood, remove the lid and stir the water. You can also add a small amount of cold water to reduce the temperature.