Wood Fired Hot Tub Water Flow Issue (Copper Coil)

I have built a wood fired hot tub with a 179 galvanized stock tank and 1/2" OD soft refrigeration copper tubing of 50ft in length. The input inlet (hot water in) is located on the top of the tank while the output inlet (cold water out) is at the bottom. In theory, this should have created a flow when heated with wood. I have gave it multiple tries by placing the most of the coiled tubing somewhat below the tanks water level then raised up the coiled tubing higher to see if that would make any flow happen. No luck so far getting any type of flow. Im wondering what i'm doing wrong here, coil of 50ft too long? Elevation of the coil too high or too low? 50 ft of coil too long? 1/2"OD copper tubing too small of a diameter.  I feel like someone in this forum might have ran into the same problem. I'm close to having the wood fired hot tub in action, any help would be appreciated! I will attach a video here to show you my set up.

I think you will fail the Heath - Robertson award!
Your are playing the  thermosyphon system.
- inlet cold is at bottom
- outlet hot should be at top [ hot water rises ]
- no part of the pipe work should be above the water level, it will not flow.
- 1/2 dia. may not allow the water to syphon up.
- 50 ft length may be too long.
- I have seen them with 11/2 inch pipe about  24ft long.
- I use 3/4 Copper as a booster for my hot water off the wood heater and a 6 fy head.

Adam Sacchetti wrote:I have built a wood fired hot tub with a 179 galvanized stock tank and 1/2" OD soft refrigeration copper tubing of 50ft in length. The input inlet (hot water in) is located on the top of the tank while the output inlet (cold water out) is at the bottom. In theory, this should have created a flow when heated with wood. I have gave it multiple tries by placing the most of the coiled tubing somewhat below the tanks water level then raised up the coiled tubing higher to see if that would make any flow happen. No luck so far getting any type of flow. Im wondering what i'm doing wrong here, coil of 50ft too long? Elevation of the coil too high or too low? 50 ft of coil too long? 1/2"OD copper tubing too small of a diameter.  I feel like someone in this forum might have ran into the same problem. I'm close to having the wood fired hot tub in action, any help would be appreciated! I will attach a video here to show you my set up.

first thought too much resistance causing the thermosipohn to not happen due to the length of the pipes and the small diameter. A 5 watt circulator pump would help a lot. Edit just reread the post and watched the video. The distance from the heat source would not help either.

I'm with John the coil appears to be too high. Reservoir level should be above the coil.

For what it's worth, a 12 volt battery and a small 12 volt pump installed on the cold side would make your system work like a charm.

From the video, you are getting some thermosiphoning, but between the small diameter and long length of the copper, and the coil positioned partly above the tub surface, there is too much resistance to get much flow. Also, if there is any point where the pipe rises and then falls, air could be trapped at the top of the bump and present a serious obstacle to flow. You need to provide a smooth path for any air bubbles to rise and escape from the tubing.

Also, the setup of the fire is not getting a lot of the heat into the tubing in the first place. You would want the fire contained all around and below the tubing, and covered above so that heat is concentrated around the tubing and cannot escape straight up without touching the tubing.

You would get at least twice as much heat into your tub if you cut the 50' of tubing in half and make two circulating coils. There is plenty of room in the fire area to double up the coil, and coils half as long would present half the drag and allow faster flow in each coil. I would put the fire as close as you comfortably can to the tub, to minimize the parasitic drag of tubing running back and forth and maximize the heated sections. A double thickness of sheetmetal with spacers would probably keep bathers safe from the fire.

I’ve got the same issue. Maybe too much coil? Zero flow at all on the hot outlet. Any advice appreciated, I may cut the tubing shorter and move closer to fire. Thanks in advance

Hi Dylan,  Welcome to Permies.

It doesn't look like the tubing coil would get all that much heat from the fire to start flow. Some of the coil sections dip down in places. How hot does the copper tubing get just out from the top of the coil?

I would move the fire closer to the tub, and take up the slack by coiling more around the firebox. Importantly, take the time to make the coils all tight to the metal cylinder and evenly sloped, with no ups and downs, just a constant slope up from the base.

Interesting....have you seen this thread? Rocket-Hot-Tub-PTJ There was talk of making a hot tub heated by rocket mass heater last year, but I don't think they actually did it. If there is enough interest this year, they might do it. All of the projects listed inPermaculture-Technology-Jamboree-July 2023  are just proposals. You know that they decide on what projects to pursue based on interest, right? There is a BOGO sale on right now for tickets to this year's PTJ Limited-Time-Buy-Free-Buddy .