In Which a Soaking Tub is Built Part 1

 

After a long winter, I decided it was time to build a soaking tub. It has been in the plans since we purchased the house, but it made less and less sense as time passed. None of the tubs in the house are suitable for a good Japanese style soak, but the cost to remodel a bathroom the way we wanted was just silly.

I've been looking into outdoor rated soaking tubs, but the decent ones are in the $2000-3000+ range and I wasn't ready to spend that much. I could try to build one, but the cost in high quality cedar planks would have been nearly as high (all those youtubers who homestead and have handmade wood hot tubs, they get their wood for free from friends or at a discount from the lumberyard and access to the best quality wood to-boot) So I went back to the good old fashioned stock-tank tub idea.

I tried to find purpose built tub heaters, but most are designed for full sized jetted tubs or are wood fired submersible units that require a pretty big tub. Not ideal for what I was imagining.

One of the common internet tub heater plans involve a heat exchanger made from coiled copper tubing placed around an open fire. Those designs typically use a passive convection circulation since the conducted heat from the copper pipe can cause issued with any type of intermediate fitting or pump. I also toyed with the classic metal-drum over a fire idea. It would require a raised platform and a wood floor to the tub do you don't sear your rear end.

I really wasn't sold on the idea of using an open bonfire to heat the tub like that. First, it requires purchasing firewood and building and tending to a fire which was a little too fussy for my tastes.

I played with the idea of a natural gas fired grill burner inside the heat exchanger since we have multiple gas hookups for a grill in the back yard. Alas, most of the standalone gas burners are designed to run on LP, and while I know you can modify the jets to convert between fuel sources, I don't feel entirely comfortable DIY messing with gas hookups and appliances. 

Then I realized that trying to build a heat exchanger from scratch was a stupid idea. There are purpose built, off the shelf, heat exchange based water heaters available on the open market from nearly all the reputable manufacturers: they're called tankless water heaters. DUH...

They are usually designed for permanent interior installation, but you can find exterior rated units that run off of natural gas. Problem with those units, for my application, is that they have a BTU/hr rating in the 15000+. Slightly overkill. They also need adequate ventilation and again, it involved DIY-ing gas appliances. You can find some sketchy LP based camping tankless heaters on Amazon, but we have the same DIY conversion to natural gas problem.

I couldn't find any reasonable outdoor installation electric units, but indoor ones are cheap and plentiful. They get a lot of bad rap due to the power requirements of instantaneously heating flowing water, but if you size it right, they can work well.

Pro tip: don't even bother with the 120V units. There is no way to shove enough power down a wire at 120V to reasonably heat water flowing faster than 0.25 GPM. You can also find units with all sorts of thermostats and fancy electronic timers and features, but those are all points of failure, especially in budget priced appliances (or high end brand name stuff too...) I looked and found a Bosch branded heater designed to be mounted under a sink for point of use applications. The unit was dead simple: just a pair of resistive heating elements wrapped around a copper loop, a bimetallic thermal overload switch, and a mechanical pressure switch to turn the heaters on and off. The 9000 W (240V) unit I purchased was the largest model of the series and, at $150, strangely priced lower than the lower-wattage units. I imagine the 50A breaker and rated wiring requirement puts off a lot of DIY/remodel types.

Luckily, I have a brand new 90A sub panel in the garage and a 50A welding outlet already wired and ready to go. Sorta. Not being a complete idiot, I decided to invest in a GFCI breaker to replace the current standard breaker. I figured the wife wouldn't appreciate fried husband soup. GFCI breakers are expensive, and 50A double pole ones are hard to find and very pricey. After some looking and some model number and revision confusion, I found the breaker I needed and bit the bullet for $70.

I initially tried to use a small impeller pump sold for moving wort in DIY brewing. The rated 2 GPM would have given me a perfect 30-40 F rise in water temperature. Instead, the expensive and useless pump moved .5 GPM, maybe? At that low flow, the pressure switch in the heater wouldn't activate and when I wedged the switch closed, the water would boil in the heat exchanger in a very sketchy puffs-of-hot-steam-out-the-hose kind of way. We used it once then decided the pump had to go. I ended up installing a 4 GPM diaphragm pump used in RVs with alleged 1/2 NPT fittings (a surflow knockoff). Yeah, no. I ended up getting it to work, but it's still kinda sketchy. Also, it isn't really rated cor continuous use, so it get stupid hot. I bought some MOSFET heatsinks and turned it into a hedgehog, which helps a lot. All that said, it does move the rated water and I am getting a 20 F rise at 4 GPM. I have it wired into a mechanical timer switch and it takes about 40 minutes to get the tub from 70 F to 110 F.

The fittings and hoses combined were the most expensive part of the project. I went with stainless ball valves and bulkhead fittings along with QD couplers and splurged on high temp silicone hose. I couldn't seem to find a reputable source of 1/2 ID PVC hose and the silicone stuff was sort of designed for the application. It is much nicer than PVC and is super flexible with fiber reinforcement. My only real complaint is that the silicone picks up dirt like a lint roller. I ended up buying some nylon sleeves and spent over an hour sleeving 50 ft worth of hose. There are some leaks from one of the QD junctions, but overall, the fittings and flexible hose makes it easy to break down the system and put away.

The $120 stock tank is from Tractor Supply. I'd never been to one before this point, and boy is it weird: fashion next to jewelry next to live chickens beside all the types of feed you could imagine opposite tools and god knows what else. Something to remember when buying stock tanks: they ship nested from the factory, so a 2'x4' tank can vary in width and length from 16"x40" to 25"x47".

 

Add some lava rock and gravel to make a base, and you almost have a hot tub.