Hot Water Heater - Tank or Tankless - Fleet Shell Build

[losttaco]

Hello, I am currently in the process of building out my fleet shell. Hot water is something I would love to have.

From my understanding FWC currently uses an "Attwood/Dometic" 6 Gallon in tank water heater.

Overland X uses the tankless "Truma" brand.


My research has kinda shown me pros and cons to both. I am trying to find some more direct advice. I was originally intrigued by the tanked heaters. Thinking it would heat up, hold temp for longer, and I would be able to hold more water.(Fill it when i fill my fresh water tank). They both run off propane and use 12 volt to ignite. I plan on using a 12-16 gallon fresh water tank. I also planned on plumbing in an out door shower. I laid out some parts tonight and the hot water should fit right under my sink.


Options right now are:

Suburban 4 or 6 Gallon Tank Water Heater

Attwood / Dometic 6 Gallon Water heater

Fogatti Tankless Water Heater


Thanks for any input, heres a few pics.

 

[Jon R]

One thing I read about on demand heaters for RVs is that they aren’t recommended if you are trying to conserve water. Each time you turn them on the water runs for several seconds before it gets hot. I have no experience myself with how much water gets wasted due to this.

 

[pvstoy]

I would prefer a tank material that does not need a anode rod. Less cleaning, less mess and no chunks to plug up faucet / shower heads.

 

[Jon R]

Current delivery required to support flow for a shower is going to be the big problem. To heat 1 gallon per minute from 40F to 110F with a 100% efficient heater would require 32,600 BTU/hr or 9750 watts. Heating 5 gallons would use about 800 watt-hours, or about 67 amp-hours, More importantly, 1 gallon per minute delivery would require 800 amps at 12 volts. Even if you back off to 1/2 gal/min it requires 400 amps. A Battleborn 100 Ah battery has a max discharge rating of 100 amps.

Another way to look at it is a full charge of a 100Ah battery stores about 4,000 BTUs. One of the ten gallon propane tanks stores nearly 1,000,000 BTUs.

 

[Wallowa]

Small points...hot water tank when drained into main tank give you a hedge in the winter against freezing lines...and as stated you carry 6 gallons more potable water...

 

[losttaco]

Current delivery required to support flow for a shower is going to be the big problem. To heat 1 gallon per minute from 40F to 110F with a 100% efficient heater would require 32,600 BTU/hr or 9750 watts. Heating 5 gallons would use about 800 watt-hours, or about 67 amp-hours, More importantly, 1 gallon per minute delivery would require 800 amps at 12 volts. Even if you back off to 1/2 gal/min it requires 400 amps. A Battleborn 100 Ah battery has a max discharge rating of 100 amps.

Another way to look at it is a full charge of a 100Ah battery stores about 4,000 BTUs. One of the ten gallon propane tanks stores nearly 1,000,000 BTUs.

Thanks for the information. I do agree an electric water heater is out of the question. I planned on using the propane units. The two that would fit the best are the Fogatti models by Recpro. one is a 42,000 but and other 55,000 btu.

From what I have found out a lot of people replace the suburban 4/6 gallon water heaters with these.

 

[Jon R]

Losttaco - you were clear that you are looking at propane heaters. I was responding to ramblinChet saying he was considering an electric heater. I should have quoted his message.

 

[losttaco]

I am working through a similar issue although I would like to build using a 12V tankless water system.

Yes, it would use a significant amount of energy but with a few large solar panels up top and maybe 200-300 Ah of LiFePO4 battery available I believe it would work. If anyone has done this I would love to learn more.

Losttaco - you were clear that you are looking at propane heaters. I was responding to ramblinChet saying he was considering an electric heater. I should have quoted his message.

Just seeing his post now. I have seen some school bus set ups with electric water heaters but they usually ran them off shore power / had a lot of solar and lithium batteries.

I think I am going to amazon one of the models above and try it out. I plan on doing some ski resort camping this winter and would appreciate anything thats not outside temp!

 

[Jon R]

You are correct, sir!

 

[MaverickMTB]

I was interested in the Truma. I liked that it’s a combination heater and water heater. I believe it holds around 2 gallons, so has at least some hot water holding capacity and is able to replenish as you draw down the hot water. I was considering ordering the “JP Products” version which appears to be a Chinese copy of the Truma Combi. My old camper had the Suburban tanked heater and it worked very well. When it was around 10 years old the tank rusted out and I replaced it with the same model. Very easy to swap out and six gallons of hot water was a pleasure to have at the end of a sweaty day. At around $400… the Suburban is hard to beat for the price!

 

[ckent323]

RamlinChet, John R, et. al,

Completing the corrected calc above for required Ah from the battery:

One gallon of water from 40° F to 110° F would require 583 BTU.

1 BTU equals 0.000293 KWh. 1 KWh at 12.8 v equals 78.125 Ah

So 583 BTU equals 0.17089 KWh and 0.17089 Kwh equals 13.351 Ah at 12.8 v.

Therefore to heat 5 gal of water 70 deg F requires 66.754 Ah of battery (or solar) power.

Note there is no time factor in these calcs except for Ah. So in order to calculate the amps needed to heat 1 gal of water in one minute we need to multiply Amp hours by 60 to get amps per minute.

The result is 13.351*60 = 801.46 Amp*min. You are not going to be able to provide that much current without melting wires.

6 gauge wire can handle about 55 amps. So the fastest you could heat 1 gal 70 F using 12.8 V with 6 gage wire is about 805 A*min/55 A = 14.64 minutes if I did the calculations correctly.

That is not fast enough for a tankless shower.

So as has already been concluded using a 12 v system to power a tankless water heater unit for showering is not going to work in practical terms.

I suppose with a large enough battery bank and an appropriately sized inverter it would be possible to use a 120v tankless shower in a camper (RV or boat). However, the efficiency of the electrical conversion is only around 50%.

0.17089 Kwh at 120 v equals 1.424 Ah or about 85 Amin

Factoring in the electrical conversion gives 1.5*85 Amin = 128 Amin

As a check I looked up electric tankless units and I find that "the average electric tankless water heater needs at least 120 amps to operate".

If the heating was cut back to say 1/2 gal per min the current required would drop to about 60 amp which may be feasible using 4 ga wire.

One gallon of propane provides about 91,500 BTU = 27 KWh of electricity. It is hugely more efficient to heat water with it.

Diesel has a higher energy density than propane and (ignoring smell and other reasons people don't like diesel) it is a more efficient fuel for heating. Indeed, 1 gallon of diesel produces about 137,381 BTU = 40.26 KWh.


It appears to this is why all the water heaters use diesel or propane.

 

[Lighthawk]

Assuming there are no dumb questions, what about using a DC to DC charger and preheating 6 gal of water while driving the last hour to camp? This assumes a tank and not on-demand.

 

[rando]

Assuming a 30A DC-DC converter at 13V, you would get around 1400kJ of energy into your water tank in an hour, which would raise the temperature about 10C. You would either need to switch the heating element for 12V version (not sure if that exists) or use a (large) inverter to power the 120V element. It does provide some benefit, but given the energy density of propane, I am not sure it is worth the effort.

 

[Vic Harder]

Assuming there are no dumb questions, what about using a DC to DC charger and preheating 6 gal of water while driving the last hour to camp? This assumes a tank and not on-demand.

I read once about water heaters that use the engine cooling system in a water/water heat exchanger to provide hot water in a camper....

 

[DanoT]

Just seeing his post now. I have seen some school bus set ups with electric water heaters but they usually ran them off shore power / had a lot of solar and lithium batteries.

I think I am going to amazon one of the models above and try it out. I plan on doing some ski resort camping this winter and would appreciate anything thats not outside temp!

Running a water system while camping at temps usually found in ski area parking lots often leads to frozen pipes. I don't run my water system in winter; instead, I heat up water on the stove and use a Hot Jugz portable shower with a low flow shower head, using less than 2 gal of water.
http://www.hotjugz.com/