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ForceFed70
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# Posted: 9 May 2017 12:12am - Edited by: ForceFed70
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Hello everyone and welcome to my off-grid HotTub build thread! Not sure if this thread belongs here or in the projects forum - Mods feel free to move.
Background:
My family owns a rustic cabin up at a mountain lake in the woods. Being that it's a mountain lake in Canada it's often quite cold up there! I want to build a hot tub for use while we've visiting the cabin. The cabin is used only about 1 weekend a month in the summer and tub will be drained and filled each trip.
About this build:
I'm sorry but this thread is going to drag on for a few months. My goal is to have this tub up in running by Fall of 2017. There is a project I must complete (Install new gas water pump, water line, water reservoir, and plumb the cabin) before the tub can be installed.
Challenges:
1)The cabin is difficult to access. There is no road leading to the cabin. You must drive up a fairly rough forest service road until you reach the lake, then take a fishing boat across to the cabin. This is a time consuming exercise and makes it impossible to transport a traditional hot tub short of hiring a helicopter or something.
2) No power at the cabin. The cabin does have a basic 12V solar power system for running lights and charging electronics, but nothing robust enough to run a pump.
3) No treated water at the cabin. We pack in all drinking water. Bathing and dish water currently comes from the lake. Hot tub will need to be filled with lakewater pumped up via a gas powered water pump.
Goals:
1) Big enough to fit my family of 4 (2 small kids) if we don't mind being cozy.
2) Big enough so I don't feel weird sharing the tub with another dude.
3) Able to heat the water in a day. If I fill it in the morning, I want to be able to use it that evening.
Here's a picture of the cabin. Plan is to put the tub to the immediate left of the deck. I'll likely take out a section of the railing there so you can access the tub directly from the deck. That stump is going to have to go..
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ForceFed70
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# Posted: 9 May 2017 12:16am
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Building the boiler is going to be a challenge for 4 reasons.
1) I want to heat a lot of very cold water in a rather short amount of time. The boiler needs to transfer a LOT of heat.
2) I don't have power for a pump! Water needs to self-circulate through the boiler using the properties of thermosiphon effect. This requires careful design. Lines must be carefully sloped - If air is trapped in the lines/boiler it won't work. Also the lines must allow for easy/high flow as the thermosiphon effect isn't very strong. https://en.wikipedia.org/wiki/Thermosiphon
3) In the winter it's likely that the boiler will freeze overnight when the fire goes out. I need to be able to easily drain the boiler without draining the tub so that it can be drained overnight.
4) Boiler need to be fairly light weight so that I can carry it from fishing boat to cabin, etc.
All of these challenges are unique to my situation and probably wouldn't apply if you were doing this in your back yard.
Here's a picture showing the intended design:
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ForceFed70
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# Posted: 9 May 2017 12:17am
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Here's a pic of the raw materials.
The tall tank doesn't end up getting used (for boiler material anyway). This is an old tank I had laying around and was going to be my boiler casing until I pulled it out of the weeds and realized it was too narrow.
The blue compressor tank will be the main body of the boiler. I picked it up from the local metal recycler for $40. It came with some bonus air pressure gauges and an air pressure regulator that I'll keep for a rainy day.
The pink steel tank is an empty helium tank. It came as part of a "party balloon kit" from Costco. My wife LOVES the fact I used it for this build instead of throwing in the garbage.
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ForceFed70
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# Posted: 9 May 2017 12:18am
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1st step - preparing the boiler body:
- Remove all compressor fittings, air gauges, etc.
- Cut sheetmetal off the top that compressor motor used to be bolted to.
- Cut top off of tank.
- Build hinge for top. To remain openable for easy access.
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ForceFed70
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# Posted: 9 May 2017 12:19am - Edited by: ForceFed70
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Here's a picture showing the forming of the copper coil.
Managed for find this length in my local online classifieds and met a nice old couple in the process. Total length used was right at 40'
I had planned to fill the coil with water then freeze before bending. Unfortunately I couldn't fit the coil in the deepfreeze and the weather has been unseasonably warm outside. Luckily, I was able to make due without. The radius of the bend wasn't too tight and only resulted in a slight ovaling of the tube.
For anyone considering doing this themselves. 3/4" type K copper was difficult to work with. It's hard to bend by hand. I'd say it's at the limit of what is usable for something like this. If your situation permits, consider using smaller diameter, or the other type (Type L?) that's a little thinner. Forming this coil wasn't too bad, but bending it later to line up with fittings in the boiler was difficult.
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ForceFed70
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# Posted: 9 May 2017 12:20am
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Test fitting the coil. Nearly perfect.
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ForceFed70
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# Posted: 9 May 2017 12:21am
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Here's the inner steel insert all welded up and ready for fitment inside.
All 3/4" black iron piping and fittings. I welded them to prevent leaks.
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ForceFed70
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# Posted: 9 May 2017 12:21am
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Here's a picture from the top showing both heat exchanger loops installed and fittings connected. Some temporary reducers attached to output for pressure test.
Here I think you'll see the idea behind the design with the center tank intended to divert hot stove gasses through the copper coil.
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ForceFed70
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# Posted: 9 May 2017 12:22am
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Alright. Time to move on to firebox area.
Here's a picture of the door I built:
- Hinge made using 1" angle iron, a spare piece of pipe, and a spare bolt. Bolt will be flipped later when I re-attach the door after painting.
- Latch is also 1" angle iron, notched so I can partially open the door for improved draft.
- Handle is 1" flat stock and pivots on the bolt holding it to the door. Spare piece of pipe and an old 351W valve spring are also used in construction.
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ForceFed70
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# Posted: 9 May 2017 12:23am
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Door open showing the firebox. Could be larger, but it'll do.
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ForceFed70
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# Posted: 9 May 2017 12:23am
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Fresh air damper.
This is built using an old piece of 3" exhaust pipe. I drilled a hole through the center and welded a nut over 1 of the holes. A bolt goes through the pipe and threads into the nut. The section of tank I cutout is repurposed as the "flapper" and is welded to the bolt.
Not shown - a small piece of rod will be welded onto the bolt head to act as a handle for adjusting the flapper position.
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ForceFed70
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# Posted: 9 May 2017 12:25am
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This is a picture of the almost finished boiler ready for a test run. Lid has been finished with a handle for opening and the stovepipe opening.
I was unable to peel the stickers from the tank. I'm hoping the heat will soften the glue and I'll be able to peel them off once lit.
I'm hoping the test run will also burn off the paint so I can repaint with a high temp BBQ paint. However, the boiler will be wrapped in a layer of fireproof insulation and you won't see much of the paint job anyway.
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ForceFed70
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# Posted: 9 May 2017 12:28am
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Test results are in!
So stoked! Unfortunately, my testing was an absolute disaster. I didn't get to record nearly as much data as I wanted, and no pictures
I should have realized. A boiler big enough to heat 300 gallons is going to heat a smaller test tank quickly! Unfortunately, I wasn't really prepared for that.
However, I did manage to take enough measurements to be able to provide the summary data below:
- Time from 1st lighting the paper for the fire in the boiler, to heating the 5 gallon primer tank, to then heating the 20 gallon test tank from 50* to 100* = 13minutes! 13 minutes to heat 25 gallons of water from 50* to 100* and that's counting the time to lite the boiler!
- 6 minutes after 1st lighting the fire, I recorded 77* water going into the boiler and 110* coming out (2gpm)for a whopping 33,000BTU/hr! It's at this point that the fire was burning it's hottest as I didn't add any additional wood since shortly after lighting.
- 30 minutes after 1st lighting the fire, the fire has burnt down to just coals. Heat output is calculated at 8000BTU/hr.
- It was cold outside, about 40* It wasn't until about 50 minutes after lighting the fire, with a very low bed of coals remaining that the water temperature in the 20gallon tank peaked at 128* Heat output required to maintain this temperature was measured at 3000BTU/hr.
I plan to wrap the boiler with fireproof insulation to improve efficiency. I believe that with the insulation and a little more time to warm up the boiler/fire I bet I'll be able to get a peak of 40,000BTU/hr out of it!
Time for some math. It takes 8.33 BTU to heat 1 gallon of water 1*
So to heat 300 gallons from 50* to 100* would be 8.33 x 300 x 50 = 125,000BTU
40,000BTH/hr output of the boiler would mean a little over 3 hours for 125,000BTU.
However, even tho the tub will be insulated, there will be heat loss. I have no idea how to calculate the impact of that heat loss, but I'm pretty confident I'm well within my goal of heating the tub in 8hrs or less. Likely it'll take 4 or 5hrs!
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ForceFed70
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# Posted: 9 May 2017 12:29am
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All cleaned up with a fresh coat of paint. Ready for install at the cabin.
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ForceFed70
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# Posted: 9 May 2017 12:31am - Edited by: ForceFed70
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Picked the tub up. It's a Rubbermaid 300 gallon "Stock Tank". Designed for providing water to livestock but I'm using it for a much different purpose
It's not quite round at 63.5x69" and 24" deep. Honestly, It's a little larger than I'd prefer. 250 Gallons would be perfect but the next size down was 150gallon and too small. I looked at lot of different tanks and styles, considered cutting open a 250 gallon water tote, etc but finally decided on the Rubbermaid. Of course, it's also the most expensive tank as well. I liked the construction, the material, and the shape with the slightly sloped sides is almost perfect.
Here's a pic. Youngest shop foreman just finished his inspection - it passed!
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ForceFed70
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# Posted: 9 May 2017 12:33am
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Picked up the materials for the skirting/surround.
Couple sheets of plywood, some pressure treated 2x4's and 1x6's, and 3" sheet of foam board.
I'm going to pre-build and assemble as much as possible.
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ForceFed70
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# Posted: 9 May 2017 12:34am
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Tub framing is done, along with foamboard and layer of plywood cut to size beneath the tub.
I'm building it such that the tub framing can easily come apart in 4 pieces for transport to the cabin. I'm hoping to pre-build as much as possible at home.
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ForceFed70
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# Posted: 9 May 2017 12:37am
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And now we're caught up to current day. Got as much done in the shop as I can. Now I need to wait until the snow is gone and I can start hauling material. Likely not any further progress until late May or Early June when I can access the cabin again.
Here's a pic of the tub pre-assembled. In this pic everything is just barely held together with only 1 out of 4 fasteners used. It needs to all come apart for transport to the cabin.
The plywood on top will be covered with a roll on decking material. I've already applied a 1st coat (not pictured) the 2nd coat will be applied during final assembly at the cabin.
Still looking for a used hot-tub cover that I can modify to fit my redneck tub.
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SE Ohio
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# Posted: 9 May 2017 01:26pm
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Nice project!
Since you probably won't be using the cover to hold heat in for long periods of time, all you really need is something to keep stuff from falling in- Might consider painted exterior grade plywood with latches to keep sheet from blowing away? A regular hot tub cover gets heavy as foam absorbs water, ends up sagging under it's own weight. Also kids walk on the covers, damaging them, and animals sometimes chew the vinyl covering for nesting material.
Good luck, and keep us posted!
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ForceFed70
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# Posted: 9 May 2017 03:29pm
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Thanks for the feedback! I was worried nobody would be interested.
I do want to insulate the cover for a few reasons:
1) Improve warmup time.
2) The firebox is fairly small with the fire likely only lasting about 4hrs when damped down. Having an insulated cover will help reduce the temp swings associated with the fire going out overnight.
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KelVarnsen
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# Posted: 9 May 2017 03:36pm
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Quoting: ForceFed70 I was worried nobody would be interested.
I'm super interested. There is a high likelihood that I will never build something like this (too many high priority projects), but maybe someday. Great job on the build and documentation.
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ForceFed70
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# Posted: 9 May 2017 08:06pm
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Great. And yeah - my project priorities are likely a little mixed up. The ole outhouse is on it's last leg for example.
What motivated me to jump the queue with this one was my wife. Not that she really wants a tub - it's that she gets bored when we go up there and is often resistant to going as a result.
Me I'm never bored - I love being outdoors, and I can download and bring movies, TV, etc to keep me occupied. My wife is a more social person who doesn't seem to get outside much- tho don't dare tell her she's not an outdoors person ;) She's not happy unless she wastes a good couple of hours a day BSing on the phone and/or facebook neither of which are available at the cabin.
Putting in a hot tub will help keep her and the kids occupied - especially for those bad weather weekends.
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SE Ohio
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# Posted: 10 May 2017 07:13am
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While not elegant, for temporary insulating, blankets, foam board, etc piled on top could work? Also you could add permanent insulation around the tub, but most loss is through the top (as you surmised).
I've toyed with the idea of adding hot tub to my cabin, too. Locally many (mostly non-working) tubs are given away on Craigslist (you haul, bring friends 'cause we don't help lift). Having an existing tub is another good starting point if one can move the heavy thing!
SE Ohio
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littlesalmon4
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# Posted: 10 May 2017 11:36am - Edited by: littlesalmon4
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We just use a piece of 1 1/2" foam insulation. We let it float on the water. For storage we leave it covered with a tarp and some spare roofing tin.
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ColdFlame
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# Posted: 10 May 2017 01:05pm
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Great project!
A couple of points RE: the hot tub cover. I agree that traditional hot tub covers are prone to failure - they end up collecting water, the vinyl covers rot, people (kids and animals in particular) often step on them, etc...
I think I'd go with a framed 2x4 w/ pressure treated plywood solution also, albeit with rigid insulation attached to the underside. I think I'd then also take some of that foil "bubble wrap" insulation that is commonly available at the big box stores and further cover the bottom of the lid so that the humidity getting to the framing/insulation is lessened, or even just use some heavy grade poly to seal it in from the bottom. It won't be perfect, but I bet it would go a long way to helping!
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DaveBell
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# Posted: 10 May 2017 07:51pm
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Some other ideas.
redneckhottub.jpg
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WV_Hottub.jpg
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ForceFed70
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# Posted: 10 May 2017 09:54pm - Edited by: ForceFed70
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Thanks all! Picked up a used spa cover today for a song. It's a little too big, but I'll cut it down and give it a shot. I plan to build a brace for underneath to help deal with snow load, and will cover the entire tub with a heavy duty tarp between trips. We'll see how it holds up. If it fails too quickly little money/time wasted and I'll have to look at a more robust lid setup.
I also plan to insulate the sides using rockwool batt insulation. It's inexpensive and won't support rot/mildew. Combined with 3" of foam underneath and it'll be a fairly well insulated tub.
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ForceFed70
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# Posted: 10 May 2017 10:02pm
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Quoting: DaveBell Some other ideas.
Thanks Dave.
Strongly considered the much simpler "Fire directly under the tub" design. But there's a couple of reason why I decided against it.
I wanted something bigger than a bath tub, and finding a suitable tub made of metal was proving a challenge. The big reason tho is because I've used a tub like this. What the pictures don't show is how much campfire smoke you end up breathing. Especially when you inevitably slosh some water over the side into the fire below. Had a nice bath and still smelt like campfire after! The damper on the boiler allows for some control over the heat output while allowing for the fire to burn longer unattended.
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littlesalmon4
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# Posted: 11 May 2017 11:31am
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You definitely want a means to damper your fire. We can literally cook ourselves out of our alumitub if we are not careful.
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ForceFed70
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# Posted: 12 May 2017 12:40am
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Quoting: littlesalmon4 You definitely want a means to damper your fire. We can literally cook ourselves out of our alumitub if we are not careful.
Very nice tub! Also a design I considered. I hummed and hawed about that choice for a while before deciding on an external boiler.
If you don't mind: How many gallons and how long to warm up?
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