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Old 01-09-2019, 01:12 AM   #11
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I agree, to limit electrical power requirements for water heating buffering water is the best way. With an LPG or a marine 4-6 gal. water heater with 750-1500W electric heater rod 8-10 gal. shower temperature water can be obtain every 15-30 min. In my marine water heater, I have a 750W rod electric heating element it is plenty for our light hot water users when on shore power. But I do enjoy speeding my water heating with the roaring 5kW diesel heater.
George, you and I have the same Isotherm 4 gallon water heater with the 750w electric element. The only difference is yours is also heated by your Espar D5 and mine is heated by my engine cooling system.

I wonder how much battery power it would take to maintain the temperature once the water was heated by my engine or your D5?

First, mine would have to cool down to below 160F from 200F+ before even needing any additional heating. What temperature do you heat yours to with the D5? Doesn't the D5 have a thermostat that heats the coolant to 180F?

Without knowing the surface area of the tank and the R factor of the insulation it's hard to calculate the BTU loss. I could try to measure the watts used to maintain the temperature with a Kill A Watt meter once it's heated with engine.

My tank is mounted inside and I believe yours is mounted outside under the vehicle, so you might experience a little more btu/hr loss on a cold day. It might be similar to the energy used by a compressor refrigerator.
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Old 01-09-2019, 01:27 AM   #12
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George, you and I have the same Isotherm 4 gallon water heater with the 750w electric element. The only difference is yours is also heated by your Espar D5 and mine is heated by my engine cooling system.

I wonder how much battery power it would take to maintain the temperature once the water was heated by my engine or your D5?
It takes less net energy to reheat the water than to keep it hot. So, the only real question is "how well insulated is your tank".
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Old 01-09-2019, 02:07 AM   #13
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I never measured how often electric heater cycles when on shore, I just keep it on. D5 keeps coolant between about 160 and 180F when running which for us is once or twice a day.

The simplest way to answer your question would be to measure energy consumed.

My tank is mounted outside in a driver side in under the floor cavity, definitely higher heat losses than from an inside mounted heater.

The recently added EasyStart Timer gives me much easier access from inside to turn D5 on, so my heating system is finished, no more checking or tuning.
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Old 01-09-2019, 02:05 PM   #14
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If you have space for a Truma Combi you might look at that as it also provides nice hot air heat eliminating a separate furnace. The hot water tank is just over 2.5 gallons.
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Old 01-10-2019, 12:28 PM   #15
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There is no great difficulty using shore power to heat your water. Lots of propane water heaters have an electric coil built in. Get one of those and simply don't fill your propane tank (or pull it out).

The difficulty comes from the stipulation the you want an on-demand unit and no a storage tank. That may prove to be difficult, since you need a lot of power to heat the water fast enough. It isn't a great idea anyway, since that style of heater has a minimum flow requirement, and they thus are hard on tank space. A hydronic system with a flash-plate instant heater is more practical, but more complex.
Thanks...doesn’t have to be on-demand, just don’t like propane..
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Old 01-10-2019, 05:21 PM   #16
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It takes less net energy to reheat the water than to keep it hot. So, the only real question is "how well insulated is your tank".
I ran into a guy using the Bosch mini-tank electric water heater. He had a very basic build (small AGM battery) but said it all worked out OK since he would heat the water while driving the van (with his inverter) and just maintain it with battery when camping. So my guess is that these units are well insulated.

It's not like this thing was heating a lot of water anyway since boondockers are generally stingy with water.
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Old 01-11-2019, 11:55 PM   #17
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Thanks, G, you always give great info...I can wait a bit for the hot water, now I just need help figuring it all out!!
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Old 01-13-2019, 04:38 PM   #18
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I used to have a Gulfstream Class B Sprinter RV, that had an on-demand water heater, cannot remember the manufacturer. The only "warm" shower I ever took was in an RV park in summer. The temp of the water in the tank has a BIG impact. Actually thought about putting a solar heater on the roof and cycling the tank water thru. That would also keep the cabin warm at night with that big thermal mass. The other issue was the rate of flow. If it was too low the heater kept turning off and back on, if it was too high the heater didn't have enough time to heat the water. Best I ever measured from it was 92°



There is a product on the market now that uses an Omic Array to heat the water, I like to think of it like microwaving the water to heat it more efficiently than a resistive coil. Check out HeatWorks, the Model 1 that they used to make may be perfect for an RV. Still, if you want a realistic shower on-demand you may wanna look into putting a second power plug on the outside of the RV for 220V so you can run a dedicated wire from the park pedestal 50 amp plug to the water heater. Use the 30 amp plug for everything else. Most places offer both.

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Old 01-13-2019, 05:07 PM   #19
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There is a product on the market now that uses an Omic Array to heat the water, I like to think of it like microwaving the water to heat it more efficiently than a resistive coil.
Well, there may be advantages to this technology, but I don't see how efficiency can be one of them (nor do they seem to claim so). Resistive heaters are, I believe, 100% efficient. Where else would the energy go?
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Old 01-13-2019, 05:13 PM   #20
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Well, there may be advantages to this technology, but I don't see how efficiency can be one of them (nor do they seem to claim so). Resistive heaters are, I believe, 100% efficient. Where else would the energy go?
The graphite heater is an array so convection heat transfer within a, no flow, tank could be improved / faster but efficiency gain will likely be undetectable, heating water with an ohmic element is 100% efficient indeed.
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