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Old 04-16-2022, 11:55 AM   #1
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What size water heater?

Hello team!

I am considering downsizing our water heater. Currently we have a 20g Raritan that is 22 years old. It’s just a matter of time before it fails. Anyway, it lives on the outboard side of the starboard motor and makes for some cramped quarters doing regular maintenance on things like the impeller and heat exchanger (just to name a couple). It is just the two of us… we never really have guests onboard and don’t have kids or grandkids. We rarely shower within and hour or two (or day or two) of each other, so it has a long time to recover. Even then, we are in the habit of not letting the water run the whole time anyway. So, would an 8 gallon heater be enough? It sure would open up a ton of space and at only 750w, we could conceivably run it off the inverter for a short time if we needed to.

What say ye’?

Thanks!
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Old 04-16-2022, 12:37 PM   #2
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Here is the one on my back burner for an upgrade.
https://www.pysystems.ca/resources/v...for-your-boat/
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Old 04-16-2022, 01:00 PM   #3
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I just swapped out my 12 gallon Raritan for an 11 gallon Isotemp. The Raritan was big enough and so far the Isotemp has been great. It's definitely better insulated and the mixing valve on the output is nice. I did upgrade to the optional 1200w heating element (Raritan was 1250w) for faster heating on generator power compared to the standard 750w.
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Old 04-16-2022, 01:19 PM   #4
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If you install a mixing valve to control temperature to 120 F a smaller tank temp can be raised to a higher temperature effectively increasing the supply of hot water. This helps in down sizing the tank.
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Old 04-16-2022, 01:21 PM   #5
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Size is one thing and water temp is another. Since you rarely have guest and never children you could consider a 6 gallon heater that is set to 180 degrees. There are some dangers such as never flush when some one is in the shower. You might find you have the same amount of usable hot water as a 20 gallon set at 120 degrees.

I have a 30 gallon set at 160 degrees. I use a mixing valve after the hot water tank to bring it down to 130 degrees. You can shower for an hour and not run out of hot water.
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Old 04-16-2022, 01:58 PM   #6
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A 10 or 11 gallon will work fine for 2 people. Especially if it has a mixing valve and you set the temp up high in the water heater. I don’t think I would go with a 6 gallon.
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Old 04-16-2022, 02:48 PM   #7
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As long as YOU shower first, size doesn't matter.

I second the tempering valve. Installed one on my Raritan and have been very happy with it.

Regarding size, it depends on where you boat. Many boats have their water tanks close to the hull. The water in the tanks comes close to the outside water temperature. That's fine in Florida. In colder climates you tend to use more hot water to temper the cold. Also, after showering does that mean there's no warm / tepid water for the better half to wash her face with in the morning?

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Old 04-16-2022, 02:53 PM   #8
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Full time cruiser friends on two other vessels have 25 litre - 6.6 gal dux 240v household units and reckon they are sufficient and, only take 20 minutes to get to temp..

Will be getting one ourselves to replace the 140 litre - 36 gal Rheem we currently have that sucks around 80amps @ 24v for an hour and a half.
Massive waste of power considering we only use hot water for washing up and hardly any in the shower.

$400 at big hardware stores
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Old 04-16-2022, 03:10 PM   #9
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I replaced my old original iron 5 gal. water heater a couple of years ago with a 6 gallon Isotemp. Starting with a Florida tank temperature of 79-80 degrees, after a 20 minute 110V heat, the system is able to deliver full blast, good pressure, four minutes of 1.5 GPM at 95 degrees, (a warm shower) before cooling back down to tank temperature. This is with no mixing with cold water at the faucet, hot side only. For our use, this is an easy, relaxing shower for each of us.

With a 40 minute heat, the system is able to provide 5 minutes and 30 seconds of hot shower water beginning at 108 degrees and still at 100 degrees at the 5:30 mark, this using about 3/4 hot to 1/4 cold at the mixer.

After a 1 hour heat, the system will deliver 8 minutes of hot shower water at 1/2 of hot & cold mixing at the faucet. This provides my wife and I with a hot shower each after the heat, and even though we are unplugged, also a warm shower for each of us the next morning.
Temperature of the Isotemp water heater is set at 160 degrees.

All the above is with plug-in only heating…no engine hookup.
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Old 04-16-2022, 04:39 PM   #10
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When folks talk a mixing valve, I am not sure what they are talking about.
I have 11 gal Seaward Model F1200 hot water heater. Also I have the engine heat exchanger piped to water heater tank.
I have ‘supported’ 4 people and still had hot water left.
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Old 04-16-2022, 04:45 PM   #11
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What size water heater?

I’m not sure I fully understand the “mixing” system or “mixing valve” thing. Can someone explain it, please?

-edit-
Nevermind… it was right there in the product description. Doh!

All Isotemp water heaters come standard with a mixing valve
Every Isotemp tank features a safety mixing valve that regulates the temperature of the water coming out of the tank so that water heated to 160+ degrees will not scald boat owners or their guests
Temperature of the hot water coming out of the tank can be adjusted to the preference of the owner by turning the mixing valve on the water heater
This feature allows the thermostat to be set at a higher temperature, while keeping the user safe from scalding
It also extends the capacity of onboard hot water by using only a fraction of the water in the tank to produce the desired water temperature
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Old 04-16-2022, 04:54 PM   #12
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Quote:
Originally Posted by Tom.B View Post
I’m not sure I fully understand the “mixing” system or “mixing valve” thing. Can someone explain it, please?
Safety device to prevent scalding

https://www.sydneyplumbinghotwater.c...mpering-valve/
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Old 04-16-2022, 05:05 PM   #13
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Isotemp 8 gallon, has a tempering valve and was perfect for my 40 footer, with 2 liveaboards. Never ran out of hot water, heated fast even with 750W and didnt tax electrics, stayex hot for a day, warm for 48 hrs depending if no electric.
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Old 04-16-2022, 05:27 PM   #14
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A mixing valve takes some cold water and mixes it with the hot water to bring it down to a safe temperature, not scalding. It will also extend the useful amount of hot water since it adds some cold to it. You can do the same thing at the faucet but you have the possibility of scalding someone.
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Old 04-16-2022, 07:28 PM   #15
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Quote:
Originally Posted by Tom.B View Post
Hello team!

I am considering downsizing our water heater. Currently we have a 20g Raritan that is 22 years old. It’s just a matter of time before it fails.

What say ye’?

Thanks!
How do you measure the amount of time left in its life? Like people, every item on your boat is going to die, you just don't know when. Does your water heater have cancer? Is there some family history of fatal illness?
I will replace mine when it fails. It is a Torrid. I think 18 gal, but I don't know for sure. It sits in a handy place, simple to get at and replace when the time comes. Hopefully it is not on the slippery slope heading towards failure, as it is not displaying any symptoms of any fatal illness.
I have a daughter its age, at 42.
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Old 04-16-2022, 07:38 PM   #16
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How do you measure the amount of time left in its life? Like people, every item on your boat is going to die, you just don't know when. Does your water heater have cancer? Is there some family history of fatal illness?
I will replace mine when it fails. It is a Torrid. I think 18 gal, but I don't know for sure. It sits in a handy place, simple to get at and replace when the time comes. Hopefully it is not on the slippery slope heading towards failure, as it is not displaying any symptoms of any fatal illness.
I have a daughter its age, at 42.
Depends on how well you can assess condition. The Raritan heaters are a lined steel tank. So if it's showing significant rust under the insulation, it may be near the end. The one I replaced was 17 years old and still working, but it was rusty and I wanted a better insulated unit. When I dumped the last bit of water out, I got some rusty sludge. So I'm thinking the interior lining was failing and it was probably near end of life.
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Old 04-16-2022, 08:58 PM   #17
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Depends on how well you can assess condition. The Raritan heaters are a lined steel tank. So if it's showing significant rust under the insulation, it may be near the end. The one I replaced was 17 years old and still working, but it was rusty and I wanted a better unit. When I dumped the last bit of water out, I got some rusty sludge. So I'm thinking the interior lining was failing and it was probably near end of life.


This… and I have done zero routine maintenance to it. Yes, that’s on me, but waiting only means that it will fail at some inopportune time AND won’t add any additional room for engine maintenance until I do. I may flush it out and see what happens. That’s free, but I still think it should be done so it doesn’t fail at the worst time.
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Old 04-16-2022, 10:38 PM   #18
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It depends on what you expect from a shower. I removed the 11 gallon Raritan and replaced it with a 20L Isotemp (that's 5.3 gallons in old money) and the two of us can take a shower without waiting between. We don't waste water, but don't leave the shower dirty or soapy either. It does have a mixing valve (more properly called a tempering valve).
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Old 04-16-2022, 11:08 PM   #19
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I've had good luck getting two "boat showers" out of a 6 gallon tank. First shower can have very hot water and the second gets comfortable shower water but not super hot. I would think an 8 or 9 gallon tank should match your needs.
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Old 04-17-2022, 01:18 AM   #20
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Our Seaward heater with six gallon tank easily provides enough for one person to shower with. The allotment can be stretched out and rationed in various fairly obvious ways. It also helps if your shower space is comfortable and warm, rather than below freezing, etc.

Of course there’s the paradox where -if the space is uncomfortable and freezing cold there’s the urge to GTFO sooner, balanced by the urge to stay under the hot water as long as possible. Compared to -having it warm and comfy where you aren’t in such a hurry so tend to linger, but also have the luxury of not getting frostbite if you ration by turning off the water for a minute while soaping up.

Not sure what the mathematical equation is for all that.

Anyway, never once has our second in line needed hot water sooner than the time it took for them to find the round tuit.
It’s always been surprisingly adequate. Including for dishes etc. and when it isn’t, just wait a little while. We are pretty thrifty with the hot water, but then I assume most boaters are.
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