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Old 01-24-2019, 08:41 PM   #1
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Water heater sizing Q&A for liveaboards.

What size water heater would be good for a water-conscious couple living aboard that doesn't want to run the engine or genny while away from shore? Power will be supplied by eight 105 Trojan batteries to a Xantrex 2000 running watts pure sine wave inverter. I could increase the inverter size or add a second with the battery bank I'm planning.



I lived with my first wife in a small basement apartment with a 20 gallon/75 liter water heater. It worked well but was 220v and recovery times were fairly short. My current home has a 39 gallon/147 liter 220v water heater. When the power goes out, all three of us can take showers before we run out of warm (not hot) water. Of course, we have a low flow shower head and take quick showers. No lollygagging when the power is out.
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Old 01-24-2019, 09:22 PM   #2
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When we (2 people) cruised the Bahamas we probably used about 1 gallon per shower. So with the typical 6 gallon hot water heater we could go two days until we had to run the engine to reheat the water. We were pretty frugal and being a bit more liberal, 6 gallons should be plenty for two if you reheat it daily.


How do you plan to reheat the water: engine, solar, genset?


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Old 01-24-2019, 09:27 PM   #3
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You will not be able to keep your water hot using batteries.

120v x 11A = 1320 watts/12v = 110A.

Trojan 105 @ 110A = 50 min to dead x 4 = 3hrs 20min. Assuming no efficiency losses.
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Old 01-24-2019, 09:36 PM   #4
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X2, day to day on the book hot water is mot a truly viable option on batteries unless you have a massive solar array and a dump load element.

We did do well with locating the 6 gallon water heater tank near the main engine. A couple hours motoring heated the tank to the point we didn't need to turn on the gen set for a hot shower after we anchored.
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Old 01-24-2019, 11:40 PM   #5
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6gallon heaters re-heat really fast due to the size. Ours is surrounded with 4" of pink foamboard and the water stays hot to pretty warm for three days aft being heated up whether by running the engine or the generator.

Trying to heat water through an inverter will eat your batteries pretty quick.
Not advised. Do the calculations.
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Old 01-25-2019, 01:55 AM   #6
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Has anyone fitted a large domestic water heater to your trawler, like 30-50 gallons? It would seem that once fully heated, and heavily insulated, you could go an entire week before running the engine or genset.
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Old 01-25-2019, 05:44 AM   #7
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Usually people can switch between dock and on the hook living and adjust habits to a point.

I find the real question is how warm/cold is the water replacing what is in the tank?

If warm as in Bahamas/Florida you can maybe expect....six gallons of hot water will be hot the first day and pretty warm the next.

If the water coming in from the cold tank is cool or cold.....six gallons isnt very hot when you replace a couple gallons when it comes in cold....and loses heat through lousy insulation.

Another biggie is insulation. I went with an Isotemp SPA 8 gallon heater with the 750 watt element and it has performed better thaan any 6 gallon, square box heater I have ever had in 2 liveaboards and several RVs.

The 750W element and good insulation will help with your battery situation....but you will still need to watch them closely and replenish them. I would rig a sensor some way to let me know the temp of the water in the heater to let me know when it is at an acceptable temperature.
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Old 01-25-2019, 05:52 AM   #8
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First, it depends on your cruising style. I move every 2 or 3 days. The engine's cooling system heats the water heater through a heat exchanger to around 170 degrees. My 20 gallon water heater will generally last 2 maybe 3 days as the water is much hotter than normal domestic hot water (so you use less). As it's waste heat that heats the water, making hot water is essentially free.

In choosing a water heater, available space and cost will likely be the determining factor. You can save some initial cost by installing a significantly less expensive home water heater, but the cost of producing electricity to run it will likely far exceed the savings versus a marine one that will heat off the boat's engine heat.

You can also run the water heater with a generator or an inverter. With an inverter, power is supplied through the batteries which are either charged by the generator, main engine, or solar. Running the water heater through the inverter / battery back makes little sense as it's more efficient to either run the water heater directly off the generator, or use waste engine heat through a heat exchanger in a marine water heater. Solar is a viable option for making hot water through the inverter. Met a couple who equipped their boat for cruising with all electric appliances, a 1300 amp battery bank, and a large solar array. They were able to anchor for months in the Caribbean and make all their electricity through solar including hot water. Now in the Caribbean you probably need less energy to heat water than somewhere in the Northern USA, but it can be done with solar.

So, in my mind, your choice comes back to available space, cost of the water heater, and cost of heating the water. Imo, heating water with waste engine heat is the cheapest longterm solution if you plan to use the boat a fair amount and tend to anchor out versus stay in marinas.

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Old 01-25-2019, 06:52 AM   #9
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Like OC Diver, my engine will heat the water in my 12 gal Raritan water heater. I really like it. With no other heating, 12 hours after, the water is still very hot, 24 hours, it's very warm, 36 hours warm, 48 hours luke warm.
Two people use a couple of gallons per shower.

I would stick to a quality marine water heater. I do save money when I can, but things that come in any contact with water seem much more vulnerable on the boat.
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Old 01-25-2019, 07:36 AM   #10
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Has anyone messed with the solar water bags? The ones you hang from railing or wherever and the heat up from the sun, and you can shower on the transom.

My 6g heater is perfect, but need the genny or shore power to heat it up. However, it stays comfortably hot for well over 24 hours, and the gen will heat the water in 30 min.

Not hooked to the engine which would be ideal, but a major job in my boat.
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Old 01-25-2019, 07:42 AM   #11
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Not hooked to the engine which would be ideal, but a major job in my boat.
Just curious why it would be a major job? Does the water heater lack the heat exchanger to heat off the engine, or is it the distance between the water heater and the engine?

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Old 01-25-2019, 08:11 AM   #12
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Has anyone messed with the solar water bags? The ones you hang from railing or wherever and the heat up from the sun, and you can shower on the transom.
I have used those. They work well for what they are. You get a limited amount of warm (usually not really "hot") water. For apres-snorkelling around the boat, they are a great, easy, cheap way to rinse the salt-water off. I wouldn't want to try to take a "real" shower with one.
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Old 01-25-2019, 08:14 AM   #13
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My boat is a planing hull, so minimizing weight is critical. I use a 3gal water heater with no connection to engine. Also it is a single engine so protecting it is primary, and have seen a few engines overheated when a leak or air pocket was trapped in heater loop. Another is engine runs too hot (185F) and I did not want to fool with some contraption to avoid scalding risk.

So I went with a simple domestic little heater. I have to run the gennie or shore power for 15min to get a hot shower. And since my water tank is also small, the water heater being small becomes a water saving feature. The girls are told that showers must be brief. And once the hot water runs out, they get religion FAST. Their next showers become way more efficient and I need not say another word.

In summer cruising, this system works just fine. Many showers taken intentionally with zero hot water and the coolness is refreshing.

In winter cruising (outside temps like 30F) it is a challenge. Boat is not insulated and shower stall is chilly to start with and man o man would a long hot shower feel good. I did set the tank t-stat up to like 150F and that helped. After I got into warmer weather, turned it back down.

In a trawler where weight is not a concern, I think 6gal would be fine with a little conservation.
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Old 01-25-2019, 08:21 AM   #14
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I wouldn't want to try to take a "real" shower with one.

Leave it in the sun longer , with the black side facing the sun.130F is possible.

The "off grid" folks sell 12V elements that will fit the std opening of a 120v HW heater element , only 25W or 50W , match it to your solar output.

If the HW box is well insulated , it might be enough

Plan B.???

Marathon sells 30G hot water heaters that are superbly insulated & "forever" plastic lined not metal lined. BIG tho.

They have 2 mounts for heating elements , so you could remove 1 and install a std 120V element and a 12v in the second.

Dockside the recovery might be slower , but it would heat from an inverter if underway and a solar panel the rest of the time.
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Old 01-25-2019, 09:11 AM   #15
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You can save some initial cost by installing a significantly less expensive home water heater, but the cost of producing electricity to run it will likely far exceed the savings versus a marine one that will heat off the boat's engine heat.

You can also run the water heater with a generator or an inverter.

In the end, cost is the determining factor. A big 40+ gallon Home Depot water heater will cost 1/2 to 1/3 the cost of a “solar water heater” with heat exchange loop of same size.

A 2-3 kW alternator on the engine, supplying power directly to an inverter then to the heater, is still heating the water from the engine. The additional diesel burn is negligible.

It’s a shame to waste all that nice hot waste water coming out of the engine though.
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Old 01-25-2019, 09:34 AM   #16
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Our boat came with a 19 gal all elec water heater - would prefer to have a loop from the engine, but this heater is not equipped. We have a very larger alt on one of our engines that powers the 2500w inverter underway, and that heats the water just fine Or we can run our quiet gen if needed.
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Old 01-25-2019, 10:29 AM   #17
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A lot of replies. Thanks. You all have giving me more to think about. We will be moving almost daily except for two to three day layovers in some areas we want to spend time in. In that case, we may be on shore power. I doubt we will be anchored out more than two days. I will have a genny, but haven't nailed down which exact model yet. Psneeld hit right on the unit I am considering 8-gallon Isotherm because of the low wattage requirements. I am hoping to get six 360 watt Mission solar panels on the boat. Others have mentioned insulation. I am planning to insulate the water heater with a blanket and also the water line to help keep heat in and condensation from forming on the PEX.
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Old 01-25-2019, 10:42 AM   #18
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In the end, cost is the determining factor. A big 40+ gallon Home Depot water heater will cost 1/2 to 1/3 the cost of a “solar water heater” with heat exchange loop of same size.

A 2-3 kW alternator on the engine, supplying power directly to an inverter then to the heater, is still heating the water from the engine. The additional diesel burn is negligible.

It’s a shame to waste all that nice hot waste water coming out of the engine though.
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Our boat came with a 19 gal all elec water heater - would prefer to have a loop from the engine, but this heater is not equipped. We have a very larger alt on one of our engines that powers the 2500w inverter underway, and that heats the water just fine Or we can run our quiet gen if needed.
It's actually quite simple (in design) to convert a domestic water heater to a marine version with an external heat exchanger. You buy or build a pass through heat exchanger such as the transmission cooler on a Ford Lehman or other modest HP marine engine. Mount it vertically on the side of the water heater as close to the bottom as possible. Connect the water heater drain to the bottom of the heat exchanger and plumb the top to the output of the water heater. Connect the other 2 ports to the hot water loop off the engine, and you're done. When the engine gets up to operating temperature and the heat exchanger gets hot, the domestic water within the heat exchanger will get hot and rise. As this water rises, cold water from the bottom of the water heater replaces it, and the upward flow through the external loop continues. This process of water heating and circulating is called "Thermal Siphoning ". Obviously you want to insulate the external loop or the loop will Thermal Siphon in reverse when the engine room cools off, cooling the water in the water heater.

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Old 01-25-2019, 11:39 PM   #19
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It's actually quite simple (in design) to convert a domestic water heater to a marine version with an external heat exchanger. You buy or build a pass through heat exchanger such as the transmission cooler on a Ford Lehman or other modest HP marine engine. ... This process of water heating and circulating is called "Thermal Siphoning ".

This is really excellent. Another approach is to buy a swimming pool heat exchanger. They are stainless of course. Mount vertically next to heater and perhaps enhance the efficiency with a small 12v/24v circulation pump. I priced them out before and were only like $300 to buy.


Hook this up to a 50gal Home Depot water heater and Bob's your Uncle!
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Old 01-26-2019, 03:43 AM   #20
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Bay Pelican went 20 years with a six gallon heater. We were anchored 90% of the time. Ran the generator or motor at least once a day. Had no hot water issues with the two of us living aboard including the occasional guest.

Our isotherm water heater had a blend feature that allowed the water in the heater to be very hot but then as hot water was called for there was a valve which blended the very hot hot water with cold water thus increasing the amount of hot water available for use.
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