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Author Topic: Running a water heater off the inverter  (Read 4115 times)
bobofthenorth
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« on: October 25, 2010, 04:32:19 PM »

They say confession is good for the soul so I thought I'd give it a try.  Today I likey broke several electrical codes from more than one country in the process of wiring my water heater off my inverter.  We recently replaced our 20 gallon water heater with a 10 gallon model.  The 10 gallon is more than adequate, it keeps up with showering with no problem, it takes up way less space in the bay and most importantly, it doesn't leak like the last one did.  So that's all good.  What it doesn't do is hold the heat.  We used to be able to go a couple of days before the water got really cold and for the way we travel that worked well.  On the rare occasions when I had a functioning generator we could run it to warm the water heater but that's a whole 'nuther story.  The new tank is often cold by the end of the day so that sux.

So the goal was to be able to run the water heater off the inverter while we are travelling and switch to shore power when that is available.  The water heater was already wired on the "other" side of the 50 amp service.  Most of the coach runs through the inverter in pass through mode on one side of the 50 amp service but the water heater and both banks of AC are on the other side.  The way I accomplished the task was to wire the water heater through a 3-way (SPDT) switch so that I can select either source of power (through the inverter or not through the inverter).  That way when we are travelling I can leave the water heater on the inverter.  When we stop for the night switching it over to the non-pass through side will effectively disconnect it from power so that we don't drain the batteries. 

I don't think I've done anything dangerous but I'm not always the sharpest knife in the drawer and I tend to think in a straight line so if  I've inadvertently done something dumb please tell me.
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R.J.(Bob) Evans
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« Reply #1 on: October 25, 2010, 04:53:31 PM »

Is there any way you could use a heat exchanger plumbed into your engine coolant lines to heat the tank while on the road? 

Glenn
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Glenn Williams
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« Reply #2 on: October 25, 2010, 05:11:54 PM »

It would seem that you could add insulation to the water heater to help it stay hot longer too.  Insulating the plumbing for it would also help.  We have a 30 or 40 gallon electric water heater at home that is in an exterior water heater closet.  I use that closet as storage for any insulation I have left over from something else.  There were times it was really packed with insulation.  It's got to make a difference.

As for the wiring, I think the biggest issue is forgetting to take it off the inverter when not running.  There a lot of things to remember when traveling.  I usually forget a couple at a time.
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« Reply #3 on: October 25, 2010, 05:23:32 PM »

"not the sharpest knife in the drawer".  i head heard that from quite a few, but you confirmation surprises me.  Just kidding.  Grin

we don't have our 12 gal on the inverter.  it and the toe-kicks are about the only thing not on the inverter.  i usually have to fire the genset every night for a couple hours to charge up, so the water heater is easily on for us to shower as we make a point to fire up the genset about 6pm.  the water is ready in less than 20 min.

having said why i don't think  the water heater is a big deal to run from the genset, i also don't think it's a big deal to run the water heater off the batteries if you have enough to handle the load.

i also decline to consider myself an expert on any of this, only my own experiences and opinions.
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Tom
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« Reply #4 on: October 25, 2010, 05:48:57 PM »

Bob, I am not smart enough to comment of the safety aspect.  I don't see anything obvious in that area, but not an expert.

However, I suspect that the element is 1650 watts and that would put a pretty good current though the switch.  Knowing you, you chose the switch with the current in mind.

The AC current is close to 14 amps.  With a 12V system that would be about 140 amps DC (factored a bit for losses).  Half of that for a 24V system.  You have eluded to the problem of running off the batteries.  Some folks can run and AC (a bit less current) off of the battery bank for quite a while.  But most folks do not have that big of a battery bank.  

I mention the DC amps to emphasize that a 12V system would need the big 300 amp alternator to cover the loads.

Your system still puts the burden of switching the source on you.  My tired brain would forget.  If the concept is safe, it would be better to use a relay and trigger it via the ignition switch.

Way over my head, but that never stops me for offering a thought Cheesy

Jim
« Last Edit: October 25, 2010, 06:05:19 PM by rv_safetyman » Logged

Jim Shepherd
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« Reply #5 on: October 25, 2010, 05:58:24 PM »

I would have to study the wiring of the inverter to comment on that -- I think it is a good thing to keep the water in the heater -- try the insulation thing -- you can't hurt anything with that and if you are in the mood to follow Jim's advice with a relay

Good luck

Melbo
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bobofthenorth
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« Reply #6 on: October 25, 2010, 06:27:01 PM »

The engine heat idea is well into the design stage Glenn but I wanted the inverter as an additional level of redundancy.  I have a flat plate heat exchanger and a recirculating pump.  The concept is to draw water from the hot water tank, circulate it through the heat exchanger and return it to the tank.  I believe that I will also be able to heat the engine using the water heater element with this system but that will just be a bonus if it happens to work.  Its too effing cold and wet here now to do all the plumbing that will be required but I've got all the bits and pieces rounded up.  Need to find somebody with a warm dry shop - you listening Clifford??

The relay is a good idea Jim, I might look into that if the basic system works.  I'm not too worried about the load on the alternator.  I realize it will be significant but the water heater is only rated for a 15 amp breaker.  I've got a 200 amp alternator which obviously won't keep up if there are other system loads but the water heater isn't going to run anywhere near 100% of the time - I'm just guessing but I'm thinking more like 5% of the time.  I'm not sure that I want the relay because I don't think we'll need to use the system every time we move.  If its just a one day move it won't matter and in warm weather we can go more than 24 hours without losing all our hot water.

I'd love to be able to fire up a generator every night Tom but we haven't had real good luck with generators.  If I had just spent all the money that I have dumped into this black hole on one brand new Wrico unit I'd have something now.  As it is all I really have is a stack of receipts.  I've actually decided to junk it completely and start over with a very simple system designed to charge my batteries.  I managed to score a brand new Leece Neville 200 amp pad mount alternator for $200 and I've got a Kubota diesel out of a yard tractor lined up to drive it.  I need to pick up the Kubota and scab up some kind of a mount for the alternator but once that's done I'll have pretty good charging capacity and if it isn't enough the engine is big enough to hang another couple 200 amp alternators off of it.  As it is I'll belt the alternator so that the engine can idle so the noise level should be considerably better than what we have got used to.

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R.J.(Bob) Evans
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« Reply #7 on: October 25, 2010, 06:42:09 PM »

I have seen entertainer coaches with "mini" electric hot water heaters for the sinks on the bus and those would be running off of inverter.

They are 110 a/c and not, on demand, but  do have a very small tank and the element pulls about 5 amp.

The one I saw was a Tiny Titan you can get it at Home Depo and it is about 159 bucks. The one I encountered put out plenty of very hot water they have a wall mount bracket.

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« Reply #8 on: October 25, 2010, 07:12:25 PM »

Bring it Bob we will be here, in late Dec Andy Wright will be here we are going to install the DD double cooler on his 8v92 and reroute his turbo that will take us about a week depending on how hard Andy works   lol.
Doyle will be here sometime in Jan for a visit no buses in the shop heck I have about 3/4 of the shop clean now

good luck
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bobofthenorth
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« Reply #9 on: October 25, 2010, 07:51:28 PM »

.... heck I have about 3/4 of the shop clean now

I don't believe you.  I need pictures.

And be careful what you wish for.
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R.J.(Bob) Evans
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« Reply #10 on: October 25, 2010, 08:26:05 PM »

I see no particular issue with running the hot water heater from the inverter, particularly as you imply it is on while the coach is running so powered from the alternator.  I also don't see a huge issue with running through a switch to select power sources - a 15 amp switch is not all that hard to come by.  About the only issue I might see is setting this up so that all power to the circuits in question are broken through a common bridged breaker - I am thinking of maybe a bridged breaker so that you are drawing from one side of the panel on half of the breaker, the other side on the other half, so that when you blow or switch off the breaker all chance of the circuit being live is deactivated.  not sure I am explaining this right...

Brian
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bobofthenorth
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« Reply #11 on: October 25, 2010, 08:47:06 PM »

I understand what you mean Brian and I don't see any way that I could implement it.  The breakers in question are in 2 separate panels.  But I agree it would be a good safety feature.  You've probably nailed one of the ways that my setup is offside with NEC.
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R.J.(Bob) Evans
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Sean
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« Reply #12 on: October 25, 2010, 09:44:21 PM »

Bob,

Your wiring is safe and legal so long as both the "panels" in question share a common neutral, such as would be found in a split-phase panel.  In this case, the conduit, chase, or cable assembly going from the panels to the switch should carry both hots, the neutral, and the ground together (code requires the neutral for any given circuit to run in the same assembly as the hot for that circuit).

If the neutrals are separate, for example because the inverter requires them to be or uses separate output and input neutrals, or because the panels are in physically different locations, then you really should run both hot and neutral to the switch from both panels and use a DPDT switch instead of the SPDT one you chose.

I agree with the earlier suggestion to use a 20-amp relay to switch the load to the inverter only when the alternator is running, with an option to disconnect it from the inverter in any case.

Lastly, I also agree with the suggestion to add insulation to the water heater.  Amazing what a simple piece of pink board will do here, or even fiberglass batts.

-Sean
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« Reply #13 on: October 25, 2010, 11:18:46 PM »

I have a similar system.  I use two 10 gal electric water heaters plumbed one into the next.  The first one is wired straight into the gen or shore line.  The second is wired through the inverter so to have hot water running down the road.  My normal way of heating the two water heaters, is I first turn on the second water heater and let it warm up.  Then I turn it off and turn on the first water heater and let it warm up and leave it on.  Then it is warming the water going to the second water heater.  I have 20 gal of hot water (not real hot so not to build up calcium deposits inside the water heater [they are 15 years old now with no problems-just drain them every so often]) that never runs out with two showers and washing dishes.  Will use the system again I like it so much.  Good Luck, TomC
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« Reply #14 on: October 26, 2010, 02:29:23 AM »

Regarding heating water whilst the engine is running:- direct heat from the engine's cooling circuit is a very good idea, but I would want to think through any possible implications of doing this in the suggested 'home brew' way of adapting the water tank from an electric water heater; I'm not saying there would be a problem doing this, but my bus has both a calorifier and an electric water heater, and the quality of the tanks is chalk and cheese - the calorifier tank is a properly designed and manufactured copper pressure vessel, whereas the tank in the electric heater is a flimsy square plastic thing with some loose bits of polystyrene around it. Also - I believe that if your hot water is to be used for potable purposes (rather than just in a closed heating circuit), it should be double-insulated from the poisonous engine coolant water - ie. the first heat exchanger should feed a closed circuit from which a second heat exchanger heats the potable water.

Another idea for heating water on the road would be to fit a second 12v heating element in to your electric water tank. 12v elements are easy to get hold of - they're widely available on Ebay for example, for use in WVO systems.

Jeremy
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