Diy Stock Tank

mwemaxxowner

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Apr 15, 2020
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My wife and I read a while back about people using stock tanks to build a hot tub. We mentioned building our own kind of jokingly.

Well, about a month ago my wife says, "When you gonna build my hot tub?" (Me) "You serious?" "Yes!"

SAY NO MORE! 😆

I spent a little time reading through some builds and came up with my own plan.

I'm using an Intex 1500 gph filter pump, intex plunger valves and inlet/outlet fittings, and a 110v heater found on Amazon.

Most of the builds I found insulate them but (IMO) very poorly. HeyWanderer has a blog and they do some great stuff. They built one and used this same heater and say it works great. However, their insulation was nothing more than a layer of reflectix and a wool blanket. Reflectix offers almost no insulating value without an air space, so it's really mostly just the insulation value of a wool blanket. They also cover it with a tarp.

I hope to have ours insulated a little better so perhaps I don't have to run the heater as much. I wrapped mine with 6 layers of carpet padding, which I happened to have in the attic after pulling out carpet up and putting LVP down. This was the best idea I could come up with to insulate a round tank. Other than several inches of spray foam, which is EXPENSIVE. Best I can find by research is an R value of 0.5 to 1 on average for a layer of carpet padding. This leaves me nowhere near the R value of a "real" hot tub, but I do believe it to be better than most of the DIY hot tubs I've found.

Once I built the deck around it, I air sealed it with foam underlayment, which I also happened to just have lying around in the attic. I used foam insulation on the side walls as much as possible also. I had a goal of putting foam insulation all around top and bottom, but that was just going to turn out to be too expensive.

I work for a lumber company, and almost all of my lumber for the deck was free culled lumber that was destined for the dumpster. Using cull stuff made some of the framing interesting but I got it. The decking wasn't free but it was an old weathered pack we couldn't sell to Lowe's so I got a steep discount. Break the pack open and most of the pieces on the interior are good.

I have R 5 foam insulation covering it right now, but it's temporary. I want to build a cover out of two layers of R10 2" foam board and plywood or wood slats. Maybe a bubble cover on the water surface as well. I think that I might fill the cabinet with blow in attic insulation or something of the sort later if it doesn't hold heat as well as I'd like. I rebuilt and restored a fiberglass boat a few years back and had to pour expanding floatation foam below the deck. That would be perfect for this application, but it's also very expensive. It would make replacing this tank in the future extremely difficult also.

I got it all hooked up and filled with water yesterday. So far it seems leak free, and I'm gaining about 1.5 degrees per hour. It's controlled by a digital temperature controller set to 100F, with a 3° differential set + and -. The heater has a flow switch and kicks in when the pump comes on. The pump is connected to the digital temperature controller, it will come on when the water temp is 97, and kick off once the temp reaches 103. The thermostat on the heater is set to 104.

Once I am certain it's all working like I want it to, and it's all leak free, I'll button the sides up with some access doors.IMG_20210110_125448561.jpgIMG_20210123_162622969_HDR.jpgIMG_20210124_171949297_HDR.jpgIMG_20210130_153028361_HDR.jpgIMG_20210130_153019573_HDR.jpgIMG_20210130_153045563_HDR.jpgIMG_20210124_172636735-ANIMATION.gif
 
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mwemaxxowner

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Thank you! It's at about 65 degrees right now. We're all dying to climb in 😆.

I think I want to add a propane on demand water heater for quick temp increases and helping it to hold temp when it's uncovered. I'm sure as it is now once we uncover it and climb in temps will start to drop. I just wanted to get it established as is first. I have plenty of propane out there for the grills. I just knew that I didn't want it to be heated solely by propane. I crunched some rough numbers on that vs electric and it was going to be much more expensive to heat it by propane alone.

And the up front cost of the electric heater vs a propane heater was less. My sister just bought a "real" hot tub and I think it was $3500. That said, it's AWESOME! But I'm all in for about $1200 here (not counting lumber costs, since most of that was free or I already had it.)

In the summer time we'll have the heater off, and this can serve as a kiddie pool, or just a place for us adults to lounge and sip a drink.

We were thinking it stinks to have our nice pool/patio area and not be able to enjoy it this time of year. Now we can!
 

mwemaxxowner

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Many thanks! It's been raining pretty much since I filled it up, so I can't tell for sure I'm leak free. Once I can verify that and I get it all buttoned up, I'll share a few more pictures 😊.

I'm thinking of getting another Stehnner pump for chlorine dosing. It sounds like I'm going to need to dose much more often than the pool. Are any of y'all doing that, and perhaps using the manual dose function before or after a soak? Or am I overthinking it, and just need to measure out the few ounces I need before or after a soak and pour it in?
 

mwemaxxowner

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We've just hit 100°F. That's approximately 74 hours to heat from 45 to 100. Having never had a hot tub, I don't have any idea how well this scores on hot tub or heater efficiency. It's 700 gallons, and 110v heating source. Rated for 2000 watts.
 
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jseyfert3

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110v heater found on Amazon.
Is that heater rated for spas or other similar human occupied devices? And is everything protected by GFCI and bonded as appropriate?

It's controlled by a digital temperature controller set to 100F, with a 3° differential set + and -. The heater has a flow switch and kicks in when the pump comes on. The pump is connected to the digital temperature controller, it will come on when the water temp is 97, and kick off once the temp reaches 103. The thermostat on the heater is set to 104.
You’ll want to crank that differential way down. 3 degrees differential for a 6 degree swing is MASSIVE. We will often soak around 100 °F, sometimes bumping up to 101 or 102 when it’s really cold, down as low as 97 on moderate spring and fall nights. It’s amazing how much difference one degree can make when you’re talking about being too hot for extended soaks. Two degrees is definitely noticeable. For a lot of evenings, especially in SC, you’ll probably find 103 is very hot (unless you’re only briefly soaking) and 97 is cold. I’d crank that differential about as small as it would go, something around 0.5 °F for a 1 degree total swing would be closer to what my hot tub usual maintains.

I think I want to add a propane on demand water heater for quick temp increases and helping it to hold temp when it's uncovered. I'm sure as it is now once we uncover it and climb in temps will start to drop. I just wanted to get it established as is first. I have plenty of propane out there for the grills. I just knew that I didn't want it to be heated solely by propane. I crunched some rough numbers on that vs electric and it was going to be much more expensive to heat it by propane alone.
Oh it’ll definitely drop, a not very well insulated, big tub with only a 2 kW heater? Yup. We used to have an Intex inflatable spa, basically no insulation except I had it on foam on the ground, could not maintain temps with a 1500 watt heater and much less surface area than yours, with evaporation leading the temp losses on an un

Do you have a bulk propane tank at your house or natural gas for heating/appliances?Bulk propane should work out cheaper than electricity for heating, 20 lb tanks not so much.

I'm thinking of getting another Stehnner pump for chlorine dosing. It sounds like I'm going to need to dose much more often than the pool. Are any of y'all doing that, and perhaps using the manual dose function before or after a soak? Or am I overthinking it, and just need to measure out the few ounces I need before or after a soak and pour it in?
Most of us hot tub folks use a SWCG such as a Saltron mini, but a Stenner pump would also work. No matter what you use, I’d probably get the hang of managing a spa first before adding automatic dosing. Automatic chlorine is very helpful but still usually requires manual dosing depending on usage, unless your usage patterns are very regular.

In any case, that looks pretty awesome! Congrats!
 
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mwemaxxowner

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Yes,hot tub heater. Yes, gfci protected and bonded.

No, I don't have bulk propane, which is why I want to heat it with electricity. If I add a propane heater, it will only be for while the tub is open, not maintaining heat full time.

I'll tighten my temperature differentials.

I'd love to go salt...except it's a galvanized steel tank. 😆 Otherwise I would have used an Intex SWCG/pump/filter like a friend of mine has on their above ground pool.

I was still tempted to try it though! If/when the tank corrodes it can be replaced, but I'd prefer it to be later than sooner.

I have a Stehnner on the pool and it has been quite helpful.
 

A.O.

Well-known member
Apr 12, 2016
225
Kershaw, South Carolina
Yes,hot tub heater. Yes, gfci protected and bonded.

No, I don't have bulk propane, which is why I want to heat it with electricity. If I add a propane heater, it will only be for while the tub is open, not maintaining heat full time.

I'll tighten my temperature differentials.

I'd love to go salt...except it's a galvanized steel tank. 😆 Otherwise I would have used an Intex SWCG/pump/filter like a friend of mine has on their above ground pool.

I was still tempted to try it though! If/when the tank corrodes it can be replaced, but I'd prefer it to be later than sooner.

I have a Stehnner on the pool and it has been quite helpful.

Pageland SC?? I just noticed this.. I'm near Kershaw not far from you at all!!

And I run my hot tub at 104 almost all year long..103 sometimes in the summer.
 

mwemaxxowner

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100 was really just my starting point. I was planning to adjust that after getting in and seeing what we like.

As it stands, I've figured out that I don't need to figure the cooling differential into my spread. That would be the temp at which it would kick on a cooling device, if I had one. The way I have it programed right now, it's just going to hover between 97 and 100. I'll move that and squeeze it down to maybe 100-102?

The thermostat on the heater is set to 104. I want to make sure my temperature controller is set a little shy of the thermostat on the heater. That way it's getting to the commanded temp and shutting off.

Yes, Pageland SC is where we call home!
 
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mwemaxxowner

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We haven't had a chance yet with work and school all week, but we're going in right after breakfast!

While I was researching stock tank hot tubs, some people were building them with outdoor propane tankless water heaters. They offer many thousands of btus more than a 2kw 110 heater, obviously.

They use D batteries to run the igniter, and the display, and have a flow switch to auto ignite it. I've ordered one and a pump for it to take over my heating while we have it uncovered when we want to soak for a while. Right now I have two 20lb tanks by the pool, and two 30 lb tanks on the camper doing absolutely nothing 🥺.

Thinking about insulation and temp drop after the heater kicks off, I am imagining I need to focus my efforts on the cover first? Heat rises, and I'll lose more through the cover, and the sides near the top than anywhere else, right?

I have a proof of concept for myself and it's all working as planned, so I'm ready to proceed with more insulation. I'm not sure whether to try blow in attic insulation or bite the bullet and go straight to two part expanding foam. It would take a lot of foam, and I'd be best to wait until summer to pour anyway. It doesn't expand as much in cool weather. If I did I would build a compartment where the pumps and valves are that it wouldn't expand into.

When I poke my head around under the deck I don't feel any heat radiating out from the tank, or if I put my hands on the carpet padding surrounding it.

I do feel heat if I put my hands where I cut some of the foam out for my bonding lugs or plunger valves. I may foam those points but I hesitate to, lest I ever need to tend to any leaks.
 
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mwemaxxowner

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Well, right now it's at 106. Lol. The thermostat on the heater is supposed to max at 104, last night I just unhooked the digital controller so I'd know we would for sure be at max temps for the test this morning. Now that I know for sure it'll reach that next time I might just set it for 103.5-104 or something like that.

I bumped it up since I don't know how quick I'll lose heat uncovered for this first try. 😬

In the past when we've had a rental or something with a hot tub we've set it to 104.
 

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mw,

Wow! Great creative design! Be careful with that temp, most commercial units max out at 104. But my guess is you don't let accepted practice influence you a lot.

Enjoy!

Chris
 
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