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Water heating, in a vehicle

Dzl

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This is not a solar specific discussion, but I'd like to discuss solutions to the energy intensive act of heating water. What solutions have you found to be most practical or most efficient for your application?

I like the idea of having multiple methods, with the primary methods be methods that take advantage of excesses or 'free energy' when available. A "3 Way" (or 4 way) solution that makes use of excess solar power when available (using a DC heating element), and heat from the engine when driving, and a third method that is on demand (not necessarily in the sense of 'instant hot'/tankless, I mean on demand in the sense of it doesn't rely upon a surplus of solar or engine heat which is not always available). Diesel hydronic heater or Propane appliance of some sort would be the two alternatives that come to mind.

I'd like to just discuss / brainstorm ideas when it comes to water heating, so the convo doesn't need to stay restricted to my specific considerations outlined above.

I've found water heaters (mostly marine) that are made to be two way (engine heat + 110v ac) and could be converted to DC, I haven't found anything that is made out of the box to be '3-way' but I'm sure there are other ways to design this.
 
There are a ton of ways to approach this. What is the primary purpose of the hot water? Showering? Cooking? Thermal storage? How much are you holding?
 
There are a few different brands and many models. Here are a couple examples of what I was looking at:
Surecal
Isotherm

I also learned there are "twin coil calorifier" which from the fittings look like there are two loops for heat inputs (maybe designed for twin engine boats or maybe designed for a second independent heat source, not sure).
Example:
Surecal twin coil
This might be a good building block for the type of system I was imagining.

The unit @time2roll linked to looks very interesting as well. Meets the "3-way" design criteria I was musing about. I wonder if the electric element is a standard element that could be switched out for a DC element.

There are a ton of ways to approach this.
Exactly the reason for the post, See what has worked for others, or what ideas people have kicking around in their minds. This is preliminary research.

What is the primary purpose of the hot water? Showering? Cooking? Thermal storage? How much are you holding?
This is just preliminary research. The contours and logic of the system/solution is what I'm most curious about at this point. Details are not all worked out. But to give a little structure to the question:
  • Capacity: something small, ~2-6 gallon range
  • Primary use: dish/hand/face washing
  • Showers: undetermined/based on feasibility and space
  • Users: 1-2
  • Small to medium sized vehicle <22ft designed for self reliance (assumption of no hookups)
  • Thermal storage: well.. A water heater is a storage vessel for heat ? so yes. But I think what you are asking is is it a way to capture energy at one point in the day when energy is abundant for use at another point? If so, yes, the idea would be to opportunistically capture surpluses (solar, engine heat) when available for use when needed.
 
This is not a solar specific discussion, but I'd like to discuss solutions to the energy intensive act of heating water. What solutions have you found to be most practical or most efficient for your application?

I like the idea of having multiple methods, with the primary methods be methods that take advantage of excesses or 'free energy' when available. A "3 Way" (or 4 way) solution that makes use of excess solar power when available (using a DC heating element), and heat from the engine when driving, and a third method that is on demand (not necessarily in the sense of 'instant hot'/tankless, I mean on demand in the sense of it doesn't rely upon a surplus of solar or engine heat which is not always available). Diesel hydronic heater or Propane appliance of some sort would be the two alternatives that come to mind.

I'd like to just discuss / brainstorm ideas when it comes to water heating, so the convo doesn't need to stay restricted to my specific considerations outlined above.

I've found water heaters (mostly marine) that are made to be two way (engine heat + 110v ac) and could be converted to DC, I haven't found anything that is made out of the box to be '3-way' but I'm sure there are other ways to design this.
I think some of the overland folks use an engine coolant loop through a hydronic heater with heat exchanger of some sort to the water tank. Triple duty of hot water, engine preheat, and allowing the engine to heat it when running itself.

I've seen at least one woman with the setup on youtube and she had it professionally installed.

Not as much of an option for a trailer of course.

I see no reason why a DC AND AC element couldn't be incorporated to have pretty much every option at once.
 
I think some of the overland folks use an engine coolant loop through a hydronic heater with heat exchanger of some sort to the water tank. Triple duty of hot water, engine preheat, and allowing the engine to heat it when running itself.
Yeah depending on how you set it up, with just a hydronic diesel heater in a vehicle you can:

Heat the engine with:
  • Hydronic Heater
  • Itself
Heat Water with:
  • Hydronic Heater
  • Engine Heat
Heat the living space (via a radiator/fan or radiant infloor) with:
  • Hydronic Heater
  • Engine heat
Ideally this is the sort of system I would like to design. Or a part of it, electrical is probably not practical as the primary source for water or space heating, but it would be nice to have as a supplement or an alternative.

I see no reason why a DC AND AC element couldn't be incorporated to have pretty much every option at once.
There is one element (you can see it of you click the surecal link above, that is a combined AC/DC heating element (but there is only one option for the combined element, it's a 300W DC and 700W AC element).
 
Ah yeah I forgot about using the water to heat the space. Also a good option if you can put a damper on the radiator airbox to prevent heating when it don't want it.

Or valves, of course. Since I'm half asleep and didn't think of that immediately.

If do both in case I leave a valve on lol
 
I’ve thought about RV hydronic in a trailer before. An anti-freezed loop through tubing under the floor heated by exchanger from whatever one chooses, OR perhaps even a dedicated heater/tank with solar, fossil, or “other” inputs as options. Nothing manufactured achieves the goal perfectly if at all. The unit posted above is the closest I’ve seen but I don’t like the forced-air component- though I suppose the fans may be less wattage than I micro circulator. Dunno.

For on-the-go the propane demand tankless water heaters are surprisingly low cost to run and many models are <$200 US$.
For multi fuel options the price goes up. Vehicle coolant heating isn’t exactly ’free’ though I suppose some efficiency is gained by using the cooling system “waste” heat.

Another way of harvesting ’waste’ watts while driving could be a 120VAC alternator dedicated to a ~10gal electric water heater. As with coolant heat (watts, BTUs) there is a cost in fuel mileage albeit probably higher for mechanical energy creation. On the other hand, at steady-state and particularly flat-ground travels and “city” or urban travels internal combustion vehicles produce more horsepower (watts) than the load of maintaining speed. In other words the heat produced at low or mid part throttle exceeds the heat needed to drive at constantly varying levels. So an alternator load can ‘use’ that tiny difference, and when it requires horsepower to turn the alternator the percentage load is probably so small an effective difference may not even be noticed in fuel mileage.
 
  • Capacity: something small, ~2-6 gallon range
  • Thermal storage: well.. A water heater is a storage vessel for heat ? so yes. But I think what you are asking is is it a way to capture energy at one point in the day when energy is abundant for use at another point? If so, yes, the idea would be to opportunistically capture surpluses (solar, engine heat) when available for use when needed.
A British thermal unit (Btu) is defined as the amount of heat necessary to change the temperature of 1 lb of water at sea level by 1°F. Using your 6 gallons let's assume a room temperature of 68 degrees F and a maximum water temperature of 200 degrees F. Six gallons of water weighs 50 pounds, so if we raise the temperature of that water by 132 degrees (200-68) we have a maximum possible storage of 6600 btus. As the temperature of the water approaches room temperature, the amount of heat that can be extracted in a given time period will begin to approach zero. Not really a substantial source of space heating except for a very small, very well insulated space.
 
I considered this for a long time but felt it just wasn’t really worth the hassle unless you find yourself in freezing weather often.

A typical 4 gallon 120V water heater is around $150 while a marine hydronic heater is around $500 plus the requisite install equipment.

If you use the full contents once a day and assuming 68F ambient temperature and 140F desired temperature that amounts to:

4 gallon X 2.47WHr/*F-Gallon X 72 F rise = 711 Watt-hours per day of use.

An additional 200 watt panel with a charge controller is probably cheaper and will work for most sunny days. I think a 400W panel will give you enough margin for most overcast days too.

I think it only makes sense if you do the whole enchilada: Hydronic heating, block heating, engine coolant water heating, etc. And only if you need the benefits of fuel heating often.
 
Since you are keeping this as vague as possible, here are my opinions: I say this not knowing what your climate is, if you are operating in a wilderness, rural or urban setting or what your vehicle looks like or how it is set up. This is how I am/would be approaching the problem based on my particular circumstances and the parameters you have offered up.

  • 6 gallons is to small to get any kind of ROI on buying, building, installing and maintaining waste engine heat unless you are moving every day.
  • 6 gallons is also to small to have any value for thermal storage for space heat if you are anywhere where freezing temperatures are a potential.
  • 6 gallons IMO is too small to bother investing resources into dc heat but is not as bad a ROI as engine heat.
  • 6 gallons is plenty of water for 2 people to have showers where you get wet, turn off and lather up, turn on and rinse off but pretty much only if you are pressurizing the tank with air and the pressure is not being maintained in the same manner a conventional pressurized system would operate where the volume being used is replaced with cold water.
I personally use this for all my water heating needs. Most everything I do I do outside. I circulate it in a clean sink or basin until the water temp is what I want. Rinsing dishes or hands I use the shower head in its holder. Showering I just run it straight out of a 5 gallon bucket. That unit is perfectly safe to use indoors with ventilation.
 
Since you are keeping this as vague as possible...
I think that we are on slightly different wavelengths here. I believe you are trying to help me solve a specific problem, for a specific context (which I thank you for) that you want/need specific details to solve with any precision, but this is not my goal of the post, and not something I can provide at this point. Down the road, that specific help would be greatly appreciated probably, right now I'm interested in learning what works for others, brainstorming ideas, and maybe learning of a few approaches I haven't heard of (or maybe learning shortcomings of things I am interested in). This is preliminary research, specifics come later.

But, I understand the desire for more detail to give relevant advice/feedback. So I can at least give some rough design goals/constraints and context that might help you better understand my mindset. First, this will be a blank canvas (DIY build), a converted ambulance, short bus, or large van, there are no pre-existing systems to design around. The build will be intended for long term, long distance travel, on and off road travel, and for the most part independent of any grid power, city water, etc. Most time spend in wilderness and small towns, with visits to cities being normal as well, just less common. Because its intended for long term living, some creature comforts are desired (like hot water) but space and available energy will be limiting factors (to stay capable of going where I want to go, staying somewhat compact is important ~20ft is the goal), but I'm used to pretty spartan travel (currently 4 months into a trip with no running (or hot) water, no refrigeration, no heat, living and doing most things outside, in a pretty cold climate. Available area for solar will be maxed out, so there will be no option to 'just add more solar' as is often recommended in other contexts.

So to summarize, (1) medium-small vehicle (2) long term, mostly self sustaining, travel (3) variable climate zones, temperature, solar potential (4) rarely count on shore connections (5) major design constraints: size of vehicle, weight, energy, $$ (6) redundancy, and (relative) simplicity/robustness, and self reliance, and suitability for variable conditions, are design goals.
 
This 200 watt, 12 volt 1.6 gallon water heater consuming only 200 watts came up in another post:


Seems too good to be true, but claims can hear water to 50° C, 122° F in 30 -45 minutes.

If this is all its cracked up to be, this would fill my requirements for my RV hot water.

For me, the only practical solutions are a propane or electric water heater.
 
I say this not knowing what your climate is
Trying to define this will probably frustrate you more than me not stating it at all ? using the past few months as an example:
Temp extremes: -15C to 45C (5F to 115F)
Climate: From Coastal California, to Sunny Eastern Rockies, to PNW cloudy/rainy, to long days in Northern Canada, to the deserts of Eastern OR/WA/CA.
Elevation: sea level to 8000 ft or so.

This variability is part of wgat draws me towards redundant, overlapping heat (and energy) sources and a multi-source hot water system (sometimes solar will be abundant, sometimes it won't, sometimes I'll be driving everyday, sometimes I wont, sometimes I'll be in climates where I could care less about how water, sometimes I'll be in areas where the daily high is below freezing (like yesterday and the day before).


6 gallons is also to small to have any value for thermal storage for space heat if you are anywhere where freezing temperatures are a potential.
Not really a substantial source of space heating except for a very small, very well insulated space.
I believe I gave the wrong impression somewhere, or something is being misunderstood. The water heater would just be used for normal water heat things, not space heating. Some of the same sources of heat could be used ot heat both water and space (excess engine heat), possibly a diesel hydronic heater, but the water in the tank (water heater) would not be used for anything beyond what water heaters are normally used for (sink + possibly shower)

6 gallons is to small to get any kind of ROI on buying, building, installing and maintaining waste engine heat unless you are moving every day.
Why do you say that? This is only something I've been pondering absentmindedly for a couple days so I may be overlooking something, but it doesn't look particularly expensive or involved, compared with other solutions. Adding an additional diesel hydronic heating into the mix is where the cost and complexity seems to increase sharply. Barring that, the water heater looks like the biggest expense (pay a $$ premium for anything with the word Marine attached to it it seems).

As to needing to actually drive everyday, yes, this is the biggest caveat to this, and why it would not be a reliable primary source of heat for most people (including me). But I do tend to move most days (the write-ups I've read have indicated ~45 minutes of driving has been enough to heat small water tanks. I would estimate, driving 45 minutes or more, would apply to me, between 25% and 75% of the time (big range I know, there is very little that is predictable in my lifestyle).

Vehicle coolant heating isn’t exactly ’free’ though I suppose some efficiency is gained by using the cooling system “waste” heat.
This is what I was thinking. Free as in the heat is generated one way or another, so using it for a productive purpose (heating water or space) would be mostly free apart from the energy consumption of any pumps or fans used. (also if its not clear, when I'm talking about free in this context I'm referring to energy/heat, not $$). Are there a hidden costs that I am not seeing?

Another way of harvesting ’waste’ watts while driving could be a 120VAC alternator dedicated to a ~10gal electric water heater. As with coolant heat (watts, BTUs) there is a cost in fuel mileage albeit probably higher for mechanical energy creation. On the other hand, at steady-state and particularly flat-ground travels and “city” or urban travels internal combustion vehicles produce more horsepower (watts) than the load of maintaining speed. In other words the heat produced at low or mid part throttle exceeds the heat needed to drive at constantly varying levels. So an alternator load can ‘use’ that tiny difference, and when it requires horsepower to turn the alternator the percentage load is probably so small an effective difference may not even be noticed in fuel mileage.
This is an interesting line of thinking, I'd like to explore more, I've never had a great comprehension of the ins and outs of alternator charging and its effect on fuel economy.
 
I think that we are on slightly different wavelengths here.

I dont think so. When I said "keeping this as vague as possible", another way I could have said it was "keeping options open". IDK if you took offense to the term "vague", it sometimes has a negative connotation for some, not my intent.

The OP was just really difficult to work with. Its boundaries were far to broad. It was like saying "I need a house". Without direction you start getting ideas for space heating, which by the sounds of it is not something you are looking for.
 
For me, the only practical solutions are a propane or electric water heater.
Electric only seems practical if you have (A) access to shore or generator power, or (B) have a abundant roof space for solar and modest hot water needs. I have heard that some people can make (B) work with small tanks and limited usage.
 
This is an interesting line of thinking, I'd like to explore more, I've never had a great comprehension of the ins and outs of alternator charging and its effect on fuel economy.
IMO large amperage alternator usage is only practical if you can add a second alternator or generator to your vehicle. My Truck has a spot for a second alternator which I could dedicate that to producing power. I also wonder if the 320 watt hours per mile an electric car may get would cut into fuel mileage with the second alternator

I don't like the idea of large amounts of energy be pulled off a single 12 volt alternator, but if a second, larger generator could be added for 24 or 48 volts to keep amps down and dedicated solely to charging, that may be great.
Electric only seems practical if you have (A) access to shore or generator power, or (B) have a abundant roof space for solar and modest hot water needs. I have heard that some people can make (B) work with small tanks and limited usage.
I agree. In my case it would be option B. That 200 watt 1.6 gallon heater would work for me. That WOULD NOT work for the other two or three in the RV. A 1.6 gallon hot water shower and then finish with the cold water would be out of the question, never mind having to space showers apart by 30 minutes each. They're much more cultured than me, and I'm pretty lonely if they don't go, and that 1.6 gallon limnit would keep them away.
 
Different question, so do you need cold water?

You cold also insulate your primary water tank with a 2 inch foam boards and heat up the whole thing with a hydronic heat loop from the engine. Depending on the volume, you only need start the engine every other day.

A water heater is essential a second small tank.

Further stick an aquarium heater pad underneath to heat with electric in case you want.
 
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