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.realgoods.com/product/solar+power/solar+hot+water/diversion+heater+elements+24v+60a+48v+30a.do)
http://www.missouriwindandsolar.com/Water_Heater_Elements.html (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?
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.
Great! Thats an answer I really like, thanks.
Quote from: Len Silva on August 29, 2010, 09:39:05 AM
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=V
2/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 (http://ourodyssey.blogspot.com)
Quote from: miles2go on August 29, 2010, 09:12:20 AM
... 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.
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... Does the thermostat need disconnecting or re-wiring.
Depends. I'd need the schematic to answer that.
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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.
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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.
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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 (http://ourodyssey.blogspot.com)
Quote from: Sean on August 29, 2010, 11:22:33 AM
Quote from: miles2go on August 29, 2010, 09:12:20 AM
... 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 (http://ourodyssey.blogspot.com)
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?
Quote from: Sean on August 29, 2010, 10:59:41 AM
Quote from: Len Silva on August 29, 2010, 09:39:05 AM
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 (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. :-[
;D
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.
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.
My 2 kids and I do quite nicely on a propane fired 6 gallon heater.
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.
Quote from: Sean on August 29, 2010, 11:22:33 AM
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.
Quote from: miles2go on August 31, 2010, 05:39:04 AM
... 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 (http://ourodyssey.blogspot.com)
If you have an inverter already, don't bother with the low voltage dedicated heating coil. What I have is two 10gal water heaters-one plumbed to the next with the final water heater wired through the inverter so I can have hot water during the day going down the road. Simple, works well, and if on only 30amp power pole, what I do is heat the inverter water heater first (the second in the series) then heat the first water heater and leave that on. Then when using the hot water, the cold water is coming into the first water heater being heated before going into the second water heater that no longer needs to have power to it. Has worked flawlessly now for 15 years, and am repeating it on my truck. Good Luck, TomC
Good advice. I flushed it well before testing it and it was clean, but I will check the anode, element, and the rest. What does TPV stand for....thermo protection valve? The pressure relief valve?
A heat exchanger is a great idea (and I would like one) but later (time and money). Powering the hot water heater through the inverter will heat the water while under way, that is enough for starters. Or is running the roof airs, two 13.5k BTU units, and trying to power the HWH too much? 20 amps each for the airs on start up, but I am not sure what the roof airs pull once they are just running. Then for 2000 watts of water heating element, about 16 amps? 20+20+16=56 amps. That is almost right at the 60 amps pass through of the 4024 when on shore power or generator. But it is inverting while underway.
I think the D50 on the 8v71 is only 270 amps or 6480 watts... thats not enough is it. 2x2400 watts for the A/C units and 2000 for the HWH is 6800 watts... thats why I need a heat exchanger. 6480 watts going in to the batteries and then the inverter from the D50 and 6800 coming out, plus what's already in the batteries, minus inefficiencies, probably will not work.
Ok, I looked at the roof airs. the locked rotor amps is a whopping 63.5. That is to start the compressor? The full load amps is 12.4 while running. The fan draws 3.0 amps running and 8.5 amps to start. The minimum circuit ampacity and circuit protection is 20 amps. How does the momentary start up of 63.5 not trip a breaker? Never mind, it doesn't, it works.
Does one try to calculate wattage using 120v or 115v? This is way too complicated.
Trace 4024=33.3 amps (I guess, 4000 watts at 120v.). Each roof air draws 15.5 amps while running, that's tight if it will work.
Don't let me have formulas anymore, I overcomplicate things. A little (miniscule) knowledge can be a dangerous thing. I suppose this is why I do need a heat exchanger after all. Woe is me, and I haven't even connected the Generator yet. :-/
Andrew.
Quote from: miles2go on August 31, 2010, 09:38:09 PM
... What does TPV stand for...?
"Temperature and Pressure relief Valve." It will be located near the top of the tank, will have an outlet directed downwards, and a little lever on it that pulls a plunger out. The discharge needs to be plumbed out the bottom of the coach. You operate the lever with the tank under city water pressure and make sure water comes out. That doesn't guarantee the valve is good, but you will at least know that it is not obstructed and the plunger is not corroded in place.
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... Or is running the roof airs, two 13.5k BTU units, and trying to power the HWH too much?
Looks like you've already answered your own question, but, yes, that's too much. You will be able to run two out of the three. The roof airs will draw from 13 to 15 each amps when running "steady state" (as opposed to starting up), and at the 120 volts that your SW4024 will put out, your 2000-watt water heater will draw over 16. The 4024 can produce a steady 30-33 amps under ideal conditions; as the temperature starts creeping up near the unit, that will actually go down. So even two out of the three might be a stretch. Which, as you surmised, is why I recommended a marine water heater with a built-in heat exchanger.
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I think the D50 on the 8v71 is only 270 amps or 6480 watts
It's actually over 7,000 watts because the 270 amps is being pumped out at over 28 volts. The 50DN is not the limiting factor here; the SW4024 is. At max continuous load of 4kW, even accounting for the ~10% efficiency loss, you'll only be using a little over 60% of the capacity of the alternator, leaving plenty left over for battery charging. BTW, we do this all the time, running two air conditioners totaling 3,600 watts or so.
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.... 6480 watts going in to the batteries and then the inverter from the D50 and 6800 coming out, plus what's already in the batteries, minus inefficiencies, probably will not work.
Just to be clear, 6,800 will not be coming out. Again, your SW4024 is rated for 4,000 watts continuous. It can supply nearly twice that as a "surge" rating, but this is for a matter of a few seconds, not minutes or hours.
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Ok, I looked at the roof airs. the locked rotor amps is a whopping 63.5. That is to start the compressor? The full load amps is 12.4 while running. The fan draws 3.0 amps running and 8.5 amps to start. The minimum circuit ampacity and circuit protection is 20 amps. How does the momentary start up of 63.5 not trip a breaker?
It
will trip a conventional circuit breaker. Air conditioners need a type of breaker known as "HACR" and if you look at the breaker I am sure you will find this marked on it. In practice, most circuit breakers sold today for residential panels are HACR type. That stands for "Heating, Air Conditioning, Refrigeration" and the breaker has a "surge" capacity that lets it carry large currents for short durations in order to start compressors.
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Does one try to calculate wattage using 120v or 115v?
You use the actual voltage you are working with. Your SW4024 is set to produce output at exactly 120 VAC. 120 is also the current North American standard for residential and light industrial voltage, although in practice voltages between 110 and 125 can be encountered. "110" is an older standard, even though old-timers tend to use that number (and it's companion "220") when talking about 120 (240) volt circuits.
-Sean
http://OurOdyssey.BlogSpot.com (http://ourodyssey.blogspot.com)
Now I know what I need to do. Thanks.