DIY water heater?

[geoffire]

Hi, I'm building an overlanding vehicle for our family of 4 and need the ability to somewhat regularly take hot showers. I'm looking to go fully electric and don't see anything on the market that would work well. There are small electric water heaters and small "instant" electric water heaters, but none of those have the ability to actually shower multiple people from non already quite warm water at reasonable power levels. Plus they are 30a 120v (or even larger 220v) so I'd need to run a very large inverter for no real reason (2000w is enough otherwise).

Here's my thought: ~3600 watts of DC heating elements in a homemade ~2 gallon hot water tank. I'm specing the electrical system (for the whole vehicle) still and currently leaning 12v, so 6 of these would do the job https://www.amazon.com/gp/product/B0182BETBE, or if I go 24v 4 of these https://www.amazon.com/gp/product/B0182BETBE. Likely I'd set it up so one element is used to maintain temp normally, but I can disable the inverter and enable the full power when we take showers (likely all of us one after the other after a hike or whatever).

I have 16 280ah EVE type cells on the way, so can power it (with ~4800 watt bms and less than 1000w peak of non-inverter powered load). Max power is quite a lot (especially for 12v), but it's only every other day or so for a short period of time and the wire runs will be very short.

Seem reasonable?
Thanks,
Jeff

 

[Hedges]

If you do a homemade tank, it's up to you to implement safety devices such as temperature/pressure relief valve and overtemperature shutoff, in addition to thermostat control. High current DC is more difficult to switch than AC.

2 gallons is 16 lbs. I would use a larger hot water tank and store energy in the form of hot water rather than (expensive) batteries. A higher temperature and tempering valve (blend with cold water) will deliver more gallons at a comfortable temperature.

I would heat the water gradually with surplus power, either excess PV after batteries charged or from alternator while driving.
An inverter to deliver 120V to a heating element seems reasonable rather than 12V, then the water heater could be off the shelf. A 240V water heater fed 120V consumes 1/4 power, so easy to implement moderately low wattage.

 

[Luthj]

You need to do the math for your energy consumption per shower, and compare that to your charging expectations. I suspect it will be higher than you anticipate. On demand heating may also take more power than you anticipate. Going fully electric and heating water is a difficult proposition. Either you need a crap-ton of solar, shore power, or major alternator charging. Or of course not showing often.

At 1 Gal per minute, you will need 4.8kw of power if going from 75F to 108F. Lower water tank temperatures will increase this further.

Assuming 4 gallons per person, that yields about 1.3kw-hr. Not a ridiculous amount of energy, but at 12V that's not a simple heater to create from scratch. 24V seems more reasonable, and you don't need to have 4 cells in parallel.


Our 4 gallon tank at 160F can provide for 8-9 gallons of shower temp water in cool temps, 10-11 gal in warm temps. 8 is enough enough for two people to have a good-great shower in my opinion. The key is a high velocity low flow head with integral shutoff.

120F+ tank temperature you must have a mixing valve on the hot water tank outlet (set to ~120F).

3.6kw at 12V is doable, but that's some major current. Making a custom tank, and connecting 4/0 cables to it won't be an easy task. Heating elements in that range aren't trivial to design either.

Resistive loads can easily be run from a modified sine wave inverter. These are quite a bit cheaper than pure sine. Its possible you could install 2 120V or 220V elements in the heater, and drive them from separate inverters.

In our vehicle we use a coolant heated water heater, with a 900W backup element. We generally get our heat from the engine when we drive (free). If on shore power the 900W element heats the tank, and we take a 20 minute break between showers for recovery. Off the grid we use our diesel fired coolant heater, which heats the water tank up in about 15 minutes. It puts about 2000-2500W into the tank (its rated at 5kw, but heat exchanger efficiency comes into play). Which means a 5-10 minute break between showers, which is typically how long it takes to towel and dress anyways.

If you are certain you want electric only, I suggest a larger hot water tank with good insulation. Then go with a modest sized element (less than 1kw), and use excess power throughout the day to heat the tank up. It would be fairly easy to trigger the element based on SOC or voltage. Say when your bank hits a certain SOC at mid day, you can use the excess for water heating. If you drive a fair bit you could also use alternator power for heating.

 

[anschluss]

Here's my recent attempt at implementing a DC water heater on a somewhat smaller scale. I am using a 20L (5 gal) PE tank with a 600W 12V DC heating element. The DC power is fed through a Victron battery protect BP-65 with an electronic thermostat switch, a water level sensor and a simple mechanical bimetal temperature switch (safety cutoff at 60 degrees Celsius) all wired in series to the "enable" input of the BP-65. I made a few pictures of the preliminary test before everything will be mounted in the RV - so far everything worked as expected:

 

[Hedges]

In our vehicle we use a coolant heated water heater, with a 900W backup element. We generally get our heat from the engine when we drive (free).

I was thinking of heat exchanger in exhaust, but liquid-liquid is simple. I've actually got one in my Honda Civic already; it is a stick shift but the replacement radiator I bought has a transmission cooler built in (not necessarily the best metallurgy for water!)

The loop off cylinder head to heater core is better because it is active even before thermostat opens. I would solder two copper tubes together (for double isolation), one for coolant loop and the other convection for the water heater.

You would definitely want double-isolated if heating potable water, but might be advisable for heating shower water too.

 

[pezzodefero]

You need to do the math for your energy consumption per shower, and compare that to your charging expectations. I suspect it will be higher than you anticipate. On demand heating may also take more power than you anticipate. Going fully electric and heating water is a difficult proposition. Either you need a crap-ton of solar, shore power, or major alternator charging. Or of course not showing often.

At 1 Gal per minute, you will need 4.8kw of power if going from 75F to 108F. Lower water tank temperatures will increase this further.

Assuming 4 gallons per person, that yields about 1.3kw-hr. Not a ridiculous amount of energy, but at 12V that's not a simple heater to create from scratch. 24V seems more reasonable, and you don't need to have 4 cells in parallel.


Our 4 gallon tank at 160F can provide for 8-9 gallons of shower temp water in cool temps, 10-11 gal in warm temps. 8 is enough enough for two people to have a good-great shower in my opinion. The key is a high velocity low flow head with integral shutoff.

120F+ tank temperature you must have a mixing valve on the hot water tank outlet (set to ~120F).

3.6kw at 12V is doable, but that's some major current. Making a custom tank, and connecting 4/0 cables to it won't be an easy task. Heating elements in that range aren't trivial to design either.

Resistive loads can easily be run from a modified sine wave inverter. These are quite a bit cheaper than pure sine. Its possible you could install 2 120V or 220V elements in the heater, and drive them from separate inverters.

In our vehicle we use a coolant heated water heater, with a 900W backup element. We generally get our heat from the engine when we drive (free). If on shore power the 900W element heats the tank, and we take a 20 minute break between showers for recovery. Off the grid we use our diesel fired coolant heater, which heats the water tank up in about 15 minutes. It puts about 2000-2500W into the tank (its rated at 5kw, but heat exchanger efficiency comes into play). Which means a 5-10 minute break between showers, which is typically how long it takes to towel and dress anyways.

If you are certain you want electric only, I suggest a larger hot water tank with good insulation. Then go with a modest sized element (less than 1kw), and use excess power throughout the day to heat the tank up. It would be fairly easy to trigger the element based on SOC or voltage. Say when your bank hits a certain SOC at mid day, you can use the excess for water heating. If you drive a fair bit you could also use alternator power for heating.

hi , do you have a diesel coolant heater or you have an air one with the radiator attached to heat the water?

 

[Luthj]

hi , do you have a diesel coolant heater or you have an air one with the radiator attached to heat the water?

I have a Eberspacher/Espar D5 Hydronic. I use an Isotemp marine water heater. The water heater has an integral coolant heating loop. If I wanted I could also install an air heating radiator on this coolant loop.




For a couple reasons my D5 and isotemp has its own loop which is isolated through a plate heat exchanger from the engine. But this isn't required.

For on demand heating with a hydronic unit, you don't need a water tank. Instead a plate heat exchanger and a tempering valve can provide on demand water heating.

 

[geoffire]

Wow, lots of good responses. Sizing up the tank to support all the showers so I don't need "instant" like heating would certainly make things a lot easier, but space and weight is very much at a premium. I'm going with only a 15 gallon main tank on this thing (we'll need to refill often when we shower or make them super short). I also considered the coolant heat exchange route (maybe with a Espar petrol heater and use it for cabin heating also), but that adds a lot of complication and it's seems that would not work for sustained "instant" like heating either.

I went a little overkill on the batteries to enable stupidness like this (and will overkill on the alternator charging as well). Maybe I'm underestimating the issues of all that current, but it seems like mostly what anschluss showed multiplied to me - I was thinking many relays and wires rather than massive ones.

 

[Hedges]

4 gallons off the shelf water heater powered by AC, set around 160 degrees or more, temperature balancing valve?
That should be minimal size and complication.
Plumbing in a heat exchanger could be left for a possible future project.

For long showers, recirculation from drain to showerhead?

 

[pezzodefero]

I have a Eberspacher/Espar D5 Hydronic. I use an Isotemp marine water heater. The water heater has an integral coolant heating loop. If I wanted I could also install an air heating radiator on this coolant loop.

View attachment 41256
View attachment 41254

For a couple reasons my D5 and isotemp has its own loop which is isolated through a plate heat exchanger from the engine. But this isn't required.

For on demand heating with a hydronic unit, you don't need a water tank. Instead a plate heat exchanger and a tempering valve can provide on demand water heating.

View attachment 41255

probably the two first thing cost more than my van.. , i have a chinese diesel heater, i know the is a kit for sale, basically one or two radiator that are heated from the air of the heater, water pump and that's it. wondering if it works in real life.

 

[pollenface]

In my travels, I used 3x 120w 12v immersion elements tied together hooked up to the starter battery with engine running. It would heat 10L of water in around 10mins, good enough for a camping shower.

Also made a foot pedal operated shower pump switch so I could have my hands free and conserve water

 

[Luthj]

A 4 gallon cube is pretty small. You can heat that to 190F or so with a <1kw element. That will yield up to 13 gallons of shower temp water best case. That would be my suggestion. No big wiring, no 300A relays or switching, and you can use your existing inverter.

 

[geoffire]

Can someone explain to me why a say 1200w AC element through an inverter would be preferable to two 600w DC elements? For one I was hoping to avoid running the inverter to avoid the idle consumption except the moments we need it (like cooking) - anything sustained like the fridge, heating (electric bed pads) and all water stuff I want on DC. For two it just seems to add complication.

Although, the "keep warm" circuit could be DC and the "instant" circuit could be AC ?

 

[geoffire]

A 4 gallon cube is pretty small. You can heat that to 190F or so with a <1kw element. That will yield up to 13 gallons of shower temp water best case. That would be my suggestion. No big wiring, no 300A relays or switching, and you can use your existing inverter.

Does this mean just drain from the 4 gallon cube and no diluting with cold water (so multiple pumps) or is this with with one pump and a temp balancing valve? I have not ran the numbers, but 4 gallons yielding 13 of shower water sounds good.

 

[Luthj]

Two 600W DC elements can work just fine, if the heater in question can take 2 of course. That's 60A each, which is a reasonable cabling need.

 

[Luthj]

Does this mean just drain from the 4 gallon cube and no diluting with cold water (so multiple pumps) or is this with with one pump and a temp balancing valve? I have not ran the numbers, but 4 gallons yielding 13 of shower water sounds good.

This is a single pump system, so the hot water tank is constantly cooling as you use water. A tempering valve will supply constant temp water until the tank drops below its setpoint.

14 gallons is a best case scenario with a 190F hot tank. If your main tank is say 65F, it will be less. You can do the math, its just BTUs. Running a 1-1.5kw heating element while you are showering will add about 1 gallon of shower heat every 4-5 minutes.

 

[geoffire]

Two 600W DC elements can work just fine, if the heater in question can take 2 of course. That's 60A each, which is a reasonable cabling need.

Anyone have any suggestions of a dual element ~4 gallon water heater?

I found there are marine Heat Exchange + 120V units which is kind of cool. Swapping that 120V element for a DC one would probably work well in my driving a lot application, only that's a bit bigger than I was wanting (and $$ for the isotherm) http://www.isotherm-parts.com/produ...re/slim-square-water-heater-4.2-gal-115v-750w

 

[Luthj]

I haven't looked to see if the element swaps, but these bosch units are pretty small. You may need to hack the thermostat for higher temps, and also add a tempering valve.

Bosch 4 Gal. Mini-Tank Electric Water Heater ES 4 - The Home Depot

The Bosch 4 Gal. Electric Water Heater has a 4 Gal. mini-tank that fits under your sink to provide hot water right where you need it. The heater plugs right into a 120-Volt outlet and has a lightweight,

www.homedepot.com

There is also this 6 gal cube unit.

Kuuma 11811 Marine Water Heater 6 Gal Front Heat Exchange + 120V Aluminum – Fredwarner Marine & Outdoors

www.fredwarner.net

 

[chrisski]

I wonder how much energy 16 gallons will take to heat. I did some math based off some unscientific data I have. To me, heating water with electric on a mobile system just does not make sense. Using propane is best, and then some other methods suggested.

For 16 gallons total for the shower, that's four gallons each for four people. I'm going to guess 1.4 kwh total. My very bad math which is based off a very unscientific test of my one liter of water boiling in an electric water kettle going from room temperature to boiling that tells me that because 1000 watts of energy to boil a liter in 4 minutes, which is about 67 watt hours per liter, 266 watt hours per gallon, or 4266 watt hours to boil 16 gallons. If it takes a quarter of that energy to get this to a comfortable temp to shower in, that is 1.4 kwh.

To put 1.4 kwh in context for a vehicle, that is more than all the usable energy in a TWO 6 volt FLA golf cart batteries. I have 4 golf cart batteries in my 5.2 kwh battery pack, of which I can use 2.6 kwh before dropping to 50%. So with my battery bank, that would take a little over half my energy storage. I think my FLA battery weighs 65 LBS. All that just for a shower.

Finding that much panel space will be challenging. The battery pack I mentioned has a max charge rate of about 876 watts per hour. So to shower 4 people that would take my solar panels 1.6 hours to charge.

To get that 876 watts I mentioned, I have 1350 watts of panels: panels rarely produce the rated value especially flat roof panels. For a travel vehicle, that is lots of panels.

There's some things that can make that easier and stay solar, and that would be lithium batteries and more solar panels, but at some point, you'll run out of room to place panels. Another is to time the showers for after the batteries have been topped off and you have the "Excess Production Potential," where the batteries will still be charged before the sun sets and the showers stop. Later in the day, this will take a bit longer than the 1.6 hours I mentioned.

EDIT: Also found on my build, that 4/0 wire mentioned is not that easy to work with. It's big and takes up a lot of space. Not like 10 AWG wire at all. Running that wire for me was about 7 feet total, and the inverter is located right next to the battery bank. I doubt the water heater will be. There's some serious limitations to low voltage DC high powered appliances. I think that's why you can't get on line and buy a 12 volt DC washing machine. Its just dangerous. A 12 VDC water heater would probably pull that much energy.

 

[Luthj]

It takes 1 BTU to raise 1 lb of water 1 degree F. So 8.3lb/gal means 8.3btu per degree per gallon.

so for 50F rise in 1 gallon you would need 50*8.33=416BTU. 416BTU is about 122 watt-hours.

The term you need to google are: From there its just converting units.
specific heat of water