I got old wood fired sauna stove for free. It has some holes so that its not usable indoors or sauna anymore, but is good enough for hot tub. That old stove needed some things: water pipe and more than 1m long chimney so that stove has thrust when wood is burning.
How does the stove work and how it heats the water. I hope you have seen stoves in sauna which have water pipes in side and behind which to lead into big water tank. Simplest way to describe it is so that cold water enters from water tank to stove where hot pipe inside of stove will heat the water up and lead it back to water tank from another tube which is placed higher than entrance pipe for cold water. This will make water circulate and heat the water in the water tank. For test run i did buy crappy hose (more plastic than rubber) we had to cool it down with snow so that it would not melt as there was air in the pipes which gives possibility for water to start boiling (means higher temperature and circulation is disturbed). Good hose is bit expensive, but seams that I need to buy good one anyway.
I got old water pump from my neighbor (thank you!) which did not had much power (400L in hour). With such climate here in Estonia (-13 degree) this pump was definitely not powerful enough, but was worth of trying. Friend came to help me again. On first testing day after stove was setup we managed to get water into hot tub only for about 20cm. Because of 2 things…
1- Crappy pump: Because water did froze before it infiltrated/leached into wood leaving holes or tracks so that it started to leak and did not stop. When we did cast the water on side planks id froze immediately. Whole hot tub has been outdoors for couple of weeks and temperature of wood was certainly lower than -10′C. (if we only could get water faster inside)
2- Side planks which were cut and measured too precisely: When you put hot tub together attaching side planks you must be sure that last plank is wider than it seams to fit place (4-6cm). At that moment we realized that side planks tenon bottom was touching floor panel and we did not had possibility to pull side planks together enough at the bottom. This resulted water to froze between side planks and tenon leaving tracks for water to leak and you are unable to pull it together enough any more (bottom).
Second day (after everything did froze during night) we filled hot tub partially, added bit snow (because it was the quicker way leveling the water line than waiting for that pump to fill the tub) and fired up the stove. It did work for 2 hours before it started to leak again! Why? ice ice ice from first testing day… we did took all the side planks off and moved them into sauna to dry out the wood. Also we did made more side planks for upcoming test run (i hope for no more).