Using low-voltage water heating elements as a diversion load.

[jtsailjt]

Originally Posted by captstu

I dumped excess solar into a water maker. Same idea

Have you found that it produces any fresh water when the normal power supply is turned off? If you have the breaker from your DC system turned off but the breaker from your solar turned on during the night doesn't the watermaker turn itself off? Then, when the sun comes up and your batteries are full to capacity, so the dump circuit is activated, does the watermaker turn itself back on? During dump conditions are there enough excess amps coming from the solar panels to power the high pressure pump to run it successfully? If too few amps are available, what effect does that have on the pump, the wiring, and the rest of the watermaker?

I understand the urge to salvage every amp onboard that it's possible to salvage, but I've never found that the conditions necessary to have excess DC power onboard seem to exist aboard my boat. If they did, I would probably turn my refrig or freezer down a couple of degrees and that would fix that "problem" straight away. For that reason I have a simple electrical coil dump load mounted upside down and suspended about 1/2" off of the top surface of my 135 gallon aft aluminum water tank. It's been there 3 years and I don't think it's ever been powered up and I suspect it may never be. Even during windy periods with the solar and wind gen and refrigeration left on while I've been away from the boat, I've never detected it getting warm, and yes I'm sure it's wired correctly. Maybe if you're going to spend a lot of time in the tradewinds and you have both a wind gen and big solar array, it might be worth getting fancy in order to turn that excess power into something useful, but for most of us it's going to a lot of expense for something that will never exist. Besides, in areas/conditions where it might be more likely to find yourself with excess power, isn't it likely that similar conditions will exist tomorrow as well so you don't really need to be pinching every last amp, especially when it adds more complication and expense to your electrical system? Just dump it into a cheap resistance type dump load and forget about it.

[jtsailjt]

Originally Posted by skipgundlach

Dual (meaning you don't lose the ability to do AC heating) elements can work if the configuration of the tank allows. My current water heater does not, so I couldn't use the one I had in my Raritan, which did.

My vendor for that part was SVHotwire...

Divert Load FAQ – svHotWire

I had that one on my boat too and if you check out the specs on it, the AC element is only 500W, about 1/3 that of a standard element. If you use a genset to make hot water, it's going to take 3X as long which defeats any conservation benefits of the dual coil.

Maybe it makes sense in the tropics, but even there I have a feeling that my refrig/freezer and watermaker will ensure that having excess power onboard is a VERY rare condition!

[Smokeys Kitchen]

Originally Posted by jtsailjt

There's a dual AC/DC hot water heater element available to do just what you are thinking about and it was installed in my water heater when I bought my current boat. Unless you're located someplace like the Caribbean I doubt that you'll spend much time with your batteries fully charged so any current is available to for your dump load. Even then, not too many cruisers report having amps to spare very often. Also, the AC element that's included in the dual element I had was such low power that it took at least an hour of running the generator just to get my water heater up to temp and now that I have a "normal" AC element it takes about 20 minutes. The only way I'd go with such an arrangement again would be if the hot water tank had two separate places to install heating elements so my ability to use AC power would not be degraded like it was with the dual element.

This.

We put a D400 wind gen on last April and instead of the heat shunt, I ordered a dual voltage heating element from eMarine Systems

https://www.emarineinc.com/

It works ok, but the heat recovery time with the new element is much slower than a dedicated single 120 volt one even when on the dock, plugged into shore power. Additionally, we spent last summer/fall coming up the inside passage, and while we received pretty good wind, the batteries were almost never 100% - so the wind gen seldom dumped into the water heater. We are considering putting the old element back and putting the heat shunt in for the wind gen.

There is a high end Torrid unit available with dual elements, but is switch controlled. If I were replacing my current unit I might consider this.

"The MVS-6 is available as a dual element, which is 120V 1500W in the first element, and the second element, controlled by a switch, is 12V."
Ian Gracey, Torrid Project Manager.

Torrid looks like they may have changed the model number, but here is the link to the new model.

https://torridwaterheaters.com/produ...-heater&page=1

[deblen]

Originally Posted by TreblePlink

Huh? I realize that heating elements change resistance a bit with temperature, but not THAT much. Here is the simple math:

10 Ohm element: with 120 volts RMS: 12 amperes. times 120 volts,
= 1440 watts.


10 Ohm element: with 12.6 volts DC 1.26 amperes. times 12.6 volts
= 15 watts.

Chris

You are correct!! I was wrong. /Len

[Cadence]

Would it not make better sense to increase the size of the battery bank? Using that power to heat water seems? The excess of power is only going to be part of the day. Storing it seems to make better sense?

[rom]

Originally Posted by Cadence

Would it not make better sense to increase the size of the battery bank? Using that power to heat water seems? The excess of power is only going to be part of the day. Storing it seems to make better sense?

yes but you cannot increase your Ah capacity indefinitely. You have to stop somewhere and decide this is about what suits you. And this limit is also dictated by how much battery weight your boat can handle.

[Cadence]

Originally Posted by rom

yes but you cannot increase your Ah capacity indefinitely. You have to stop somewhere and decide this is about what suits you. And this limit is also dictated by how much battery weight your boat can handle.

I realize that. Heating water with a few watts a few hours a day seems somewhat a lost cause. JMHO

[foufou]

Theoretically you'd get more by diverting the waste heat of the photovoltaic panels to the water heater. Waste heat is about 4 or 5 times the energy of the electricity produced by the panels. Not so easy to actually accomplish

[rom]

Originally Posted by foufou

Theoretically you'd get more by diverting the waste heat of the photovoltaic panels to the water heater. Waste heat is about 4 or 5 times the energy of the electricity produced by the panels. Not so easy to actually accomplish

and you would then increase the solar panels' efficiency at the same time by cooling them down ... oh, and at the same time you may warm up salt water before it enters the watermaker. Yes there is still room for progress in efficiency !

[foggysail]

Originally Posted by zboss

I think what appeals to me is just having the 12 volt element hooked directly to the batteries through the breaker panel. Turn it on when you have excess power available or if we absolutely need to run it at the same time as my honda generator is charging the batteries. It'll run fine off shore power obviously. No need to complicate the system by running more wires and worrying about diversion loads.

Have you ran the numbers? You may find that using 12v batteries to heat water does not make sense but I don’t feel like running numbers to prove it other than to point out that a 1000 watt heating goal requires over 80 amperes from your batteries. Good luck!

[Seriously Slow]

We recently installed a system to use our inverter to feed 120v to the water heater. It has a 15 min. timer on it so we can't overdo it and drain our bank. The heater takes about 20 min to provide enough hot water for dishes and 2 showers. That's all we need. I only gets used when the batts are close to full, the sun is really overhead and we have not motored that day. We have not had it for long but it is very promising.
The timer operates a relay that switches the 120v to the element from the inverter. It is about 1/3 solar while on and 2/3 batteries so the draw should be OK for a short time before the solar is back to fully charging the batts.
We have a generator if conditions do not cooperate.
It also was reasonable in cost. The only parts needed were the timer and relay plus wire and connectors. If we only get a few extra showers it will still be worth it.

[Scott Berg]

Torrid water heaters will make a dual element model so you can have both a 400-600watt 12v element AND a 1500 - 3500 watt 120/250v element. I set these up for customers often

[Cadence]

Originally Posted by Scott Berg

Torrid water heaters will make a dual element model so you can have both a 400-600watt 12v element AND a 1500 - 3500 watt 120/250v element. I set these up for customers often

That's still a hell of a drain on the batteries at 12V. 33+ to 50.

[Stumble]

Originally Posted by Cadence

Would it not make better sense to increase the size of the battery bank? Using that power to heat water seems? The excess of power is only going to be part of the day. Storing it seems to make better sense?

If you batteries are already full, which is the only time you need a diversion load, having more batteries would actually make the problem worse not better.

Note this is not an argument for smaller battery banks...

A larger bank is inherently more efficient because it suffers less from the effects of Peukert's law. Basically there is less resistance from pulling power out of the batteries and less resistance putting it back in. So if you had a larger bank you would have lost less power to resistance while pulling power out, and lost less power as you put power back in. So under the same draw a large battery bank actually has less power taken out of it, and also needs less to put it back in.

In this specific, and rare, case a large battery bank just means you would need the diversion load sooner and for longer than with a smaller bank.

[foggysail]

Originally Posted by Cadence

That's still a hell of a drain on the batteries at 12V. 33+ to 50.

YEAH! And I ran the numbers using a 40 degree water temperature change in a 6 gallon, 400 watt heater. Try 1.4 hours!