If you're going to run it while driving, why not just run it on 110v via inverter? It will likely run fine on a cheap msw inverter.

 

I have 300aH of lifepo4 and a 2000w Kisae inverter.

It would take around 141ah to heat water with the 110v element it comes with for 2 hours a day vs around 20ah using a 12v element, hence why I'm debating trying this "cheap" water heater out while swapping to 12v.

 

My experience is that you only need about 15-20 minutes to heat 2.5 gallons of water. If you need to run the element for 2 hours a day, that's a lot of hot water.

 

I figured, worst case scenario 🤷‍♂️

So I suppose the general consensus so far is run it how it comes and see how I like it before converting it to 12v?

 

That would be my vote. I have 210 AH of LifePo battery (300W solar) and can do hot water. Needed to put hot water heater on a switch and add alternator charging to keep the batteries from deficit during trips.

 

It looks like the Stiebel has a 2400 watt heating element. What is the wattage of the 12V Missouri Wind & Solar unit you are proposing? To get the numbers you are using it is likely in the 200W-400W range. Yes it will use fewer AH in 2 hours but it may take longer than 2 hours to get the same temperature at the lower wattage. On the other hand, it will probably take far less than 2 hours at 2400 watts to reach the desired temperature.

 

Is there a 12v element that fits this? I can't tell if it's the standard size or proprietary?

 

I would think so but if not it’s certainly easy enough to make it fit with off the shelf pipe adapters

 

I saw some metric numbers on a parts web site, if that's the case I think sticking with 120v would be the easiest route.

 

This one says it will fit a 1" standard US pipe thread

https://www.amazon.com/DERNORD-Immersion-Submersible-Element-Stainless/dp/B0182BETBE/ref=sr_1_3?dchild=1&keywords=12+volt+water+heater+element&qid=1619894346&sr=8-3

 

There is no free lunch when it comes to heating water with 12 volts compared to 120 VAC...

To heat 2.5 gallons of water from 60F to 110F at 100% efficiency takes ...
(2.5 gal)(8.34 lb/gal)(1 BTU/lb-F)(110F - 60F) = 1040 BTU, or 305 watt-hrs whether you use 12 volts DC or 120 VAC.

For a 12 volt supply:
(305 watt-hrs) / (12 volts) = 25 amp-hr out of the 12 volt battery

For a 120 VAC supply
(305 watt-hrs) / (120volts) = 2.54 amp-hrs at 120 VAC, which is 25 amp-hrs at 12 volts

The only real difference is that with the 120VAC system you have to account for the inefficiency of the inverter, so with a 90% efficient inverter, the 25 amp-hrs becomes 27.8 amp-hrs.
So, with a 12 volt heating element, 25 amp-hrs to heat the water.
With a 120VAC heating element, about 28 amp-hrs to heat the water.

Seems like if you can find a 12 volt heating element, it gives you a bit more flexibility with no need to have the inverter involved. You can also play with heating element size - a 300 watt element will take about an hour to heat the water and draw 25 amps. A 600 watt element will take half an hour to heat the water and draw about 50 amps -- and so on.

edit: corrected the bad calc for the 600 watt heater just above.😞
Gary

 

Hi Gary, I know you know what you meant but this could be confusing to some people if the amp-hours don't have a voltage specified.
That may have been the original misunderstanding.

So, with a 12 volt heating element, 25 amp-hrs to heat the water.
With a 120VAC heating element, about 28 amp-hrs to heat the water.

So, with a 12 volt heating element, 25 amp-hrs @12v or from the batteries to heat the water.
With a 120VAC heating element, about 28 amp-hrs @12v or from the batteries to heat the water.

28amp-hrs @ 12v = 336w
28amp-hrs @ 120v = 3360w

 

Right -- probably did not say that too well.

Gary

 

Thank you all! I understand what went wrong now.

The Stiebel comes with a 1300watt heating element. It is arriving tomorrow, but on second thought I may just get a Rheem 2.5 gallon performance point-of-use, it's MUCH more compact. The casing of the Stiebel makes it a bit bulky. But I'll wait to see it in person.

Even if I'm not getting the same amazing deal at the Stiebel, the Rheem is still less than half the cost and easily swapped over to 12v.

 

The Rheem 2.5 gallon is the one I put in. It has a drain on the bottom to “winterize”. Takes about 20mins to heat the water on the 120v.

RV8Rs Camper Van Buildout

Hello Builders & Conversion DIYers I have been outfitting a 2018 EXT for the last 6 months. Here are some photos

www.promasterforum.com

 

If you wanted to try the inverter 120v

Johnson Pump has 500W 120v tanks that also are engine heat exchangers. The 500w would be gentle with alternator charging to inverter

AquaH Marine Water Heaters | SPX FLOW

AquaH - is an innovative series of AISI 316 stainless steel water heaters, with a storage capacity of 22 up to 80 litres (5.3 to 19.3 gal.). Dual heating as standard - electrical heating and indirect heating based on a second heat source, for example the cooling system from the Marine engine or...

www.spxflow.com

Then if you did not like that you have the option of hooking up engine heat exchange - many on here like that system.

Inverters can be 90% or a bit more efficient (IIRC my Victron spec is 93% efficient).

I could not see the 12v heating element you were thinking of. If running off alternator then where everyone above has used 12v, I do not understand as the alternator will supply 14v I think. I am no expert in electrical but in my mind a 300W 12v heater formula on alternator “math” would be 300W divided by 14v = 21.4 amps. Our PM alternators do not provide 12v.

Regardless; I believe a good efficient inverter to 120v is a better way to go (easier to negate the distance of the wire in 120v & more plug & play). Sure you will loose 10% efficiency with an inverter, but 14v is also 17% more than 12v.

If I was looking at doing what you are thinking of I would try out the Johnson Pump 500W 120v 5.8gal

 

If you wanted to try the inverter 120v

Johnson Pump has 500W 120v tanks that also are engine heat exchangers. The 500w would be gentle with alternator charging to inverter

AquaH Marine Water Heaters | SPX FLOW

AquaH - is an innovative series of AISI 316 stainless steel water heaters, with a storage capacity of 22 up to 80 litres (5.3 to 19.3 gal.). Dual heating as standard - electrical heating and indirect heating based on a second heat source, for example the cooling system from the Marine engine or...

www.spxflow.com

Then if you did not like that you have the option of hooking up engine heat exchange - many on here like that system.

Thank you for the suggestion, I have not seen those yet. The dimensions seem to be a touch to big, but I'll check.

 

The inverter inefficiency on a large load is not going to be much different than the voltage drop from such high 12V currents. Sometime a little better and sometime a little worse, but when you start taking about 30+ amps of 12V current the losses in the wiring start getting pretty large too. It all depends on the lengths and sizes of wiring because the current has to get the inverter too, but those runs tend to short and very large gauge.

 

Harry, you just described my plumbingless van. 😊

We are headed toward Colorado tomorrow. Since we don’t know our route for the next couple weeks, we are carrying 20 gallons of water.

WordPress.com

msnomersvan.wordpress.com

 

Well, I received my hwh today and it actually fits pretty perfectly. I'm surprised, maybe for the deal, we'll go with this until I want to replace it with a calorifier.

 

I saw a video a while back about this. I follow this guy and he is very knowledgeable. Lot of cool hacks. Even has a 12 volt 3D printer.

 

I've watched his videos before. Thanks for the heads up though!

 

Hi,
On using a 12 volt DC heater vs using an inverter with a 120 VAC heater.

I'm not seeing why the 12 DV heater is not a better choice?
An example.

Examples:
Use 12 volt DC heater:
12 volt, 1000 watt heating element, wire runs to water heater 6 ft each way (12 ft round trip)
amperage = 1000 watts / 12 volts = 83.3 amps
Using BlueSea Circuit Wizard,
#3 AWG gives 2% voltage drop (0.24 volt drop on supply plus return)
#6 AWG would be miniumum gauge required for 83 amps, and gives 4% voltage drop

Power loss in the wiring for #4 AWG = (0.24 volt)(83 amps) = 20 watts
Power loss if #6 (min for ampacity) = (0.48 volt)(83 amps) = 40 watts

This gets harder if the water heater is further away from the battery, but does not seem like it gets out of hand -- if you move the example water heater (say) 10 ft away, you would have to go up to #1 AWG to keep the 2% voltage drop, or #4 AWG for 4% voltage drop.

Use 120 volt AC heater:
120 volt 1000 watt heating element, wire run 3 ft (6 ft round trip) to inverter, tank, inverter efficiency 85%. Ignore voltage drop along the 120 VAC wire from inverter to hot water tank.

There would be some voltage drop on the wires from battery to inverter, but they might be pretty large gauges for other loads, so, ignore these losses.

Power loss in the inverter depends on how good it is -- looking at some of the data and tests out there, the better inverters are around 90% and the not so good ones more like 80%. Say 85% as an average.

Power loss is about 15% for the inverter
Power loss = (1000 watts) (0.15) = 150 watts
Not counting any voltage drop losses in the wires.


Seems like for this example, the losses with the inverter setup are about 150 watts and losses for the 12 volt heater setup are more like 20 to 40 watts.
Seems like the only downside of the 12 volt setup is having to use some heavier wires, but the changes don't seem that dramatic. And, if a lower wattage heater is used, the gauges would go down, but heating would take longer.
The inverter also has the issue of idle power use if there is no other reason to leave it on except the hot water heater.

Maybe I'm missing something?

Gary

 

I consider a 12v water heater with a thermostat a battery, charge it up, once it's heats up it would just cycle on and off like a 12v frig.

 

100-200 watt element and connect it to the "load" terminals on your solar controller. And as Phil said, an inline thermostat.

 

A consideration that may affect some is if the RV (camper van) will be used sometimes with shore power at typical campgrounds. In those cases, having power provided directly from 30-Amp 120V service would be preferable to having to go through a converter to first drop to 12V.

It’s not a big issue for most on this forum that avoid campgrounds, but for the few like me who frequent KOA-type from time to time, it should be considered.

This becomes even more of an issue if trying to power a large 12V air conditioner and or charge a large battery bank at same time. Converter capacity should be considered, although in case of hot water, extra energy for heater can be provided short-term from batteries.

I’ve thought about this because we would likely use campgrounds when touring at least every 2 or 3 days/nights to dump tanks, refill water, and charge batteries as necessary.

 

@el Jefe Typo I'm sure you meant 100-200 watts?

 

Thanks Phil. Corrected.

 

I plan on doing this with my next van. Since it's 12V, you don't need to bother with an inverter. The great thing you can do is wire up a relay from your battery monitor and instead of turning your battery to float when your battery is fill, you divert the energy to the water heater as a 'dump load'. You can also have it on a switch to turn it on demand.

 

I have a Stiebel-Eltron tankless water heater at my cabin and in my experience it takes a lots o' electricity to get it to work right. Then you also have to calibrate flows to get the right amount of hot. I have used it for 10 years and it has had plenty of finicky moments where it crapped out at important times.
So recently I bought a Joolca Nomad and plan to use it instead - at my cabin and also in the van:

Joolca

 

I have a Stiebel-Eltron tankless water heater at my cabin and in my experience it takes a lots o' electricity to get it to work right. Then you also have to calibrate flows to get the right amount of hot. I have used it for 10 years and it has had plenty of finicky moments where it crapped out at important times.
So recently I bought a Joolca Nomad and plan to use it instead - at my cabin and also in the van:

Joolca

We'll see what happens 🤷‍♂️

 

Here's a data point from someone else on a Bosch 2.5 gal water heater.