Safety & best use
Getting the best out of your stainless steel heater coil
Safety should be paramount in the planning and use of your heater coil. We would like you to take away a few key safety points:
The warm water return tube from the coil MUST be below the water level in your hot tub at all times. If the water level in the hot tub falls below this level (for example when people exit the tub) then the water within the coil can heat excessively leading to an unsafe situation and the potential for injury through scalding water or steam.
Think carefully about how you will attach your coil to your hot tub and plan it out thoroughly. Make sure to leave enough space above the warm water return so that it is still below the waterline after anyone exits the tub. If the waterline drops below the warm water return then this could create the potential for injury through scalding water or steam.
The coil MUST be filled to the brim with water at all times. Including when people exit the tub and the water level drops.
Water within the coil must be able to circulate freely with the hot tub at all times.
Do NOT obstruct the water flow to or from the coil with a valve or similar.
Fire and alcohol mix poorly. Please take care!
The coil should be supervised at all times when in use or cooling down - particularly when children or animals are in the vicinity.
Plan the placement of your coil carefully so that the area around the coil is free of obstructions or flammable materials.
Ensure that your coil is stable and will not tip over.
Take any and all measures to mitigate any danger from fire.
Before using your hot tub heater coil for the first time it is worth conducting a brief common sense safety check of the setup and the surrounding area.
Wood, even seasoned wood, will release water when burned. Damp or unseasoned wood will release a lot. If you see water running down the side of your coil, it is almost certainly condensation.
Safety & the thermosyphon effect
Your heater coil works using the thermosyphon effect. When it is correctly connected to a supply of water, the fire within the coil will draw cooler water in from the bottom of your tub (cooler water is denser than warmer water and will sink to the bottom). The fire in your coil will heat the water up, making the water less dense. This water will rise through your coil and leave as warmed water back into your tub.
However, some things can interfere with the thermosyphon effect, or make it less efficient. These include:
pockets/bubbles of air trapped in the coil or attached hose
making the water go through long lengths of hose to get into, or leave, the coil
hose that sags or bends excessively between the coil and the tub.
Please note that your cold water inlet and hot water outlet feeds must be below the water level in your hot tub at all times. If the water line in your tub falls below the level of your warm water outlet feed from your coil water will stop circulating through the coil. This water will continue to heat up. This is called 'dry firing' the coil, which is extremely dangerous and can result in scalding hot water or steam coming out of the warm water outlet. Do not allow dry firing to occur and plan your setup accordingly to prevent this from happening.
If you have a stocktank style hot tub, we would advise attaching it to your coil in the position shown in the photo above, with the hot water outlet below the upper ripples in the tank. The warm water return tube from the coil MUST be below the water line at all times. If the water level in the hot tub falls below this level (for example when people exit the tub) then the water within the coil can heat excessively leading an unsafe situation and the potential for injury through scalding water or steam.
Unlike an electric heater, any wood-fired heating option for a hot tub will not have an inbuilt thermometer. We would advise buying a pool/spa/hot tub thermometer to allow you to monitor the temperature of your hot tub so you can bank/extinguish your fire if need be (or add more cold water). These thermometers are easily found online.
Note that if you do not have a circulating water pump, the temperature may vary considerably in different parts of your tub (especially in wider or deeper tubs), with the water next to the warm water outlet from your coil being the hottest part of your tub.
Getting the best out of your coil:
Keep your hose lengths as short as you can. Use only as much hose as you need to have a straight hose connection to your coil. Don't let the hose sag.
Minimise any chance of leaks. We provide jubilee clips to help secure your hose to your coil (and to your brass connectors, if you purchase them). Use plumbers' PTFE tape on any threaded connectors to ensure they stay water-tight.
Avoid getting air bubbles in your coil or hose. You can do this by minimising leaks (see above - if water can get out, air can get in). It also helps to have a very slight upward incline in the hose from the top of your coil back to the tub, as air bubbles are more likely to rise up and back into the tub.
If you think you've got an air bubble in your hose or coil, gentle tapping or shaking can help to dislodge it.
Your cold water inlet should be as low as possible in your tub. Your hot water outlet should be as high as possible in your tub (while still below the water line at all times). Having a very low cold water feed and a much higher hot water return is really important for efficient circulation.
DO NOT DRY FIRE THE COIL! Never start a fire in a coil that isn't properly attached to a water-filled hot tub, or allow a fire to keep burning in a coil where the warm water outlet has dropped below the water line. This causes the coil and any attachments to get extremely and rapidly hot and any water that is in the coil (or later comes into the coil) to boil, steam and pressurise your coil. THIS IS EXTREMELY DANGEROUS.
Other ways to improve your heating times:
Have a good cover for your hot tub and use it while you're heating the water.
Insulate your hot tub, if you can.
Have good airflow underneath your coil (we sell stands that help with this, or you can build your own).
Burn properly seasoned dry wood or similar. This will burn hotter and produce less smoke.
Coils with fireguards help to contain the heat and speed up heating times. We sell coils with fireguards, but people also make their own version in a variety of ways.
If you really want to speed things up, or you're trying to heat a very large amount of water, a pump will also help. Remember to keep the pump running while the fire has heat to it, so as not to create any build up of steam and/or pressure.