Off Grid, Wood Fired Hot Tub Review

Wood Fire Off Grid Hot Tub

Off Grid, Wood Fired Hot Tub Review – I had the pleasure last weekend of getting away with the family. It was actually another anniversary of my Mother’s 21st birthday – strange how I’m in my forties yet she never gets any older! Anyway, with these mathematical improbabilities aside we went away as a family to a house in the country that happened to have a hot tub. We had a great time.

The tub was a wood fire off grid hot tub. I must admit, my experience with wood fired hot tubs has been pretty limited. I have done a couple of designs and builds that have used “coil” fire heat but the principle is a little different.

This particular hot tub was something that you would classify as being totally “off grid”. No electric. No pumps, no jets, just a tub and a wood fire heater. There wasn’t even an electric thermometer! It was a floating ‘thing’ This was going to be interesting….

Wood Fire, Off Grid Tub – The Appearance

On first glance, it looked pretty neat. There was a large chimney protruding from the tub to take any smoke well above you. There was a nice heat guard around this so you would not burn yourself or damage the cover.

The cover was a regular cover for a hot tub and the tub itself was encased with a wood finish.

Inside there was the formed plastic shell you would expect to find. Build quality was good and the fire or furnace portion was particularly well made.

I was impressed. No idea how much this cost of course, but it certainly looked the part.

wood fired off grid hot tub 1
wood fired off grid hot tub

Building the Fire

Apart from the fact you are on your hands and knees trying to light a fire, then stocking up the wood, keeping it going long enough to heat up, it wasn’t that bad. Yes, it was a hassle and not like pressing a couple of buttons on your air source and coming back an hour later.

This was involved!

With the fire roaring it was time to monitor the temperature – with the floating “thing” also known as a thermometer.

The Heat Up Time

The very kind owner or custodian of the property had clearly built a fire and heated it up before we arrived as it was holding at around 34C or 93F when I opened up the tub. It was going to be interesting to see how long it actually took to get to temperature.

I did build quite a big fire using some big logs but was pleasantly surprised that the tub actually got up to around 39C or 102F in about 50 minutes – not bad at all. This is much quicker than you can do on electric, and is comparable to propane or natural gas and my big air source heat pump on a good (warm) day would just about do that too.

Pretty impressed with the heat up time to be fair although this was not from cold of course.

wood fired off grid hot tub.jpg 2
wood fired off grid hot tub

Checking the Temperature

With the lack of electricity, there wasn’t even a battery powered thermometer so we were going old school relying on reading the good old mercury. Not too much of an issue apart from the heat loss every time you open the cover to check – not the most convenient of ways of checking the temperature – especially if it floated into the centre of the tub and you needed to fish it out. I much prefer looking at my App or even the Topside control on the wall….

We got to temperature and the tub was ready for us to get it. I must say, I’m a bit funny about getting into other hot tubs now, especially communal / rental ones so I left the actual “testing” to my kids and the other members of the family.

Localised Heating

The first thing that they noticed was the localised heating around the area of the fire. Pretty obvious I guess.

The principal that they use to heat the tub is based on convection currents. Warm water rising to the tub of the tub drawing in cooler water to the heating cavity. However, according to my “testers” there were hot and cold parts of the tub, it was not uniform.

Also, the actual vents where the hot convection currents came out of got incredibly hot – like “going to burn you if you touch them” hot. This off grid hot tub is interesting to say the least and could definitely leave its mark, on your leg…….

No Temperature Regulation – Overheating!

What I hadn’t thought about to be honest was what happened next. I guess with a regular hot tub and our DIY built tubs, we have a thermostat that is controlling whatever kind of heating we are using.

We can get up to temperature then regulate things so it actually remains at that temperature.

However, with this off-grid, wood fire hot tub, there was not regulation of heat. As time went on and the fire continued to burn, the water got hotter and hotter. It continued to heat and the point at which some of the “testers” called it a day, it was up at 44C or 111F – far too hot for me!

What is pretty clear here for the off grid hot tub was that if you have misjudged the size of the fire, this thing was going to continue to heat and heat – you could actually end up scalding yourself! I’m sure you would be getting out before that, but it is seriously hot in the mid 40s for a Hot Tub.

The “testers” called it a day and we let the fire go out.

wood fired off grid hot tub
wood fired off grid hot tub

The Next Day…..

I opened up the cover of the tub to check if it had held the temperature. It had retained some temperature yes, but I was greeted with what I can only describe as a stale dirty water stench. It was horrific, verging on disgusting and I don’t have the best sense of smell as it is.

The lack of chemicals here coupled with no filtration really came into play. Warm, stagnant water yuck.

Nobody was getting into this tub as it was – time to empty it and quick!

Not the most economical in terms of water usage as we were about to dump 800 litres, just over 200 gallons of water into the flower beds.

Empty the Tub.

After we eventually found the drain leaver – it wasn’t very obvious and we didn’t have the manual for this model, we did manage to drain the tub. However, this process did take us well over an hour in the end. Then we had to clean it down before we started to fill it up.

Time time time. Everything was taking time and lots of it!

The Fill Up

As with any hot tub that you are filling with a hosepipe, it takes time. At least an hour, usually more. The problem that you have here is that you have to have lots of spare time. You have to invest time to empty, which took about an hour. Time to fill which is another hour. Then you have to factor in the time to build a fire. Then there is the heat up time from cold. All of this is lots and lots of time just to be able to get into the tub.

Whilst it might be cheaper to run a tub this way, a person’s time certainly has a cost to it – I’ll let you decide what this is worth to you. For me, this is a no go – I just don’t have the time to do this on every occasion I want to use an off grid hot tub.

The attempt to heat it up again

Given the stench that was coming from the water, there was no way that anyone was going to get in the tub – so we changed the water as already mentioned. However, trying to heat up probably around 800 litres or 211 gallons of water with a fire from cold was a challenge.

In fact, it was too much of a challenge and after about 3 hours we gave up – it was just not heating at any rate that was going to get the tub ready for the bathers. We had started a little late in the afternoon, but we ran out of time – there is a running theme here…

In Conclusion

I’m going to be totally honest here. In my opinion, this is not a hot tub. It is an outdoor bath! Yes, perhaps I am biased with my pre-conceived concepts about what a hot tub should be given what I design for customers but the fact that it is essentially a single use body of water means in my eyes, this is a bath.

I think the “wood fire thing” could actually work and if you were not a big fan of jets; the lack of which though my mother described as being “boring” whilst she was in there. That aside, heating with wood did work from a mild temperature and it was faster than on electric for sure – certainly in this day and age it is far cheaper to run a tub this way. Given a little more patience on the heating from cold, we would have got there eventually.

For me, the lack of sanitation, not even chemicals as it could run on UV and active oxygen or a salt system. Ok, you will need some power for this, but the lack of sanitation and filtration in general made it single use only. It was disgusting and un-usable the next day and the family members had showered before use!

This type of off grid wood fired hot tub can definitely be improved on in terms of the design and I know you can buy models with filtration. With some sanitation and filtration, not only would that help prevent the localised heating but making it a multi-use tub would get this in my mind back to being classed a hot tub rather than an outdoor bath.

That said, if you don’t have lots and lots and lots of time on your hands, this is not for you.

Happy Hot Tubbin’

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Thanks - Andi


Hi, Andi here. I own and also write all of the articles and info pages on the site. Some years back now, I built my own hot tub but struggled to find the information I needed. So, once my tub was complete, I started this website to help others in their own pursuit of hot tub and plunge pools DIY building information.

Today, I've helped over 1000 DIY customers just like you all over the world build hot tubs and pools. Have a good look around the site, there are lots of resources here. Please do get in touch if I can help you. - Cheers, Andi

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