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Old 02-28-2014, 02:03 AM   #1
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On demand hot water?

Is there any reason why a person would not want to use an on demand type hot water heater for their boat? I'd like to have some hot water, but I really don't need very much of it at all, hand washing, dishes... Pulled out the generator, electric stove, electric water heater. Probably put in a propane stove, water heater if necessary, but the on demand heater seems like a good idea to me. Feel free to tell me why I am wrong in wanting something that simple cause I don't know better, just yet.
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Old 02-28-2014, 02:26 AM   #2
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Is there any reason why a person would not want to use an on demand type hot water heater for their boat? I'd like to have some hot water, but I really don't need very much of it at all, hand washing, dishes... Pulled out the generator, electric stove, electric water heater. Probably put in a propane stove, water heater if necessary, but the on demand heater seems like a good idea to me. Feel free to tell me why I am wrong in wanting something that simple cause I don't know better, just yet.
On demand is nice for light usage- as long as you're dockside. Once you are away from land, you're SOL, as you have no genset, nor do you have the heat exchanger found in a marine water heater. Propane is an option, albeit a spendy one.

Any reason why you're shying away from the standard marine water heater?
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Old 02-28-2014, 02:40 AM   #3
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Don't want to get into "fried my engine cause of my water heater" type issues that I have been reading about. Hot water is just not that important on this boat. I just want some at times. I'll have a fairly decent amount of batter power on board, couple big D's and a few deep cycle. I don't see why I couldn't run an on demand system off battery power seeing as how the alternator should be charging the batteries as we go along anyway. I could be wrong, thats why I asked is all. On demand systems are pretty cheap as well. My parents run an entire house off battery power in baja, find it hard to believe I couldn't run a boat the same way...
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Old 02-28-2014, 06:48 AM   #4
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The small propane units do work, but have a problem when showering.

It tales observed water flow for the HW burner to come on , so when taking a sailor shower , wet down , stop the water, lather up, scrub , then turn on the water for a rinse.The burner will be off while scrubbing so the first shot of water will be cold. None of the propane units are marine grade , there are loads of thin parts to each.

An all conditions vent must also be installed

IF you have enough propane a shower/tub diverter can be installed and the heated water piped back into the FW tank rather than running down the drain.

The Std metal HW heater with an engine loop is fine and will seldom if ever do harm to the engine. And its nice not to have to light a device ,just to wash your hands at night.
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Old 02-28-2014, 07:02 AM   #5
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I'd like to have some hot water, but I really don't need very much of it at all, hand washing, dishes... Pulled out the generator, electric stove, electric water heater. Probably put in a propane stove, water heater if necessary...

Just put the kettle on...

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Old 02-28-2014, 07:05 AM   #6
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Don't want to get into "fried my engine cause of my water heater" type issues that I have been reading about.
I don't understand this? Energy-wise, there are very few free things in life, and hot water heated off a running engine is one of them. And I've never heard of an engine being damaged by an attached hot water heater.

Getting back to propane, FF covered a bunch of the idiosyncrasies of on demand water heaters. Another consideration is that a lot of standards have been developed over the years to protect against explosions from leaking propane. One of them, embodied in the ABYC standards, is that BOTH the intake air and the exhaust need to be connected exclusively to the outside of the boat, and away from other openings to the boat. That way of the burner malfunctions in some way (like it doesn't light), the spilled propane will dump outside the boat, not inside. An on demand heater doesn't meet this requirement, and would likely get flagged by an insurance surveyor. Consider this not only for the safety of your own boat, but for the safety of all the boats around your that are at risk if yours has a problem.
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Old 02-28-2014, 07:22 AM   #7
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I looked into this option and will be doing the on demand. Although I have lots of space top side for the exterior tank and bottles and 1k gallons of fw to run. I would think the need would dictate what direction you would go.
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Old 02-28-2014, 07:55 AM   #8
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I installed a Takagi TK3 tankless water heater in our home
4 years ago. We Love It. We are Never out of hot water.
Temperatures are easy to adjust. Intake air and exhaust travel
thru 4" stainless steel combo pipe. Easy to install, but it does
require both propane and 120 volt power. They are rather expensive
and when I'm ready to drop that extra cash, we will have one on
our trawler.
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Old 02-28-2014, 08:52 AM   #9
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You are in San Diego. Hang a Stearns Sunshower up for warming in the sun. They work. Years ago we had a couple for hosing the sand off the kids when they came back to the boat from the beach.

Amazon.com: Stearns SunShower 5.0 with 4-5 showers (Capacity- 5 gallons): Sports & Outdoors
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Old 02-28-2014, 09:51 AM   #10
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Hot water is just not that important on this boat. ...
That says it all. For some of us it is important. Therein lies the difference.
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Old 02-28-2014, 10:22 AM   #11
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Don't want to get into "fried my engine cause of my water heater" type issues that I have been reading about. Hot water is just not that important on this boat. I just want some at times. I'll have a fairly decent amount of batter power on board, couple big D's and a few deep cycle. I don't see why I couldn't run an on demand system off battery power seeing as how the alternator should be charging the batteries as we go along anyway. I could be wrong, thats why I asked is all. On demand systems are pretty cheap as well. My parents run an entire house off battery power in baja, find it hard to believe I couldn't run a boat the same way...
That water heater needs a lot of electrical power. You would need a bilge full of batteries and a humongous inverter to power it and the engine's alternator would take many hours to recharge the batteries. It's simply not practical on most boats unless you have a genset.

There is nothing wrong with heating water using engine coolant and it's "free".

Look around at other boats, especially production boats. These people have figured out what works and what doesn't. They have already considered what you are proposing and figured out that it's not the best solution.
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Old 02-28-2014, 10:24 AM   #12
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That says it all. For some of us it is important. Therein lies the difference.
Have it or don't - whatever you like. Just don't burn down the dock and the rest of our boats in the process......
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Old 02-28-2014, 10:27 AM   #13
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I don't understand this? Energy-wise, there are very few free things in life, and hot water heated off a running engine is one of them. And I've never heard of an engine being damaged by an attached hot water heater.
I'll second that. It is hardly a concern. There are many engine installations that use the "waste heat" from coolant to perform work elsewhere and none of them are a threat to the engine, except on boating forums.
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Old 02-28-2014, 11:13 AM   #14
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Had a propane one on our last boat and it's in a dumpster in Norfolk. Would never have another one. Problem was trying to control temperature because the burner is either on or off. Most miserable showers ever.

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Old 02-28-2014, 11:29 AM   #15
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Water heaters plumbed to engines can cause engine cooling problems. Rare, but I have seen it happen.

But just a few common sense precautions eliminate the risk. Just make sure circuit is bled and hoses warm with engine. And keep a close eye on coolant level until engine has gone through a couple thermal cycles. Critical when adding the WH or if coolant is drained for any reason.

If you pay mind to the above, it will not hurt your engine. Way better system than propane. That stuff is dangerous in a boat no matter how you do it.
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Old 02-28-2014, 11:59 AM   #16
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I don't see why small electric tankless heater would not work for sporadic needs if boat can generate 25A 120V AC or 3kW over required time of use. Here is the smallest, simplest, and cheapest example I could find in 1 google search: Stiebel Eltron 220816 Mini 3 Point-of-Use Tankless Electric Water Heater - 3.0 kW, 0.40 GPM
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Old 02-28-2014, 12:10 PM   #17
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Well this all good information thanks guys. The water heater that I just pulled to check out its condition was a 3 gallon capable of being plumbed to the engine coolant. The PO had hoses on the tank, but it looks like it was never plumbed to the engine. I'm gonna guess that he could not figure out where to tie it in at. I would also have to look around a bit to figure it out. The tank looks nice inside its rusted box! Of course I can't vouch for the internals of the tank, when I drained it lots of white chunks came out, looked like a crack head was living in my ER... I think I'll try and reuse this tank for now, if I can figure out how to plumb it to the engine.
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Old 02-28-2014, 01:21 PM   #18
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I can't speak to shipboard use, but I know something about it on the land-based side and some things are just down to the laws of physics.

Bottom line is that it takes quite a bit of energy to rapidly heat something, water included. To heat it rapidly enough to allow a shower...?

On land that often means up-sizing your gas manifold and piping quite a bit. Or, hooking into a fairly significant electrical circuit; at least 30-35amp and maybe more than 50amp, depending on how much flow you want.

FWIW, electric is more efficient at pretty much 100% or as close to it as physics lets you get. Gas is more like 80% or so, although since you'd be talking propane on a boat and not NG, you'd get more out of it. Plus of course, if you go electric on a boat you've got the generator to supply electricity as long as you have fuel.

I'm neither arguing for or against anything here...
As I said... I can't even begin to speak to what works on boats instead of land since I am still pretty much at the beginning of my learning curve.


I will say this however...
If you have room for it a solar hot water panel might be something to look at. I know that Heliatos makes them for marine use and I doubt they are the only one.

I grew up with solar hot water and it is amazing how well it works.
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Old 02-28-2014, 01:55 PM   #19
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Just put the kettle on... -Chris
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You are in San Diego. Hang a Stearns Sunshower up for warming in the sun. They work. Years ago we had a couple for hosing the sand off the kids when they came back to the boat from the beach. Amazon.com: Stearns SunShower 5.0 with 4-5 showers (Capacity- 5 gallons): Sports & Outdoors
Either of the above covers 100% of our hot water needs. KISS for low demand users.
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Old 02-28-2014, 02:49 PM   #20
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......... FWIW, electric is more efficient at pretty much 100% or as close to it as physics lets you get. Gas is more like 80% or so, although since you'd be talking propane on a boat and not NG, you'd get more out of it. Plus of course, if you go electric on a boat you've got the generator to supply electricity as long as you have fuel............
I'm not buying that for a minute. Short of actually burning dollar bills, running electricity through a resistor to make heat is the most expensive way to make heat. People heat their homes with gas, oil or heat pumps because they are all less costly ways of doing it. Electric resistance heat is the last resort. Nobody uses an electric water heater if gas is an option.

You may make the argument that 100% of the electricity used to heat water (or a building) goes into making heat, but how was that electricity produced? In most parts of the country it is produced by burning fuel to make heat to make steam to turn a generator to produce electricity. Then, it is delivered over hundreds of miles of wire with some loss to be converted back to heat.

Now think of the your last sentence - You advocate burning diesel or gasoline to produce heat to rotate a generator to produce electricity to run through a resistor to produce heat to heat water. Think about that for a minute.

Simplest and most efficient - put the flame under the water and be done with it. Only when that cannot be done should another solution be considered.
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