Making a Dump Load with a Hot Water Tank Heater

September 18, 2015 · 10 comments

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Can a standard off-the-shelf tank style electric hot water heater can make a dump load?

Electric tank water heaters are an excellent choice to make a dump load for your solar, wind or micro hydro power system as they are easy to get, cheap, already insulated and have holes to mount low voltage water heating elements.

You can usually find a pretty good looking and quality electric hot water heater just about anywhere in the world that can be used either as:

  • a pre-heater for your main hot water heater if you are dumping power from a normal sized solar system or
  • your main hot water heater if you have a large, consistent supply of electricity such as a good quality water turbine or high power wind turbine.

Even though an off-the-shelf tank style hot water heater can make a good dump or diversion load there are a few concerns such as:

  1. The water heating elements in a standard tank heater are made for high voltage (either 120 or 240 volts)
  2. The thermostats in a standard electric water heater are not made to handle DC and will either arc and stop working or catch fire
  3. When the tank of water becomes too hot, where do you send the excess electricity?

We will answer the above questions in a few minutes but let’s discuss the most common question I get about using tank style water heaters for a dump load/water heater.

The first question I get when folks decide to go this route is:

Why can’t I just use the output of the inverter to dump electricity (when the batteries are full) into the existing water heating elements using the factory thermostats?

While it is possible, it is likely not a good idea.

The first problem is that if there is ever a problem with your inverter, your dump load is non functional and it only takes one really long overcharge for your battery bank to be damaged.

The second problem is hot water elements are usually high wattage 2500-5000 watts which could be enough to consume all the electricity your inverter(s) can produce, depending on the size of your system. If you use your entire inverter’s capacity, there will be nothing left over for your loads like refrigerators, freezers, computers, dishwashers, and all the other goodies we have grown accustomed to.

The third problem with using AC from your inverter to operate your dump load is your will need to set your system up to turn the dump loads on at a particular voltage (your bulk voltage) and off at a particular voltage (something lower than your bulk voltage). This is how old solar charge controllers used to operate. They had a hysteresis (difference) of a few volts and would turn the solar array off at about 14.4 volts and back on at roughly 13.2 volts in a 12 volt nominal system. There are two problems with this:

a) The voltage is always changing between the on and off voltage and we have learned that batteries will last much longer if the bulk voltage is held steady with pulse width modulation (PWM) rather than going up and down constantly.

b) Your 2500-5000 watt element(s) are a huge load on a battery bank unless it is huge making the heating elements cycle on and off sometimes many times per minute. This is very hard on your inverter and will make your lights blink on and off as well as make your refrigerator and freezer compressors unhappy.

How can I keep the battery bulk and absorption voltages steady so my batteries will be happy and last longer?

This is where PWM charge controllers came in. PWM stands for pulse width modulation meaning the load (or solar array) is switched on and off several times per second in trying to hold the battery voltage as steady as possible. PWM is much easier to understand than most people think.

If your batteries only need 100 watts to hold the voltage at the bulk voltage but your solar array is making 1000 watts, PWM will make sure the circuit (solar array to batteries) is connected 10% of the time and open circuited 90% of the time just like the first of the three PWM examples in the diagram below.

pulse width modulation (PWM)

EXAMPLE #1 Your solar array produces 1000 watts but your batteries only need 500 watts to hold the bulk voltage. The PWM solar controller simply turns the solar off 50% of the time and on 50% of the time. 50% of 1000 watts is 500 watts. The only issue with PWM is it is fast. It does its switching hundreds or thousands of times per second.

EXAMPLE #2 Imagine you want to dump 1000 watts of electricity but your dump load is a 3000 watt dump load. A PWM dump controller or 12 volt relay driver (built in to most quality inverters and charge controllers)  turns the dump load (or solid state relay) on 1/3 of the time and off 2/3 of the time. 1/3 of 3000 watts is 1000 watts. Of course it does this hundreds or thousands of times per second.

The downside of pulse width modulation:

Standard relays (coil type) cannot handle that type of switching. They will be destroyed. Only solid state relays (SSRs) can handle this type of rapid on/off switching. Another issue is PWM controllers will cause noise in your phone lines or other sensitive electronics.


 

So the question remains…

How do I use a standard hot water heater (tank style) as a dump load that will make free hot water?

The best method is described below. There are other methods but this is the most reliable and accurate.

STEP 1  Purchase the following:

* Two (or one if you are only using one half of the capacity) low voltage 12/24/48 volts DC water heating elements large enough (enough wattage) to dump your excess power. dc water heating elements for making a solar hot water tank

They are usually available in the following configurations:

  • 12 volt system: 450 watt and 900 watt X 2 elements = 900 or 1800 watts maximum at 15 volts DC
  • 24 volt system: 720 watt, 900 watt and 1440 watt X 2 elements = 1440 or 1800 or 2880 watts maximum at 30 volts DC
  • 48 volt system: 1440 watt X 2 elements = 2880 watts maximum at 60 volts DC

You will notice the voltages above are somewhat higher than the nominal battery voltages. This is because you typically dump excess power at approximately 14.4-15.0 for a 12 volt DC system, 28.8-30.0 for a 24 volt system and 57.6-60.0 for a 48 volt system.

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water element adaptersAdapters to convert a threaded DC element to a square unit with four bolt holes if necessary.

All low voltage elements used for diversion loads have a 1″ male national pipe thread (MPT) and sometimes they even have a gasket as well. Most of the time there is no gasket.

Although many hot water heaters have a female pipe thread (FPT), some have a square hole with a threaded bolt hole in each corner.

If you have the square hole type you will need one adapter, gasket and bolts per low voltage water heating element.

water heating element types

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A typical SSR (solid state relay) used in a dump load* Two solid state relays (SSRs) that are rated for DC at the amperage (depending on the size of your element(s)) and voltage (more than 15,30,60 volts DC) you will be using. You will need one SSR per heating element.

The SSRs also must be able to be triggered (turned on) at 12 volts as we will be using one of the 12 volt relay drivers already installed in your charge controller or inverter.

You can usually get DC SSRs that are good for 40 or 60 amps at up to 160 or 220 VDC that are triggered by 3-24 volts on places like eBay.

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* A charge controller or inverter that has a programmable 12 volt relay driver such as one made by:

  • Outback Power (all of their inverters and charge controllers have one),
  • Magnum Energy (all of their inverters have one or two)
  • Schneider Electric (all of their inverters and charge controllers have one),
  • Morningstar Corp and a few others. Some call them voltage controlled relay drivers or voltage controlled relays or voltage controlled switches.
  • Another option is the Morningstar RD relay driver. (It has four different channels or relay drivers that can be turned on and off at certain voltages and even temperatures. They are even programmable through a serial cable from your laptop or desktop computer.) If your equipment already has relay driver’s you will not need the Morningstar RD.

morningstar relay driver is a great unit for building a dump load.

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* A dump/diversion controller such as Morningstar’s TS-45, TS-60 or Schneider Electric’s C-40 or C-60 for a secondary dumping system for when the water is too hot in the water tank or the original diversion system malfunctions.

dump controllers to protect your batteries

or

* Use another of the programmable relay driver’s (in your existing equipment) in conjunction with a solid state relay or standard relay.

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* A secondary dump load such as an air heater/resistor. You may have to use multiple units and place them in a metal vented electrical box or purchase a manufactured unit.

backup air heaters

 


Now before you start to think this is going to be a massive and confusing project, relax and only worry about one item at a time.

For all the next steps listed below we will include wiring diagrams and explain things as clearly as we can.

STEP 2 Remove the high voltage and high wattage AC elements and replace them with DC lower wattage elements.

 

 

 

STEP 3 Connect the negative lead from your battery/DC breaker panel to one side of each heating element.

 

 

 

STEP 4 Mount one solid state relay (SSR) to the outside of your hot water tank for each element you will be using. SSR’s can get hot so it is a must that each SSR be mounted to a large piece of aluminum or heat sink.

Aluminum heat sinks are great for keeping SSRs cool.

I apologize this article is not yet complete. I clicked the publish button by mistake. I will finish as soon as possible. Thank you for your understanding….

Leave a Comment

{ 10 comments… read them below or add one }

Sean S. October 28, 2016 at 2:04 am

Hi, I am setting up a test project with a home built wind turbine and will collect data from turbine power production versus wind speed, temp to a data logger. I have a 48 volt three phase 2000 watt PMG I ordered from china and fitted it to the turbine. As I don’t have it connected to any battery charging system or grid tie it was suggested I look into hooking the three phase up configured to produce either 120 V or 240 V to dump load to stove elements or if possible a water tank. I will be testing in various locations so mobility from one location to another will be a consideration. What would you suggest I do or is this even possible?

Reply

Jody Graham October 28, 2016 at 7:36 pm

Hi Sean,

Thanks for contacting me. Are you able to reconnect the windings of the alternator to series wye? Is that how you plan to get 120VAC 3 phase out of a 48VAC alternator or am I misunderstanding something?

Right now I am assuming it is configured for 48V wild 3 phase AC.

You could rectify the 48VAC to DC with a 3 phase bridge rectifier as found in old Ford alternators and use a couple of 1000 watt 48 volt water heating elements or air heating elements to make things simple.

Or you could stay with the 3 phase and use 3 elements wired 3 phase. I could draw it out for you if you wish. I would just need to know if the alternator is wired delta (3 wires) or wye (3 or 4 wires).

If you have more info let me know. It sounds like a fun project.

I hope I have answered your question but please tell me if I didn’t. Jody

Reply

Chupacabra May 16, 2016 at 7:00 pm

The other concern is a light bulb can blow if the input voltage is too high. A blown bulb is open circuit, and no longer provides any form of dump load protection to your battery bank or windmill.

Reply

Jody Graham May 16, 2016 at 9:06 pm

Hi,
You are absolutely correct. That is one of the reasons we NEVER recommend incandescent or any light bulbs as a dump load. Thanks for your comment…Jody

Reply

Brady Sheridan January 29, 2016 at 12:24 pm

I would be really interested in the finished product, wiring diagram etc. as i am a newbie to the solar area. Great articles on this site, I really enjoy them! A couple of things i am curious about is whether or not there is a “fuse” somewhere in the diagram and where, as well as why you use 2 SSR’s, one for each element?
Brady

Reply

Jody Graham February 3, 2016 at 1:17 pm

Hi Brady,

Thank you for you comment. I will work on a wiring diagram as soon as I can. I am struggling with a chronic illness that slows me down. You would definitely use a fuse or breaker for each element/dump load. Just make sure they are rated for DC current and not just AC as most are. I would use one SSR per element. That way you can set your system to turn each element on and off independently depending on how much solar, wind or micro hydro you have available at the time. Will try to finish this article with diagrams very soon.Jody

Reply

Gene November 8, 2015 at 6:49 am

Hi Jody, just want to add to my previous comment/question that using my grid tie inverter for off grid would only be done if the grid was down for an indefinite time. My home electric would be disconnected from the grid. I have installed a battery back up to activate my PV’s, now I just need to divert the excess AC to be ready for a power grid outage should this ever happen.

Reply

Gene November 8, 2015 at 6:23 am

My 4.5Kw PV is grid tie and 300 ft from my main breaker panel. Your article is a great way for off grid where PV supplies DC directly to battery bank via charge controller/inverter. Here is my idea for using a 120v AC hot water heater as a dump load. My grid tie inverter trips off at 134 vac single phase and can also be adjusted. If I install three 10 amp 120v AC switching SSR’s staged to dump excess solar AC at say 125v, 128v and 131v this would reduce the large voltage drop of a single SSR. On sunny days my excess solar is 500W to 3Kw. Do you think this is a workable way to dump excess AC to a 120v hot water heater?

Reply

alan ali September 18, 2015 at 10:52 am

What about using a pwm as a dump load controller feeding a dedicated 220v inverter which is in turn hooked up to your hot water heater? There are cheap 220v inverters (modified sine wave) on ebay.

Reply

Jody Graham September 23, 2015 at 2:07 pm

Hi Alan,

Thank you for your comment. Powering cheap (or high end) 220 volt inverters with PWM (pulse width modulation) to operate off the shelf heating elements would never work. If you could get a PWM dump controller that would dump that much amperage (unlikely) the inverters would never turn on and off at many times per second. If you take a normal 12/24 or 48 volt to 220 volt AC inverter and connect it to battery voltage for a 100th of a second, the inverter would not even have time to boot up before the power is removed again. Try connecting your inverter for a second and then disconnecting for a second and back on for a second and see what happens. The inverter will sense this as unstable input voltage or low voltage and shut down.

The other problem is most of the cheap 220 volt inverters on eBay are 12 volt. If you have even one 2000 watt heating element that is 167 amps your dump controller will have to be rated for. I know of no such product. 40, 45 and 60 amp models are the norm.

Anyone else have any opinion?

Thanks…Jody

Reply