Adding second electric water heater?

[KRum]

I have a Atwood 10 gallon gas and electric water heater... The electric portion is rates at 1,440 watts...

When boondocking this would take a hefty toll on my inverter (2,000w) and battery bank...

 

I was thinking of adding a HotRod hot water conversion kit that is only uses ~400 watts... and use this during the day (Basically turn on after the morning shower - or add a timer to run in the afternoon after the batteries are topped off and the solar is not being utilized)...

 

I understand the time needed to warm the water would be increased... But with 650 watts of solar it would not draw from the batteries as much as the 1,440 watts?...

[SWharton]

Why don't you just use the propane side of the water heater???

[oldjohnt]

Mornin K, you ask "But with 650 watts of solar it would not draw from the batteries as much as the 1,440 watts?..."

 

Of course you're correct, a 400 watt 120 VAC heater will draw approximately (give or take and subject to battery voltage and efficiency) somewhere around 35 amps from your battery bank WHILE RUNNING, while a 1440 watt would draw somewhere around 130 amps (subject to battery voltage and efficiency) WHILE RUNNING.

 

HOWEVER the 1440 watt will heat 10 gallons of water to x degrees much sooner then the 400 watt and NOT run as long!!!!!!!!!!!!!!

 

As I see it (Electrical NOT Mechanical or Thermo Engineer) it takes x amount of energy to heat 10 gallons of water from x to y degrees, and you can use 400 watts for a longer time orrrrrrrrrrrrrrr 1440 watts for a quicker heat up. THEREFORE I THINK IT TAKES ABOUT (efficiency factors) THE SAME AMOUNT OF ENERGY FROM YOUR BATTERIES TO HEAT 10 GALLONS OF WATER FROM X TO Y DEGREES.

 

THAT BEING SAID, if your solar panels can deliver 650 watts into your batteries (that's ONLY at ideal angle bright sunlight of course) but you're ONLY drawing 400 watts to heat water THE BATTERIES CAN KEEP CHARGED UP IN GOOD SUN AND ANGLE CONDITIONS However, if you draw 1440 watts your solar cant keep up and your batteries will suffer some degree of discharge before the heater shuts off.

 

SUMMARY its my opinion ifffffffffffffffffff there's no hurry to re heat the water, Id opt for the 400 watt heater for a longer heat up time versus the 1440 watt which (although will heat it faster) will draw your batteries down to some degree. A reason is its better to never draw your batteries down (possible with 400 watt heater under ideal sun and angle conditions) anymore or more often then you have to as the 1440 watt heater would necessarily do.

 

I have a 400 watt electric and dont use LP when connected to shore power UNLESS we need 2 quick back to back showers in which case I fire up the LP for its quicker recovery. If I were dry camping and using solar and inverter I would probably fire up the LP Gas to reheat the water versus using my batteries....

 

That's my story n IMa stickin to it. True, an Engineer (especially Engineer and Attorney) may overthink and over answer a question, but I'm trying my best to help and educate to the best of my ability SO THERE TAKE IT OR LEAVE IT LOL use the 400 or 1440 and/or batteries/inverter as you see fit or others tell you.

 

John T NOT a Solar Expert so see what those gents have to say, they may agree or disagree as to which way to go.

[Dutch_12078]

I suspect the 400 watt heater will use more energy overall than 1440 watt heater due to the heat loss during the much longer heat up time. Whether the difference would be significant or not is an unknown though. I think the real question comes back to SWharton's query, "Why don't you just use the propane side of the water heater???", and nearly eliminate the battery draw down.

[Yarome]

Going along with what John and SWharton have already said, heating with LP "on demand" is going to be your most efficient and reliable source while boondocking. It is true that it will require x amount of energy to heat water from temp a to b... however... heating with a 400 watt element will actually require more energy due to the fact that (I don't know the technical term) you will also have "sympathetic" heat loss (cooling around the outer edges) taking place over the greater length of time required to reach temp with the lower rated heating element. Using the 1440 watt element will get you to temp much quicker.. hence.. less energy loss.

 

It will undoubtedly take a toll on your battery bank, but either taking it out all at once and gradually replacing it with solar or taking it directly from solar a little at a time really makes no difference in the whole scheme of things. By the end of the day you'll actually end up with less juice wasted to inversion and cooling.

[Yarome]

I suspect the 400 watt heater will use more energy overall than 1440 watt heater

 

Ya beat me by a 1/4 minute.

 

X2

[pugsly]

We use LP as the primary source of energy for our little water heater. We have an electric element as well, but that only gets used when we have shore power (same thing for heat).

 

The amount of gas it takes to heat up that water tank really is minimal. The only thing that really puts a dent in our propane supply is running the forced air heat.

[Kirk W]

The amount of gas it takes to heat up that water tank really is minimal. The only thing that really puts a dent in our propane supply is running the forced air heat.

My thoughts exactly. The next biggest propane user is your cook stove.

[oldjohnt]

Mornin Yarome, "heating with a 400 watt element will actually require more energy due to the fact that (I don't know the technical term) you will also have "sympathetic" heat loss (cooling around the outer edges) taking place over the greater length of time required to reach temp with the lower rated heating element. Using the 1440 watt element will get you to temp much quicker.. hence.. less energy loss."

 

FWIW I AGREE

 

During the LONGER time it takes for the 400 watt heater to bring the temp up, you are giving up some small degree of heat to the environment as the heater IS NOT AN ADIABATIC (call it a perfect insulator, spelling???) WALL. That's why I had that small "fudge factor" in my response IE "ABOUT (efficiency factors) THE SAME AMOUNT OF ENERGY"

 

My dual electric/LP water heater doesn't consume near the gas as comfort heating when its fairly cold, or if the wife fires up the oven so its NOT my major energy concern.

 

Have fun yall

 

John T

[Ronbo]

Use the waste not function of your charge controller to turn on a relay for the water heater. It would only energize the element of the batteries are in float mode and there is still power being produced by the solar system.

[skp51443]

The diversion solar controllers sound like a really good idea, just send all your excess power to a 12 volt heater in the water tank and get some use out of the otherwise wasted power. The downside is that once your water is as hot as you want it and the sun is still shining you need another load for the system, usually a fan-cooled resistor bank. That can be inside (for heating) or outside the RV but it adds to the cost and back when I looked at it the expense would have paid for years of propane use.

[Darryl&Rita]

Were I that worried about solar water heating, I'd skip right over the electric option to heat the water directly with a solar loop collector. Less conversion issues, and better efficiency.

[oldjohnt]

MnTom "The water doesn't really care how long it takes, but the amount of energy required is the same no matter what."

 

EXACTLY plus in addition as Yarome pointed out and I agree, since the water heater storage tank is NOT a perfect insulated no heat loss Adiabatic wall, and is constantly giving up heat to the ambient surroundings, the faster you bring the water up to temperature the less time for heat to escape, so the higher watt heater has at least that advantage. BUT Id still opt for the 400 watt since he only has 650 solar watts so it theoretically (subject to sun angle and intensity) can prevent any battery discharge, unlike a 1400 watt heater.

 

Very fun sparky and energy chatting with you, even if this may bore others to tears lol. Its the old conservation of energy theory and heat lost = heat gained which is about all I remember from Thermodynamics (that and entropy and enthalpy or something like that lol)

 

John T

[oldjohnt]

Mntom " I really don't know how to calculate that... ^_^"

 

NEITHER DO I today, but I bet around 50 years ago when taking Thermodynamcis I could have lol. Regardless, the heat loss that occurs in the time difference between what a 400 watt versus a 1400 watt heater requires to bring the temperature up, I BET IS SMALL. Indeed if we knew perhaps things such as the R value of the tanks insulation and the volume of water and surface area and ambient temperature we might make a stab at it???????????? Hey one Prof used to tell us all we needed to remember was heat lost (out of the tank) = heat gained (to the ambient surroundings) piece of cake right????

 

Keep your water nice n warm yall

 

John T

[Kirk W]

Nobody has even considered the cost of the needed propane to do this job, versus that of adding a second water heater or any of the other possible answers. To me the thread should demonstrate to KRum the thought that we are creating a very expensive and possibly inefficient answer to a fairly simple problem. And reading through the posts, I don't see where anyone has figured into their response the temperature of the supply water, which also plays a major role.

 

 

If the cost of installation isn't a factor in all, perhaps a better answer might be to think about a solar water heater to augment the electric one?

[KRum]

I am not wanting to add a whole second water heater... I just wanted to add the Hotrod (http://www.campingworld.com/shopping/item/hott-rod-water-heater-conversion-kit-6-gallon/47672) heating element to my present 10gal gas/elec unit as the Hotrod is rated at a much lower wattage (400) than the 1,440 the Atwood comes with...

My goal was to utilize the excess solar electric capacity that is not being used... If my 650 watts of solar gets my battery bank full by 11am the rest of the afternoon a wanted to have a relatively low draw heating element to warm the water that could make use of the solar that is not being utilized... My investment would be ~$100 in the Hotrod kit... The electric would be free...

As for the sympathetic thermodynamic issues... I am content to limit my understand of that concept to my beer cozy... ;-)...

[oldjohnt]

K Rum, The 400 watt model of a so called "hot rod" I installed screwed right in where the drain plug is on my Atwood, HOWEVER, unlike my buddy's hot rod (different brand then mine) my temperature sensor and controller fit right onto/against the safety relief valve on the outside versus his having to cut out the insulation on the interior and place his sensor/controller against his tank there. Mine installed in a matter of minutes and has performed flawless so far (knock on wood). While my 460 Amp Hours of battery storage, four rooftop solar panels and 2000 watt PSW Inverter could heat my water, I chose to use LPG to heat water when boondocking BUT OF COURSE TO EACH THEIR OWN I did NOT perform any calculations when I made that choice, just did it lol.

 

John T

[Yarome]

The R value of your tanks insulation will play a role, but the main heat loss will be through your water lines (un-insulated) even though the water may not be moving. Heat will "bleed" up those lines at a rate of 20x that of heat transfer in air. With a 10gal tank.. that's roughly 40% more water volume than the 400 watt elements were really intended to heat to temp. If you're not running your inverter full time already, then you also need to take into account the increased overhead of your inverter.

 

I understand the desire to fully utilize solar production. Your intent may not be to reach temp, but only "any increase is better than none", but personally, I wouldn't choose to utilize excess in that way. It's building in potential setbacks (when it's "on" it's "on" and won't care if it's drinking juice from excess solar or your battery bank) which would seem to defeat the purpose. You're also betting on utilizing that heated water. If it fully cools then you're still loosing the excess solar production, but at greater expense.

 

Just taking a different tack that might spark some ideas... what I do with mine... I wired a switched outdoor 12v socket. I run an independent 12v battery that I use for remote pumping, UV water purification, work light, etc. When I notice my battery bank topping out I'll just throw the switch and dump any excess solar out to the other battery. Granted.. I don't do it daily. Only when I'm around in the afternoon and when I remember, but it's enough to keep the exterior battery charged when needed. It is a necessity to me.. it gets used.. and it's efficient in that it doesn't require any additional expense or power overhead (inverter).

[skp51443]

You can also see some heat loss through the propane burner's tube since it is not insulated. Folks that never use the propane side of the heater have blocked the tube to stop this heat loss. If you go that way make sure the burner can't be activated until you have removed the blockage!