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.

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.
Bill

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.

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.

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?
Bill

Billylll said:

...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

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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.

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.

Larry M said:

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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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

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?

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.

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.

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.

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.

Good point!

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.

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.

Ted

O C Diver said:

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

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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.

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.

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.

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.

Like Pau Hana, we have a 30 gal. WE never have an issue with hot water. Well, maybe one issue. You guys with 6-12 gal ones must not have a teenage daughter with long hair. She can run out the 50 gal one in our house!

ActiveCaptain said:

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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You cut your available hot water by 50% and it will be missed. Seems the effort required for install vs. less hot water for the life of the unit, I'd go through the install. It there something I'm missing as it seems to me that if installing requires all the dismantling then so would removing the older one? Is there a problem matching the dimensions of the old one?

11 gal will do fine with showers and dish washing .

You might miss the 20 if you do constant loads of laundry .

THD said:

Like Pau Hana, we have a 30 gal. WE never have an issue with hot water. Well, maybe one issue. You guys with 6-12 gal ones must not have a teenage daughter with long hair. She can run out the 50 gal one in our house!

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That's the real issue...

If only 6 gallons...the water hogs get what they get and the next in line gets a hot shower before being late for the dinner reservation... because the quick recovery time.

Active Capt....I have thought about both issues and have often thought a small several gallon point of use one in line with the feed to the master shower might be a solution to total hot gallons when needed and space requirements...a bit more to think about but just only for the times those couple extra gallons are needed. The small ones can just be plugged into a wall outlet somewhere near where they are mounted.

I do agree that if you don't live aboard or have no or limited AC power..storage size becomes a much large consideration and probable winner in the debate.

FF said:

11 gal will do fine with showers and dish washing .

You might miss the 20 if you do constant loads of laundry .

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Ask the admiral how she does laundry. Lena only uses the cold or warm water setting when doing laundry on Hobo, which is typically under way or when the generator is running.

Raritan's published recovery rate for the 12 or 20 gallon units is 13 gph.

Jeff..don't be crazy! The biggest you can get and fight the install issues. Cruising is not camping and you will miss the HW capacity you have now if you cut down. You live aboard for much of the year, you do laundry aboard, occasionally have visitors...go BIG !

My 11 gallon unit which the PO installed in the Bahamas due to availability recently went out. That unit served them well as liveaboards there including laundry. However, I found it to be insufficient during the cold winter. It seems the colder water entering reduced the capacity significantly. I replaced with a 19 gallon like the boat came with and all has been well.

BandB said:

It there something I'm missing as it seems to me that if installing requires all the dismantling then so would removing the older one? Is there a problem matching the dimensions of the old one?

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Yeah, I didn't explain everything. We're back in our house in Maine so I pulled the falling-apart hot water tank but put the water system back together so the boat is still usable if we want. Now I'm looking at getting the new tank and re-installing it. So re-installing a duplicate 20 (which I can get) would be painful, at least until my scars heal.

We've never run out of water with the 20 gallon unit. Just before I pulled it, I wanted to purposely run the hot water out to make draining it easier. So I turned off the breaker for the heater. My wife took a long shower. Then I took a long one too - I was turning into a prune after about 15 minutes and decided I had enough. So I just ran the hot water on the sink overboard - lasted another 10 minutes. We must have a pretty great shower limiting sprayer because the numbers seem magical and we're happy with the shower quality.

Laundry is 15 gallons per load but its never all hot - perhaps we use 5 gallons of hot water per load. The recharge time is less than the water use too. It's unlikely that we'd need to be taking multiple showers with the washing machine running. Running the washer/dryer at anchor would require the generator so the hot water heater would be live too.

It's still not an obvious choice - 11 or 20. I'm thinking about getting an 11 and living with it for a year. Then if it's terrible, I'll pull it and go with the 20. The scars will be long gone by then (I honestly did lose a fair amount of blood in the removal...).

AC,
I think you are going the right direction in installing the 11g. unit, If you have to conserve water a bit (but most likely you will not need too) you will adapt. On Volunteer I went from a instant Paloma unit the our guests always complained about the hot/cold cycling to a 12g marine water heater. We had the same guests aboard every year for 10 days and they really appreciated the storage unit. Most times, we had our family of four
(myself and three girls.. two with long hair) and two dirt dwellers that were used to endless hot water.
I at times in the first couple years exercised the right as Captain to shut off the pressure pump when their showers ran excessive.. and had to explain the concept of needing to conserve the 400 gal of fresh water we carried (in later years I brought on line a unused additional 200 gal. tank to up capacity to 600 gallons).
As far as hot water, when I knew late in the afternoon that showers were going to be non stop I brought the generator on line and ran it to help produce endless hot water. As Captain I always took the last position and never lacked for hot water.

When just the family was aboard we could do showers, dinner cleanup and still have some hot water in the am without excessive gen. time... on 12 gallons.

HOLLYWOOD

ActiveCaptain said:

Yeah, I didn't explain everything. We're back in our house in Maine so I pulled the falling-apart hot water tank but put the water system back together so the boat is still usable if we want. Now I'm looking at getting the new tank and re-installing it. So re-installing a duplicate 20 (which I can get) would be painful, at least until my scars heal.

Snip

It's still not an obvious choice - 11 or 20. I'm thinking about getting an 11 and living with it for a year. Then if it's terrible, I'll pull it and go with the 20. The scars will be long gone by then (I honestly did lose a fair amount of blood in the removal...).

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The solution is real obvious to me. Assuming the boat came with a 20 gallon unit, put another one back. Second obvious solution is to pay someone to do the bleeding and only do the parts (plumbing and electric) that don't leave scars. While I try to do everything I can (doing an engine swap currently), sometimes it makes more sense to use your brain and your wallet.

Ted