In fact,we already answered this question in above question.While copper tubing is a popular material for plumbing and heating systems, but it may not be the best choice when it comes to making a wood-fired hot tub coil. Here are some disadvantages of using copper tubing for this project compared to stainless steel tubing: Corrosion: Copper is prone to corrosion over time, especially when exposed to high temperatures in burning fire. And also,most of copper tubes are thin thickness.These factors could lead to corrosion leaks when used as hot tub coil after some time. Heat resistance: Copper has a lower melting point than stainless steel, which could be an issue when the wood-fired hot tub coil gets too hot after long-time firing. It could also warp or deform under high heat with long time. Cost: If you still insist on make coil heater by copper pipe,we suggest the copper pipe`s outer diameter should be equal to or more than 25mm,and its wall thickness should be more than 1.5mm in order to avoid deform when firing.But cost is a big problem,copper tubing with big thickness is generally more expensive than stainless steel tubing, which could make the project more costly overall. Overall, while copper tubing may seem like a viable option for a wood-fired hot tub coil, but it may not be the most practical or durable choice. Stainless steel tubing offers better heat resistance, strength, low cost and overall durability for a project like this.