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Old 05-30-2014, 08:39 AM   #1
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What size hot water heater?

I'm replacing my hot water tank after 12 years. This process got me thinking about size we really need.

Without saying what we have, what size to you think works in gallons for 2 people with a washer/dryer for normal full-time cruising.
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Old 05-30-2014, 08:49 AM   #2
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I can only say that an 11.5 gallon Atwood has served my wife and I better than a 6 gallon unit. We have never run out of hot water since the Atwood was installed. Now on my new to me Gulfstar trawler where I am installing a Splendid washer/ sort of dryer I'm not sure if the 10.5 gallon water heater currently installed will suffice? I'll let you know but I'm at least a month away from being able to cruise or use this boat.
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Old 05-30-2014, 09:08 AM   #3
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We have a 12 gallon Raritan, with heat exchanger, (really 10 gallon capacity) with a washing machine, that we use, and never feel like we need to upsize. The recovery rate is pretty quick at 13 gph. Since there are only the 2 of us, with only the occasional guest, I adjusted the thermostat up which helps extend the capacity.
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Old 05-30-2014, 09:20 AM   #4
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Bay Pelican has had a six gallon unit for the last 16 years. We are now replacing it with another six gallon unit. Never had a problem. Have a Splendide washer/dryer and take showers on board. If you move a lot and have the space an eleven gallon unit might be useful as the engine will heat up the eleven gallons which will then last. If you are heating by the generator or shore power, then the 11 gallon unit just takes up space. Shore power would keep a tankless unit hot as it is constant. Using a generator the 11 gallon unit will take approximately twice the time to heat the water as the six gallon unit.
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Old 05-30-2014, 09:24 AM   #5
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In both my boats the water heater has a heat exchanger hooked up to one of the propulsion engines. I have often thought it might be a better idea to hook the heat exchanger to the generator. I'm not sure what the pro's and con's are of doing this?
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Old 05-30-2014, 09:51 AM   #6
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...I have often thought it might be a better idea to hook the heat exchanger to the generator. I'm not sure what the pro's and con's are of doing this?...Bill
I wouldn't suggest using your generator. By doing so, you would have added 11.5 gallons to the fresh water side of the generator and you will be taking heat away from it. Your warm up time on the genset will increase significantly.
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Old 05-30-2014, 09:54 AM   #7
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The real question is how much do you need at a time?

Numerous articles both marine and RV suggest 2 theories...

One where you have plenty for 2 showers with 12 gallons (but...if you take long, hot showers, 12 may not last long enough for both and recovery time is longer than you can wait for the second shower)...

and the other is 6 gallons where the first to shower will get a feel for when the water is getting cooler and finishes up quicker....then the timed recovery for the second shower is easier to figure out.

I'm not sure which size is better...but as long as you don't take scalding, long showers....6 gallons seem to do many boaters well enough and the recovery time is often less than 15 minutes (some manufacturers post their recovery times in the manuals).

Go to 220V/dual element heaters and all bets off.
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Old 05-30-2014, 10:10 AM   #8
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I wouldn't suggest using your generator. By doing so, you would have added 11.5 gallons to the fresh water side of the generator and you will be taking heat away from it. Your warm up time on the genset will increase significantly.
I have thought about what your saying but the generator has a thermostat so in theory warm up time shouldn't matter. I'm certainly not saying I am correct.
Now recovery time for the generators freshwater side might initially take longer? One area I know where my limitations are is mechanical so I'm just asking for the pro's and con's if I were to use the generator to help supplement the water heaters heat exchanger. I'm not sure that I have added 11.5 gallons of water to the fresh water side all I have added is a heat exchanger that might hold a gallon?
Thanks for your input.
Bill
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Old 05-30-2014, 10:43 AM   #9
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By using the generator plumbed through the water heater's heat exchanger you are taking heat from the generator to raise the temperature of the 11.5 gallons of water in your Attwood. The cooling system capacity on a Northern Lights 6 Kw, 3 cylinder generator is ~0.5 gallon and you are going to exchange the heat at the water heater via that 0.5 gallons?
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Old 05-30-2014, 10:46 AM   #10
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A couple points: 1. Usually the coolant for the water heater is taken off the engine on the bypass loop, the coolant that is circulating even when the tstat is closed. So a water heater on a gen would cause the engine to be slow to heat up.

2. Gens are usually pretty small and don't make much heat, so the time it takes to warm the water might be pretty long. Depends on tank and gen size, and load on gen.

3. The point of using the main engine is to avoid the need to run the gen. If you are going to run the gen anyway, the incremental cost of adding elec WH load is quite small.

4. many gens (especialy small) do not have convenient taps for coolant heat extraction.
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Old 05-30-2014, 11:39 AM   #11
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The PO of our boat installed a 30 gallon water heater, so we're rather spoiled in that regard. We've had 6 folks onboard (4 adults, 2 teens) for weeklong cruises, and never has a problem with hot water. This includes showers, dishes, and use of the washer/dryer.

When it goes out it'll be replaced with a tank of similar size.
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Old 05-30-2014, 11:49 AM   #12
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AC I would go with the biggest HW you can install. Plumb for heat exchange from one of your engines and then of course electric. The reason I say this is that it is nice to be able for you and the admiral to take back to back showers without running out of hot water.
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Old 05-30-2014, 11:54 AM   #13
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I would agree with biggest you can install...above 25 gallons or so...below that and unless someone has some published numbers...the consensus I've read is really think recovery time...a small water heater has the water warming up much faster...

Taken to the extreme would be one of those small couple gallon point of use ones...they can almost stream warm water...just a suggestion that lends credit to thinking smaller if already limited by size/type....

Total hot water volume vs recovery time....make one think and roll the dice which works better for them.
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Old 05-30-2014, 11:57 AM   #14
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Good point!
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Old 05-30-2014, 01:55 PM   #15
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We use a 20g capacity WH. We are four (sometime five) on weekend cruises and I had no issue. When anchored, we run the generator about 10hrs over a 24hrs period.
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Old 05-30-2014, 07:22 PM   #16
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20 gallon unit on my boat. Plan to replace it with the same size while doing the motor swap.

Size may also matter depending on how you use your boat. If you're traveling most days, the larger unit may allow you to avoid the morning genset run as there still is plenty of hot water left from the previous night. The larger unit may also allow you to make all the hot water required and then shut it off if you are generator or shore power limited. Other than space and money savings, if you are mostly making how water off waste engine heat, hard to see significant downside to more capacity.

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Old 05-30-2014, 10:12 PM   #17
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Quote:
Originally Posted by O C Diver View Post
20 gallon unit on my boat. Plan to replace it with the same size while doing the motor swap.

Size may also matter depending on how you use your boat. If you're traveling most days, the larger unit may allow you to avoid the morning genset run as there still is plenty of hot water left from the previous night. The larger unit may also allow you to make all the hot water required and then shut it off if you are generator or shore power limited. Other than space and money savings, if you are mostly making hot water off waste engine heat, hard to see significant downside to more capacity.

Ted
Absolutely right. I replaced our old ~ 6gal cylinder with a 42 litre Isotemp (=~ 10gal), unit some time ago, and we don't have the capacity to heat electrically out on the water as we don't have AC except at the dock, and then only to batt charger and the water heater, which we give a quick pre-heat before we depart, while loading up. We never run out of hot water doing this, as engine running takes care of all the hot water needs. So I also vote for the largest that will comfortably fit. Then AC top-up heat if you have a genny is at least optional. We like peace and quiet, and have 12v, charged by solar and AirBreeze only on the pick.
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Old 05-30-2014, 11:22 PM   #18
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Thanks for all the input and experiences. I just pulled out our 20 gallon heater and am considering an 11 gallon one. It's more about the space - it's crazy difficult to use the existing space for another 20 gallon duplicate - it requires disassembling much of the water system and pressure tank in the engine room. A smaller unit wouldn't require so much effort. I can't do much to change the location either.

I'm still not sure which way to go based on this thread. I'm going to have to give it some more time to figure it out.
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Old 05-31-2014, 01:03 AM   #19
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The largest that will fit. No such thing as too much hot water capacity. It's one of those little luxuries that will make for smiles and happiness.
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Old 05-31-2014, 01:48 AM   #20
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AC, based on our experience with two of us, 10 gal (42L), would probably be all right, because you could do a heat top-up at anchor via the genny, I'm sure you have. Whereas we don't have the option of heat via AC at anchor. So as long as you keep showers fairly short, you should be ok.
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