Heating a hot tub??

[Jolly Roger]

Here’s a different one from the norm.
When I rebuilt my aft cabin head, I managed to squeeze a full-size bathtub in as well. Actually, it’s a real hot tub, with a separate 10-jet 12-volt blower.
The bath maker supplied a 120-volt heater, but it took ages to heat the 50 gallons in the tub and soon burned out. The boat’s water heater is only 6 gallons, so that’s not enough to fill the bath. I’ve been told I need a 240-volt heater, like a HydroQuip HeatMax RHS 5.5Kw.
The question is: how can I hook up my two 120-volt shore power leads, to give 240 volts to a heater? Both leads are separate systems with separate breaker boards, each one running an AC unit and a few other things. The tub will only be used when twin shore power is available and water from a shore spigot.
Advice greatly appreciated to finish the project.

[bensolomon]

I can't help with the electrical, but I wanted to tell you how awesome that looks! It's nicer than my tub at home.

[Jolly Roger]

Thanks Ben, it took some doing, and I can't use it now until I solve the voltage problem.

[rslifkin]

Getting 240v from twin 30A connections might be iffy, but it's possible. It'll only work if both connections are out of phase.

With the kind of power draw you're talking about here and the other loads mentioned, it may be worth upgrading the boat to a 50A 125/250V shore power feed (and carrying adapters for when 2x 30A is all you can get).

[Jolly Roger]

Unfortunately, the little private marina I’m in doesn’t have a 50amp outlet, so I’m stuck with trying to join two 120-volts. Even if it did, are they not still only 120-volts?

[rslifkin]

Originally Posted by Jolly Roger

Unfortunately, the little private marina I’m in doesn’t have a 50amp outlet, so I’m stuck with trying to join two 120-volts. Even if it did, are they not still only 120-volts?

The 4 wire 50A plugs give you 2 out of phase 120v legs. So you get 120v from L1 or L2 to neutral, but L1 - L2 gives you 240v (same as typical US split phase house service).


2x 30A will give the same thing if the 2 plugs you're connected to are 180* out of phase (which they may be if the marina is fed with split phase service). But if you have 2x 30A on the same phase, you won't get 240v from them (you'll have 120v to neutral from either one, but 0v between the 2 hots).

[Shrew]

It would be cool if you could rig a manifold and run the AF from the generator through the manifold to then heat the tub water. Similar to how you can heat water in a water heater using the coolant circulated through a running engine.

[rslifkin]

Along that line of thinking, a hydronic diesel heater could also be used (and would enable fast heating without needing huge amounts of electrical power).

[Franziska]

Originally Posted by rslifkin

Along that line of thinking, a hydronic diesel heater could also be used (and would enable fast heating without needing huge amounts of electrical power).

This, or perhaps the cooling water from the main engine via a heat exchanger.

Or an on demand propane water heater.

Not sure your neighbors are happy if you draw the entire electric power of the pontoon. They might have a main breaker somewhere. [emoji1787][emoji23]

[smac999]

Look at diesel water heating like a webasto

Could also be used at anchor. If you have enough water.

[Jolly Roger]

Thanks for all these ideas fellas, and I’m not closing any down, I don’t understand some of them anyway.
I will check to find out if the marina 120-volt is out of phase. Power drain would not be a problem in my marina situation. I now understand how this is done in a house for a washer/drier or AC. thanks rslifkin.
I would like to learn more about a diesel fired 12-volt water heater, but I can’t find one. Would such a thing need outside venting? I have 375 gallons of diesel. Could someone give me a link please?
I’ve looked at regular propane tankless heaters – too big and need venting.
The main engine already heats fresh water through a six-gallon hot water calorifier tank, for normal washing and showers. But it would need a much bigger tank to be able to fill a 50-gallon bath and maintain a constant temperature. There’s no room for a bigger size tank on the boat.
I’ve even considered changing my 6.5Kw 120-volt generator to 240-volt output, which is something I understand can be done. But I don’t understand the complexity. Also, I don’t like the idea of running the main engine or generator while I’m having a nice quiet soak.

[smac999]

Most Likly you could just find a higher wattage 120v heater.

A 3000w 120v heater is going to be the same power as a 3000w 240 heater. Just less common to find.

You probably had a 1500w?


If the boat or shore has elci / gfci breakers. You can not get 240v from the dock. It will just trip.


Your current gen can probably be changed between 120/ 240 with internal jumpers. But that wound requiring changing the wiring configuration on the boat too.

[Stu Jackson]

Originally Posted by Jolly Roger

......................

I would like to learn more about a diesel fired 12-volt water heater, but I can’t find one. Would such a thing need outside venting? I have 375 gallons of diesel. Could someone give me a link please?

........................

from a search on boat hydronic heating:


https://panbo.com/ode-to-hydronic-bo...arine-service/


try a search which also lists manufacturers' websites

[bob_h]

The fact that you have two separate 120V outlets doesn't mean you've got two separate 120V 15A _circuits_. Think, going into the bathroom and plugging in two different hair dryers. Gonna blow a fuse, probably. Even if there are two different outlets on the wall. First, find out how much power is actually delivered to your slip. If you've got the power available, then I'd find a way to plumb in a _second_ 120V heater. The heaters typically aren't very big. That way, you can power them both up, if there's power available, and just use one if your visiting another marina that won't accommodate that. And you don't need to mess around with split-phase stuff. Which might work today, and suddenly fail when somebody needs to rearrange the circuit breakers on the marina's panel. And I expect that somebody here could tell us just how many electrical code violations this idea would entail, and what your insurance company might think of this project.

[Jolly Roger]

You all make good points, and I’ve also been thinking.
I’ve found that I can manage to squeeze in a larger water tank, from five gallons to eleven, and heated by two 120-volt immersion heaters, instead of one at present. Between the engine and the immersion heaters I think they will heat eleven gallons to 180F in about half an hour. I will then open the bath hot water tap and run it into the bath. I will leave the heater and engine running to continuously heat more water and leave it running into the bath. It remains to be seen how long the water will run warm, and I hope this might be a good way towards the bath’s capacity, but it can run until it either fills up, or I need to cool it to 104 F. At this point I will switch off the engine and using two change-over valves, transfer the flow through a slow 12-volt pump circulating water from the tank into the bath and back again. I will then leave the immersion heaters on, which will be regulated by a thermostat on the water flow pump.
This is where I jump in the bath, switch on the ten jets 12-volt blower and open my pint of Boddingtons pub ale.
If this works there are a few advantages: First, a new water tank is $300 less than a heater, and I will have twice as much warm water after we have been for a sail, by running the engine in and out of the marina. Second, I don’t pay for electricity on my dock. I also don’t have to mess with any shore power and risk burning down the marina.