110v PLUS 24v on the SAME water heater

[miles2go]

Hello all. Would this idea work and be viable? I found a decent second hand water heater. Used as a holding tank with no power ever hooked up to it so the elements have not been used. The GE 30 gallon water heater is $60, used, but good condition. It is 220v., dual element, 4500 watt elements, 30 gallon, 30" tall (will fit in the bay), 6 years old. 

I would like to replace the upper 220v. element with a 110v. element of 1500 or 2000 watts ($10-$20), the lower element with a 12v./24v. element of 600 watts ($60-$100), maybe more wattage is needed? The lower element would work while underway but put a drain on the batteries while boondocking. The upper element would kick in when demand brings the water temp down and be powered by the inverter or shore power. I am not sure if it wouldn't run all the time if the 600 watt 12/24v. element doesn't work well enough. I have a converter for 12v but the chassis and house system are 24v. So I would wire the water heating element at 24 V for 20 amps.

http://www.realgoods.com/product/solar+power/solar+hot+water/diversion+heater+elements+24v+60a+48v+30a.do
http://www.missouriwindandsolar.com/Water_Heater_Elements.html

I would appreciate some input from the board on this idea. Would the wattage be sufficient? Is it a crackpot idea not worthy of the effort. Is there a benefit worthy of the effort. Does the thermostat need disconnecting or re-wiring. Would it make sense to cut power via an inline disconnect to the 110v. element or the 12/24v. element when not wanting to power them. Or has this been thought of before and is useless complication?

It would, on surface investigation, get me into hot water for about $180. Like most, the bus has a tight to non-existent budget. 110v. water heaters are not as easy to find as 220v., and propane is a lot more work. Although I have already run propane lines I am shying away from cutting a hole in the roof or the side for a vent for propane use.

I also found a used 110v. 10 gallon for $40... but it doesn't sound like enough capacity (teenage daughter)?

I need a nudge in the right direction please.

Andrew.

PS Why is the 12/24v. element so much more expensive than 110v. element? Is it a different, more expensive metal?

[Len Silva]

No need to go to great lengths here.  Simply wire the 220 volt, 4000 watt element to 110.  It will draw 2000 watts and work just fine.

The 12/24 volt elements are so expensive just because of the limited demand.
The existing thermostat will not work with the DC elements.  The website does not discuss thermostats.  I would skip the low voltage idea.  Since you already have an inverter, it's just over complicating things for you.

[miles2go]

Great! Thats an answer I really like, thanks.

[Sean]

No need to go to great lengths here.  Simply wire the 220 volt, 4000 watt element to 110.  It will draw 2000 watts and work just fine.

Better re-think that.

If the element draws 4,000 watts on 240 volts, it will draw a mere 1000 watts on 120 volts.  The formula is P=V²/R, where P is power in watts, V is voltage in volts, and R is the resistance of the elements, which is fixed.

Because the relationship is to the square of the voltage, doubling voltage quadruples power, and halving voltage quarters it.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

... 220v., dual element, 4500 watt elements, 30 gallon ...

I would like to replace the upper 220v. element with a 110v. element of 1500 or 2000 watts ($10-$20), the lower element with a 12v./24v. element of 600 watts ($60-$100), maybe more wattage is needed?

Well, here's the math.  One watt is ~3.4 BTU/hour, and a BTU is the amount of energy required to raise one pound of water one degree Fahrenheit.  There are about 8.35 pounds in a gallon, or around 250 pounds in your 30-gallon tank.

To raise that amount of water one degree will require 250 BTU.  A single 2000-watt element, assuming 100% efficiency (and electric water heaters are pretty efficient) could produce 6,800 BTUs in an hour, which would raise the temperature of your 30 gallons by about 27 degrees.

The 600-watt, 24-volt element could raise that same amount of water at most 8.1 degrees.  If you filled the heater at a campground with, say, 65° water (a common temperature for city water supplied underground), you'd need to drive six hours to get to shower temperature (barely).

IMO, you are much better off using 120-volt elements and a large inverter while under way.  Better still is a heat-exchanger type DHWH.

... Does the thermostat need disconnecting or re-wiring.

Depends.  I'd need the schematic to answer that.

Would it make sense to cut power via an inline disconnect to the 110v. element or the 12/24v. element when not wanting to power them.

I would use a pair of relays so that the water heater will only be energized when either shore or alternator power is present; you don't want this thing accidentally running from batteries, whether you use a DC element or an AC one through the inverter.

I also found a used 110v. 10 gallon for $40... but it doesn't sound like enough capacity (teenage daughter)?

FWIW, we have a 12-gallon DHWH and it is more than sufficient for us, and has worked fine even with three aboard.  My first RV had a 6-gallon LP unit that was adequate for two if we were careful.  No teenagers, though.  Just make her go last...

Remember that a 30-gallon unit will use more energy on average, and you'll be hauling around 250 pounds of "unusable" water everywhere you go (the 30 gallons is not part of your usable tank capacity).  Ten gallons would be 165 fewer pounds of dead weight.

One trick is to turn the thermostat up as far as it will go.  Hotter water lasts longer (mixed with cold at the valve, of course).  If the safety aspect of this concerns you, get an anti-scald valve for the shower.

PS Why is the 12/24v. element so much more expensive than 110v. element? Is it a different, more expensive metal?

No, it's just the law of supply and demand.  240- and to a lesser extent 120-volt elements are commodity items, manufactured in the millions.  12- and 24-volt elements are specialty items, manufactured in the thousands.

-Sean
http://OurOdyssey.BlogSpot.com

[Sean]

... Does the thermostat need disconnecting or re-wiring.

Depends.  I'd need the schematic to answer that.

OK, more info.  I'd need the model number to be sure, but most GE electric water heaters use a Thermodisc type 59T thermostat.  This thermostat is strictly mechanical and is not sensitive to input voltage.  The one concern I would have is that it is not DC rated and I don't know what, if any, arcing problems there might be if you tried to crank 25 amps of DC through it.  !2 volts is out altogether, as 50 amps would exceed the contact rating, not to mention the internal wiring.

Note also that the controls for two-element tanks switch between elements and do not run the elements simultaneously.  That said, there will be two 59T's on the unit and they can be wired independently, so long as the inputs both go through the same 66T thermal safety disconnect.  if you wanted to use two separate voltage inputs, you'd need another 66T, and I'm not sure where you'd put it.

That's even more impetus to stay with a straight 120-volt solution.  Moreover, you'd need to have a single 120-volt feed to the unit, because again it needs to pass through the thermal safety.  So switching that input between shore and inverter would be one of the two relays I mentioned earlier.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[desi arnaz]

ok, i may  be stupid but why waste your $$$ and your weight?  i paid $52 for an instant hot water heater, not the greatest but it works great. hot showers hot dish washing what else do you need?

[Len Silva]

No need to go to great lengths here.  Simply wire the 220 volt, 4000 watt element to 110.  It will draw 2000 watts and work just fine.

Better re-think that.

If the element draws 4,000 watts on 240 volts, it will draw a mere 1000 watts on 120 volts.  The formula is P=V²/R, where P is power in watts, V is voltage in volts, and R is the resistance of the elements, which is fixed.

Because the relationship is to the square of the voltage, doubling voltage quadruples power, and halving voltage quarters it.

-Sean
http://OurOdyssey.BlogSpot.com

You got me Sean.  I would have done the math but Cody got me all bored and I didn't feel like it.

[cody]

 

[miles2go]

Smaller tank = Less water = will heat faster using less energy and consequently renew more quickly. Mixing cold with a higher temp will stretch it further, just don't scald yourself. That makes sense without even considering the wiring nightmares Sean outlined.

Thanks Sean, et al. Ok then. The "changing everything because I thought 24v. would be better" is out. 30 gallons may be too much water. Look for something between 10 and 20 gallons.

Or... it sounds like just using the 10 gallon may be the way to go immediately for the least expense ($40 used) and quickest satisfactory (hopefully) result. Use a heat exchanger in conjunction with the 10 gallon later as money and time permit.

You certainly must not be stupid Desi. Where to find a Desi Arnaz $52 instant HWH? Sounds like a bargain, and it works great? Wow. Is that propane or electric?

Thanks, smaller tank, simple 110v, modify later... sometimes you can't see the forest for the trees.

Andrew.

[desi arnaz]

propane. i found it on e bay,it was an off brand with a 3 1/2  inch outlet.   they don't have 3 1/2 pipe in the USA ,only china so i had to do some metal work but it works great.

[happycamperbrat]

My 2 kids and I do quite nicely on a propane fired 6 gallon heater.

[miles2go]

I read somewhere that propane recovers more quickly than electric. Anyway, I bought the used 10 gallon electric yesterday, got it for $35. It has one 2000 watt element and is 110v. Tested it before I install it and it was nice and hot in less than an hour. I'll install it today. Thanks for all the input. Your advice saved me time and money, AND needless complications.
Andrew.

[Paso One]

IMO, you are much better off using 120-volt elements and a large inverter while under way.  Better still is a heat-exchanger type DHWH.

Going with the heat exchanger is so reliable and easy to install I often wonder why more people don't take advantage of the heat the engine produces.  To heat their Domestic water.

[Sean]

... I bought the used 10 gallon electric yesterday, ...

I recommend before installing that you first remove and inspect both the element and the anode.  The element may need cleaning if it has a scale build-up, and the anode may need to be replaced, all of which will be easier with the unit out.  Also, if there is a scale buildup in the tank, this is a good time to flush it.

Now will also be a good time to flush the unit with plenty of fresh city water before installation.  This is a good time to inspect and test the drain valve and TPV, which may also need to be replaced.

I would brush any corrosion off the electric terminals with a wire brush before installation, and put dielectric grease liberally on the terminals when making the connection.

-Sean
http://OurOdyssey.BlogSpot.com