removing hot water heater

rogerbum":3o634koi said:
oldgrowth":3o634koi said:
Roger – I believe there is a slight flaw in you last paragraph (it just takes about 4 times as much time to heat the water). It seems as though something is missing here. If that were true, then we could use one watt of energy and given enough time have all the hot water we want.

Using a little logic/common sense because I don’t have training in this area:
When we cut the voltage in half we have to decrease the resistance by one fourth in the heating element in order to create the same amount of energy/heat.

Now let’s assume the 240v element is designed to operate at 440 degrees in order to heat the water to its desired temperature in a reasonable amount of time. A thermostat will shut it off and let’s assume that is at 140 degrees. Now using a 240v heating element with 120 volts will give one fourth the amount of energy/heat that 240 volts does. One fourth of 440 is 110. A heating element operating at 110 degrees will never heat the water to the desired cutoff temperature of 140 degrees.

Now my thought process could be wrong, so if it is tell me where.

Dave dlt.gif
www.tolandmarine.com
Click to expand...

Dave,

You are both correct and incorrect so let me explain. You are correct in stating that there's something missing from my statement that it will take approximately 4 times as long to heat the water. It will take a little longer due to heat loss from the water heater. If there was no heat loss (e.g. the impossible "perfect" insulation), it would take 4 times longer. But there will be some heat loss so it will take a bit longer (depending on the percentage of heat lost relative to what goes in). Also, there's another complicating factor which is that the rate of heat transfer from the element to the water is related to the temperature difference between the element and the water and the temperature difference will be lower for the lower power element. The rest of your logic above is wrong. In a really well insulated system we could in fact heat water with 1W - it would take a really long time, and it would probably cost a hell of a lot of money to create the system with such fantastic insulation (probably a NASA like project).

The heating elements are not designed to operate at a certain temperature - they are simply resistive elements and they will continue to get hotter as long as current flows through them and there is no heat loss. E.g. the logic about the temperature of the heating element going down by a factor of 4 when the power goes down by a factor of 4 is completely screwed. The temperature at which a heating element in a water heater will operate is for the most part limited by the temperature of the water and the rate of thermal conductivity between the element and the water.

E.g. you can think of the cold body of water as "sucking away" heat/temperature from the heating element. The heating element will be above the temperature of the water but not by too much since the water serves as a heat sink for the heating element. Since water boils at 212F, the temperature of the heating element should never get too far above 212F until the tank goes dry. Also the temperature of the heating element is probably limited by an independent thermostat as a safety feature (at least that's how I'd design a water heater - I don't really know if this is the case). How much is "too much" and "too far" in the above sentences? I don't know since I don't really know the rate of thermal conductivity between the element and the water and even if I did it would require me to go back and look up some long forgotten physics to do the correct calculation.
Click to expand...

Roger – thanks for your explanation, it almost makes sense to me. I figured you would talk about the heat loss from the rate of heat transfer from the element to the water in relation to the temperature difference between the element and the water.

I also know a heating element is not per say designed to operate at certain temperatures, but I believe it has a certain optimal temperature it reaches with a given size, material, resistance and voltage. Then any increase in temperature above that is negligible as current continues to flow through it. By increasing or decreasing the voltage with the resistance, size, and material being fixed, seems to me, will change the temperature of the element.

I do have some lingering questions though based on my observations.

1. If the heating element in a water heater continues to get hotter as long as there is current, why does a heating element on a portable space heater not continue to get hotter? They seem to reach a certain redness/brightness, then not get any hotter. If you increase the voltage or reduce it, the element changes its color/heat output.
2. Now look at a light bulb with an old style dimmer switch on it and the light produces less heat/brightness as the voltage is reduced. The element is in a vacuum so you cannot attribute the heat loss to air sucking the heat away.
3. If your statement that a heating element will continue to get hotter as long as current flows through it, why doesn’t the filament/element in the above light bulb not burn out when left on at full power?
4. Dosen’t the light bulb and space heater operate on the same principals as a water heater?

The reasons for these questions are so someone does not spend money on something that does not have a chance of working.

Dave dlt.gif
www.tolandmarine.com
 
Not sure what temp the element gets up to or even exactly what the thermostat is currently set to on the tank (factory setting for now)...

47 minutes after running the generator and listening for a throttle change, it stepped back down to the lowest setting running with the ECO throttle switched to ON. At that time, there were no other appliances or accessories operating so I am making the assumption that the thermostat kicked in and switched off the heat. Now I would be happier with 27 minutes but if I can still run chargers for the batteries at the same time, it is worth running a little longer. If it took 2 hours I would call it a mis-judgment and throw it as far as I could.

Sure enough there was enough hot water to run a shower at that time and without mixing cold into the flow, I could not comfortably leave my hand in the stream. So 625 watts into a 240volt, 2500 watt element seems to do the trick in this configuration. I did think about finding a similar element for the factory tank but I guessed that 6 gallons would take too long to heat with that little power input. I think I made a good choice.
 
oldgrowth":1hdmjbki said:
Roger – thanks for your explanation, it almost makes sense to me. I figured you would talk about the heat loss from the rate of heat transfer from the element to the water in relation to the temperature difference between the element and the water.

I also know a heating element is not per say designed to operate at certain temperatures, but I believe it has a certain optimal temperature it reaches with a given size, material, resistance and voltage. Then any increase in temperature above that is negligible as current continues to flow through it. By increasing or decreasing the voltage with the resistance, size, and material being fixed, seems to me, will change the temperature of the element.
Click to expand...
That's mostly true since the heating element is connected to a heat sink (e.g. the water tank) and there's some constant heat loss. However, once the heat sink (water) get's warmer, the element temperature will continue to rise for a given current. The element temperature is a combination of the power that's going in and the power that is flowing out. The power that's flowing out is related to the temperature difference of the element and the heat sink to which it's connected. So as the heat sink gets warmer, difference in temp decreases, the power loss is less and the element gets warmer (creating a bigger temperature difference and more heat flow). In practice, these factors keep in balance so that the temperature of the whole system rises smoothly rather than discontinuous explanation I provided. Eventually, the water gets warm enough and the element is turned off by the thermostat. Otherwise, the temperature of both the water and heating element would continue to rise until either the water boiled off or other heat losses (through the pipes and insulation) limit the temperature.
oldgrowth":1hdmjbki said:
I do have some lingering questions though based on my observations.

1. If the heating element in a water heater continues to get hotter as long as there is current, why does a heating element on a portable space heater not continue to get hotter? They seem to reach a certain redness/brightness, then not get any hotter. If you increase the voltage or reduce it, the element changes its color/heat output.
Click to expand...
Assuming you are not talking about a space heater with a fan - now you've hit on another source of hit loss (one I left out in my simplified explanation) - radiative heat loss. First, if you are talking about a space heater with a fan, the heat loss is mostly due to transfer to the air which is flowing over the heating element. In such a case, the temperature the element obtains will be limited by the combination of power in (from the electricity) and power out (warmed air moving across the element). But on a radiant heater, the heat loss is not so much due to movement of the air but due to radiative heat loss.

What is radiative heat loss you ask? It's light (both visible and invisible - most invisible infrared light) leaving the element. In this case the primary source of heat loss is that generated by photons (light) leaving the element. On elements where radiative heat loss is the dominant source of heat loss, the temperature of the element will rise with increase current/power and the temperature rise is directly reflected in the color of the element (orange, red, white). In such a case, the temperature will be still be limited by a combination of the power in vs power out, it's just that the power out is dominated by radiative heat loss as opposed to conductive heat loss.

oldgrowth":1hdmjbki said:
2. Now look at a light bulb with an old style dimmer switch on it and the light produces less heat/brightness as the voltage is reduced. The element is in a vacuum so you cannot attribute the heat loss to air sucking the heat away.
Click to expand...

A light bulb loses heat primarily through radiative loss. That's the heat you feel when you put your hand near it. It's essentially the same as a radiative space heater. In fact some people use light bulbs as space heaters (common in chicken coops and some well insulated dog houses).

oldgrowth":1hdmjbki said:
3. If your statement that a heating element will continue to get hotter as long as current flows through it, why doesn’t the filament/element in the above light bulb not burn out when left on at full power?
Click to expand...
Because of radiative heat loss and because the element is designed with a high enough resistance that the power in vs. radiative loss out is such that the temperature will not melt the element. However, hook any light bulb up to a high enough voltage and you can drive the temperature up to a point that it will melt/burn out the element. It's in a vacuum since that prevents oxidation of the element by air which would burn out the element faster.
oldgrowth":1hdmjbki said:
4. Dosen’t the light bulb and space heater operate on the same principals as a water heater?
Click to expand...
Yes in terms of power in, no in terms of the dominant source of heat loss from the element. Heating elements in space heaters operate at around 1800°F and elements in incandescent light bulbs operate at 3100–5400°F. At such very high temperatures and with no direct thermal connection to a heat sink, radiative heat loss dominates. The heating element in a water heater probably operates at a much lower temperature and has a direct thermal connection to a heat sink (the water tank).
 
Roger – I kind of followed most of your explanation but I did not understand the explanation for the following.
rogerbum":30ojqdni said:
oldgrowth":30ojqdni said:
2. Now look at a light bulb with an old style dimmer switch on it and the light produces less heat/brightness as the voltage is reduced. The element is in a vacuum so you cannot attribute the heat loss to air sucking the heat away.
Click to expand...

A light bulb loses heat primarily through radiative loss. That's the heat you feel when you put your hand near it. It's essentially the same as a radiative space heater. In fact some people use light bulbs as space heaters (common in chicken coops and some well insulated dog houses).
Click to expand...
In the light bulb example above when the voltage drops, all the other factors drop except the resistance of the element. If the voltage drops low enough, it will not give off any perceptible heat or light. It seems to me it would be the same for the heating element, regardless of its application.

After a little research I came up with the following.

Using the following formula to find the resistance of a heating element.
Resistance = Voltage squared over Wattage.
We know volts = 240 and watts = 1200. The current is 5 amps.
240 squared divided by 1200 = 48 ohms - the resistance of the heating element.

To find watts from a known voltage and resistance use the following formula.
Watts = volts squared divided by resistance.
Reduce the voltage to 120 volts, resistance remains the same because we still have the same element.
120 squared divided by 48 = 300. Current is 2.5 amps (see formula below)
Reduce the voltage to 12 volts
12 squared divided by 48 = 3 watts. Current is .25 amps (see formula below)

Knowing the resistance of the element we can figure the amperage
Amps = voltage divided by resistance.
240 divided by 48 = 5 amps
120 divided by 48 = 2.5 amps
12 divided by 48 = .25 amps

Looking at the above information, I would guess there has to be some internal lose in addition to the losses Roger identified. I don’t think 12 volts will ever under any condition heat the element any perceptible amount.

I believe using a 240v heating element with 120 volts will end up using more power than using it with 240 volts because of the internal loss as well as the loses Roger identified. It seems to me, using an element that draws the most power a battery or generator is capable of producing would be the most efficient.

To figure the most efficient you need the least resistance you can get by with in the heating element.
For a 1000 watt generator look for an operating draw of 900 watts.
120v squared divided by 900 watts = 16 ohms. So you need a heating element with 16 ohms of resistance.
120 divided by 16 = 7.5 amps
That would be the most efficient for a 1000 watt generator.

I don’t know how to measure heat other than BTU and 1 watt = 3.4121 BTU per hour.
Using the 240v element with 120 volts = 1,023 BTU per hour.
Using a 900 watt 120 volt heating element produces 3,070 BTU.

Roger - am I wrong somewhere here.


Dave dlt.gif
www.tolandmarine.com
 
oldgrowth":3rbq3v7x said:
Roger – I kind of followed most of your explanation but I did not understand the explanation for the following.
rogerbum":3rbq3v7x said:
oldgrowth":3rbq3v7x said:
2. Now look at a light bulb with an old style dimmer switch on it and the light produces less heat/brightness as the voltage is reduced. The element is in a vacuum so you cannot attribute the heat loss to air sucking the heat away.
Click to expand...

A light bulb loses heat primarily through radiative loss. That's the heat you feel when you put your hand near it. It's essentially the same as a radiative space heater. In fact some people use light bulbs as space heaters (common in chicken coops and some well insulated dog houses).
Click to expand...
In the light bulb example above when the voltage drops, all the other factors drop except the resistance of the element. If the voltage drops low enough, it will not give off any perceptible heat or light. It seems to me it would be the same for the heating element, regardless of its application.
<remainder clipped>
Dave dlt.gif
www.tolandmarine.com
Click to expand...
Dave,
I general all of what you wrote this time is correct. I will point out that that lack of any perceptible heat or light is not equal to the lack of actual heat or light. What's really at issue is the dominant source of heat loss from the element (heating element or light bulb). The power in and the resistance behavior is essentially the same it's just that how they lose the heat is different. However, even a light bulb will lose some heat through conduction and a water heater will lose some heat through radiative losses. It just that at normal operating temperatures for each device (high for a light bulb element and lower for a water heater element), the dominant sources of heat loss are different.

You are correct that the most efficient water heater element would be operated at close to max available current if the definition of most efficient is to get the maximum rise in temperature of the water per unit of power. The reason for this is that since heat energy is always leaking out of the water heater (through conduction and radiative loss), the quicker you get the water to temp, the less heat is lost to other places. However, as a practical matter water heater elements are only available in certain watt ratings for certain voltages so one can't necessarily tune the resistance of the element to maximize efficiency. Also, my guess is that since the water heaters are fairly well insulated, the loss is relatively minimal. I know that when I had the 6 gallon water heater on for an hour or so in my Tomcat, the water was still quite warm 6-8 hours after I turned the water heater off. [/i]
 
Here's a less technical consideration regarding the factory installed water heater: it takes up the whole cabinet, but it works. If plugged in to shore power, it takes about 15 minutes to heat the 6 gallons. If we use the generator, it takes the same amount of time, but the Honda 2000 is revved up (a 1,000 watt generator wouldn't handle the load). Once heated, we have hot water for the day, depending on use. 8 hours after turning the water heater off, we still have warm (not hot) water; warm enough to shower if you don't turn on the cold water.

Having been an RVer for decades, I understand the reason for the 6 gallon water heater: it has been a standard in the RV industry (even though most larger RVs have now gone to 10 gallon heaters)... economy of scale. You buy what's readily available and affordable.

I think a 2 or 2.5 gallon water heater makes more sense in this size boat, because those of us who shower in there are not letting the water run while you shower. It would also make more sense to put the water heater near the shower and sink (under the galley cabinet) so you wouldn't waste water, getting the warm stuff to where it's needed. These were suggestions I made to the powers-that-be at the factory a couple of owners ago... you can see where that went.

I understand why some folks would prefer the storage space, especially if they don't shower in the boat. But, with a smaller water heater unit closer to where you use hot water, you'd have a win/win.

With any of these types of water heaters, there is a diminishing returns with the hot water if the tank/heating element can't keep up with water usage... if the water heater isn't plugged in and turned on, by the time you've used a good portion of the hot water, more cold has come into the tank and lowered that water temp. This is more noticeable with a smaller capacity tank; likewise with a heating element that takes longer to heat.

We won't be pulling the water heater (or Wallas) out of our boat - we're in the camp that uses the shower and hot water to wash dishes. It works, we like having hot water available. If our water heater were to die, I'd consider installing a smaller unit.

We've spent extended time cruising on our boat. Extra storage space could be put to use, no doubt; but we get along fine by making good use of the storage space we have. No easy task when you consider the cat gets the biggest storage cabinet (under the forward dinette seat). :wink:

Just another consideration beyond heating elements, watts, and amps (oh, my!).

Best wishes,
Jim B.
 
JamesTXSD":1xjatikr said:
...regarding the factory installed water heater: it takes up the whole cabinet, but it works. If plugged in to shore power, it takes about 15 minutes to heat the 6 gallons...Once heated, we have hot water for the day, depending on use. 8 hours after turning the water heater off, we still have warm (not hot) water; warm enough to shower if you don't turn on the cold water.

...I understand why some folks would prefer the storage space, especially if they don't shower in the boat. But, with a smaller water heater unit closer to where you use hot water, you'd have a win/win.

...We won't be pulling the water heater (or Wallas) out of our boat - we're in the camp that uses the shower and hot water to wash dishes. It works, we like having hot water available. If our water heater were to die, I'd consider installing a smaller unit.

Extra storage space could be put to use, no doubt; but we get along fine by making good use of the storage space we have.
Click to expand...

X's 2 :thup
 
Whew, I'm kind of glad that the hyper-technical dialog on thermodynamics between Dave and Roger has wound down and we are back a level mere mortals can comprehend!

We took out both the water heater and the Wallas, and have no regrets whatsoever.

As Jim points out in another thread, he really doesn't need cabin heat on the Tropical Tip except for maybe 10 crappy days in January or February...we need cabin heat here on the Upper Left Coast pretty much year round, even in the damn middle of July sometimemes...as a heater, I trust most sane people would agree that a dedicated forced air heater like the Webasto, with its heat vent placed down low where it belongs, kicks sand in the face of the Wallas combo cook-top / heaters. We sold the Wallas for $1,200 and the cost of the Webasto, purchased from Interstate and delivered to my door, was $960. Installation complete by Triton Marine was $977, since I lack David's ability to do any of it myself. So this upgrade - and it is a major upgrade - netted out costing us $737. Our cheap little Gasone butane stove, which we already had, is better for cooking to boot. It heats up and shuts down instantly and it is easier to finely control cooking temperatures. This was really a no-brainer.

Now, the water heater. This one I can see reasonable people disagreeing on, and I will not say I will never install some kind of water heater. We found a 120 volt ac water heater not very useful for us, and since I had to take it out anyway to replace the fresh water pump that was behind it, I just decided to leave it out. Since we almost never used it anyway, we sure have not missed it. If we are at a dock with shore power, there are always showers available. If we are anchored out, we can easily heat up water for a PTA bath or hair washing...with less wasted water to boot.

Still, an on-board water heater that was not 120 volt ac would be nice. Since nobody has talked about a diesel fired unit with a small tank and 12 volt pump or a 1 lb propane cylinder fired tankless water heater, I am assuming that these critters do not exist, at least at a reasonable cost (hey Dave, here is a niche you should scratch!).
 
There are 1lb propane tankless water heaters on the market but they are for outdoor use and I think there is a least a brat or two that run them in the cockpit. I would be all for it in a 22 but we already have the head and want to put it to good use. If there was an installible forced air heater that could also heat water and make sense for our boats, I would be first in line.

Here is an example of the propane, portable units

http://www.zodi.com/
 
Got terribly unlucky and our new tank turned out to have a tiny leak in the lower tank itself that was sneaking water slowly and mysteriously onto the floor and into the carpet and pad. We thought it was condensation perhaps but I found it by digging in cabinets and a bright light. New tank is in now and I hope to have better luck with this one. No warranty coverage because of the non standard boat install.
 
Aurelia........

I read your article on installing that little water heater. I was going to give you a heads-up on mine but the last piece from you told the story. After one day mine sprung a small leak internally but I have it in the crawl space in my house. No damage done other than a 100 mile trip to Home Depot to get another under warranty. The 2nd one is now 2 yrs. old and doing great.
I use mine as a initial heater on a real long hot water run. By the time the 2.5 gals is gone the main heater is sending plenty more.

Jack in Alaska
 
Guess what, second tank has developed a leak as well. Maybe I am just on the wrong track with this model. Now I have to tear it out and either replace it or leave it without. I still think the boat should have at least a small tank but I will not put a big 6 gal model back in that cabinet because the usable storage space we got is better than more hot water we don't really use. Guess how many showers were taken on the boat during our last 17 night trip. One.

Anyone have any favorite tanks in the 2-4 gal range? I am in the market.
 
Aurelia.
Last week there was a brand new 4 gal hw htr. at a silent auction. Went for $50. Nothing wrong with it.
You are having hell with your system for sure.
Jack
 
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