What size water heater?

[mvweebles]

It "hurts" if power generation is borderline.



A bigger than needed HWS ( we have 37 gallons) requires more power to keep it hot and, if you miss a heating cycle it takes even more power / longer time to get it back to temp



We very rarely used the Genset for our first 5 years out here, the 2.5kw of solar could do the HWS in the middle of the day and the batts would still be at 100% by day's end.



Now we have the big lifepo4 bank that can handle big load without voltage sag we run an electric kettle , an induction cooktop and a piemaker as extras.

Every 3rd day we make water, all off of battery.



Solar no longer keeps up and Genset is coming into play.

But if we get rid of the 10+ year old 37 gallon has that takes 1.5 hours to "top up"

And replace with 10 gallon or smaller that takes 20 minutes to top up I reckon we'll be back to excess solar production at days end

Simi - what if you reduced size of heating element on your water heater? I know it would take longer to top-up, but would it be more efficient?

Curious if you've done the math. You're pretty good at the electron mass-balance stuff.

Peter

 

[motion30]

I installed a 3 gallon whale water heater. It makes hot water in literally 5 minutes, perhaps 7 minutes in the dead of Winter

 

[Simi 60]

Simi - what if you reduced size of heating element on your water heater? I know it would take longer to top-up, but would it be more efficient?

Curious if you've done the math. You're pretty good at the electron mass-balance stuff.

Peter

Already has the 1.8kw vs 3.6kw standard element.
Guessing 3.6 would be done in half the time but the inverter may have something to say about it.


As the existing unit is likely nearing end of life it'll make sense to go smaller if not needed
Usage is washing up daily and yachties showers when needed, so not necessarily daily.
And as our future cruising ground will push us closer to the equator, hot water for showers becomes less of an issue - added bonus is solar production will increase as well.

 

[DegoRed]

This thread got me curious as I had no idea what size I had. Looked through my phone and found it, 30gal and it's 40 years old and still working flawlessly. Could see going smaller, but how small? Funny thing on my RV they have those on demand deals with maybe 1 gal tank, and it never runs out of hot water, so I'm not sure what's the answer.

 

[Tom.B]

This thread got me curious as I had no idea what size I had. Looked through my phone and found it, 30gal and it's 40 years old and still working flawlessly. Could see going smaller, but how small? Funny thing on my RV they have those on demand deals with maybe 1 gal tank, and it never runs out of hot water, so I'm not sure what's the answer.

And this is my point… Could I live easily with an 8g unit? I don’t know since I have had a 20g all these years, but maybe I am only using less than 25% of its capacity.

 

[Mischief Managed]

We have an 11 gallon water heater and routinely have guests aboard that use the guest shower. With 4 people showering and doing dishes etc., 11 gallons has been plenty. In think two people could easily live with 8 gallons. As others stated a tempering valve is a smart addition, especially for safety if you heat the water with engine coolant. I just added one to my boat over the Winter.



While I had the plumbing apart to add the tempering valve, I added extra long silicone flex lines between the boat plumbing and the water heater, so that changing to a different brand and style water heater will be very easy when ours fails, and I added valves for by-passing and draining the water heater to make Winterization very easy.

 

[O C Diver]

In reading through many of the responses, there is a pont that seems to be missed. A smaller water heater will work well if you're plugged into shore power or running the generator. It's very different if you normally want to heat water from an engine loop while cruising and need to have water capacity at anchor without being dependent on the generator making more. For a boat that runs the generator much of the time, I'm sure a 5 gallon water heater would be sufficient.

Ted

 

[psneeld]

This is like a battery capacity and electrical demands question asking how big a battery bank should I have?

How big a water heater? Depends on demands like people cleaning, washing dishes, washing clothes, any at the same time or consecutively, durations, how often and how reheated, how hot, etc...etc....

Like the battery question.... people make a list of averages and factor in time of use and when the demands are placed and come up with an estimate of how much hot water is needed at any given time.

It's easy for anyone to sit there and say we take showers back to back and never run out...but even that has some pretty big variables in just those 2 showers.

Or instead of complicating the question, many people have lived with a 6 gallon for years. So that's really good enough. When they replaced some replaced with the same and are happy and some upgraded and were happier.

So like many answers to many questions here.... don't go smaller unless willing to get detailed and calculate carefully or wind up possibly changing the way you use hot water.

 

[Tom.B]

In reading through many of the responses, there is a pont that seems to be missed. A smaller water heater will work well if you're plugged into shore power or running the generator. It's very different if you normally want to heat water from an engine loop while cruising and need to have water capacity at anchor without being dependent on the generator making more. For a boat that runs the generator much of the time, I'm sure a 5 gallon water heater would be sufficient.



Ted

This is the case. We don’t have a way to easily use the engine to heat the water on this boat. So we will rely totally on shore, genset, or maybe even inverter (which honestly isn’t a high priority, but with a 900Ah bank and 3000W inverter we could possibly do… however we are not currently wired to do so) to heat the water.

My main concern, and why I started the thread, was to get an idea of capacity and to see if 8g will be enough for two people. It sounds like it will. With the added benefit of creating some much needed space outboard of the starboard motor and maybe decreasing some of the starboard list.

 

[OldDan1943]

You want a brief answer?
8 gal should be enough
How are you going recharge the HW? Generator? Solar panels to charge the batteries?
What is the problem with heating water with the main engine? Chances are there is small heat exchanger on the main engine or heater exchanger eternal to the HW tank. Research the engine brand and and diagram of the heating loop. When you discover the loop, remember to put isolation valves between the main engine and the hot water tank.

 

[kthoennes]

A couple years ago we replaced our original-to-the-boat 11 gallon water heater with one of these:

https://www.partsvu.com/albin-pump-marine-premium-water-heater-8g-120v.html

It's worked out very well. We're two adults and two kids, 12 and 14 and it's plenty for us. We're not fanatically cautious with water but 8 gal has always been enough. I'm also amazed at how long the water stays warm. If we run the engines and go for a run on Saturday afternoon, the water is still warm (although not hot) on Sunday. Our old water heater was very tired and had developed a bad stinky smell and even after I changed the anode I was always cleaning crud and rust bits out of the faucet aerators. The old one was a Seaward 11 gallon in a rectangular housing. I was worried the gallon size reduction would be a problem but our heater is located in a kind of odd, very tight horizontal space under the salon dining table and I wanted to free up some space so I could move around down there. The new one is round but much more easily fits the space. Of course in our current location we only use the boat during warmer months, so there's no desire to take long hot showers in very cold weather for example. But to my surprise, 8 gallons has been plenty for us.

 

[Tom.B]

You want a brief answer?
8 gal should be enough
How are you going recharge the HW? Generator? Solar panels to charge the batteries?
What is the problem with heating water with the main engine? Chances are there is small heat exchanger on the main engine or heater exchanger eternal to the HW tank. Research the engine brand and and diagram of the heating loop. When you discover the loop, remember to put isolation valves between the main engine and the hot water tank.

The genset would probably be the primary way to reheat the water. It isn't that I CAN'T hook it to the starboard engine... I just don't know if I WANT to. Setting aside that it adds several more points of failure. Access to the front of that engine where the fresh water pump is located is pretty limited. Besides, I don't see it gaining much more than having hot water for a few hours after we get to an anchorage (side note: we don't anchor out THAT much, and when we do it isn't so long that we would need to take more than one shower). Our two big Volvos don't get very hot at idle, so starting the engines to heat water wouldn't do much when we could just start the generator, use the AC power it provides, and use a lot less fuel to heat the water more thoroughly. Otherwise, we'd probably be on shore power.

 

[dannc]

Has the OP measured how much time one is spending in the shower and estimating how much water is being used?

In our house, we have low flow shower heads/wands that use, worst case, a bit less than 1 GPM. I know because I have measured the water usage. On the more restrictive streams the shower head uses far less water but I have not measured it. Figured knowing the worst case number was good enough.

IF someone took a 10 minute shower, WITH the most amount of water setting on the shower head that is only 10 gallons. Most likely, the 10 minute shower would be 5 gallons, or less, if one switches the spray pattern around during the shower. If the 10 minute shower was a 5 minute shower, even less water is being used.

Then the other question is how much hot water is actually used? 25-33% would be my guess depending on the temperature of the cold and hot water. So if my 10 minute shower uses 10 gallons, then roughly 6-7 gallons were cold water and 3-4 gallons were hot. If the shower was 5 minutes, then we would be looking at 5 gallons total with around 3 gallons of cold water and 1-2 gallons of hot water. IF one used the shower head setting that used the most water.

Some of the boats we have been on have used a shocking amount of water, hot or cold, that could have easily been minimized with a better shower head/wand.

I would go measure how much water is used by the current shower head, and if more than 1 GPM, replace it Then measure how much time one spends in the shower to figure out how much water is being used, hot and cold. Then decide on the capacity of the hot water tank.

Later,
Dan

 

[psneeld]

The second shower uses more hot water because the temp of the heater is now down... depending on season and where the tank is, that drop can be significant, then so would be the second shower usage if back to back with the first.

 

[Tom.B]

Has the OP measured how much time one is spending in the shower and estimating how much water is being used?

No... I was told there would be no maths

 

[OldDan1943]

Math??
Go read the plate on the FW pump. It will tell you the capacity or gph.
Anything beyond that is an estimate.

 

[dannc]

Math??
Go read the plate on the FW pump. It will tell you the capacity or gph.
Anything beyond that is an estimate.

The numbers on the FW pump might not tell how much GPM is flowing out of the shower head. A clock, a bucket and a few minutes, is all it takes to figure out how much GPM is flowing from the shower.

Later,
Dan

 

[dannc]

No... I was told there would be no maths

Not much math if one uses a clock and a bucket.

Later,
Dan

 

[OldDan1943]

I used a 5 gallon bucket and a watch.

 

[sunchaser]

Tom
If it were my decision (been there done that) I'd get the biggest 1500 Watt heater that will easily fit. But not an aluminum tank like a Seaward. Go for glass lined like a Torrid.

A "big" heater will help load the genset up, always a good thing. A "big" tank will allow you can to forget about buckets and stop watches. Just jump in the shower and wash up, our 17 gallon unit can handle 4 in quick succession. With our off engine heat exchanger we're never out of hot water while cruising and genset off.

 

[Tom.B]

Not much math if one uses a clock and a bucket.

Later,
Dan

LoL… yea… there is that. Thanks. [emoji16]

 

[Tom.B]

Tom

If it were my decision (been there done that) I'd get the biggest 1500 Watt heater that will easily fit. But not an aluminum tank like a Seaward. Go for glass lined like a Torrid.



A "big" heater will help load the genset up, always a good thing. A "big" tank will allow you can to forget about buckets and stop watches. Just jump in the shower and wash up, our 17 gallon unit can handle 4 in quick succession. With our off engine heat exchanger we're never out of hot water while cruising and genset off.

Well, a lot of this is about gaining space and losing starboard weight. I will look at the Torrids though. Thanks.

 

[clburti]

We had a six-gallon tank on our sailboat and we used it as you described for 18 years....worked fine for the two of us and adequately for an occasional guest couple.

 

[DC36Monk]

“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.“

Why would anyone shower two days in a row when there on the boat? Maybe two weeks in a row, but……

 

[Simi 60]

Tom


A "big" heater will help load the genset up, always a good thing. :

And for us, a small heater can be done in 20 minutes using solar panels and the rest of that lovely "free" electrickery can fill the batteries.
No Genset required.

The big water heater we have now can still be done by the sun, but it takes 1.5 hours.
Less time putting power into the batteries
The downside to the new big LFP bank is we now use more electrick things, so we now have to use the Genset an hour or more a day playing catchup, something we never needed before.

 

[rslifkin]

“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.“

Why would anyone shower two days in a row when there on the boat? Maybe two weeks in a row, but……

Getting the sunscreen off before going to bed makes sleeping on a warm night much more pleasant.

 

[Tom.B]

Why would anyone shower two days in a row when there on the boat? Maybe two weeks in a row, but……

This is true!

 

[RonR]

We are looking into replacing our 20 gallon tank. Currently its a standard home unit and the outside is rusting. The PO said he put it in 10 years ago. It had a fancy marine unit that would take heat off of a main or the generator. His thought was " no way I will run a detriot to make hot water, and if I am running the generator to heat water I might as well just use the power". We have a 15k gen set, most often we are hooked to shore power. If we are on the hook for a weekend we run the gen set for about 1 hour while we cook on the range, and thats long enough to make hot water.
I am thinking about replacing out rusting unit with one of these.

https://www.homedepot.com/p/Westing...tainless-Steel-Tank-WER020A1X020N10/305554770

We do like a hot shower in the PNW winters..

Thoughts?

 

[rslifkin]

A shore power trip in a thunderstorm the other day provided a good first test of heat retention for my new water heater. 11 gallon Isotemp Spa heater, factory thermostat setting is 167*. All of the connections on the unit and all plumbing that gets noticeably warm to the touch near it is insulated, but I haven't added any insulation to the heater itself.

Power was off for just over 19 hours (no hot water used during that time as nobody was on the boat). When I got to the boat to sort out the power issue, hot water out of the tap was still coming out at the full mixing valve setting of 120* or just under. Once I got power back on, it took 40 minutes for the tank to get back to full temperature and cycle off. A little math says that means the tank was still 135* or a little warmer. Losing about 30 degrees in 19 hours with the engine room temp hovering around 60* is very good, I'd say.

Definitely good enough to get things hot, take a couple of quick showers in the evening and still have at least some hot water for dishes, etc. the next day before things get another heating cycle.

 

[Tom.B]

A shore power trip in a thunderstorm the other day provided a good first test of heat retention for my new water heater. 11 gallon Isotemp Spa heater, factory thermostat setting is 167*. All of the connections on the unit and all plumbing that gets noticeably warm to the touch near it is insulated, but I haven't added any insulation to the heater itself.

Power was off for just over 19 hours (no hot water used during that time as nobody was on the boat). When I got to the boat to sort out the power issue, hot water out of the tap was still coming out at the full mixing valve setting of 120* or just under. Once I got power back on, it took 40 minutes for the tank to get back to full temperature and cycle off. A little math says that means the tank was still 135* or a little warmer. Losing about 30 degrees in 19 hours with the engine room temp hovering around 60* is very good, I'd say.

Definitely good enough to get things hot, take a couple of quick showers in the evening and still have at least some hot water for dishes, etc. the next day before things get another heating cycle.

Thanks for this!