On demand hot water

It looks like we'll need to take out a rusty electric hot water tank fairly soon. Rather than installing a new replacement unit I'm considering a propane on demand type of setup. My only experience with these was on one of my dad's boats which was fitted with a somewhat tempermental Paloma heater.
Hopefully technology has progressed in the passed thirty years and I wonder what the collective wisdom here would suggest? We're a cruising family of four, three of the family are girls which may go a long way to explain just how important hot water is to us...them.

some people swear by them...but they do seem consume a lot of propane if you plan on extensive gunkholing.

We were just cruising along with a couple (gunkholing) who had one of those. They swore by the unit. They also had propane fridge and cooking, and had 12 20 lb tanks aboard so the fuel issue never was a problem.

jleonard said:

We were just cruising along with a couple (gunkholing) who had one of those. They swore by the unit. They also had propane fridge and cooking, and had 12 20 lb tanks aboard so the fuel issue never was a problem.

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Sounds waaaayyyy too similar to an LNG Tanker...

Just kidding ...but that's a lot of bottles to find storage for on our 40 somethings!

Propane is wonderful as a heat source.

Not much difference in consumption weather its an RV flame unit keeping 10 gal of water hot , or the Bosch or Paloma demand unit.With high HW needs.

Tho the RV units can have a 120 element , to work free at dockside, or when 120 is handy.

The demand units DO have a problem if used for a sailors (little water) shower .

Flow causes the burner to come on or turn off, there is no storage, there is a minimum flow to operate. .

So when showering you wet down , stop the water flow to lather and scrub ,for your rinse cycle.there will be a delay , a douche of cold water , as the flow resumes and the burner turns back on.

Solution is OK ,blow the water , but there is also my way.

Install a home bathtub/ shower selector .

When not in the shower position the water is directed to the bathtub fitting , which simply is piped to dump the heated water back in the FW tank.

With a bit of practice only unheated water is diverted , so little propane is used for nothing .

psneeld said:

Sounds waaaayyyy too similar to an LNG Tanker...

Just kidding ...but that's a lot of bottles to find storage for on our 40 somethings!

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That was on a 37 ft boat! All stored outside...and yes the boat was a mess but they were happy. I guess that's the point.

Here are some on-demand units.

Search - tankless water heaters - Camping World

Someone is bound to come along and mention these are RV units and not marine units. To that I'll capitulate to check your insurance policy and make sure there are no exclusions. If you install them safely there should be no ill effects.

A word of caution is to size these units appropriately. Go small and they will not keep up, oversize by too much and they may not activate. 3 ladies on the crew makes hot water more important than a radar or chart plotter.

a few of the on demand units declare that they may be used on boats.

Several, including Paloma say specifically - do NOT install on boats. If that statement is printed on the faceplate of the unit, most surveyors will comment, which will raise a flag for the insurance co.

The newest units utilize a number of safety features which include: no standing pilot, CO detection, sensors for flame out and water flow which instantly cut the gas feed.


Be sure to vent the flue directly outside, also that combustion air is generous - to mitigate the CO problem - i.e. dont mount the unit in a closet, or in a tightly closed shower-stall.


It is interesting how small a unit will be sufficient - unless you shower and wash dishes simultaneously, a 6 liter unit will probably work well.


We manage to heat water, cook, and run a LP fridge all summer on 3ea - 5 gallon propane bottles with plenty of leftover.

"Install a home bathtub/ shower selector .

When not in the shower position the water is directed to the bathtub fitting , which simply is piped to dump the heated water back in the FW tank."
...That is bordering on genius!
Thanks all. - Boyd

In many juristictions it is illegal to install gas hot water heaters on boats; some insurance companies won't allow it even if it is technically permitted. Whatever the status where you boat, there are enough concerns to warrant careful consideration before going down the propane HW path.

ITR's WaterMaker is a diesel instantanous HW solution. I don't have it myself (though it is on The List !) so cannot vouch for how well it works, though ITR has a good name. See it here: The Water Heater by ITR | International Thermal Research

Here's a link with more info than the one above;

ITR diesel hot water heater

I would not be so concerned with insurance companies or local laws concerning safety issues, I would be concerned with my actual safety and the safety of my family.

If one can find a propane powered water heater, refrigerator, etc. that is designed and approved for use on a boat, and one installs (or has it installed) it according to the manufacturer's instructions and ABYC and USCG (for the USA) standards, and operates and tests according to the instructions and standards, that's fine.

A camper is not the same application as a boat. A camper is open on the bottom and any leaking propane blows away. A boat has a hull that collects and concentrates the leaking gas.

Couldn't agree with you more.
Our last sailboat was powered by an Atomic 4. Great engine but being gasoline fed, we were very safety conscious. That being said, we already run propane to out stove. First thing I did when I bought our current boat was to install new lines and regulator to replace what was already there.

110 volt instant hot water heaters would seem to be an alternative. Do any forum members have experience with these?

BTW: In my grandparents' day, gas-powered water heaters were called "geysers". Gramps' house had a huge one next to the bath that started up with a whoomph! - frightened me then and still worries me now.

Shoalwaters said:

110 volt instant hot water heaters would seem to be an alternative. Do any forum members have experience with these?.

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No experience, but of course, you're going to need electrical power to operate it. Most electric water heaters are 240 volt because of the power needed to heat water quickly.

Standard (not "on demand") water heaters designed for boats usually have a heat exchanger to use engine coolant to heat the domestic hot water when the engine is running. Mine does and it works quite well.

rwidman said:

No experience, but of course, you're going to need electrical power to operate it. Most electric water heaters are 240 volt because of the power needed to heat water quickly.

Standard (not "on demand") water heaters designed for boats usually have a heat exchanger to use engine coolant to heat the domestic hot water when the engine is running. Mine does and it works quite well.

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Not only 240 volt but high amperage draw as well. They are also severely restricted on flow rate. Most are less than 1/2 gallon per minute. I do have experience with these.