In Blog, Inverters, Micro Inverters & Optimisers, Solar Panels

The Death of Hot Water Timers and the Fronius Hot Water Relay

Hot Water Timers and Hot Water Diversion has been a topic surrounding solar systems since the option became available. The idea is simple; when installing solar you remove your electric hot water system from your off-peak meter, and instead run it on your main meter – T11 (which is what the solar is connected to) – and time it’s heating to coincide with the solar production hours. This used to be done simply with a hot water timer – the dumb method, but now can be done using an inverters “Load Management” function – the smart method. There are other options, but we’ll touch on them in future posts. This post simply aims to cover both methods, and why, as a result of increased feed-in tariffs, Solar hot water timers, and the Fronius Hot water relay, are dead.

The Solar Hot Water Timer

With a hot water timer, installers set the timer up to come on for a set amount of hours during the period when they expect solar production to be at it’s highest, and when they expect the homeowner will be sending the most power back to the grid. The problem here is, it’s not guaranteed that period will always be the time of highest production due to rain or clouds, and the homeowner may have different usage habits on different days. If the period where the timer flicks on, is not when enough surplus power is being generated, the hot water will be heated from the grid, at the increased T11 rate. In this case, you can often lose money installing timers, hence, the dumb method. Surprisingly, many sales companies in Brisbane are still selling hot water timers as a load control. I’ll give an example later in this post of why it’s not an ideal method.

The Fronius Hot water Relay

Now for the smart method; using the Load Management Function of an inverter. The best one on the market is the Fronius Inverter’s Data manager card connected with a third party relay. For simplicity, we refer to this as the Hot water Relay, and it’s the method we use (well, used, but I’ll get to that). The way it works is when a Fronius hot water Relay and Fronius Smart Meter are integrated into a solar system, they have the opportunity to smartly decide when the hot water unit turns on. The hot water relay is able to use the information from the smart meter to identify when a system is sending a large excess back to the grid. When that excess exceeds the kW demand of a hot water unit, it tells the hot water unit to switch on. When the excess is under the kW demand of the hot water unit, it turns off and tries again later. Below is a nice example of a hot water relay in action:

A Fronius Relay working well

The green is our solar production, and the filled in grey is our household load. The large spikes are the hot water unit turning on, and you can clearly see the hot water relay controlling the unit to only turn on when the solar production is at its highest, and when it knows it won’t need to draw from the grid. This is very beneficial on cloudy days, on days that may have morning rain, or basically for any household that has varying day by day usage habits. However, the hot water relay, like a hot water timer, still requires excess solar production equal to the full capacity of the hot water unit (meaning a 3.6kW hot water element will require the full 3.6kW of solar production all at once), and as such on rainy days, when the excess is not there, it will need to draw a portion from the grid to ensure the hot water unit receives heating just like a hot water timer.

Below is a good example of the hot water unit working well early, but then it hits a time when it can no longer wait for excess solar production, and it needs to draw from the grid. The unfilled section is bought from the grid at the highest rate ( T11, 27c/kWh) :


Similiar to a hot water timer

Who killed the Fronius hot water relay?

Using a portion of T11  power from the grid used to be fine, as it is far superior to a hot water timer, and even with this occasional draw from the grid at the increased T11 rate, they still made financial sense. A hot water timer may help a hot water unit achieve 50% coverage from solar, while a hotwater relay may achieve 70%. They both cost the same, and the hot water relay is actually more reliable. However, as good as relays are, since July 1st 2017, they rarely make financial sense.

Feed in tariffs

On July 1st, 2011, feed-in in tariffs greatly increased, in some cases from 6c per kWh to 14c per kWh, so the math behind hot water diversion becomes more complicated. This below is an example of an Excel spreadsheet calculation of a hot water relay’s payback period on a 6c Feed In Tariff – what it used to be. Most of the values we can read directly from a power bill, but the section we estimate is the percentage of the excess hot water kWh draw that will be covered by the solar + relay combo. In the below case we estimated the hot water relay will account for 70% of the heating, and the remaining 30% resulting from rainy days, higher base load etc, will need to be drawn from the grid. The calculation importantly takes the feed in tariff into account, as we have to remember the excess used to heat the hot water unit could have been sent back to the grid for 6c per kWh:

Hot water relay

(EXCEL SPREADSHEET CALCULATIONS – 6c FEED IN TARIFF)

$34.20 better off a quarter than the off peak rate, $136.80 better off a year.

However, the calculation becomes worse the higher the feed in value becomes. Below is the exact same estimate, but calculating off a 14c Feed In Tariff instead (Origins Solar Boost rate as of July 1st 2017):

Solar hot water relay

(EXCEL SPREADSHEET CALCULATIONS – 14c FEED IN TARIFF)

A whopping $2.40 a year in savings, you’d have to be a pretty good (corrupt) salesman to convince someone to proceed with this kind of payback period. And just for an extra laugh, let’s look at a timers current payback period:

Hot water relay calculations

(EXCEL SPREADSHEET CALCULATIONS – USING A TIMER & 14c)

 

I’m probably being generous anticipating a hot water timer to cover 50% of the hot water heating, but even if it does exceed expectations to say 60%, you’re nearly always guaranteed to lose money if you install a timer.

Is the Fronius hot water relay dead?

No, I’d say it’s more like semi-retirement.
(Incidentally, its role has been replaced by the Fronius Ohmpilot, but that’s for another post).
But there’s still plenty of situations where the Fronius hot water relay may be of use. For example:

  • Feed in rates are not guaranteed
  • Off peak rates may increase
  • If you have a 10kW system, your solar may cover more than 70% of the hot water heating.
  • It may not be viable to connect your hot water system to off peak.

However, it’s still important to have a chat with a solar consultant, work out your household loads and times they’ll come on, run through your bill, and work out if removing an off peak meter in favour of a relay is worth it. This is what we’re here for, and we’re constantly re-evaluating market changes to ensure when we come out to quote, we are offering the best, up to date advice.

And remember, if anyone tries to sell you a hot water timer, run for the hills.

 

Ben Neville

Technical Sales Support

MC Electrical

 

 

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34 Comments on The Death of Hot Water Timers and the Fronius Hot water Relay

JD said :Guest Report 3 months ago

Thanks Ben for your reply to my earlier query re pool heater, that tallies with where my thinking has got to also (but with more technical nous!). I think I'll stick with the timer on the heater (which is separate to the filtration timer) and override it (i.e. switch the heater off) if it looks like it will be a (rare) Queensland sun-free day! And also adjust the timer so it runs at peak solar output times generally.

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Ben Neville said :administrator Report 3 months ago

Hi JD, Thanks for the kind words. While you could use an appliance relay to regulate a pools' heating and filtering, it's generally more suited to a timing method. The reasoning is the relay parameters are yearly, while the variances in the pool heating and filtering requirements are seasonal (more heating in winter, less in summer, etc.) This means you would need constant parameter changes, depending on the circumstances and time of year. I guess you could manually override the relay during rainy days, but currently there is no automatic feature for this yet. So yes, it's possible, but it's generally not suited to the heating variances of a pool.

    JD said :Guest Report 3 months ago

    Hi - great post, and I'm just trawling through all of the various articles on your site - thank you for making this information so clear, and available! I am just getting my head around usage/consumption kWH etc. The main energy usage culprit in my home (most probably) is the swimming pool heat pump which we keep running for c. 6-8hrs/day throughout the (Brisbane) winter (more or less). So my (slightly off topic) question is, is there a relay type system which would switch the pool heater (and its pump) on when there is solar power, but not when it's rainy etc (pool temp is not so critical as a shower, won't be swimming anyway...) Very indulgent, but when the capital outlay on the pool was so much it's nice to get use out of it. Adding PV should help ease the environmental conscience... Thanks!

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    Mark C said :administrator Report 3 months ago

    Hi Sheila. It's not a new concept. It's called a solar hot water diverter. SolarHart has just integrated a diverter into the tank. Fronius has one called the "ohm pilot" (i have it at my place) and CatchPower is another brand. It's a great concept as long as the diverter is warranted for at least 5 years, and I'd always buy a stainless steel tank. Work out how much it will save you. (Around about 20cents per kWH x how many kWh you in hot water per quarter. Your solar may only heat 80 percent of your hot water and the rest will draw from the Grid. I would tend to keep the tank and diverter as 2 separate devices and I'd recommend a Catch Power (cheaper) or Fronius (premium).

      Mark C said :administrator Report 3 months ago

      Hi Sheila. It's not a new concept. It's called a hot water diverter. SolarHart has just integrated a diverter into the tank. Fronius has one called the "ohm pilot" (i have it at my place) and CatchPower is another brand. It's a great concept as long as it is warranted for at least 5 years. And work out how much it will save you. Around about 20cents per kWH and work out how many kWh you in hot water per quarter. Your solar may only heat 80 per cent of the

        Sheila Donaldson said :Guest Report 3 months ago

        Hi Mark, Wondering about your views on the Solahart Powerstore Hot Water system, which as a variable heating element and hence claims it can use any amount of spare PV solar power. Also we are in Ergon territory - current FIT is a measly 7.8 c/ kw. Many thanks Sheila Donaldson

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        Ben Neville said :administrator Report 7 months ago

        Hi Edd, One of the setups via the energy manager, is to provide some form of heating regardless of the weather conditions. This means, if you've had a cloudy miserable day and not enough solar excess was found, the relay will still trigger the hot water unit to heat. The graph shows this happening, using solar where it can, and drawing the rest from the grid. This is important to ensure the household still has hot water. An Enphase MicroInverter setup does not have a load management function as far as I'm aware. A catch power would be an ideal solution in this scenario, but more on that in another blog!

          edd said :Guest Report 7 months ago

          Hello Ben.... I dont quite get this sentecne "Below is a good example of the hot water unit working well early, but then it hits a time when it can no longer wait for excess solar production, and it needs to draw from the grid", so the SHW will not do as what has been set up through the energy manager ? why will it force to run ? do Enphase Micro Inverter have their own smart relay too ? or they need to pair up with some third party relay Thankyou

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          Mark C said :administrator Report 9 months ago

          Hi Sean, I'm not sure, but probably - I've only installed one (at my place). Best to contact Fronius.

            Sean said :Guest Report 9 months ago

            Is it possible to alter the threshold at which the OhmPilot starts feeding the surplus to its loads ? - I have one, along with an 8kW Primo, and have just installed a Victron grid parallel self consumption ESS system, which is working well - apart from the OhmPilot gradually increasing its diversion, and thus increasing the battery loading. I can alter the point at which the Victron ESS starts feeding from the battery, but I cant see if it's possible with the OhmPilot - anyone offer any wise words ?

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            Mark C said :administrator Report 12 months ago

            Hi Chris. I have the Fronius ohm pilot at my place. Not only is it expensive to purchase, but it is also expensive to install if your hot water is remote from your switchboard. We had ours connected wirelessly and had some issues so I hardwired the ohm pilot back to the smart meter. Once it is working, it is a great product - rock solid. I can't see the economics working out in most cases.

              Chris N said :Guest Report 12 months ago

              Are you able to give any update on the OhmPilot and temperature sensor you mentioned in your blog? It is certainly a more costly addition than a "smart" relay, but it maximises usage even on cloudy or rainy days or even early/late hours of the day.

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              Ben Neville said :administrator Report one year ago

              Hi Quido, I believe we are on the same page here. Timer and Relays do not "ramp up" depending on the spare capacity, they require solar + grid equal to the full capacity of the hot water unit. Your Sunmate is different technology, similar to the Ohmpilot or Catchpower (a blog for another day). Because of this type of technology, your hot water diversion is able to cover a higher % of hot water heating, and therefore make more financial sense. You are also in a State with a favourable FiT to off peak ratio, where here in Brisbane the FiT equals the off peak rate - making no financial sense even with 100% hot water coverage.

                Quido said :Guest Report one year ago

                Forgot to say I've got 7.8kW of panels on a 6kW inverter which is probably the minimum right size for the 100% diversion-powered HWS scenario to be viable incl. winter months. But a smaller system could still do it at say 90% with minimum grid topups during consecutive cloudy winter days.

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                Quido said :Guest Report one year ago

                The misleading failure of this article is that it calls the Fronius relay 'smart' and that it assumes that under the diversion smart scenario the "excess solar production [must] equal to the full capacity of the hot water unit (meaning a 3.6kW hot water element will require the full 3.6kW of solar production all at once),". In have had an AustralianWindAndSolar Sunmate 2.0 diverter installed for 2 years on a 3.6kw heater with 315L capacity (young family of 4) and never had to boost from grid import, i.e. 100% covered by my excess solar production. The diverter has a wireless meter in the meter box and ramps up from 50W to 3600W depending on spare capacity. I understand the Ohmpilot now does the same, 2 years later. So even on a 11c FIT and say 19c off-peak here in VIC I'm still approx. 8c better off diverting into the 16kWh hot water "battery". Diverter cost was around $800.

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                Ben Neville said :administrator Report one year ago

                Hi Dez, Thanks for the kind words. Electric hot water is still viable compared to gas, even if you remove solar from the equation. Only recently we had a councilman out to check the gas mains, and he advised against gas hot water due to the costs alone. Also, if off peak rates rise and your situation deems load diversion viable again, you'll be prepared with an electric hot water system along with a solar system to make the change. So, an off peak electric hot water system is not only cheaper, it gives you more options in the future.

                  Dez said :Guest Report one year ago

                  These blogs are really very good. I've got a question re solar relays vs gas instant. Given that feedins are up, would electric hot water be viable at all? Would it make more sense to just go a large solar array, export and use credits against gas bill?

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                  Mark C said :administrator Report one year ago

                  Hi Dave. I'm not a fan of solar hot water. The plumbers I know don't like it for reliability, and that has always been my experience. Either way, my take is that if you use that roof space for another 4 panels, and then gleaning Solar excess with the likes of a Catchpower or Ohmpilot, would not only be cheaper, but it would be making use of 1 reliable type of technology for all your energy needs. (We don't often see booster elements in QLD.)

                    Ben Neville said :administrator Report 2 years ago

                    Hi Pete, Just curious why you were looking towards a relay when you already have a solar hot water system? As per my understanding, if there's not enough sun for the solar HWS to heat, then a relay hooked up to an inverter is also not going to find enough excess solar. So any boosting you do will still be from the main tariff regardless of if you have a relay installed or not. I believe you're far better off having the booster connected to a night rate tariff, so if the sun is lacking that day, the night boost will give you the heating you require at a much better rate. I'm unfortunately not sure about the charges to have that work done, might be worth an inquiry with Essential Energy and an electrician. Those are also some hefty supply charges, if you didn't already have solar hot water, a relay in your circumstance would make sense just to avoid paying those charges.

                      Pete said :Guest Report 2 years ago

                      Hi Ben, Out of curiosity, how do the numbers stack up with Essential Energy which currently has a 14.85c/day daily supply charge for a controlled load meter. This works out to be $54.75 per year just to have the meter whether it's used or not. I have solar HWS and have the booster on for around 6 months of the year consuming around 1600kWh. The booster is currently connected to a single tariff meter. I don't have solar yet, but are planning on getting a 5kW system in the very near future. I was originally looking at the fronius relay, but I'm not sure now. I assume I can get a controlled load meter, but I'm unsure what the up front cost would be.

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                      Ben Neville said :administrator Report 2 years ago

                      Hi Mark, This is something we thought about as soon as the tariff increases came through last year - our existing customers. For those who were installed well prior to the price increases, the relay installations on the old 6c FiT would have paid for themselves by now. You were installed just before the FiT increase, so you fall in that transitional period and would have a combination of both exceptional payback period, and minor payback period. Fortunately in your situation, I can see by your solarweb data you are utilising your relay effectively, even throughout winter. You have a north facing array, with a minimal base load during times when the relay wants to heat. I estimate the relay is saving you $90 a year, based on a 10.6c FiT, a 28c T11 charge, a 18.7c T31 charge, and 500kW hot water usage a quarter. The savings between May - July last year would have been even higher. Going forward we are advising against hot water relays for our customers, unless their situation is ideal and they meet some of the criteria listed in the closing lines of my post.

                        Mark C said :administrator Report 2 years ago

                        Hi Sean, if you are talking about Origin's Solar Boost 20c feed-in tariff, then only use a relay if you can not get an off-peak tariff. If you can get an off-peak tariff of around 20c, then use that instead. Then 100 percent of your water will be heated with cheap power. With a "Fronius relay" controlling hot water, you only heat water with solar when there is enough excess solar. On rainy days you revert to using expensive T11 grid power.

                          Sean said :Guest Report 2 years ago

                          I don't currently have a PV system but certainly plan to shortly (6.6Kw). I log both my consumption Tariffs with an Efergy and PVOutput (this has been eyeopening and very educational in itself!). The numbers show pretty accurately my smallish HWS costs me 50c daily, $185 yearly including all supply and taxes. Would you recommend the relay if selecting 'Solar Boost' ? P.S. Would love to see more articles like this from people who do this stuff day in day out.

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                          Mark C said :administrator Report 2 years ago

                          Hi Sean, Yes, it's about $550 for a single-phase Fronius Smart Meter and relay. Add about $300 if your switchboard is 3 phase. However, in reality, I would always recommend a smart meter anyway. Having consumption monitoring will help you understand your usage patterns and will help size a battery. A single-phase smart meter is about $330 installed. So you are only looking at about $220 to add a relay. It's not so much about the initial outlay, it's just that using low off-peak rates 100% of the time is often better than using 14c solar some of the time.

                            Sean said :Guest Report 2 years ago

                            So many numbers to crunch. Roughly speaking whats the going rate to install the Fronius Smart Meter + Relay? $5-600ish?

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                            Ben Neville said :administrator Report 2 years ago

                            Hi Jaime, You have a good size array that should produce well, but being east/west your winter months will yield noticeably less. However, it's lucky you anticipate less daily usage in those months as this is when your array will also be producing the lowest. To keep relay contributions high, my advice would be to closely monitor your system, and make sure any daytime usage habits you have allow a window for your hot water unit to heat completely from the solar. As the blog form isn't the best for calculation formatting, I'm going to e-mail you my calculations so you can play around with a few projections.

                              Dave said :Guest Report 2 years ago

                              Hi Ben very informative article, however, would it not still be wisest to get Solar Hot Water Installed, as this alone will cull up to 80% of existing hot water heating costs. The smaller (ours are 2.4kw) booster heating elements only activate if required, so, the sun actually does all the work. The 3.6kw draw for Hot Water units is a myth, as most full electric only units have dual elements of 3.6kw or even 4.8kw units. For any more Solar Hot Water questions, please contact me. Cheers Dave BSE

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                              Jaime Workman said :Guest Report 2 years ago

                              Hi Ben, my array will be E/W with 1/3 of the panels east and 2/3 west. We have changed the design slightly and will increase to 8 kw. My water heater element is 3.6kw. We use about 20kwh during spring and autumn and 35 in winter and summer if we run AC. Our hot water usage ranges from 3 to 5 kwH per day. The water heater is 315 litres. I expect to use about 60% of the power during the day, though the percentage may be 70% in summer and 59% in winter. Thanks for your advice.

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                              Mark Blayney said :Guest Report 2 years ago

                              Thanks but this article has left me more than a bit confused !? You guys installed my system last May with the Fronius smart meter and I now get an 11 cent feed in tariff with AGL. So should I disable the meter or change the settings or what ?

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                              Ben Neville said :administrator Report 2 years ago

                              Hi George, thanks for your comment. I think your setup is a case in which a Fronius Relay works perfectly. Your inverter capacity is large enough, and your hot water kW draw is low enough to not be effected as much by rain or high base loads. This is where you could see the relay and smart meter combination account for over 85% of the hot water heating - making it worthwhile. A lot of the data and assumptions we made were based around a 3.6kW element and a 5kW inverter capacity, so my percentages are going to be lower than yours and therefore less financially viable. Happy to hear your setup is removing your bills!

                                George Roumeliotis said :Guest Report 2 years ago

                                The fronius relay installed here is feeding into a 1.7 kilowatt heater element of our hot water system. The solar system consists of 30 Jinko 270 W panels into 8.1 fronius inverter. By the way maximum p v output often exceeds panel specs 8268 W being the maximum. Our hot water system has been set to activate when there are 1500 Watts of excess and stay on for 30 minutes even if the panels drop below the threshold. End result in our case is that it comes on twice a day and still supplies all the hot water for the house. The panels are arranged in 2 strings of 15 North and 15 West both arrays function almost identical except for the expected bias 2 North in the morning the west in the afternoon. So far hot water and electricity bills have been eliminated. We use under 20kw a day... Not sure of this empirical outburst helps but the fronius smartmeter/relay has been fantastic here.

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                                Lindsay Mathieson said :Guest Report 2 years ago

                                Nice article, thanks. I bet it makes even less sense when you add batteries to the equation … I think people get obsessed with squeezing every last cent of usage out of their solar, without considering the cost of doing so.

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                                Ben Neville said :administrator Report 2 years ago

                                Thanks Jaime! Do you have a general idea of how your panels are going to be arranged on your roof? What do you expect your base load to be, and what size is your hot water in terms of it's kW draw? I can calculate your data, but the factors I've mentioned heavily decide the percentage of heating the relay will cover. If you are north facing, have a low base load, and a 2.4kW hot water unit, I'm calculating at least $68 a year savings for you (80% hot water heating covered by the relay). However, if you expect to use a lot of power in the day, have an inferior panel layout, and a 3.6kW element, you could be looking towards only $35 saving a year (70% hot water heating covered by the relay). The percentage of hot water coverage is the main component that influences the calculation, and is important to get right. I can show you my workings if you like, but any extra info you can provide on your base load, hot water capacity and panel layout will be helpful.

                                  Jaime Workman said :Guest Report 2 years ago

                                  Well written and thought provoking article. I wonder how those stats figure out for me? I am planning to install 7 kW of solar and use about 3kWh of hot water in summer and 5 in winter so I am well under your sample illustration. Feed in tariff is .10 and economy tariff is .175 and regular tariff is .285. I think in my case the Fronius smart meter with relay would save me a hundred dollars a year or so, but am I figuring correctly? We might lose some export in winter and are limited to 5kwh.

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