14 replies to this topic

[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.

 

 

Edited by ckent323, 27 November 2021 - 08:39 PM.

[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/

 

Edited by DanoT, 30 November 2021 - 04:52 AM.