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Discussion Starter · #21 ·
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.

Awesome! I love how compact it is. I WISH I could just find one locally. They're all out until June. Amazon and sears website are the only places I can get one.
 

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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


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
 

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Discussion Starter · #23 ·
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


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.
 

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Thank you for the suggestion, I have not seen those yet. The dimensions seem to be a touch to big, but I'll check.
They are in the same price range as Isotherms. I think Isotherms are good also, but 700W heating element IIRC.

I edited my above post incase you didn’t see my edits.

Good Luck with your choice !!
 

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Discussion Starter · #25 ·
They are in the same price range as Isotherms. I think Isotherms are good also, but 700W heating element IIRC.

I edited my above post incase you didn’t see my edits.

Good Luck with your choice !!
Thank you.

Now I'm wondering if I scratch the mounting position idea and just get what you suggested or the isotherm and see if I can install it in the driver's side open area of the engine bay, under the headlamp.
 

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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


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
Actually the resistance is fixed, so if it is 300W at 12V, it draws 25Amps, meaning the resistance is 0.48 ohms. More likely it is designed to operate at 13.2 - 13.8 volts, but the point is the same.

At 14V it will draw (14/0.48) 29.1667 amps, for power of (14 x 29.1667) 408W, so it will be faster to heat water, so long as the element can handle the current flow without burning up.

This is why a 220V element will only produce 25% of the power at 110V even though we might naively expect it to produce 50% of the power.
 

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Thank you.

Now I'm wondering if I scratch the mounting position idea and just get what you suggested or the isotherm and see if I can install it in the driver's side open area of the engine bay, under the headlamp.
The Hot Water Tank is my most indecisive decision on van build #1 (I did it twice). I am building van #2 when it finally arrives & still can not decide about hot water. Both builds all we have for plumbing is a sink 😳. We typically heat water in a pot using propane to wash dishes 🙄. We can & have (if 120v is not available), just plug our Honda generator into the shore power & heat our 2.5G tank in less than 20mins (but then we have the generator running for 20mins).

Van #2 I am reserving a space for a HWT but will not buy or install one until I have decided. I am still on the fence, Isotherm / 120v / Propane tank / or what we do now heat from a propane stove.

In the end of my “years” of pondering what is interesting is this thought; For me the decision’s largest factor isn’t even about the type of heat, but about “water conservation”. That is the entire install must be very conservative of water consumption
 

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Thank you.

Now I'm wondering if I scratch the mounting position idea and just get what you suggested or the isotherm and see if I can install it in the driver's side open area of the engine bay, under the headlamp.
I am pretty sure it will not fit there. It is not designed for exposure to the elements FWIW. Just run coolant lines to where you can install it.
 

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Actually the resistance is fixed, so if it is 300W at 12V, it draws 25Amps, meaning the resistance is 0.48 ohms. More likely it is designed to operate at 13.2 - 13.8 volts, but the point is the same.

At 14V it will draw (14/0.48) 29.1667 amps, for power of (14 x 29.1667) 408W, so it will be faster to heat water, so long as the element can handle the current flow without burning up.

This is why a 220V element will only produce 25% of the power at 110V even though we might naively expect it to produce 50% of the power.
Thanks for that @jracca !!

That is why my gut says short 14v runs to the house battery & inverter & then distribute to the hot water tank in 120v. I would take the inverter efficiency penalty & keep it 120v if considering this (which would also give me redundancy with shore power or a 120v generator for 1/2hr or so.
 

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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.
 

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In the end of my “years” of pondering what is interesting is this thought; For me the decision’s largest factor isn’t even about the type of heat, but about “water conservation”. That is the entire install must be very conservative of water consumption
Exactly. Water is usually the limiting factor in camping.

Skip using a "water system" completely.

Use paper plates and use them for camp fire kindling vs washing dishes.

Try to not "cook" - instead re-heat pre-cooked food.

A coffee cup of slightly soapy water can clean utensils.

Use 3 liter or similar size "bottled water" from a store to deal with having a clean water source, storage, heating and dispensing (showering).

Create a location for these "cases of water" vs tanks and heaters.

Heat in a microwave oven or a sous vide water heater.

Saves tons of room and the costs is much lower. Risk of leaks is small.
 

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Discussion Starter · #33 ·
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.
 

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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.
 

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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
 

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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.
 

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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.
 

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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.
 
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