Best hot tub water heater for conversion?

[Kevin Warnock]

I want to be able to soak in a jacuzzi style bathtub on my coach, with the water depth to the top of the tub. This requires about 75 gallons of hot water at 104 degrees. I plan to chemically treat the water like hot tub water so that I can drain the water into the dedicated supply tank, and reuse it for the next soak. I have a separate shower for actual washing, and I would not use any soap in the tub. I would hop in after a traditional shower, so the water won't get that dirty.

My question is what's the best way to heat that amount of water? I was thinking of a Webasto water heater, and just piping the tub water through it, without a heat exchanger. But aren't Webasto's designed for a closed loop system, not one like I am describing? There is a Webasto on EBay right now for $300 which seems OK.

What about a heat exchanger? Does that slow down the water heating a lot? I don't plan to drink the tub water, but someone might try down the road, for example in a dire emergency like after a natural disaster. I wouldn't want a Webasto malfunction to kill someone. Where do I get the heat exchanger and what do they look like? What are they called specifically so I can search on EBay?

Other options are a propane home water heater as the tank and water heater in one. These are about $400 new from Sears. It would be a pain to locate and vent a residential full size water heater in my coach.

I have also considered a propane on demand water heater, but they are costly for one with high heat output. Seems like a used Webasto is the cheapest, most mobile ready solution, but I would like your feedback.

Thanks

[BG6]

I think you should forget the idea of saving your Jacuzzi water.  First, no matter how clean you are, the water will get dirty, ferment in the tank and come back smelling like used gym socks.

Second, pumping hot water into plastic storage tanks can result in degradation of the plastic, so eventually the tank will fail.

If you're going to want to soak while on shore hookups, you don't need to save it anyhow.  If you are thinking about boondocking with your hot tub, you can use the tank idea IF you use a really good water purification system and let the water cool before putting it back into the tank.

BTW, when you look at the amount of water you need, don't forget to subtract for the volume that YOU will fill!

Now, after all that, to answer your actual question, just about any small water heater will do what you need.  Remember that you are recirculating the water, so that it will warm more on subsequent passes through the heater.   Check with hot tub shops to see what they recommend for small tubs -- if you call around, you may even find one, cheap, either used or not big enough for the original buyer.

[buswarrior]

Hello.

I'll participate in your dream project!

Any water that is hot, yet cool enough for you to sit in...if that harms your plastic tank, well, that's a pretty poor plastic tank!

In the name of safety, I'd take care to not over engineer a heat source that would be able to harm either you or the rest of the plumbing.

In the spirit of the project, you may plumb a dedicated Webasto right into the tub plumbing and then control the temp down to your needs using simple thermostatic controls and or a mixing valve.

You might consider for efficiency of the whole bus conversion project, that in order to employ the Webasto for more than the hot tub, you may consider a flat plate heat exchanger, so that you may plumb the Webasto into your engine coolant loop, and other needs(hot water heater, engine pre-heater, radiant heat) and have a dedicated loop to the hot tub exchanger, used only when tubbing.

The bus conversion is yours, and as long as it meets your needs and desires, never mind what the rest of us say...

Do it your way!

happy coaching!
buswarrior

[pvcces]

1967_MCI5a , 75 gallons of water would weigh about 625 lbs. If the cold water was 50 degrees and you heated it to 104 degrees without losing any of the heat while it was warming, you would need to use about 34,000 btu heating the water.

Since an electric home water heater would probably use a little over 13,000 btu per hour, it would take close to three hours to heat the water. A gas heater would heat it quicker, but it seems to me that allowing for a hot tub in your bus would be difficult to do.

For example, if you had 50 amp park service, it would take a full hour drawing 100% of the power from that service to heat your water. I suspect that propane heat would be the easiest and least expensive to set up, but it could be done with a Webasto.

Good luck.

Tom Caffrey

[niles500]

Check out Sean Welsh's system on Odyssey - HTH

[Busted Knuckle]

you should check out Sean & Louise's "Odyssey"
here's a link!

http://ourodyssey.blogspot.com/search/label/Tour%20of%20Odyssey

FWIW    BK  

[Sean]

As long as my name is being bandied about...

Our YachTub-brand hot tub holds between 300 and 350 gallons, depending on how many people are expected to get in it.

Our Webasto DBW-2010, which is rated at around 45kBTU/h (probably a bit less than that, since the rating is for a 0.40 gph nozzle, and we have a 0.35 nozzle installed), will heat that amount of water from ~65°F to 102°F in a little over three hours.  Perhaps a bit longer if it is also doing other work, like heating the bus.

We use a conventional coil-in-tube type heat exchanger; if I were to do it over again, I'd use the flat plate style.

The YachTub control box carefully adjusts the amount of water circulated through the tub side of the exchanger to maintain safe water outlet temperatures, so you can't get scalded in the tub.  Might heat a tad faster without that safety.

Note that the Webasto is not dedicated to the hot tub alone -- it also provides all our domestic heat and hot water (when parked -- we get that for free when driving from the waste heat of the Detroit), and can be used to pre-heat the Detroit if needed on those cold mornings.

The hot tub exchanger is plumbed on the "summer loop" portion of the coolant circuit, so it is always in the loop.  It's in the tankage bay, so any heat that would be wasted coming off the exchanger and the plumbing is keeping the bay and tanks warm.

BTW, if we can get parked within 150' of a spigot, we'll have the tub up and running at Arcadia this week.

-Sean
http://ourodyssey.blogspot.com

[Rick Brown]

I've got a Precision Temp propane fired tank-less hot water heater in my coach for domestic hw.  Somewhat pricey at $600 when I bought it a number of years ago and somewhat tricky to drain for freeze protection.  Supposed to be good for 40K btu/hour.  If I wanted a hot tub associated with my coach I would use that or something similar dedicated to the task.
-RickBrown in Reno, NV

[Sean]

I've got a ... propane fired tank-less hot water heater ...  Supposed to be good for 40K btu/hour.  If I wanted a hot tub associated with my coach I would use that ...

Wow, that's a lot of propane.

Don't forget to do the math:

It takes 8.34 BTU to raise one gallon of water one degree Fahrenheit.  In the hypothetical 100-gallon tub being discussed, to go from, say, 65° to 102°, an increase of 37°, would require 8.34*100*37 or about 31,000 BTU.

Since Propane has about 90,000 BTU per gallon, you'd use a third of a gallon every time you started the tub, and perhaps again as much due to losses in operation.  So you could use your tub about six or seven times on a BBQ tank.

Diesel, BTW, has 130,000 BTU in a gallon, and most of us carry a lot more diesel than LP.  Right now, even with motor fuel tax, diesel is also generally cheaper at retail.

Our tub, at 300 or so gallons, requires nearly 100,000 BTU to heat up, and that's every day we use it -- the water temp tends to drop back down to ambient overnight.  We use a little over a gallon of diesel to do that with our ~40kBTU/h boiler, which gives you some idea of the losses involved.

-Sean
http://ourodyssey.blogspot.com

[boogiethecat]

Maybe think about putting a water-to-water heat exchanger in your engine cooling system to heat the hot tub tank to temp while you're driving...that way the bulk of the heating work comes almost for free from your engine, then use a smallish hot tub heater for 'makeup' to get it to exactly the right temp during use.
I used to do the shower on my first bus that way- a "tempering valve" kept the shower at 110 no matter what the engine temp was, and it worked great.  I also did camp showers that way- had a little heat exchanger built into my toyota so I could pull up to a campground, hook a water hose to the faucet, turn the toyota on at idle, and provide infinite hot showers to all my friends for as long as I chose to idle the car.  It made a lot of people happy!!!

FWIW if you install a good filter and circulating pump in your tub system and then use hydrogen peroxide for sterilization at 15-20ppm, you'll never have any alge, bugs, growths, or nasty smells ever.
I have a 2500 gallon tub that's been at 100 degrees for 12 years now, sterilized solely by peroxide at 12ppm, and it's been the most non-maintainance system I've ever had.  MUCH nicer than chlorine or bromine as the byproducts are simply water and oxygen! For a smaller tub it's easy to calculate how much to put in, and it becomes simply "this much per day" thing.  You can get way out of range (ie too much) and it won't hurt a thing.  There are testing kits available to tell you what your concentration is, and after a few weeks you won't need to test anymore as you'll just figure it out. It's too easy!!!

Cheers

[youknowwho]

So if you have a dedicated tank that holds X amount of water for your hot tub and its treated water like used in a regular hot tub and filter then a loop with a FPHE hooked into your engine coolant system would be ideal. You could heat it up before reaching your destination and start use immediately. Hook it into your Wabesto loop also but separate from your  engine system with a FPHE.

[Ray D]

You need the aqua-hot, unlimited hot water and I have about $1,000.00 in it, got it surplus at Northwest RV in Eugene, Or.

Ray D