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Old 11-26-2021, 05:40 PM   #1
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12 volt hot water heater

Hi everyone.
I am up to the hot water tank phase in my school bus build.
I am using a Electridacus BMS and have 4 300w panels, 2 each in parallel.
I am planning on installing this Richmind 2.5 gallon hot water tank pictured below in the bathroom.
I removed the AC heating element, and replaced it with a DC 12 v 300 watt element for my BMS.
My question around the thermostat which needs to be rewired from AC to DC.
Can I just remove the Ac hot and neutral wire [discarding the green ground], and replace them with a DC hot and ground ?
Meaning, in the picture, the AC Black hot wire is wired into the thermostat, the AC White neutral, is wired directly to the heating element. The thermostat then is wired to the second pole on the heating element.
I would disconnect those two, and replace them with the DC wires.
As I am reading this post here: someone did precisely that, I just wanted to double check this is the right way to do this so I dont fry my thermostat.
I purchased a on/off switch and relay as well as was recommended in the link.

Thanks everyone in advance!
Peter
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Old 11-26-2021, 07:20 PM   #2
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DC is much harder on AC rated switches. AC switches used in DC service must be de rated to last any length of time. How long would it last? Maybe long enough to test proof of concept.
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Old 11-26-2021, 08:11 PM   #3
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Internal Wire Size

I'm familiar with the Richmond Essential 2.5 gal. The single pole, 120v unit is rated at 12A/1440W. Which is safe to run on 14g wire.

A 12v/200W element is rated at 16.6A (20g wire). Swapping the element to 12v @ 300W yields a 25A load. (10g wire)

Simply replace the heater's factory wiring sections to 10 guage wire. The thermostat will still receive 25 Amps at full load. (Which may or may not survive, idk) Check the thermostat for excess heat, if it dies, replace the thermostat with a single pole, rated at 25-30A.

I concur with s2mikon about the DC on AC switches. Please let us know how well this performs.
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Old 11-26-2021, 09:41 PM   #4
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Thanks guys,
One question.
Re " if it dies, replace the thermostat with a single pole, rated at 25-30A."
What is a single pole? Do you mean thermostat?

Peter
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Old 11-26-2021, 10:21 PM   #5
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1P vs 2P

In case the amperage (above 12A) overheats the factory Thermostat.....

Two pole =
(2) 120v (split phase) making 240v
Neutral not required.
Most (dare I say all) whole-house water heaters have two hots and no neutral.

Single pole =
(1) 120v, accompanied with a neutral (aka grounded conductor).
Smaller units, like yours, have only one hot. Similar to a 1440w hairdryer.

Both utilize a ground for safety.
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Old 11-26-2021, 10:29 PM   #6
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Thanks!!

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Old 11-27-2021, 12:40 AM   #7
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If I were you I would use a DC rated relay with a 120vac coil (I think itíll be easy to find) for the heating element and let the existing thermostat turn the relay on and off. Itíll save the thermostat and make it all work long term. You wonít have to change much, just wire the existing AC out of the thermostat to the coil of the relay, neutral to the other side of the coil, and run the DC for the heaters to the contacts of the relay. Itíll save the thermostat and is likely the right way to make it all work long term.
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Old 11-27-2021, 09:53 AM   #8
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I think you could use any DC continuous rated 4 prong relay that is the correct amp load. Bosch or hella lighting relays come in 30 amp. Use the thermostat to control the #85 terminal to ground and #86 to the water heater on / off switch in the wall and #30 from solar and #87 to the heating element. Clear as mud??
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Old 11-27-2021, 10:05 AM   #9
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Here's the way Whale Seaward wires their 12v water heater.Screenshot_20211127-090352.jpg
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Old 11-27-2021, 10:29 AM   #10
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Quote:
Originally Posted by s2mikon View Post
I think you could use any DC continuous rated 4 prong relay that is the correct amp load. Bosch or hella lighting relays come in 30 amp. Use the thermostat to control the #85 terminal to ground and #86 to the water heater on / off switch in the wall and #30 from solar and #87 to the heating element. Clear as mud??

It would be easier if you used a relay with a 120vac coil instead of a 12vdc coil.
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Old 11-27-2021, 11:12 AM   #11
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This is the diagram I linked to earlier:

So were saying, Im going to run 10g wire [distance is about 20'] , using a 25a fuse [at the DC breaker panel] to the on/off switch hot, then to the relay pictured here. Finally to the thermostat and the new DC 12v 300 watt heating element.
Thank you for the detailed diagram above btw.


P
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Old 11-27-2021, 03:13 PM   #12
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Use the thermostat to run a relay that handles the amps.
Jack
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Old 11-27-2021, 03:53 PM   #13
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Just a reality check here.

WHO would use DC for heating? AC I get (free in a campground). DC for solar or even from your alternator? Seems like a basic misunderstanding of electricity.

What are you using for cooking? Propane almost universally. A great fuel for heating, DC not. Free AC electric? Then go electric stove, cook top and hot water.
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Old 11-27-2021, 04:09 PM   #14
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Quote:
Originally Posted by matthews2001 View Post
Just a reality check here.



WHO would use DC for heating? AC I get (free in a campground). DC for solar or even from your alternator? Seems like a basic misunderstanding of electricity.



What are you using for cooking? Propane almost universally. A great fuel for heating, DC not. Free AC electric? Then go electric stove, cook top and hot water.
I'm swapping the 1500w 120VAC hearing element to a 600w 12vdc element. The more I move to 12v system the more electricity I save. I don't plan on staying in campgrounds all that often. And for me the 12v is the secondary or tertiary power for water heating, behind the engine coolant while the engine is running and the Webasto Scholastic for initial heating. The 12v will run as a diversion load on the DC side of the charge controller. If you go 120vac you have to deal with inversion losses.

There are plenty of reasons not to use propane for heating or cooking. The question wasn't, "should I use DC for water heating.", Obviously they're aware of the limitations. Seems like a basic misunderstanding of the question at hand.
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Old 11-27-2021, 06:49 PM   #15
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Quote:
Originally Posted by matthews2001 View Post
Just a reality check here.

WHO would use DC for heating? AC I get (free in a campground). DC for solar or even from your alternator? Seems like a basic misunderstanding of electricity.

Basic misunderstanding of electricity? in what sense?


The main disadvantage of DC (or AC--just electrical heating in general) is that storing it and generating enough off grid, to heat what you need to heat is often not practical or cheap compared with easier to refill, easier to store, more energy dense . But that should apply as much to AC as DC. But this is more a system design / energy storage factor than a misunderstanding of electricity.


I think what you are saying (and I understand the logic) is that because using stored electrical energy for heat off grid is hard to do sustainably, an AC element makes more sense since, most people will only be using electric for heat when connected to shore power. This is reasonable, and makes sense in many cases. But devil's advocate/different perspective if you will use electrical for heating off grid sometimes, a DC element could make more sense even if you will be using AC/shore 90% of the time. My thinking is that when on shorepower efficiency doesn't matter, so the conversion is not a big deal, but when offgrid, the inefficiency of using the inverter to convert from DC to AC for a high draw load would be a significant factor.


I think that sometimes people misunderstand the reason for avoiding electrical heating/cooling as an efficiency thing. But I don't think that's quite accurate, electric is fine for heating and cooling, maybe even more efficient (?) the bottleneck is generating and storing enough electricity in a usable electrical form onboard for it to be possible. But IF you can generate it (excess solar) no reason not to direct it towards heating water a small amount of water. There are a few examples of people doing this successfully.



At least that is my take on it, i'm not convinced that i see all the factors/understand what i'm talking about though
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Old 11-27-2021, 10:12 PM   #16
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Quote:
Originally Posted by s2mikon View Post
DC is much harder on AC rated switches. AC switches used in DC service must be de rated to last any length of time. How long would it last? Maybe long enough to test proof of concept.
.
a)
POC -- Proof-Of-Concept, an engineering term.
.
b)
I think using AC-rated switches for DC is test-pilot territory.
Why?
Alternating current cycles OFF sixty or so times each second... fuzzy.
Direct current is always ON... and angry.
.
Do some welders use DC?
Some do.
Maybe you experienced a form of that by touching a wrench across a battery?
How was it the second time?
.
To interrupt (successfully) a DC circuit, I think a DC-rated switch is probably a good idea.
To prolong the duty-cycle of the switch, I think I would have minor juice to the switch for it to activate a relay for the heavier juice.
.
c)
Fuse early, fuse often.
.
In America, we have well-tested building codes for plumbing, HVAC, electric, foundations and walls and rooves, you name it.
I often leisurely peruse the codes for an enjoyable afternoon.
.
.
Disclaimer:
You are responsible for your decisions.
Concerning electricality, I know neither diddly nor squat.
.
.
After saying all this, I realized I could do one of my 'Contrarian Views':
.
In our rig, we carry water in retired five-gallon stainless-steel Pepsi kegs.
A quick puff from the air-compressor pressurizes the keg, furnishing a nice water-flow through the spritzer... a stand-still house kitchen-sink sprayer.
.
For showers , we use a dedicated three-gallon Torpedo keg.
We heat that soon-to-be shower-water using one of our 'sous vide' circulation-heaters designed to maintain my set temperature.
Our 'sous vide' circulation-heaters are 120vac through an inverter.
.
Some brainiac determined the amount of photovoltaic required to heat a three-gallon shower.
I think it came out to about fourteen minutes.
.
.
In other news:
Somebody was considering a heated floor.
Should they go water-loops, all-electric, diesel-fired, roof-mounted solar?
.
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Old 11-28-2021, 12:03 PM   #17
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So I am not looking to slurp power from a campground, my bus is completely OFF GRID.
DC is the preferred power source for all off grid applications.
AC is used for strictly play-time.

Nothing above has anything to do with the basic question I had originally, which was, how to wire up a DC heating element to a DC solar system replacing the AC heater element, while still using a AC heater thermostat.
The web site I linked to twice, shows a basic wiring diagram, which goes like this.

Starting at the DC panel.

25a fuse to 10g wire to on/off switch.
On/off switch to DC Relay
DC relay to Heater Thermostat.
Heater Thermostat to heating element.

The key in not frying the thermostat seems to be in staying at or under 300watts.

What threw me of in the post above, is why would anyone suggest to use a AC relay.

I thick I have enough info from what everyone has commented on to give this a go.

But going forward, I would be eventually using the 36volt diversion from my electrocadus system to power this heater, not the 12volt. The 12v is a proof of concept test for me. I can then use the 12 element for the 2.6 gallon heater, and use the 36 volt for the 6 gallon heater. As I,m not sure what my how water needs will be, I wanted to test bosh scinarios.
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Old 11-28-2021, 12:56 PM   #18
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Dc kills ac contacts if theyíre not rated for dc.

Your ac thermostat is not likely rated for dc and will likely die if you run the dc through it.

If you run ac through the thermostat and use the ac to power a relay with dc contacts that turns the heater on and off youíll likely have a better experience.

Essentially replace ac heating element with a relay that has a 120vac coil. The ac thermostat will turn the relay on and off. The dc rated contacts on the relay will handle the current of the heating element and everything will live happily. Itís really the right way to do this and relatively simple to implement.
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Old 11-28-2021, 01:21 PM   #19
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RE:
>Essentially replace ac heating element with a relay that has a 120vac coil.


Can you link to one here? I cant visualize what your talking about.


Thanks,
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Old 11-28-2021, 02:07 PM   #20
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Peterjk, I just sent you a PM.
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