electric water heater on aux

jtdiesel65 · January 2012

Is anybody using an electric water heater off an aux? I was thinking of putting an electric tank in line before my gas tank heater and setting up an aux circuit that would come on at float. The idea being that it would make a difference even if the water were preheated to some degree. It's pv and aux options are sw4024, classic, or morningstar relay driver.

waynefromnscanada · January 2012

Re: electric water heater on aux

What do you have for an off grid system - - PV watts, battery AH etc? It might be worthwhile if you had lots of both.

jtdiesel65 · January 2012

Re: electric water heater on aux

It's 4080w and 1550ah. But I was thinking of only using it when Charge controllers go into float mode and probably a tank on the smaller side with a smaller watt element as a preheat. So perhaps a 20Gal with 2000w element.

mike95490 · January 2012

Re: electric water heater on aux

jtdiesel65 wrote: »

....So perhaps a 20Gal with 2000w element.

I'd guess that the tank is too small, go for an 80 gallon, and they usually only have 1,000w elements

techntrek · January 2012

Re: electric water heater on aux

Most 40-80 gallon tanks have 4500 watt elements @ 240 volts. The largest "point of use" water heater I found on Home Depot's site has a 20 gallon tank with a 2000 watt element @ 120 volts.

waynefromnscanada · January 2012

Re: electric water heater on aux

Here in Canada, the 20, 30 and 40 gal tanks all have 3000 watt elements, and our 60 gal and above have 4500 watt elements. All are designed so that only one element is ever on at once - - if it has an upper and a lower element. Our gallons by the way are larger than American gallons. Ours are 4.454 litres/gal.
Also, if you want to run on a lower wattage, there's a couple of ways you can go. One is to change the element (you can also get 1500 watt, 120 volt elements), or you can run a 240 volt element on 120 volts and get 1/4'th the consumption and therefore 1/4 heat. For example, running a 4500 watt 240 volt element on 120 volts will make it 1125 watts. And consider adding, not one of those thin electric water heater "blankets", but rather at least one foot of fiberglass insulation to hold the heat in. It makes a huge difference. I've used a thickness of 2 feet around and on top of my water heaters for 30 years or more, and wouldn't do without it.

jtdiesel65 · January 2012

Re: electric water heater on aux

I guess the situation I'm really aiming for is a 40Gal tank but with smaller element. We're planning to put in an outdoor wood boiler. This will eliminate propane use in winter. We would rather not burn wood in summer. So I'm trying to come up with ways to eliminate propane use in summer. I was thinking the domestic hot water heat exchange tank for the boiler could be outfitted with electric elements. In summer, extra pv could be used to run these elements to warm the water before it enters the propane heater. I'm not sure if this is even practical.

8.3 lbs/gal *40 gal*1btu/lbs*.000293kwh/btu= .09727kwh to heat 40 gal one degree
.09727kwh/degree*10 deg =0.9727 kwh to heat 40 gal 10 degrees

8.3 lbs/gal *40 gal*1btu/lbs*10 degrees=3320 btus to heat 40 gal 10 degrees
91600 BTUs in a gallon of propane
1 gal/91600 btu*3320btus = .0362 gal propane to heat 40 gal 10 degrees

hmm, that sure doesn't seem like it's worth it.

BB. · January 2012

Re: electric water heater on aux

Just installing solar thermal water panels is usually a better deal. They are around 50-70% or better efficiency vs the ~15% or so efficient solar electric panels.

It is usually just too expensive and "too little" heat is collected from solar PV powered systems to heat hot water. Using heat pump type water heaters can be done with larger installations (multiple kWatt arrays).

-Bill

jtdiesel65 · January 2012

Re: electric water heater on aux

I never really considered water panels due to the orientation and location of the house. But just thinking about it, they probably could be located where the wood boiler would be going and perhaps even tied into the same lines as the boiler.

BB. · January 2012

Re: electric water heater on aux

And depending on your needs and where you live, for domestic hot water, you will probably need around 40 to 80 square feet of hot water panel per person... Usually a lot smaller than the size of PV array needed for a home.

-Bill

solar_dave · January 2012

Re: electric water heater on aux

I don't know where you live but in Northern climates the evacuated tube type Solar water heaters are suppose to work very good even in winter. I have a 4 X 10 regular panel here it supplies all my hot water 11 months out of the year and a good assist on the one month.

n3qik · January 2012

Re: electric water heater on aux

I have a dump load for my solar setup. With a 40 Gal tank, can get up to a 20 deg. rise in water temp during the summer. The max temp the tank reached was about 102 degs.
Attachment not found.

As you can see, total DC watts was 861,489 and 357,242 watts ended up in the water pre-heater.

sub3marathonman · January 2012

Re: electric water heater on aux

waynefromnscanada wrote: »

Here in Canada, the 20, 30 and 40 gal tanks all have 3000 watt elements, and our 60 gal and above have 4500 watt elements. All are designed so that only one element is ever on at once - - if it has an upper and a lower element.

Also, if you want to run on a lower wattage, there's a couple of ways you can go. One is to change the element (you can also get 1500 watt, 120 volt elements), or you can run a 240 volt element on 120 volts and get 1/4'th the consumption and therefore 1/4 heat. For example, running a 4500 watt 240 volt element on 120 volts will make it 1125 watts.

I have what probably is a basic question to someone who knows, but if you make the water heater 120 volts, won't that throw off the balance between the two 120 volt legs that make the 240 volts? Or would you do one element on one leg, and the other element on the other leg? But since they don't come on at the same time, it seems that wouldn't help either. Or do off-grid people not have to worry about balancing the loads?

And, to show how little I know, why is it 1/4th the consumption instead of 1/2?

BB. · January 2012

Re: electric water heater on aux

For on utility grid applications, people do not normally worry about balancing 120 volt loads on 120/240 VAC split phase power. As long as the maximum current does not exceed 80% of the main panel's current rating (for one leg), everything is OK.

Of course, I would not suggest going out of your way to put all of the loads on one leg. You may have slightly higher losses (power = I²*R) with vs a better balanced set of loads. Doubling the current on one leg will give your 4x higher losses (should still be a relatively small amount of losses for a properly designed wiring system).

For Off Grid power, you will have the issue of inverter's maximum power on one leg. As an example, the Xantrex XW can only supply around 70-75% of its rated output on one leg (as I recall). You can reconfigure an XW to be a 120 VAC only unit so it can output 100% of power on 120 VAC too (if you do not need the 240 VAC support).

-Bill

electric water heater on aux

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