Water Heater Diversion Load HELP!

johnnysolar · September 2013

I have recently completed the installation of a 4.6 kW (18 panel) ground mount of Suniva 250's. They are feeding an XW MPPT 80/600 charge controller charging 8 450aH Rolls s600 deep cycle batteries. The inverter is a Magnum MS4448PAE 120/240 off grid inverter. My reason for using the 80/600 was the long distance between array and mech room. I didnt want to bury a bunch of #4 and there is a possibility that this system will someday become grid tied if the utility changes their attitude towards solar. The 80/600 is the only inverter that would accommodate my situation.

The system has reliable grid power but the owners wanted it for other reasons. They(2 people) are using it everyday and it has become apparent that the array spends a good deal of time float charging the batteries and we are not taking full advantage of the solar power. These folks are "night owls"

I'm sure you have heard it all before. I want to heat water with the PV system. I know that I need to do a dump load but I don't know how. Any guidance and technical support is greatly appreciated. It's tempting to just swap the 240v 4500w elements of their existing heater out with 120v 2000 watt elements one on L1 the other on L2 and heat the water with AC power off the inverter. I would put the WH on a timer so the water heater would only come on at noon and heat for an hour. I would add some aftermarket insulation to the tank. What's the downside of doing this.

If I have to do a DC diversion load style water heater setup I will have to trench pipe into the house as the batts/inv/CC are in a shed behind house and I know the wire from the diversion load controller to the element will be quite large. My battery/inverter wires are 3/0.

I would like to know the best way to do this from a cost standpoint and a technical standpoint. Which equipment should I use? How should I install it? This is for a customer so I need it to be safe/professional/code compliant. Thanks for all your help people. This is not my first install but definitely my first dump load. I have been stalking around on this forum for a few years and today I finally joined, hopefully I will be giving advice some day too! So glad this forum is here.

Cariboocoot · September 2013

Re: Water Heater Diversion Load HELP!

Welcome to the forum.

My suggestion would be to use the charge controller's AUX function to switch on the electric hot water heater when the batteries are at Float. For charge controllers that don't have this function you can achieve a similar action using a Voltage controlled switch to activate a relay to turn on the heater.

Of course water heaters have thermostats and if they do not need to heat the water when the power is available nothing happens. The two elements do not come on at the same time, btw, but in sequence. There would be no advantage to switching to 120 VAC elements over running the 240 ones. The Magnum can supply 240 Volts, but at less Wattage. You might want to swap the elements for 3500 Watt ones.

You do not want to try and do this with DC direct from the array or controller; switching high Voltage/current DC is much more difficult than AC.

johnnysolar · September 2013

Re: Water Heater Diversion Load HELP!

Thanks for the quick reply. If I was to do the setup you recommended would it be wearing out the inverter quicker? I don't want to put unnecessary load or strain on the system. Does the excess power pass right through when battery is in float or does it still pull off the battery?

Cariboocoot · September 2013

Re: Water Heater Diversion Load HELP!

An inverter rated as 4400 Watts should be able to output that amount continuously for a very long time. Some things will affect that, for example how low the input Voltage goes under the load (more current pulled on the DC side = greater heat) and how well-ventilated the area where it is installed (heat is always the enemy of components). A cheap inverter will not manage this as well as a good one. Magnum should have no trouble.

The other good thing is that a water heater will not pull 3500 Watts continuously; once the water is warmed up it shuts down. This may not be the best for fully utilizing the power potential of the panels, but it's better than not using it at all.

Power will only be taken from the batteries if the load demand exceeds the panel output at a given point in time. In other words so long as the panels can manage > Watts than the load the batteries will continue to float.

How is the water being heated normally? If you are supplementing another thermostatically controlled heater then using an electric tank in this way as a "pre-heater" can be quite a bonus.

animatt · September 2013

Re: Water Heater Diversion Load HELP!

Depending on the users I might venture in with smaller 220v elements. Something like 2000w or 2500w elements. This gives more working room for the inverter if it is called on to do some other work. Not sure it is an issue in this case with these night owls. The 2000w would be a bit gentler, but it would need longer run times. Down side could be non collected pv power.
Enough solar to collect 4000w but only floating then only using 2000w of energy, so only collecting roughly 2000w. Good side being that less optimal weather the array should still supply most of the 2000w. Basically keep the batteries floating. I think the 2000w element would be more conservative of a plan. Less overall reward but also less risk of negatively affecting users.

Also this would work really well if all the extra solar was going into a preheater tank. The preheater tank can fluxuate more so more overall btus can be stored. It could be a rare thing when the preheater tanks were already at temps that shut thermostat off. So when etc electric was around could always go into heating water. Heating main tank the tank maybe already up to temp so no more room to store heat.

Matthew

Cariboocoot wrote: »

An inverter rated as 4400 Watts should be able to output that amount continuously for a very long time. Some things will affect that, for example how low the input Voltage goes under the load (more current pulled on the DC side = greater heat) and how well-ventilated the area where it is installed (heat is always the enemy of components). A cheap inverter will not manage this as well as a good one. Magnum should have no trouble.
The other good thing is that a water heater will not pull 3500 Watts continuously; once the water is warmed up it shuts down. This may not be the best for fully utilizing the power potential of the panels, but it's better than not using it at all.

Power will only be taken from the batteries if the load demand exceeds the panel output at a given point in time. In other words so long as the panels can manage > Watts than the load the batteries will continue to float.

How is the water being heated normally? If you are supplementing another thermostatically controlled heater then using an electric tank in this way as a "pre-heater" can be quite a bonus.

animatt · September 2013

Re: Water Heater Diversion Load HELP!

Something like this for the heater elements.

matthew

solarvic · October 2013

Re: Water Heater Diversion Load HELP!

Preety high price for 3 used elements, just over $70.00 @ the opening bid. solarvic

animatt · October 2013

Re: Water Heater Diversion Load HELP!

I was not suggesting to buy that exact auction. Just the size. It was the first to pop up in a search.

Straight google search for 2000w 220v electric heater element did not show very good results. I got lots of coffee heater elements and others heater elements. It could be i am in mexico and google.com.mx definately varies from straight google.com

I would probably suggest the largest in physical size that is 2000w/220v(if cost is close). More surface area for the element. Not so much about efficiency as electric heating is right arounf 100% but more about scaling and longevity of the element.

but your point is well made. that is expensive.

If you have elements around like 4500w/220v and feed it 110v. it would effectly turn it into a 1125w/110v element. Kind of getting low on wattage but feeding 2 elements at the same time may work. Top element could use one side of the 220v and the bottom element could use the other phase.
matthew

Cariboocoot · October 2013

Re: Water Heater Diversion Load HELP!

An assortment of elements: http://www.plumbingsupply.com/elements.html

You ought to be able to buy most of the common ones locally.

johnnysolar · October 2013

Re: Water Heater Diversion Load HELP!

The water is currently being heated by a 40 gal electric water heater. I usually get elements locally too. Thanks for all the good info guys. Sounds like the best plan of attack is to mount a relay in the house and control it with the XW CC aux output based on a slightly higher than float voltage. I'll have to bury another wire but at least I won't be trenching and insulating pipes for burial. I like the idea of preheating a dedicated solar storage tank so I have tons of room for the btu's. Maybe they will have room for an extra tank. I actually have most of my background in solar hot water, but since the drop in PV prices I've found myself doing lots of solar electric.

My biggest worry was that heating water with the solar was going to wear out the inverter or battery prematurely but no one seems to be too worried about that so I wont either. I could probably use the relay to turn other things on too like mini split or even a space heater or is that going to far?

Cariboocoot · October 2013

Re: Water Heater Diversion Load HELP!

johnnysolar wrote: »

My biggest worry was that heating water with the solar was going to wear out the inverter or battery prematurely but no one seems to be too worried about that so I wont either. I could probably use the relay to turn other things on too like mini split or even a space heater or is that going to far?

The water heater should be activated based on the charge stage, so if it is in Float it comes on. Doing it by Voltage alone gets a bit tricky as the load of the heater could take away too much power during Absorb. The best system would be to use a MidNite Classic which has a special "waste not" function designed to do just this: divert surplus PV power when available. But that would involve changing out the controller and it's probably not practical to go to that extreme.

As for using the relay to activate other things, you'd have to switch what was being controlled somehow based on another state indicator. Somewhere logic would come into it: "IF water > C AND charge state = Float THEN control 'X' output". Gets complicated. If the two major opportunity loads together don't exceed the PV & inverter capacity you could have them run simultaneously as needed.

Photowhit · October 2013

Re: Water Heater Diversion Load HELP!

I think the Xantrex XW CC can do much the same thing, but I'm not familiar with it, a 240w 2000w element is available on amazon for about $9. I'm using a 3600 watt 240v element on 120v (runs around 900watts) and it works if slow. I'm currently around during the day and have been just turning it on manually. I have an hour timer to install, but haven't yet, Thinking it might be good to do it an hour at a time, and then not having to worry about leaving it on...

Vic · October 2013

Re: Water Heater Diversion Load HELP!

Last time that I looked, the XW SCC STILL lacked an Aux output=active when CC is in Float.

When contacted S/X's crack Tech Support Team, they wondered just what other CC had this "useless" function, and wondered also just what use it could serve.

After explaining which CC did have it, and just how it might one of the most useful CC Aux outputs, they mentioned that it was very unlikely that their CC would ever have this (useless) function.

They have often seemed unimaginative over at brand S, at least to me. Vic

johnnysolar · October 2013

Re: Water Heater Diversion Load HELP!

After reading about the AUX function in the manual it seems like I can only trigger it by array voltage, battery voltage, heat sink temperature or battery temperature. Seems there is some truth to Vic's slanderous X comment. Still, I had to use the XW for the long wire runs and I really didn't anticipate a need for a diversion load water heating setup when I designed this thing. Perhaps I will call the good folks at tech support tomorrow and get there 2 cents. Maybe they will want to turn it on and off manually. Thanks fellas

NorthGuy · October 2013

Re: Water Heater Diversion Load HELP!

johnnysolar wrote: »

After reading about the AUX function in the manual it seems like I can only trigger it by array voltage, battery voltage, heat sink temperature or battery temperature. Seems there is some truth to Vic's slanderous X comment. Still, I had to use the XW for the long wire runs and I really didn't anticipate a need for a diversion load water heating setup when I designed this thing. Perhaps I will call the good folks at tech support tomorrow and get there 2 cents. Maybe they will want to turn it on and off manually. Thanks fellas

Note that you have XW SCC80-600, which is relatively rare. Other people probably talk about XW SCC60-150, which is very common. Unfortunately, I don't know about XW SCC80, but Dave Sparks who frequently posts here, may be able to tell you if there are tricks about it.

johnnysolar · October 2013

Re: Water Heater Diversion Load HELP!

how would I go about contacting this Dave Sparks

Vic · October 2013

Re: Water Heater Diversion Load HELP!

johnny,

Go to this page, and on the upper left, there is a "..send private message" button:

http://forum.solar-electric.com/member.php?2991-Dave-Sparks

Try that. Vic

BB. · October 2013

Re: Water Heater Diversion Load HELP!

You can send Dave Sparks a PM or Email via the forum:

http://forum.solar-electric.com/member.php?2991-Dave-Sparks

Or email him through his website:

http://www.sierratel.com/offgridsolar/

-Bill

niel · October 2013

Re: Water Heater Diversion Load HELP!

just click on anybody's username you wish to pm and it will list a bunch of stuff including private messaging on the left.

Water Heater Diversion Load HELP!

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