Possible do "dim" water heater? - Page 1
Forum section: electronics / general

author: thielj
author_rank: Newbie
timestamp: June 02, 2025, 01:05:45 pm
content: We have an electric water tank / heater running at 220-240V 5A. Is it possible to "dim" or restrict it to the 600W surplus PV energy we usually have during mid-day peaks? I don't think the heating element is easily swappable.

author: darkspr1te
author_rank: Frequent Contributor
timestamp: June 02, 2025, 01:36:59 pm
content: the simple way is a electronic timer that comes in during those times.There are systems like HomeAssistant which can be programmed to monitor power produced/battery etc and sweitch items on/off. Some inverters may have support for this option already built in (example is victron systems with AC2 out) and these secondary power outs can be configured to operate under certain conditions (free power times/max solar etc)darkspr1te

author: MarkF
author_rank: Super Contributor
timestamp: June 02, 2025, 01:52:11 pm
content: If I remember correctly, here in the states waters heaters have two elements. One low power element to maintain the temperature and a second high power element for fast temperature recovery. You might try disabling the high power element during the times you want to limit it's power usage. But it shouldn't be ON unless you're using hot water.If you want to bite the bullet, you could replace the tank heater with one of the instant ON water heaters that don't have a tank and only heat when hot water is called for.

author: brucehoult
author_rank: Super Contributor
timestamp: June 02, 2025, 01:54:01 pm
content: Quote from: darkspr1te on June 02, 2025, 01:36:59 pmThe POINT was that the water heater wants ~1000W but they only have 600W of power available.You can use a high power SCR or triac with PWM, and this is more linear to control than trying to adjust the voltage, but it won't be cheap.Everything is easy if you have even quite small battery storage, because then you can save up solar energy for 10 minutes and then run the heater at normal power for 5 minutes, or whatever.

author: brucehoult
author_rank: Super Contributor
timestamp: June 02, 2025, 01:57:26 pm
content: Quote from: MarkF on June 02, 2025, 01:52:11 pmThose commonly require around 8kW and 30A 240V supply if you want more than a dribble of hot water.That's the exact opposite of what OP wants.

author: Marco
author_rank: Super Contributor
timestamp: June 02, 2025, 01:59:31 pm
content: Triac dimmers are cheap as chips.https://www.amazon.co.uk/Eagle-13A-Plug-In-Dimmer/dp/B015GDBVGMDunno what the effect will be on the thermostat though, ideally it's behind the relay switched by the thermostat ... but that requires a little more modification.

author: themadhippy
author_rank: Super Contributor
timestamp: June 02, 2025, 02:34:42 pm
content: QuoteIf its a bog standard immersion type heater the stat is nothing more fancy than a mechanical bi metal switch in line with the element

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 02, 2025, 02:39:01 pm
content: Quote from: Marco on June 02, 2025, 01:59:31 pmYup, that dimmer should work and the thermostat shouldn’t give a hoot about how much energy is going into the heater. It just turns on below setpoint and turns off above. An oversimplification to be sure but it’s still bang-bang control.Don’t forget the timer or what ever mechanism would work to bypass the dimmer when full power is available.

author: johansen
author_rank: Super Contributor
timestamp: June 02, 2025, 08:26:14 pm
content: Quote from: thielj on June 02, 2025, 01:05:45 pmYour inverter may not like reducing the typically 4500 watt heating element down to 600 watts with a triac dimmer.Supplying the hot water heater 80 volts ac may work, you would need to find a 600 watt rated transformer, dual 40v secondaries are not that hard to findAlternatively look for a 3/4” npt cartridge heater and put it in the bottom of the tank where the drain port isWiring both elements in series and running it from 120vac may be the cheapest method to drop the watts to 600

author: David_AVD
author_rank: Super Contributor
timestamp: June 02, 2025, 08:47:13 pm
content: The OP has a 1200W 240V heater, not 120V and certainly not 4500W. Several replies have misread / ignored the stated specs.Dropping the element voltage to from 240V to 160V AC would get close to the 600W target and avoid the peak current that a conventional phase angle dimmer would involve.I swapped out my 3600W tank element with an 1800W one for the same reasons the OP has, but I have enough solar year round to do the 1800W + house load except for rainy days.Reducing the element wattage and making it up with a longer heat time does work, but there must be some lower limit where it's just not enough power to heat the water to the required temperature.

author: Bud
author_rank: Super Contributor
timestamp: June 02, 2025, 10:19:51 pm
content: Water heater must maintain specific temperature to prevent bacterial growth. Google that up.

author: brucehoult
author_rank: Super Contributor
timestamp: June 02, 2025, 11:01:53 pm
content: Quote from: Bud on June 02, 2025, 10:19:51 pmThe implication is this is something productive to do with excess solar energy on good days, not the main or only source of power for water heating.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: June 03, 2025, 03:22:59 am
content: Quote from: thielj on June 02, 2025, 01:05:45 pmI have a PV diverter that does this via the usual triac "dimmer" method. There are also PV diverters that turn on for some whole mains cycles and skip some whole mains cycles. Finally, you can do high speed PWM but it is a much more complex circuit to achieve this.The simple devices appear to rely on the electricity meter taking a "net" measurement over a few mains cycles (mine does, and I suspect most do).It's not particularly kind to the grid, I believe the non-PWM dimmers might be outlawed in EU already.

author: MF-jockey
author_rank: Regular Contributor
timestamp: June 03, 2025, 12:48:59 pm
content: With a 240V to 160V 4A autotransformer, the heating output can be reduced to approximately 600W.Either the heating output is sufficient for your lifestyle, or a corresponding switch for full voltage is provided.A used 400V to 230V >=4A transformer may be cheaper to obtain and will also work; connect the 240V to the 400V winding and the heating cartridge to the 230V winding.An isolation transformer also works but is larger and has more losses.

author: Marco
author_rank: Super Contributor
timestamp: June 03, 2025, 01:27:48 pm
content: Quote from: Geoff-AU on June 03, 2025, 03:22:59 amCan the heating element take DC? Then a constant on time fixed frequency buck regulator behind a bridge rectifier can handle it, same as in induction cooktops.PS. oops, buck boost I guess.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2025, 01:42:40 pm
content: Quote from: brucehoult on June 02, 2025, 01:54:01 pmThat depends on how it's metered. If you don't get paid much for the export, then you don't gain anything, over just letting it run at full power.

author: brucehoult
author_rank: Super Contributor
timestamp: June 03, 2025, 04:58:24 pm
content: Quote from: Zero999 on June 03, 2025, 01:42:40 pmThe POINT,again, is that OP is wanting to send surplus solar energy to contribute to their water heating, presumably BECAUSE it is not worth exporting it.And they don't have enough of it to run the water heater at full power. If they ran the heater at full power in that moment then they'd have to buy grid power for the difference, which might not be at a cheap price at that time of day. (though in heavily solar-ized places such as Australia and Netherlands the grid price is zero or negative at peak solar generation time on good days)

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2025, 05:23:11 pm
content: Quote from: brucehoult on June 03, 2025, 04:58:24 pmExactly and that's exactly why simple PWM won't work.If the heater uses 1200W, yet the inverter produces 600W, then turning the heater on at 50% duty cycle, will result in 600W being drawn 50% of the time and 600W being generated 50% of the time. If you don't get paid for the 600W you generate, but still have to pay for the 600W being drawn, you lose.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 03, 2025, 05:35:06 pm
content: I think utility meters, even those that meter export vs. import do at least a few seconds of averaging. Reactive loads are not counted as import for X milliseconds and export for Y milliseconds each cycle. Using a phase angle dimmer to limit the power to what your PV provides may not make the power company happy due to the harmonic currents but they usually don't charge you for it. At least at the residential level. Industrial customers would get dinged for the terrible power factor. Using slow PWM of course won't work, but that is not what was suggested.If the OP is in an off grid situation, then the inverter needs to supply the harmonic currents. In that case you have to make sure that the peak doesn't exceed the inverter capability.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2025, 06:03:59 pm
content: Quote from: ejeffrey on June 03, 2025, 05:35:06 pmThat's really the question. If the meter averages over a few cycles, then PWM 50% say at 5Hz, 5 cycles on and 5 cycle off, then it should be fine, but sub-Hz maybe not.Heck, I wonder if a simple diode will do?

author: Marco
author_rank: Super Contributor
timestamp: June 03, 2025, 07:20:43 pm
content: The smart meters in my country average over 15 minutes, but the way things get logged and averaged are a legal issue.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2025, 09:46:02 pm
content: A flip-flop triggered by the mains frequency could be used to fire a TRIAC to get 50% duty cycle at half the mains frequency. Perhaps a circuit similar to the one linked in the thread below, run off a capacitive dropper, with the trigger connected to the mains via a large value resistor and clamping diodes. I don't have time to draw the schematic at the moment.https://www.eevblog.com/forum/projects/help-adapting-a-discrete-toggle-latch-circuit/msg5928328/#msg5928328

author: Freesurfer
author_rank: Regular Contributor
timestamp: June 03, 2025, 11:49:32 pm
content: Using a diode to draw current for just half the period is not good. While it can work, it will introduce a DC offset in your system, which depending on your equipment, could saturate transformers and inductors and generate more heat. Especially for a relatively high power load like a hot water tank.And as others have pointed out, drawing the full power for half the amount of time, can cause the system to feed back current for half of the time, while consuming the other half. If it is a battery that absorbs and delivers this current, you will end up "microcycling" the battery, causing unnecessary wear that eats through the cycles of the battery quickly.If it is the grid that offloads it, and even if you don't get charged for it, it's not really doing any "good", you are relying on the grid compensating for poor PF for free.We need to remember that solar panels generate a steady DC current, while an inverter will draw a waveform that looks more like a rectified sine to power its AC loads. So DC current and AC RMS current are not directly comparable. A good inverter will smooth the current draw to some extent but not fully.That said, I am in a similar situation looking for a way to give finer control of the power consumption for a hot water tank. My approach is to replace the existing 2kW single element, with a new dual element of 700 + 1300W which has the same size and threads as the original one. Most of the time, hot water tanks have standard sizes and threads. Then through different combinations of running the elements individually, or in series or parallel I can achieve multiple power levels. In this case: 0W - 455W - 700W - 1300W - 2000W.

author: Psi
author_rank: Super Contributor
timestamp: June 04, 2025, 01:13:49 am
content: Quote from: Zero999 on June 03, 2025, 05:23:11 pmHe could maybe pull power off the panels on the DC side if he has enough voltage to PWM the element directly with DC.Doing this he could run the PWM at a much faster frequency and so the capacitor size needed to smooth that into average current is much less.But this does create some issues. You can't run DC through a thermostat switch designed for AC or you will fry it from all the arcing.Also you would have to be careful not to generating PWM noise on the panel as the inverter wont like that.But I would expect that the inverter MPPT would simply adjust itself and, from its perspective, the panels would be producing less energy due to the amount being pulled away as DC. But it might require some creative control logic for the PWM frequency.It has the advantage of being more efficiently due to not wasting heat in the inverter, but yeah, its definitely a non-standard setup and may have issues I've not thought of.I've seen some people replace one of the elements in a hot water cylinder with a lower voltage one so they have one element for AC input and one for DC input. Usually this is mains vs battery voltage, but no reason you cant have one element for solar DC and one for mains AC.Insert warning about hot water cylinder explosions being extremely violent, see mythbusters.So don't mess with any of the safety release systems and make sure you always have a thermal cutoff somewhere for all power you feed into it.

author: mikerj
author_rank: Super Contributor
timestamp: June 04, 2025, 06:37:50 am
content: Quote from: brucehoult on June 03, 2025, 04:58:24 pmI don't think I've ever seen a thread where so many people have either not bothered to read the OP, or read what they wanted to read rather than what they wrote.The only potential issue I can see with a triac style dimmer is whether the inverter is OK with the inevitable harmonics. OP, do you need the ability to run the element at full power sometimes or will it purely be used as a 600W sink?

author: Zero999
author_rank: Super Contributor
timestamp: June 04, 2025, 07:01:53 am
content: Quote from: mikerj on June 04, 2025, 06:37:50 amI suggest you take a bit of time to read the replies properly. Powering the heater with a surplus is great, but using a much more powerful heater, then "dimming" it down is not the optimal approach, especially when such methods i.e. PWM, TRIAC control can introduce other problems and might not play well with the metering.The OP needs to purchase a lower power heater, or perhaps connect two in series, as has been mentioned in some of the replies, you didn't read.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: June 04, 2025, 07:47:02 am
content: Quote from: Marco on June 03, 2025, 01:27:48 pmOnly responding to this tangent because it touches on a safety issue. Yes, the element is fine with DC but the thermostat isn't. Switching DC is more demanding than switching AC and you can't use an AC-rated switch (or thermoswitch) to switch DC, it can fail and catch fire because a DC arc is much harder to extinguish than an AC arc.

author: Xena E
author_rank: Frequent Contributor
timestamp: June 04, 2025, 09:17:29 am
content: Quote from: thielj on June 02, 2025, 01:05:45 pmTell us about your system, people here are arguing over assumptions.Quote from: Geoff-AU on June 04, 2025, 07:47:02 amMarcos suggestion is the most efficient, but yeah, it's a world of pain switching DC.Water tank heaters are available for a number of working voltages/powers so a simple system could be arranged for whatever the need and system DC voltage, if they are prepared to attempt to change the heaterhttps://www.amazon.co.uk/DERNORD-Submersible-Element-Stainless-Heating/dp/B09V466KC6/ref=asc_df_B09V466KC6?Phase angle controlled dimmer type idea would work, kinda, but would give the inverter a really hard time.I dont know what level of experience the OP has but the following is just to give ideas...I have a small PV system that is stand alone and charges batteries.The exess is dumped at DC into a standard domestic open vent watertank immersion heater, the DC is nominally 48Volts but then goes through a boost converter, (that is separate from the main power inverter), into the element and is regulated so that the panels are used at their optimum.At best it has been delivering @ 500Wh during periods of dump.The Element voltage is then ≈100Volts, (the element is rated 3kw at 240Vac).The element is otherwise unused in the heating system.The thermostat is used to control the converternotswitch the DC, the hot water is bulk heated by a gas boiler but is timed to come on late afternoon. This time of year the boiler doesn't see much work, savings have been worthwhile, around £300/year on the HW.It was not expensive to realise but also not trival to design,X

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: June 04, 2025, 10:09:03 am
content: You can easily buy a number of off-the-shelf systems to do this. It is very common. You don't need to work all this out from first-principles and home brew anything. (or get drawn in to long threads of people arguing about how they would home-brew such a system)https://www.immersun.co.uk/products/power-diverterhttps://www.myenergi.com/product/eddi/https://www.marlec.co.uk/product/solar-iboost/https://www.solaredge.com/en/products/residential/smart-energy/solaredge-home-hot-water-controller

author: Xena E
author_rank: Frequent Contributor
timestamp: June 04, 2025, 10:15:44 am
content: Quote from: DenzilPenberthy on June 04, 2025, 10:09:03 amWholehearted agreement here...But some of us with the abilities like to save money...Some because it is fun...Some because its our job!Regards,X

author: Seekonk
author_rank: Super Contributor
timestamp: June 04, 2025, 05:39:08 pm
content: Well, I haven't seen it mentioned if he is grid tied or this system is off grid. Sometimes I totally forget that this is an electronics forum with all the buy it now solutions.I heat water with PV DC voltage right from the array which also goes to a charge controller. I look at the voltage and when the array goes over power point it gets dumped into the water heater. The charging to the capacitor bank is adiabatic keeping the differential voltage as small as possible. Power point is adjusted with temperature and is pulsed PWM. This method works happily with any MPPT controller as long as the capacitor bank is isolated from the array with a diode. I've build dozens of designs and can't see why this isn't popular with the solar world which thinks water heaters have to have their own array. Some designs include arc interrupt that turn off for a short period after 10ms of being on. This allows daisy chaining of heaters with standard AC thermostats. Just a picture to show how simple these are.

author: Zucca
author_rank: Unknown
timestamp: June 10, 2025, 04:29:34 pm
content: Why not use a VFD?Anyway in my case I just heat/cool my home above/below the desired indoor temp.Works well for me as I have to deal with excess solar energy only sporadically.

author: elecdonia
author_rank: Frequent Contributor
timestamp: June 10, 2025, 09:24:59 pm
content: Quote from: brucehoult on June 02, 2025, 01:54:01 pmThis would work if the PV system has sufficient battery storage.Another idea: Does the inverter in the PV system provide both 240V and 120V outputs?A water heater which consumes 1000W at 240V will consume 250W when operated at 120V. Would this be sufficient?If so, then something as simple as a contactor which can select 120V or 240V to feed the water heater would do the job.Or, if everything in the home and the PV system is 240V only, then the idea suggested below of using a step-down transformer would be the next best choice. It should be relatively easy to source a 240-to-120V transformer. Although this limits the water heater to receiving 250W, perhaps that would still be adequate?

author: max_torque
author_rank: Super Contributor
timestamp: June 20, 2025, 01:50:44 pm
content: The problem with all these "use excess solar in summer" problems is one of economics, not of physics!I ran the numbers, and it's really, really, really hard to build a system that will ever payback finanically. Now of course, you might want to still do it even at a financial loss for other reasons, but it's worth sitting down and working out the financials are!The only way i could make even remotely viable was using second hand parts in a very simply system. So i have a second hand 240 to 110v transformer that cost £20 and a change over relay, and when my battery system is fully charged AND there is lots of solar i simply use the changeover contactor to drive the single std 3kW immersion heater element (normally connected across 240v of course when using grid power) via the transformer. This "derates" the element to deliver roughly 800w of heating power to the hot water tank.I looked at lots of other options, but they all come with serious limitations, either cost or presenting a very poor power factor etcThe "ideal" solution would be a dual input, active PFC filtered buck converter that could modulate the output voltage to control the input current, but this as mentioned is expensive and has other limitations, such as requiring a different control mechanism / thermostate etc etc

author: Seekonk
author_rank: Super Contributor
timestamp: June 20, 2025, 05:22:31 pm
content: Agree somewhat.I entered solar thru some back door because to be allowed to enter the solar there must be an intelligence test you have to fail. The solar world is the dumbest group of people I have ever experienced. They just can't heat water efficiently. Or anything else.The cheapest and most efficient way is to divert excess power directly from the panel voltage. More thana dozen ways to do that and yes, out of stuff from the junkbox.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 20, 2025, 10:57:28 pm
content: Thinking laterally, what about connect some other load in series? Maybe there's something else that the extra solar power can be used for.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 22, 2025, 09:42:25 am
content: Adding an autotransformer (or even a variac) is a solution that works, and those have already been mentioned, and those things do cost a fair bit of money.But what about going the other way? Invest some money into the solar installation itself, so you have have plenty more then 1kW of "excess power" for the water boiler. Solar installations have become quite cheap over the years, and if you have plenty of power you also worry less about having enough power during cloudy days, and it improves your winter situation.

author: woofy
author_rank: Frequent Contributor
timestamp: June 22, 2025, 10:57:04 am
content: Hard to help when the OP has not clarified anything, or even logged in since the original post.

author: 5U4GB
author_rank: Super Contributor
timestamp: June 23, 2025, 07:46:43 am
content: Quote from: thielj on June 02, 2025, 01:05:45 pmThis is exactly what solar diverters do, they use a triac or similar in a standard dimmer circuit to restrict how much power the heater uses. However you need to take into account the fact that your load can be several kW, not a 50W lightbulb.

author: 5U4GB
author_rank: Super Contributor
timestamp: June 23, 2025, 08:04:28 am
content: Quote from: elecdonia on June 10, 2025, 09:24:59 pmYou could also use a smaller cylinder as thermal storage. So get something with a 600W element, use the surplus solar to get it as hot as you can, then pump the very hot water from that into the main cylinder. This works if the main cylinder is set up to handle solar-heated hot water from things like heat pipes, you're just doing it via one level of indirection.

author: Zero999
author_rank: Super Contributor
timestamp: June 23, 2025, 08:33:36 am
content: Quote from: Zucca on June 10, 2025, 04:29:34 pmThat will work, so long as its designed to work with a single phase motor. Connect the heater between two phase on the output.

author: johansen
author_rank: Super Contributor
timestamp: June 23, 2025, 11:30:53 pm
content: Quote from: Zero999 on June 23, 2025, 08:33:36 ammost cheap vfds only measure the current on two phases. probably phases 1 and 2, but sometimes 1 and 3 because there is more room on the board to do it that way.as long as the current is below the trip point, they do not care about imbalance.

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 24, 2025, 11:00:46 pm
content: Quote from: johansen on June 23, 2025, 11:30:53 pmCheap VFD’s don’t measure even two motor phases, they just have a shunt in the dc bus, measure the current there and extrapolate. No output ground fault functions or output phase loss/imbalance is a good indicator of cheapness.

author: johansen
author_rank: Super Contributor
timestamp: June 25, 2025, 04:02:40 pm
content: Quote from: WattsThat on June 24, 2025, 11:00:46 pmi've not found that level of cheap yet lol.