Hot Water Heating From Batteries

[Marinepower]

I would like to do some limited hot water heating off a 12v, 400ah lithium bank.

I know heating water takes a lot of energy. In my case, however, the tank is only 20 litters. Also, our water is typically luke-warm from the engine's heat exchanger from anchoring the night before. We are generally only looking to raise the tank temp from luke-warm to shower temp ( aprox. 25C to 40C, a 15 C difference).

So 20 litters raised 15 degrees should only be about 0.33 Kwh, if i have the calculations right.

Is anyone doing this on their boat? Or am i missing something here?

Also, currently there is a 700w AC heater element in the tank. Should I change this out to DC element to avoid the inverter efficiency losses?

Thxs for any advice from those who have tried this.

MP

 

[zanydroid]

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[pollenface]

I got 3x 12v 10a immersion heating elements from ebay and soldered them together for a single 30a immersion element (about 360w) nothing flash but it would heat a bucket of water from the alternator warm enough for a camping shower in about 10-15mins.

 

[Vigo]

I think your math is right and trying to avoid the conversion losses is more trouble than it's worth in my opinion. Console yourself that even though the inverter making the ac might be giving away something like 10% (or whatever it is), putting the heat in the water is nearly 100% efficient when almost nothing else we do with ac power is! It's usually far worse!

 

[chrisski]

My tank is 3 or 4 liters bigger at 6 gallons, and took three times the energy,1.27 kWh, to heat. Seem a bit more than you calculated. The inverter does lose 12% - 15%

RV electric water heating on Solar—Lot of energy little return

I ran my water heater today off solar, and as many know, takes a lot of energy. Specifically took 1.28 kWh at 1602 watts at which is 48 minutes for my 6 gallon tank to get warm from a water temp of 60 degrees. This time seems in line with my 8800 BTU water tanks claim to heat 7.4 gallons an...

diysolarforum.com

 

[wholybee]

I am curious what kind of tank you have and how well insulated it is? I have a Raritan 6 gallon tank. It is very well insulated. 15-20 minutes of running the engine it is hot. By hot, I mean it will burn you if you don't mix it with cold water. And will still be hot the next morning, but not burn you hot. The next evening it is warm, but still perfect for a shower. I would think that starting with a tank that doesn't lose heat will save more electricity from heating than anything else.
From an electricity usage standpoint, my 1500W AC element gets it hot in 30 minutes. So 750Wh. It won't get "burn you" hot off electricity because the thermal switch turns it off. That doesn't account for losses.

 

[Marinepower]

Thank for your reply Wholybee.

My tank is a Sigmar unit and it's much like you describe. Smoking hot from the engine and will stay hot for about 24 hours. But in practise, we use hot water at night for dishes ( which brings cold water into the tank) and by the morning it's just tepid.

We don't need the tank to be scolding hot, just hot enough so that if you have a short shower with no cold water mixed in, it's relatively comfortable. So that's about 40c.

 

[Vigo]

This is just my curiosity (dont own a boat, my RV does not have an engine coolant heat exchanger but i have considered implementing one DIY) in asking, but is there any kind of thermostat controlling how hot the engine coolant is allowed to heat your water tank? Thermostatic mixing valves are fairly cheap on Amazon (some under $40 us) and if you're not already, you could allow the hot water tank to get as high as 180 degrees but set your output to ~120f, which would 'grow' the tank capacity a bit, but also extend the timeline until water dropped below 120f.

 

[wholybee]

This is just my curiosity (dont own a boat, my RV does not have an engine coolant heat exchanger but i have considered implementing one DIY) in asking, but is there any kind of thermostat controlling how hot the engine coolant is allowed to heat your water tank? Thermostatic mixing valves are fairly cheap on Amazon (some under $40 us) and if you're not already, you could allow the hot water tank to get as high as 180 degrees but set your output to ~120f, which would 'grow' the tank capacity a bit, but also extend the timeline until water dropped below 120f.

Mine has no mixing valve, although some do. In my case it isn't a cost issue, but the small space and complexity. While a mixing valve isn't complex, i would basically need to replum my whole boat to accommodate it. I've worked it out in my head already.
But the same effect is done manually at the tap I turn on the cold water, then the hot to bring it to the temp i want.
The coolant loop comes off the engine before the thermostat, so it begins heating the tank even before the thermostat opens. Also, the thermostat on marine engines is quite low compared to a car or truck. I think mine is 130F or so.

 

[Vigo]

Gotcha. Limited (or no?) benefit to a mixing valve then if you can't really heat the tank past 130f without huge 'scope creep'.

Is there any way to easily reduce the 'heat loss' of the tank by improving the insulation of the tank, or the space it's in?

 

[gmakhs]

i use my hot water boiler 20lt, and run it through a Victron 1200 inverter.
The water is warm after 30 min and the rated load is 550 watt.... Easy peasy

For better Exonomy of hot water u have an auto mixer that Mixes the water on the output of the boiler and keeps it at 40 Deegres celcious
it works great.

 

[wholybee]

Gotcha. Limited (or no?) benefit to a mixing valve then if you can't really heat the tank past 130f without huge 'scope creep'.

Is there any way to easily reduce the 'heat loss' of the tank by improving the insulation of the tank, or the space it's in?

It would depend on the boat. I have a fairly expensive tank, supposedly the best insulated available. It holds heat well and doesn't cook my food which is stored directly above it. With my old tank that storage space got quite warm.

 

[TimE]

We have a 27 litres hot water tank with a 1200w 220v immersion heater installed. The boat AC wiring is 220v. We have 740w of solar installed. To heat hot water we use a very cheap 110v inverter. If you feed a 1200w 220v immersion heater with 110v the immersionn heater element only takes 300w. Once our batteries have gone to float around noon, I switch on the 110v inverter. I have a change over switch installed in the AC supply to the immersion heater that allows me to switch between 220v or 110v. The 24v feed to the 110v inverter has a mechanical 1 hout timeclock installed so I can turn the 110v inverter on for 1 hour increments. I normally run the 110v inverter for a couple of hours per day so we have enough hot water for a couple of showers and the washing up. Works great.

 

[svsagres]

So I have a little Isotemp hot water tank on my boat. It's heated by 120VAC. It also has a heat exchanger in it for engine coolant, but our engine is raw water cooled, so no coolant loop to tap. Since it's off the main breaker panel, which is downstream of our 2kVA Victron, we can technically heat our hot water with battery power. Doing so uses about 20% of our battery capacity.

We actually find ourselves using this more than you'd think. This past summer, I was anchored out for a few nights, and knew the next night I was going into the marina. I still had about 60% on our 460Ah battery, so I figure "why the hell not?" and heat up a tank so I can do dishes without boiling water on the stove.

The Isotemp tanks actually heat the water up to about 80C or so, then use a thermostatic mixing valve to drop the output temperature to a safe temperature. This allows it to heat up to engine temperature, and also significantly increases the amount of available hot water given the tank size. They're also very well insulated, and a hot tank will easily last more than a day, if you don't actually use it up.

 

[heavy-impact]

Why would you heat "hot" water?