Design concept for pizza oven / hottub stove combo

[bigger_blue]

I am trying to come up with a solution for my backyard that will allow me to heat my hottub as well as double as a pizza oven. I have looked at some commercial options and most seem to have a dual firebox setup. The bottom serves for firing a few hours to heat the tub, and then the top half should hold enough heat to make pizza. At least, in theory.

Below is the design I have come up with to do this as a home build incorporating as much wisdom from these forums as possible. The bottom firebox holds a metal spiral which heats up the tub with the siphon effect. There is a hole in the top with metal grating so the hot gasses heat up the dome as well as the oven floor. An hour before making pizza the fire in the dome on top of the grating can be lighted, to really boost heat in the top section. Interior dimensions will be 70cm (27.5") with a dome height of 42cm (16"). The door opening will be around 27cm (10.5 inches).




Does anyone have experience with such a design? It seems a little unconventional at best, but I'm hoping to kill two birds with one stone. I envision downsides can be longer firing times, draft issues for the fire box on the bottom and perhaps more? Any thoughts on this build are highly appreciated.

[SableSprings]

First-Welcome to the forum community as a member. I have worked With a system using a live fire to heat water and a separate system that uses a wood fire to heat a hot tub. Here's my thoughts for you to consider:

1) Creosote buildup on water pipe is a major problem. The water does heat up & circulates into an insulated water tank very well, but the coil of copper tubing stays much cooler than the gases coming off the active fire & creosote builds up quickly (and thickly) on the pipe.

2) The fire chamber below the oven means you will need a much more complicated draft & exhaust system as well as a separate support system for the oven's cooking floor (above the firebox). Concrete & bricks act as a heat sink unless they are insulated from the heat source. So in your design (as I see it illustrated), your firebox would not only be heating the oven chamber but the entire structure. You would need to insulate the firebox and oven from the base. The design for this type of baking system is called a white oven (if you never intend to build a fire in the cooking chamber), a gray oven is one that can be heated indirectly or by firing directly in the oven, & a black oven is on that is only heated by firing in the cooking chamber. The majority of ovens built & documented on this forum are black ovens.

3) My experience with heating a hot tub with a wood fire is that it takes a very long time (& a lot of wood) to heat 200-300 gallons (~1,000 liters). Yes you'll get supplemental heat but I'm not sure you'll get up to hot tub temps very quickly & you'll need to really insulate the line going out to the hot tub as well as expecting major creosote buildup as noted before. My other problem with the hot tub was circulation within the tub. The hot water from the firebox tended to layer on top & if it got too hot, you just have to wait for it to cool down...

Hope some of my experiences with these type of systems helps...it can be done, but there are issues that you do need to be aware of...

[SableSprings]

Just another thought Bigger Blue...I also recall that the stove's copper coil in the water tank system didn't get drained properly for one winter in Canada. The ice formation in the pipe split it and it was really difficult to clean off the creosote and patch the copper coil

[deejayoh]

I think you're going to end up with neither thing working very well... If you want to heat a hot tub with wood, I suggest looking into a snorkel stove (google it, easy to find). I've used one at two different locations 1) a killer hot tub built on the beach on a remote San Juan Island, filled with salt water via a pump and 2) at a "high camp" ski resort with no electric service. They work great! Heat up the tub in a couple hours, will get it so hot you need to add more water!

[fox]

Also consider a rocket stove for hot tub heating, rocket stoves can also be used to heat a pizza oven.

[bigger_blue]

Thanks all for the valuable input! It is great to have some critique on the design from more experienced people. The creosote buildup on the heat exchanger is something I had thought about. The heat exchanger would be fitted with two flanges so it can be removed every now and then for cleaning.

However, I had not yet considered a rocket stove. This concept seems very promising, and will also help massively with the creosote buildup on the spiral heat exchanger.

Edit: I have updated the design with the rocket stove. The hot gasses will pass through the grating in the base of the oven allowing the dome to heat up while heating the tub. I would still have the option to light a wood fire in the dome itself, this will help massively with heating the floor of the oven.



Love to hear your comments once more

[Petter]

If you do a rocket, you need to harvest the heat some dinstance up the rider tube. Otherwise, the heat exchanger will cool the vortex and complete combustion is hindered.

Look at this site
https://batchrocket.eu/en/
Everything you need.