Electric 110 on demand vs. Traditional Whale Water heater for geeks

I posted about needing to replace my existing water heater and I am considering an on demand 110 electric water heater. Here are my thoughts on this. please share where my thinking is incorrect. I am open to redirection. I used Chat GPT for help with this, so if I'm wrong, I blame technology.

Now for the Geeks who want to debate energy suck, here is some data.

We did some measuring, and we will need 11 gallons of hot water a day and that makes it easy for the calculations. As the replacement would be an 11-gallon whale.


Assuming I am at anchor and not running the motors (no heat exchanger) or the genset, just operating on solar and batteries.
And assuming I will use the hot water at 3 or 4 different times during the day.
And assuming I replace the heater with one like the original 11-gallon tank

To calculate the energy needed to heat all eleven gallons in the Whale water heater:

First, let's calculate the energy required to raise the temperature of all eleven gallons by forty degrees Fahrenheit. (40 degrees seems adequate if we are starting with 80-degree water)

The mass of one gallon of water is approximately 8.34 pounds.
The specific heat of water is 1 BTU per pound per degree Fahrenheit.

Bu using the formula:
Energy = Mass X Specific Heat X Temperature Change
For eleven gallons:
The mass of eleven gallons of water is approximately 8.34 pounds per gallon, so 91.74 pounds.
The energy required to raise the temperature by forty degrees Fahrenheit is:
91.74 pounds X 1 BTU/pound/F X 40 = 3669.6 BTU’s

Converting BTUs to kilowatt-hours, knowing that one kWh equals 3,412 BTU’s

Energy in kWh = 3669.6 BTUs / (3412 BTUs / kWh)~.098 kWh

So, it would take approximately 1.075 kilowatt-hours to heat eleven gallons of water by forty degrees Fahrenheit. Assuming zero loss in efficiency. The above DOES NOT TAKE IN TO ACCOUNT The Whale has a typical efficiency rating of 90% and the on demand is 99% efficient.

With that data, if I wash the dishes and need one gallon of hot water. With the original heater I will have to heat all 11 gallons and use one, then later in the day I will want a shower from all that work washing those two cups and two plates and I will need to heat all 11 gallons again. How is this possibly more efficient?

Additionally, if I am keeping the water warm all day I use additional energy and that is calculated below.

The energy required to maintain the temperature depends on factors like insulation, ambient temperature, and how often the heater cycles to maintain the temperature.

Let's assume a reasonably well-insulated tank and typical conditions. The standby loss for an electric water heater might be around 0.1 to 0.2 kWh per hour. If we assume the heater cycles periodically throughout the day (let's estimate this at 0.15 kWh per hour):

Standby energy per day = 0.15 kWh/hour X 24 hours = 3.6 kWh/day
So, to keep the water warm all day long, the water heater might use an additional 3.6 kWh.

- To heat the eleven gallons will take 1.075 kWh
- To keep it warm all day will take approximately 3.6 kWh

For a total of 4.675 kWh a day

I am new to this solar and battery energy calculating, so please be kind in the smack down you are about to give me.

Your power consumption numbers seem very high for the 11 gallon heater. My 11 gallon Isotemp with a 1200W element runs 2 - 3 minutes per hour to maintain ~165* in the tank. Assuming no hot water is used that means it's using somewhere in the range of 1 - 1.4 kwh per day. Using some hot water will increase that of course. You're also assuming the heater cools down quickly when you turn off the power. If I haven't used much hot water I still have plenty for a shower after 24 hours with the heater turned off. The Isotemp heaters are pretty well insulated (which is why I bought it), but at some point when I look to run mine on inverter / battery / solar I'll likely add more insulation to it which will further reduce the standby power consumption.

In theory you could also use a heat pump water heater for higher efficiency if a suitably small one could be found.

In general an on demand water heater will be more efficient. But on a boat my biggest concern is actually powering one. To use 1 gal/min of hot water continuously and heat it by 70* (assuming cold weather so the tank is only 50* and you want 120* at the tap) you'd need about 38,000 BTU/hr output from the water heater from what I can find. That means your water heater is going to consume 11 KW whenever you're using hot water. That's basically an entire 50A 120/240 shore power hookup just dedicated to heating water. As an example, Rheem sells a 13kw tankless water heater (240V). That's 54 amps at 240V. And that heater is rated at 1.97 gal/min for a 45* temp rise, 1.37 gal/min for a 65* temp rise, so we're still talking about a very small tankless setup.

Effectively, if you're not using propane to heat water, an on demand heater just isn't viable on a boat as the short-term power draw is impractically high even if the total power use per day would be lower.

OK, BUT if I only cruise in warm weather, so the starting temp is 80 degrees and I am shooting for 120 degrees, I only need 40-degree rise, therefor 21,700 BTUs and I use a 110 Rheem capable of producing 1 GPM which is what we need, and it uses 3500 watts of 110 volts. What happens then? This is where My brain can't keep up.

is it just sucks up all my capability for that time it's on? like I have to shut off the refrigerator, lights, music in order to heat water?

Thank you so much for helping me understand the shortfalls of this. I have read where people have used these on boats, not sure about if they ran the genset while making water. I assume that changes the equation entirely. I have a 240 washer/dryer and a 240 stove.

Suggestion: why not just plumb to engine and/or generator?

Comment: Airstream recently started putting on demand in their trailers. Feedback has been generally negative. Reason is they waste a lot of water. Perhaps depending upon location you could reduce this but probably still an issue unless you need hot water at only 1 tap.

Also these are propane based as it would likely overload your inverter. Especially if any other systems are running.

On my new to me GL33 with engine plumbed in water was hot next day.

Relevant thread

READY said:

Suggestion: why not just plumb to engine and/or generator?

Comment: Airstream recently started putting on demand in their trailers. Feedback has been generally negative. Reason is they waste a lot of water. Perhaps depending upon location you could reduce this but probably still an issue unless you need hot water at only 1 tap.

Also these are propane based as it would likely overload your inverter. Especially if any other systems are running.

On my new to me GL33 with engine plumbed in water was hot next day.

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My goal is to be silent at anchor and not have to run the genset. But it sounds like I need to go with the original anyway. Rats! installation is going to be a total S@#% show.

Have you considered a smaller 6 gallon model? Probably more than enough if you only use 11 Gals all day. Still might need Sawzall to get yours out but maybe easier refit? You could possibly relocate the heater as it would make life easier for you or the next owner. Would also be more efficient.

READY said:

Relevant thread

Point of Use Water Heaters

A few bits of info first about our trawler usage. Never on shore power, about 80% anchored often for days at a time, located in 45 deg. N, 1600W solar, 700Ahr AGM (actual), 9kW genset, 15 gal hot water tank, all electric boat. We do not use the genset except when solar does not get us to 100%...

www.trawlerforum.com

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Ken,
Thanks, that was very helpful, and a good read.

READY said:

Have you considered a smaller 6 gallon model? Probably more than enough if you only use 11 Gals all day. Still might need Sawzall to get yours out but maybe easier refit? You could possibly relocate the heater as it would make life easier for you or the next owner. Would also be more efficient.

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Thats a good idea. I went right to on demand. I will check the dimensions and see if the 6 gallon will make it easier for refit.

I was also thinking the 11 gallon tank may be a bit much considering what you described for usage. 6 gallons will usually support 2 showers but if you want a buffer isotemp makes an 8 gallon tank. Depending on your solar situation if you have excess power you could dump it to the water heater. Either via an inverter or a little more fancy by swapping out the AC heater element for a DC element. I believe you can find 750 watt DC element on Amazon or search off grid living type ecommerce sites.

My buddy has an on demand propane hot water heater in his boat. Neither of us would recommend going that route. I don't remember the model but it's a water waster because the water is often too hot or too cold and it varies by water flow. So you waste water in the shower trying to get it warm enough or letting scalding water go down the drain while you wait for the water to cool down.

Demand heaters are very efficient, not much thermal loss. But the power demand can be huge when running depending totally on flow and the supply temperature. You are heating water "on the fly" as it goes by. The power needed is dependent on the temp rise and the flow rate. There is no reserve "thermal bank", if someone turns on a faucet (hot or cold) the temp will change.

Rays53hatt said:

OK, BUT if I only cruise in warm weather, so the starting temp is 80 degrees and I am shooting for 120 degrees, I only need 40-degree rise, therefor 21,700 BTUs and I use a 110 Rheem capable of producing 1 GPM which is what we need, and it uses 3500 watts of 110 volts. What happens then? This is where My brain can't keep up.

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By my math 21,700 BTU will require close to 6400 watts assuming it's electric resistance heat and not a heat pump. The 3500W / 120V Rheem unit is only rated for a 0.5 gal/min at a 48* temperature rise. That's probably not enough for a good shower.

That makes Sence! Thank you so much for helping me understand this. I really appreciate it. I will start cutting the old one out and figuring out what I will have to to remove in order to get another one back in. And I agree, 6 gal should be enough for us. We measured and we use just over a half a gallon of hot water per minute while we’re taking a shower. So that would make for a 10 minute shower for my girlfriend. Who has long thick dark hair.
Ummm now as I’m thinking this through,probably as big as I can fit.

Rays53hatt said:

That makes Sence! Thank you so much for helping me understand this. I really appreciate it. I will start cutting the old one out and figuring out what I will have to to remove in order to get another one back in. And I agree, 6 gal should be enough for us. We measured and we use just over a half a gallon of hot water per minute while we’re taking a shower. So that would make for a 10 minute shower for my girlfriend. Who has long thick dark hair.
Ummm now as I’m thinking this through,probably as big as I can fit.

Click to expand...

On the size thing, you can get more effective capacity by running the heater at a higher temp and adding a mixing valve on the output. You'll get more 100* shower water from a heater that started with 6 gallons at 160* than one that started with 6 gallons at 130*.

I use a hydronic loop from my heating system to heat the water tank.
For those that have diesel available, and 12 volts to run the hydronic burner, I suggest that it could solve your problem of how to heat water when on the hook, no engine running required. With a muffler on the exhaust it’s almost silent.

I have two 11 gallons H/W tanks. Both used to be heated from one engine while running, now just one, otherwise 120V AC.
A while back I came across 12V element for hot water tank. Thought what if I changed one element to 12V and run on a timer to keep water warm. Just a thought in make work plans. So far we have hot water when needed as the GEN is run to top up batteries and heat water at same time.

Install a pair of these in parallel https://a.co/d/6ekea7o.