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Old 06-12-2018, 09:02 PM   #1
Graphite Dave
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Default Simple Shower Warm Water Heater

In the sold Sprinter I had a 5 gallon SS beer keg with a 450 watt heating element to provide warm water for a shower. Have made some improvements for the Transit build. To eliminate the space required for the beer keg I had a custom 14 ga. SS tank built to also be the support pedestal for the portapotti and located the tank/potapotti in the shower enclosure. That used wasted space. The 120 volt AC heating element was changed from 450 watts to 625 watts. The Sprinter used a "6 gallon" propane to electric conversion kit. The Transit kit is the "10 gallon" kit. The tank is a non pressure tank with holes for filling and for the pump on the top of the tank. Pump is a 12 volt DC submersible centrifugal pump. Pump discharges into a garden hose with a on/off nozzle. I do not need hot water at the sink. This design does not have any plumbing or hot/cold water mixing. All the water in the tank is at the correct temperature. Tank is not insulated so warm water is not stored.

At start:

5 gallons of water.
Air temperature = 74 degrees.
Water temperature in tank = 74 degrees.
Single 255 amp-hr battery at 100% SOC.
Weather was clear with full sun.
Single 300 watt solar panel with MPPT controller.
No other electrical running.
Thermostat to control the heating element.
Indicator light to show when power applied to heating element.

Tested the system today with these results:

9:55 AM start with 5 gallons of water at 74 degrees.
10:38 AM power off to heating element. Water temperature 97 degrees and SOC at 92%.
2:30 PM the house battery SOC was back to 100%. Used 20.4 amp-hrs of the house battery.

The test result shows the house battery/inverter can be used to heat shower water on a clear day. On no sun days the 120 volt AC power from the vehicle powered inverter would be used with the engine running or shore power if that is available.

For one person shower less water could be put in the tank to reduce the heating time. A 12 volt DC heating element could be used instead of the 120 volt AC unit I use.
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Old 06-14-2018, 07:50 PM   #2
Graphite Dave
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Default Re: Simple Shower Warm Water Heater

A couple more tests were done on the 5 gallon shower water heating.

Start at 10:00 AM with SOC @ 100%, air at 68 degrees, water in tank @ 67 degrees.
10:46 AM thermostat stopped heating with water @ 93 degrees and SOC @ 90%.

So water gains about 1/2 degree for each minute with 625 watt heating element and 2 1/2 degrees for each 1% of SOC of 255 amp-hr battery capacity.

Amperage from the battery when heating was 58 amps DC. 625 watt heating element would use 52 amps. The 300 watt solar panel was outputting 10.4 amps. So without solar amps would have been 68.4 amps. The Magnum owners manual states inverter efficiency at peak output is 87%. Running the inverter at 60- 70% of max. output probably has efficiency less than the 87% at full load.

At 90% SOC I used about 25.5 amp-hours of the 255 amp-hr battery. 58 amps x 46 minutes/60 = 44.5 amp-hrs. Inverter eff. must be about 25.5/44.5 = 57%. Good argument to use a 12 volt heating element so inverter is not required.

Heating shower water with the house battery is possible on a clear day without running the vehicle engine. On days without sun the water heater can be powered from the vehicle powered inverter with the engine running.
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Last edited by Graphite Dave; 06-14-2018 at 07:53 PM.
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Old 06-14-2018, 10:02 PM   #3
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Default Re: Simple Shower Warm Water Heater

How much water does an average shower consume?
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Old 06-14-2018, 11:09 PM   #4
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Default Re: Simple Shower Warm Water Heater

Quote:
Originally Posted by Bobnoxious View Post
How much water does an average shower consume?
You can do a Navy shower with about 2 1/2 gallons.

My shower routine is unusual because I use the shower space for multiple purposes. Do not want to waste all that space for just a shower. So first I need to remove two shelves, two storage containers and three towel rods at the top of the shower. That leaves the shower enclosure emptied with the portapotti still at the bottom. Portapotti remains in place when showering. Do not want shower door or a curtain.

https://sprinter-source.com/forum/sh...&postcount=150

Next I put down a towel on floor in front of the shower to catch any overspray. There is very little. Then I kneel down in front of shower and put my head in shower to do my hair. When hair done I get in the shower between the portapotti and back shower wall. Get wet using a full flow (not spray) radiator fill valve that has a on/off flow lever. Soap body and then rinse off.

Works well to get clean but certainly not like a shower at home. So amount of water used depends on how you shower.
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Last edited by Graphite Dave; 06-14-2018 at 11:13 PM.
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Old 06-16-2018, 04:47 PM   #5
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Default Re: Simple Shower Warm Water Heater

Completed all the testing so this morning dumped the grey water tank so I could pump the shower water tank into the grey tank to rinse it out with the 5 gallons of fresh water.

Was amazed about how long it took to drain the shower water tank. Should have timed it. Suspect the 5 gallons is good for 4 Navy showers.
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Old 07-23-2018, 02:54 PM   #6
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Default Re: Simple Shower Warm Water Heater

Coming at this from a different direction and using your system as a calorimeter - we get:

93f-67f=26f delta

5 gal * 8.34lbs/gal = 41.7 lbs water

BTU = H2Olbs * Tdelta = 41.7*26 = 1084.2 BTU = 317 watt hrs (energy transfered to H2O)

Your electrical input was:

68.4 amps *12 v * 46 min/60min/hr = 629 watt hrs

317/629 = 0.503 = 50% efficiency (very close to your estimate)

If you were to measure how fast the temperature drops after removing the power we could add an estimate of heat loss from the keg.

Last edited by rollerbearing; 07-23-2018 at 03:00 PM.
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Old 07-23-2018, 03:21 PM   #7
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Default Re: Simple Shower Warm Water Heater

Quote:
Originally Posted by rollerbearing View Post
Coming at this from a different direction and using your system as a calorimeter - we get:

93f-67f=26f delta

5 gal * 8.34lbs/gal = 41.7 lbs water

BTU = H2Olbs * Tdelta = 41.7*26 = 1084.2 BTU = 317 watt hrs (energy transfered to H2O)

Your electrical input was:

68.4 amps *12 v * 46 min/60min/hr = 629 watt hrs

317/629 = 0.503 = 50% efficiency (very close to your estimate)

If you were to measure how fast the temperature drops after removing the power we could add an estimate of heat loss from the keg.
System is not designed to retain heated water. Turn on water heater when you want a shower, shower and then let heated water return to ambient temperature. Can not insulate the SS tank because it also serves as part of the shower pan. Top of tank is sloped to direct water to the pan drain. Reason for the tank location was to elevate the portapotti to the correct height and to use the wasted space under the portapotti. Tank could be located someplace else and insulated if a person wanted to retain the heat.

I do have a indicator light wired into the element power. Light is on when the element is powered and off when element is off. Use light to tell me when warm water is ready to be used.
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Old 07-23-2018, 03:27 PM   #8
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Default Re: Simple Shower Warm Water Heater

I read post again and saw you used a 625 watt heating elememnt.

Using that:

625 watts * 46/60hr = 479 watt hrs electrical input

317 watt hrs/479 watt hrs = 0.66 = 66% efficiency

I would want to confirm this more accurately, but what this tells me is that the bulk of you inefficiency is from heat losses and not inverter losses. This is assuming that the element is truly operating at 625 watts.
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Old 07-23-2018, 03:30 PM   #9
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Default Re: Simple Shower Warm Water Heater

I realize you are not retaining the heat for later use - simply wanted the number in order to estimate where the inefficiencies are in the heating up process. The above post I just made maybe solves that question. Absolutely not a criticism of the system.

What I would conclude though is that it probably isn't worth worrying whether or not the element is 12v or 120v.

Last edited by rollerbearing; 07-23-2018 at 03:50 PM.
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Old 07-23-2018, 03:50 PM   #10
Graphite Dave
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Default Re: Simple Shower Warm Water Heater

Quote:
Originally Posted by rollerbearing View Post
I realize you are not retaining the heat in use - simply wanted the number in order to estimate where the inefficiencies are in the heating up process. The above post I just made maybe solves that question. Absolutely not a criticism of the system.

What I would conclude though is that it probably isn't worth worrying whether or not the element is 12v or 120v.
Agree. Does not make any difference if heating element is 12 volt DC or 120 volt AC. In my case I already had 120 volt AC available from the vehicle powered inverter so used that. 120 volt AC also allows use of shore power if it is available. 12 volt would be more efficient because it eliminated the inefficiency of the inverter.

The efficiencies are not important. The change in mpg when powering from the alternator would be very small. The shower is not used every day.

The other advantage of the system is the elimination of plumbing and hot/cold water mixing. Cost is considerably less than any alternate system (if you already have the vehicle powered inverter).

All choices have advantages and disadvantages.
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Last edited by Graphite Dave; 07-23-2018 at 03:52 PM.
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