12 volt hot water heater

[Peterjk]

Maybe this is the simplest solution? Use a DC thermostat? Looks like these are available at 120, 140, 170 and 180.
I ordered a 120 to test with.
https://windandsolar.com/water-heater-thermostat/

[rffffffff]

This is a suitable relay for phase one:

Baomain Power Relay MK2P-I AC 110V Coil DPDT 8 Pin with Plug-in Terminal Socket https://www.amazon.com/dp/B01N3KO8X1...ing=UTF8&psc=1

I would probably buy a better brand from mouser or someplace like that but the existing thermostat wired to the coil on this will open
And close the contacts, which are rated for 28vdc @ 10a.

Actually, that’s not enough for this application, you need 30a but you get the idea.

[djdalfaro]

I'm pretty sure this is the way I'll be rewiring my Kuuma 6gal. It came with some chinsy heat limiter and thermostat, so I'll replace those with a standard 59T/66T combo unit (59T 4300 66T 4401 120-180°F). I'll be going with a 12v Dernord double-U 600W heating element which is actually (2) 300W single U elements wired in parallel. I'll disconnect one of the connecting bars and wire them as shown.

A couple of points.

I've show the 66T disconnected from the 59T. This is accomplished by removing the connecting bar and using wires as needed. I've put the 59T on the low load (85-86) side of the relay because it as it is the switching unit and I'm using an AC switch in DC environment, I want to give it the best chance possible for a long life. So with the relay coil resistance the 59T should only see about 150mA.

I have the 66T on the high load (30-87) circuit because I feel more comfortable leaving it directly connected as it's only purpose is to prevent overheating (fire hazard). In normal service it SHOULD never trip so it shouldn't be damaged by the DC. You could move it to the low load side if you are ok without that protection. Additionally, I'll wire it after the heating elements so that the voltage across it should be very low even if the current is 25A per pole.

[kazetsukai]

Originally Posted by matthews2001

Just a reality check here.

WHO would use DC for heating?

From my perspective, the OP's idea sounds very promising. If I were to do it again, I'd likely do something similar. I have a Girard tankless propane, which lasts for months on a single tank doing dishes and daily hot showers. But it has its share of severe drawbacks.... interested in how this one turns out honestly.

The question is, will that heating element consume too much energy to be practical or not... will it get the water hot enough, etc. The latter I think is a safe bet. It will take a little longer doing so, but it should get a couple gallons up to temperature. The amount of time it spends on is the problem- will maintaining that temperature be too costly a draw or not?

Lets do an estimate, say this thing is running 25% duty cycle. For 6 hours a day this 300W element is on... That's 1800Wh in a day. With the OP's 1200W of solar, I'd say he could average something like 4000Wh of yield without much difficulty on decently sunny days, with That's 1/2 his energy budget... not what I'd consider comfortable territory. I'd say you'd want a draw like this taking at most 1/4 or 1/5 your daily yield to be really in the safe zone.

Now I'm not sure how long this element would be running for in a day... and that number is going to vary depending on how much hot water is used. That's the experiment. Maybe another member here can chime in on how much energy their tanked heater uses...

[rffffffff]

Originally Posted by kazetsukai

From my perspective, the OP's idea sounds very promising. If I were to do it again, I'd likely do something similar. I have a Girard tankless propane, which lasts for months on a single tank doing dishes and daily hot showers. But it has its share of severe drawbacks.... interested in how this one turns out honestly.

The question is, will that heating element consume too much energy to be practical or not... will it get the water hot enough, etc. The latter I think is a safe bet. It will take a little longer doing so, but it should get a couple gallons up to temperature. The amount of time it spends on is the problem- will maintaining that temperature be too costly a draw or not?

Lets do an estimate, say this thing is running 25% duty cycle. For 6 hours a day this 300W element is on... That's 1800Wh in a day. With the OP's 1200W of solar, I'd say he could average something like 4000Wh of yield without much difficulty on decently sunny days, with That's 1/2 his energy budget... not what I'd consider comfortable territory. I'd say you'd want a draw like this taking at most 1/4 or 1/5 your daily yield to be really in the safe zone.

Now I'm not sure how long this element would be running for in a day... and that number is going to vary depending on how much hot water is used. That's the experiment. Maybe another member here can chime in on how much energy their tanked heater uses...

If you look at electrodacus bms and the diversion feature of the charge controllers it’s really interesting. Essentially you can automatically only use excess solar with charging as the priority. No downside to overall capacity and if the water isn’t hot enough you can just add panels until it is.

[matthews2001]

My comment was in reference to the cost of solar panels. Highly expensive way to heat anything. As a former solar engineer in the 1980s if you wanted hot water you used solar hot water panels. The cost even now of using solar electric panels to heat anything is HIGH. ANY bus probably has propane for cooking. He could save money by buying less panels.

And really it is not about HIM it is about newbies considering a bus conversion. You never need solar panels unless you are a full timer else that 'investment' in panels just sits all unused months of the bus.

Solar electric panels are great for battery charging, tv computer,, inverter electric fridge led lites. But lets be real you will always have a generator on a bus for microwave use and quick battery charging if you get dead batteries.

Want 2 gallons of hot water? Get a solar shower, one of those black plastic bags to hang in a tree or the side of your stationary bus for a couple hours...$10?

BTW there are heat pump domestic hot water heaters nowadays, expensive but can make sense in a house...

[rffffffff]

Originally Posted by matthews2001

My comment was in reference to the cost of solar panels. Highly expensive way to heat anything. As a former solar engineer in the 1980s if you wanted hot water you used solar hot water panels. The cost even now of using solar electric panels to heat anything is HIGH. ANY bus probably has propane for cooking. He could save money by buying less panels.

And really it is not about HIM it is about newbies considering a bus conversion. You never need solar panels unless you are a full timer else that 'investment' in panels just sits all unused months of the bus.

Solar electric panels are great for battery charging, tv computer,, inverter electric fridge led lites. But lets be real you will always have a generator on a bus for microwave use and quick battery charging if you get dead batteries.

Want 2 gallons of hot water? Get a solar shower, one of those black plastic bags to hang in a tree or the side of your stationary bus for a couple hours...$10?

BTW there are heat pump domestic hot water heaters nowadays, expensive but can make sense in a house...

I’d have to do the math a bit but I think this calculus may be changing. I can get 250w panels for $50-75 used as much as I want where I am. The hydronic panels are expensive and heavy, although I have a bunch of used ones that I got relatively cheap too I would hesitate before mounting something that heavy on a bus, especially because it needs to be angled to the sun where pv can get away with flat mounting.

I think on a bus the real estate and practicality matter more than cost a lot of the time and although I’m still pondering a hydronic panel mounted vertically on the side of the bus I think it’s probably a bit extreme for most to go to those lengths especially considering the need for an insulated storage tank in addition to the dhw tank, heat exchanger etc.

Throwing 4 extra pv panels on the roof and adding a dc heating element seems a lot more practical.

[joeblack5]

Depending on the size of your inverter you could use the electrodacus to turn on the water heater. In that case an ac relay or ssr is good enough. Then you stay with cheaper conventional parts and you can use it also Incase you are on grid ..

Good luck,
Johan

[kazetsukai]

Originally Posted by matthews2001

My comment was in reference to the cost of solar panels. Highly expensive way to heat anything.

Depends on what you're looking at. Panels are actually quite cheap compared to lets say, batteries. Solar electric energy -> heat is quite difficult as you say though...

Originally Posted by matthews2001

But lets be real you will always have a generator on a bus for microwave use and quick battery charging if you get dead batteries.

Ehh, the biggest problem with microwave oven is usually inverter continuous load rating, once you have something which can serve that load its not as difficult to satisfy. And microwaves are not the biggest draw imaginable out there, typically aren't run for much more than 10 minutes at a time. The convection oven we run off solar pulls 1800W and typically needs to run for anywhere from 30 minutes to two hours. The biggest consumer by far is the mini split heating or cooling by attrition alone...

Originally Posted by matthews2001

You never need solar panels unless you are a full timer else that 'investment' in panels just sits all unused months of the bus.

Definition of need may vary... The expanded capabilities of a rig with solar vs without solar is pretty extreme, even with modest solar. A couple panels and you can keep the lights on inside indefinitely without having to run a genny or plugin.

Originally Posted by matthews2001

And really it is not about HIM it is about newbies considering a bus conversion.

What about newbies that want to full time?

The biggest problem with the OP's idea I can see is that once they run out of that 2 gallons, that's it. Its going to take a while to get piping hot water after that because the heating element is 1/6th as powerful. Really depends if that buffer is big enough for their use case or not...

[djdalfaro]

So I did some napkin math today.
Assuming 6 gal gallon tank (my setup), 1 inch foam insulation, 600w element, and taking the water from 70° to 180°.

About 1650wh and 2.75hrs initially, but after it's heated it only takes 28w to maintain temp. Throw in some inefficiencies and it becomes 2200wh and 50w. That's not an unreasonable ask considering it equates to 18 gallons of 105° shower water.

Even less of an ask if I do the initial heating with my engine or Webasto.

[bus-bro]

As was said earlier, you only charge the 12 volt water heater element as a diversion load. That is: you heat water when your batteries are charged.

That assumes you have sufficient solar installed. You can heat water from your batteries if that is to be your priority come shower time.

[dzl_]

Originally Posted by matthews2001

My comment was in reference to the cost of solar panels. Highly expensive way to heat anything. As a former solar engineer in the 1980s if you wanted hot water you used solar hot water panels. The cost even now of using solar electric panels to heat anything is HIGH.

The cost of solar has dropped continuously and substantially over the years
The (shitty resolution) chart above shows cost per watt 1977-2013
I believe I read since 2010 alone, cost per watt has dropped 80%

Further savings can be found if you find a good deal locally, depending on where you live, there are many.

ANY bus probably has propane for cooking. He could save money by buying less panels.

True, propane is definetely a reasonable option, there is good reason its the default solution.

You never need solar panels unless you are a full timer else that 'investment' in panels just sits all unused months of the bus.

...
But lets be real you will always have a generator on a bus for microwave use and quick battery charging if you get dead batteries.
[/QUOTE]
Different strokes for different folks, I personally would not consider omitting solar from a build. I don't like RV parks and I don't like generators (the noise or the ongoing cost/dependence on fuel). Economics is one (complex) design consideration, but its not the only one. Solar is probably a higher upfront cost than a generator, but then is 'free' going forward, and allows a lot of independence so long as the sun is shining. Whereas a generator will be cheaper upfront but it costs you every time you need to run it, and you are self sufficient only so long as you have fuel, one advantage is that you don't have to worry about sun.

As to needing a generator for microwave use or quick battery charger, the former is not dependent on a generator with a decent sized solar system and battery bank, and the latter is a 'nice to have' but not a necessity. There is some point where you do need a generator, when you start going down the road of all electric appliances, A/C, etc. And heating water with electricity is sorta at the edge of that line.

To give some rough numbers to the conversation, a 2.2kW Honda generator is ~$1200. 3kW (10 x 300W) discount panels from a reputable supplier near me can be had for $1200 another $500 for adequate mid range charge controllers that can handle 3kW solar, round up to $2000 to account for odds and ends. The difference in cost is not prohibitive particularly if you have other reasons for wanting solar, or when you factor in the cost of fuel or rv parks. The real limiting factor with solar is roof space, its not hard to max out the available real estate.

But circling back, I do really think it comes down to your priorities and wants, and the economics will depend somewhat on your particular situation

[matthews2001]

Just saying I thought hot water rv bus applications was solved: $400 2 gallon lp/(electric 110/220 water heaters (when in a campground(I dont go to those either but nearly everyone else does!)). You know, lighting solved with Led's ...
blah blah. Most all buses have propane for cooking...

Would love to hear how it goes.

This is my favorite DHW! No standby losses, can pipe the Bus with just cold piping!!

[dzl_]

Originally Posted by matthews2001

Just saying I thought hot water rv bus applications was solved:

Never underestimate the DIY crowds propensity to rehash and rethink and re-argue the same problems 100 times over (usually leading to nothing more than beating a long dead horse, but sometimes leading to some cool creative solutions, some stupid creative solutions, some brilliance, or iterative improvements).

I think you are right that a small propane/120v combo conventional water heater is probably the simplest/most sensible solution for most use/cases.

But for someone that does have abundant solar, a dc heating element could be a good way to harness otherwise wasted PV energy.

$400 2 gallon lp/(electric 110/220 water heaters
...
Most all buses have propane for cooking...

You are right, with propane already on board for cooking, using it for space and water heating seems like the reasonable solution, for most situations.

[djdalfaro]

Just finished rewiring my water heater... Well mostly finished, I need to find a socket terminal for the loose wire to connect to terminal 86 and rewire it in red.

[DeMac]

Originally Posted by djdalfaro

Just finished rewiring my water heater... Well mostly finished, I need to find a socket terminal for the loose wire to connect to terminal 86 and rewire it in red.

Why us the Earth ground terminated with the DC Neutrals?
Combining the two Bonds the Earth Ground & DC Neutrals, sending current to the exterior chassis of the appliance and backfeeding anything that is tied to the same Earth ground.

[djdalfaro]

Originally Posted by DeMac

Why us the Earth ground terminated with the DC Neutrals?

Combining the two Bonds the Earth Ground & DC Neutrals, sending current to the exterior chassis of the appliance and backfeeding anything that is tied to the same Earth ground.

Can you explain this to me more in depth? Electrical theory is not my forte.

[DeMac]

The green jumper is to be used for bonding the access-panel-cover to the cabinet. The grounding terminal is bonded to appliance's skin.

Equiptment ground should not be terminated with the DC (share current) via the hi-temp limiter.

[djdalfaro]

The hi-temp limiter 66T is the bottom unit. It is not connected to the appliance ground, it is connected to the relay to switch off the 6awg DC- of the heating elements. The appliance ground is connected to the ground of the control thermostat 59T (top unit), which is connected to the ground DC- 6awg of the relay. Neither the relay nor the thermostat share any current with the appliance ground.

[DeMac]

is the Ground or planet Earth. UL requires the GND terminal to be bonded to the chassis of the appliance. If the manufacturer placed the label next to the terminal, it is an appliance Ground. The word Ground is NOT synonymous with the words Negative, Neutral, or DC.

Just trying to help. Leave it that way, if you like. DC wont electrocute. The water heater will not be Grounded.