Using a Hot Water Tank as a Dump Load

September 18, 2015 · 26 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.

Standard off-the-shelf electric water heater

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 as shown below.

Aluminum heat sinks are great for keeping SSRs cool.

Leave a Comment

{ 26 comments… read them below or add one }

Hugh Piggott May 9, 2019 at 8:26 am

I have dealt with these issues in many different ways over the years but have found that I am very happy with using a phase-control AC SSR to manage the AC heater and driving this by a circuit that is activated by the PWM pulses to the DC load. http://scoraigwind.co.uk/tristar-follower-to-control-ac-heaters/

This arrangement needs very little special wiring, works with normal water heaters and their thermostats, and is “fail safe” because the Tristar controller is always there doing its job if the AC heating is not needed.

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Jody Graham May 9, 2019 at 2:06 pm

Hi Hugh,

It is a pleasure to hear from you. You are a legend in these parts. You and I have spoken over the years as I used to build water turbines like the “Stream Engine” with Paul Cunningham. Paul got to be a little more than I could cope with so I left and went out on my own years ago. He is one of the smartest fellows electrically I have met and he taught me so much about electricity, efficiency, water turbines, solar and simple things like voltage drop and really gave me an excellent background in AC and DC electricity for which I am grateful.

I am also grateful you left a comment as your “Tristar Follower” with the phase angle control solid state relay (SSR) is one of the coolest things I have seen in awhile. It looks like it makes perfect sense but I can’t wait to build one and try it out. Do you mind if I refer to your website from this one for this or other information? I will always give you credit.

For those of you reading the comments please take the time to go to http://scoraigwind.co.uk to learn more about wind, solar, microhydo and anything electrical. Hugh is always trying new and better ways of doing things and he documents his projects well. Plus he provides all of this for free.

I also want to say “Thank you for all the changed lives you have created as a result of your research and homemade wind turbines”. I know you have brought electricity to many in 1st, 2nd and 3rd world countries and made their lives that much better”….

Regards,
Jody

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JT May 1, 2019 at 4:46 pm

I tried a simple setup.
2 arrays of 4 275w in series connected in series into a 120vac 2000w element
Measured 132 vdc at 14.5 amps
This is a dedicated setup to heat water during off season ( spring , summer, early fall) when my wood boiler is down and not heating my domestic hot water.
Simple and self regulating without controller , batteries etc
I do have a thermostat controlled relay to prevent over heating.

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Jody Graham May 9, 2019 at 2:46 pm

Hi JT,

For a dedicated solar hot water system this is an excellent idea. You have the potential of 2200 watts (8 modules @ 275 watts) and you are able to make 1914 watts of heat (132VDC x 14.5 amps). That is an efficiency of 87% with no charge controllers or other special components. That is excellent. Really excellent. Good work, and when the hot water gets too hot (which probably never happens) you have a thermostat and relay to disconnect the solar modules.

A couple of my thoughts. If you are using an AC thermostat and you are putting DC through it to trigger the relay, expect the thermostat to wear out prematurely, catch fire or have the contacts of the thermostat weld together. Or get a DC thermostat which you may have done already.

If you have any need for extra electricity you could divert the solar modules (once the water is hot) into an MPPT tracking controller and charge a 24 or 48 volt battery bank.

You have done really well matching the MPPV of the modules with your heating element. That didn’t happen by accident I’m sure.

I think you have done a great job. Well done and thanks for sharing…Jody

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Philip Kudsk July 20, 2017 at 4:12 pm

I have a 400 watt wind gen and would like to just make hot water .Could you give me a sample of how to wire it up ? Possibly just as direct as possible using a off the shelf element to supplement the hard wired upper or lower element. Can I run the water heater with only 1 element wind powered and one hard wired shut off ,,if it takes 10 extra hour to heat up water that is fine by me. I can turn on the hard wire to help speed it up if need be.

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Jody Graham May 9, 2019 at 3:25 pm

Hi Philip,

You can make hot water with your 400 watt turbine but the rated voltage of the turbine would decide how we could do it.

If it is a 12 volt turbine you could purchase a 12 volt water heating element from any RV store in different wattages. I would match it up with a 300-400 watt or so element.

If your turbine is 24 or 48 volt you will need to purchase an element rated 300-400 watts at your turbine’s voltage.

You could simply hook the positive and negative from the turbine to the element. The danger of this is if the element fails, you will have no load on the turbine causing it to overspin and be destroyed. If it is an AIR303, AIR403, or AIRX they have an internal voltage regulator but it is still a good idea to have a backup dump load connected (or ready to be connected) to keep the turbine safe. This would require extra components that are not cheap.

I have rarely seen a hot water heater element fail as long it is ALWAYS in water so your setup will likely be fine as is. Keep us updated…Jody

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Aaron Martin May 31, 2017 at 9:11 am

The disadvantage is that over time, heat escapes through the tank wall and the water cools down, activating the heating system to heat the water back up, so investing in a tank with better insulation improves this standby efficiency. Additionally, when heavy use exhausts the hot water, there is a significant delay before hot water is available again.

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Jody Graham May 9, 2019 at 3:51 pm

Hi Aaron,

I hear you on the standby heat loss. The more insulation the better. However we are just discussing the ways of heating water as a dump load with extra power that would otherwise be lost.

A tank is really the only way we could heat a decent amount of hot water over a long period of time depending on your power system. Tankless is NOT going to work.

True it will likely take awhile to reheat the water but this too is dependent on the amount of power your system creates and needs to dump as the batteries fill up.

Thanks,
Jody

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Warren Brubacher March 23, 2017 at 4:30 pm

Hi,
I wonder if you will be finishing the article : Making a dump load with a hot water tank.
I run a 24 volt micro-hydro home power system.
Look forward to reading the rest of the article.
Thanks,
Warren.

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Bobby February 24, 2017 at 9:44 am

Thanks for making a great site with lots of information.

I use a 5 gallon cat litter bucket with a lid. It’s filled with water to the top and the 48vdc probe is screwed into the top. Works great for my wind turbine, 48vdc, 8-6 volt battery system with a charge controller. Checked it working the other day and it is set to dump at 57 plus vdc. The water got to hand washable temp.
I am watching for a used 5 gallon water tank on CL. Lots of YouTube videos on this subject.

I treat this as a hobby and I use it to run my entertainment system at night for about 5-6 hours with a quality inverter.

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Andy February 8, 2017 at 12:54 am

Hi
Any chance you could share how you are dealing with the temperature in the tank and preventing it from getting too hot?
What 12v thermostat could I use? I am really hurting to find something online.
Thank you

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Jay December 31, 2016 at 9:34 am

Can you forward me any diagrams for hooking up 2 water heaters with 2 elements in each. I have 2 c40 and 1 ts-45. Running a 1kw turbine with a power centre (Berger) and 500w solar.
Thank you

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Jay December 31, 2016 at 9:37 am

Sorry spell check. It’s a bergey turbine. I only have solar running for now until I can figure out best way to wire up and size of elements to use.
Thank you for any help.

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Peter December 17, 2016 at 7:36 am

Could you answer one question about setting up a hot water heater as a dump load? The only thing I don’t understand is how do you keep the water heater from over heating? Do you need a special DC thermostat or something to turn off the elements when it gets up to temp?

Thanks! Love your site. Reference it all the time when looking for tips for my off grid projects…

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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?

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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

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Sean S. October 29, 2016 at 4:15 pm

Thanks for the response Jody,

Your assumption is correct that the PMG is a 48V three phase wild AC 60 hrz and has three wires coming out of the back of it. In order for ground to be established I was going to tie a ground to the PMG motor mount and then to a pin into the earth.

I looked at arrangements to configure a delta with the three phase which would provide the option to get the optional 120V or 240V however it would depend on what kind of heating elements I would choose, right?. However if you have a better suggestion like the wye configuration I am open to that.

You say I could just keep it simple and use three elements wired in three phase.

Is that like regular oven stove elements like 1200 watt or 1500watt or possibly even larger for 240 like 2500watts? I am up north in northern canada so we have very cold tempuratures which causes complexities like adding a hot water tank to the mix.

Yes that would be great to have a drawing for this wiring diagram.

Thanks

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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.

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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

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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

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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

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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.

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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?

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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.

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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

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William Bayne December 9, 2016 at 2:05 pm

Hello Jody,
Wishing you good health in your illness… and hoping you have the time to finish the instructions/wiring diagram.
Best article I have read thus far on how to do this – but I could surely use the finishing touches.
Good thoughts to you,
Bill

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