design input on a small water boiler to heat my 200 gallon outdoor tub in the winter.

[ken morgan]

Ok so my homemade wood burning stove modeled after a fisher stove is almost finished...sucks when you only have a few weekends to work on stuff. Next project is a wood fired boiler to heat a 200 gallon outdoor tub. Over the summer I built a outdoor tub that is designed to look like a hot springs. I took a 200 gallon insulated stainless milk cooling tub and poured a pad and then built up the surrounding area with excess lava rocks form my property.

originally I was planning on using a propane convection water heater similar to whats on my cabins bathtub already. But the cost of propane would be very prohibitive for regular use. once a month type thing would be fine, but I want to set this place up for my retirement. At that time sitting in a hot tub in the middle of winter with some wild turkey in one hand a good cigar in the other and a good view of Mt. Fuji is one of those life goals and if I can only afford it once a month due to excessive cost then i will be one stinky mother between baths

so I had several thoughts in mind, one was a simple plate steel wood stove in the same vein as a fisher but with a sealed 5 gallon reservoir on top and an inlet plus outlet that went to the tank and let natural convection action move the water. the other possibility was same basic design but with a stainless steel 1" pipe running back and forth inside the stove at the top. which would be preferred and why? all design inputs are welcome, and I hope to have photos of my current woodburner as I finish it this weekend to post up when I get back.

 

[TimberWolf530]

Here ya go. Should be a simple build. You can pay me a small design fee if you wish, but that's not really necessary. I'm just happy to help.

 

[ken morgan]

central indiana huh? sure you didn't come from some place like Ohio! Ha ha ha! originally from Sullivan county myself!

 

[NSMaple1]

Whatever you end up building, it will be very prone to corrosion if made of steel. No way around that - open oxygen rich water makes rust wherever it has the chance. And not sure either method you mention will get the tub hot enough fast enough, vs. the wood burned & heat lost off the stove.

I wonder if a used LP water heater could be re-purposed to burn a wood fire inside it instead of using the LP burner? It would have a built in chimney & water compartment not quite so prone to corrosion. But would still be prone to corrosion - as a wood fire will condense creosote badly if its temps are below 140°. And a hot tub heater would see that constantly - unless the heating unit was separated from the tub water by a heat exchanger.

Would it be full all the time? Or only before use? Prone to freezing?

Random stuff....

 

[ken morgan]

Whatever you end up building, it will be very prone to corrosion if made of steel. No way around that - open oxygen rich water makes rust wherever it has the chance. And not sure either method you mention will get the tub hot enough fast enough, vs. the wood burned & heat lost off the stove.

I wonder if a used LP water heater could be re-purposed to burn a wood fire inside it instead of using the LP burner? It would have a built in chimney & water compartment not quite so prone to corrosion. But would still be prone to corrosion - as a wood fire will condense creosote badly if its temps are below 140°. And a hot tub heater would see that constantly - unless the heating unit was separated from the tub water by a heat exchanger.

Would it be full all the time? Or only before use? Prone to freezing?

Random stuff....

thanks for the Thoughts NSMaple1, initially it would be filled & fired up on the weekends when I arrive Friday night, and then be shut down and drained by sunday night departure. I could make the area that touches water out of stainless if necessary. once running I would adjust the burn for the quickest and hottest burn till the water hit temp, and then turn the wick down to hold temp. if it were an in house stove it would be at the over heating stage for the flue but that would not be an issue due to its location. once I retire up there I would guess it would still only get used once or twice a week. overnight freezing might be an issue, don't know till I try it. the tub is well insulated as it was a milk chilling tub from a dairy farm with about 3" of foam insulation sandwhiched between stainless steel both inside and outside. i made a cover for the top to keep trash out that is also insulated so until I try it I will not know.

To date I have filled it with a hose from the gas fired water heater and it held temp pretty well overnight (80 deg daytime and 65 deg night time temps)

I do not think that a heat exchanger out of a gas fired water heater would work very well in this instance as the space between fins is very tight and would probably ash/creosote up really fast. thats why i was considering just a couple of stainless pipes running back and forth through the top of the box as a really basic heat exchanger

 

[M.R.]

Here's mine Chitty Chitty Bang Bang w/ years of prototyping into it...
Added on to the back end of a Sweet Home stove
it'll handle 34" lengths of wood, although it's easier to load & control the output by double stacking 16".
Where the 5" SS pipe is now was replaced a 4" steel pipe
With this alone (4" pipe) & hot fire would perk a 55 gal tank like a coffee pot.
Towards the back is three 2&1/2" pipes....
Half mooned closed on top of the cold in side then
the hot out side is half mooned closed on the bottom.
Note this is an Open system not pressurized........
On top around the 8" SS is a 12" SS 18" high
Making a 2" water jacket, this is the second system &
can be tied into the other.

 

[Ted Jenkins]

Here are some of my thoughts concerning this system requirements. I would bet it really looks like a hide away hot springs. From my experience you do not want a open system or in other words the water that you are sitting in is not the water that is being heated. MR's system might work well for what he is using it for, but your system needs to have an eye for efficiency. It will need to be well insulated so as as much heat goes to the hot tub as possible. The system will never need to be drained as the hot tub would likely take at least a week to freeze in sub zero weather even at 20 or 30 F below zero. Unless you forget to cover it when not in use. You could weld up some stainless steel tanks and then build your heater around them or us stainless tubing. You will need at least two pumps and a couple pressure relief valves. Do not for get thermometer gauges. I used auto style pumps which are good for at least 250 to 300F. You will have to adjust the pumps or use a waste valve to regulate how much flow you are getting. If cost is important just use mild steel since it will only rust where the flame it touching the steel as the inside will not rust because it is a closed system. If you use a complete closed system you would likely have a section of tubing where as the water could be circulated with out exposing any body parts that would be sensitive to 200F. Enjoy Thanks

 

[ken morgan]

View attachment 599100Here's mine Chitty Chitty Bang Bang w/ years of prototyping into it...
Added on to the back end of a Sweet Home stove
it'll handle 34" lengths of wood, although it's easier to load & control the output by double stacking 16".
Where the 5" SS pipe is now was replaced a 4" steel pipe
With this alone (4" pipe) & hot fire would perk a 55 gal tank like a coffee pot.
Towards the back is three 2&1/2" pipes....
Half mooned closed on top of the cold in side then
the hot out side is half mooned closed on the bottom.
Note this is an Open system not pressurized........
On top around the 8" SS is a 12" SS 18" high
Making a 2" water jacket, this is the second system &
can be tied into the other.

that looks pretty cool, bet you can get some serious hot water out of that unit!

 

[ken morgan]

Here are some of my thoughts concerning this system requirements. I would bet it really looks like a hide away hot springs. From my experience you do not want a open system or in other words the water that you are sitting in is not the water that is being heated. MR's system might work well for what he is using it for, but your system needs to have an eye for efficiency. It will need to be well insulated so as as much heat goes to the hot tub as possible. The system will never need to be drained as the hot tub would likely take at least a week to freeze in sub zero weather even at 20 or 30 F below zero. Unless you forget to cover it when not in use. You could weld up some stainless steel tanks and then build your heater around them or us stainless tubing. You will need at least two pumps and a couple pressure relief valves. Do not for get thermometer gauges. I used auto style pumps which are good for at least 250 to 300F. You will have to adjust the pumps or use a waste valve to regulate how much flow you are getting. If cost is important just use mild steel since it will only rust where the flame it touching the steel as the inside will not rust because it is a closed system. If you use a complete closed system you would likely have a section of tubing where as the water could be circulated with out exposing any body parts that would be sensitive to 200F. Enjoy Thanks

You are correct in so far as the design of the hot tub. it is setup so that I can drink a beer and watch the animals in the neighborhood while reflecting upon the grandeur of having a semi dormant volcano within a couple of miles of my house ha ha ha . So you are suggesting a closed system loop with a heat exchanger that transfers the heat to the Tub? or ust a closed box with pumps to transfer the hot water form the boiler to the tub? sorry for being so obtuse

 

[Ted Jenkins]

A closed system has some pros and also some cons. Could your system freeze? A closed system would void that problem away, but would need one more pump. A open system if it needs cleaning from mineral build up or then you are looking at a total rebuild. Most systems do not in an effective way transfer the heat from the fire box to the water. I would guess if 50% of the heat from the fire went to the tub that would be doing great. It looks like the ones I have seen are getting maybe 10 to 20%. I hope that you let us know how the build is going. Drinking beer while it is snowing sounds pretty nice even though I do not drink beer. How about adding some solar heating to the system so you would not have to start from zero ever time you want a drink. My neighbor used electric heat and was able to plan carefully how he was going to use it. Thanks

 

[ken morgan]

was finally able to get my stove finished and some photos of it.

 

[ken morgan]

this is a basically a little smaller on the outside than a Fisher "Momma Bear" due to the need to match to the japanese fire bricks (weird sizes) but internally it is only slightly larger than a "Baby Bear" due to the same issue the bricks are basically 220 x 105 x 54 in mm or 8 3/5" x 4 1/10" x 2 1/10" so the interior dimensions are really small. just test fired it yesterday up at the cabin to heat the paint up and get the smell out of it prior to moving it to the house. with a 50/50 mix load of soft and hardwood it burned for right at about 4 hours with full heat output and 2 hours with just coals. that was with the primary air-feed and secondary air-feed (re-burn valves) wide open. I think that at the reduced levels and the re-burn set correctly it should burn for a full 8 or 9 hours.