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Old 24-07-2021, 03:13   #1
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Question Hot water heater through an inverter?

I have a 2014 Oceanis 41 and it has a (factory installed) hot water heater which gets the power from the engine at the moment.

The person who will install our new inverter says it's not a smart idea to put the hot water heater through the inverter and keep it as it is which means we'll have hot water only when we run the engine or through shore power.

What do you guys think?
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Old 24-07-2021, 03:41   #2
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Re: Hot water heater through an inverter?

Calculate the energy required to heat the water and compare that to your installed battery capacity. You can then draw your conclusion on the feasibility with some facts in hand.
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Old 24-07-2021, 05:18   #3
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Re: Hot water heater through an inverter?

Not much information to give specific guidance, but there are three possible issues. First, is the inverter large enough to pass the relatively large load of a resistance heating element? Second, can you generate enough power to replace the power used by the water heater? Third, battery capacity supplying reserve power?

Many water heaters have 1500w heaters - if yours has a 1500w heating element, you would need at least a 3000w inverter to carry this and other loads. A few have only 750w but even that is a large draw as it takes roughly twice as long to heat.

Let's say for ease of discussion you need just 3000w per day of heating power. That's in the neighborhood of 250ah at 12vdc. That's a lot of power to generate, store, and manage.

Obviously, this becomes a relatively sizeable power system involving large battery banks,, large alternators, large solar arrays, large inverters, etc.. As a result, most vessels using resistive heating elements for cooking for water heating use a generator for power.
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Old 24-07-2021, 05:21   #4
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Re: Hot water heater through an inverter?

In the general case inverters don't run high need items like water heaters, stoves, and air conditioners. Both the size of the inverter and the size of the battery bank become prohibitive, as the calculation sailormed describes will reveal. But then, many (most?) of us have sources of energy other than the engine and shore power. In my case, it's a large and a small generator, for others it's solar or wind. A few may use propane. All are energy sources that can be used as you wish.

It may be time for you to run an energy review and budget. If you want a hot shower after sailing and on the hook (I'd certainly want one) then start thinking about an energy source that will allow you that luxury.
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Old 24-07-2021, 08:43   #5
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Re: Hot water heater through an inverter?

Your electrician is right. Have you considered solar showers? They are brilliant in temperate climates and so effective they must be handled with care in the tropics.
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Old 24-07-2021, 09:06   #6
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Re: Hot water heater through an inverter?

I had a raw water cooled diesel in a previous boat, so the water heater was not attached to the water heater. I used various other methods, such as solar showers, etc, and had good results. However, I also installed a larger inverter, and very small electric water heater, with a capacity of 2.5 gallons. This took very little time to heat, and yes, I would run it off the inverter. Worked fine, but do the calculations mentioned up thread to see how you will manage the power requirements. I had never seen such a small water heater, before, but I think it has applications on boats. It was, and still is, made by Rheem, and is called an under-sink water heater, I believe.
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Old 24-07-2021, 09:32   #7
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Re: Hot water heater through an inverter?

My hot water tank has a 750 watt element and only takes about 1/2 hour to get enough hot water for dishes and a quick shower. That's only about 30 amp hours so should be pretty doable with a decent solar/battery setup.
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Old 24-07-2021, 09:57   #8
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Re: Hot water heater through an inverter?

I have done it but only when the solar has completely topped off our batteries and all excess solar is being dumped. Why not run the inverter judiciously and heat some hot water? If the tank is well insulated it should stay warm for evening showers unless you are in the Arctic at which time you have other heaters turned on which could do the same.
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Old 24-07-2021, 10:07   #9
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Re: Hot water heater through an inverter?

I find our 6 gallon hot water to last about 3 days, rarely stay one place much longer, I would hesitate connecting to inverter.
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Old 24-07-2021, 10:07   #10
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Re: Hot water heater through an inverter?

I wired a Honda 2000 gen into a customer's boat, it took 1/2 hour to produce hot water for his morning shower.
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Old 24-07-2021, 10:10   #11
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Re: Hot water heater through an inverter?

Quote:
Originally Posted by Dougtiff View Post
I wired a Honda 2000 gen into a customer's boat, it took 1/2 hour to produce hot water for his morning shower.
I carry a Honda for backup, heats water and charges batteries faster than running main engine.
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Old 24-07-2021, 10:37   #12
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Re: Hot water heater through an inverter?

There are several threads here that have done this to death. The answer is YES provided you have a reasonable battery bank and inverte.
First, the battery bank probably needs to be at least 300 Ah at 12v or equivalent.
Second, the inverter needs to be continuous rated for at least the rating of the heating element. No margin necessary over that as the load is pure resistive.
Third, use common sense, and use the inverter to heat water in bursts, monitoring the battery voltage. We use timed 15min bursts for our 1200W heater from our 360Ah bank which drops battery capacity by 10%. That gives us a couple of warm showers. Unless, you have a massive bank, you will not be able to regularly get a large tank up to shore supply temperature.
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Old 24-07-2021, 10:55   #13
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Re: Hot water heater through an inverter?

"What do you guys think?"

I think you should listen to your installer. Any appliance that produces heat uses lots of juice. High power electric gear for mid-size sailing yachts is practical for short use times: like an anchor winch (and there are many problems with those), for boats with a gen-set, and for those that rarely leave the dock and shore power.

For example: A 1,500 watt resistance element (the smallest size available) will draw about 140 amps @ 12v when run through your inverter @ 90% efficiency. If you don't know volts from amps, that is a large! current.

Heating water by running your engine for a short time is actually very efficient. You will have a comfortable shower in a half hour of engine run, burning a 1/2 qt. of fuel, and charging your batteries for other uses at the same time. In real world cruising, rarely a day goes by that we don't fire up the old Volvo for one thing or another.

IMO: Hook up the element in your water heater to your shore power and forget about using it when you are away from the dock.
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Old 24-07-2021, 12:00   #14
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Re: Hot water heater through an inverter?

I may work with a large enough battery bank and inverter along with a well insulated water heater. Get it up to temp. with the engine so your just maintaining the temp. with the AC. Start the engine when showering.
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Old 24-07-2021, 12:48   #15
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Re: Hot water heater through an inverter?

Providing your batteries can handle it and have excess solar why not?

On our boat we have a 120v 2.5 gallon 1750w water heater. No provisions to heat from engine coolant or shore power. 300ah lithium batteries and 2kw inverter.

To heat that tank of water it uses 30ish ah or 10% of our batteries. Which is easily topped up by the solar or alternator.
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