As part of the solar install, I had to learn far too much about how hot water systems work - what options you have and how to get the most value out of each option. You probably don't want to know about hot water systems, but I want to tell you, and it's my (well, Angela's) blog so you can't stop me.
But read this before you click away!
You should read on for the next paragraph or so though, especially if you have an old (20+ year) electric hot water cylinder (HWC). It will suddenly fail one day, and you'll have to make a quick 4-figure-sum decision about what to replace it with. You need to do your homework now so you are ready to make an informed decision when it happens.
Get a HWC timer, right now
Hot water used to cost me 30% of my power bill. I added a $200 timer and now it's 0% (and nothing to do with the solar panels on the roof).
There is no need to run your HWC 24/7, and there are plenty of deals that give you cheap or even free power in off-peak times. It's an absolute no-brainer to fit a timer and heat your hot water when it's cheapest.
Pick one out (see links below) and get it fitted next time you need a sparky to visit.
I've also set up my beer fridge, towel rails and underfloor heating to only run when power is cheap or free.
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A typical day with the HWC running 9pm to 7am. 2 hours or so is all it takes to heat the day's water. The HWC spikes well beyond what the solar panels generally give us. |
Wrap your old HWC, right now
It's worth mentioning that you can wrap your elderly HWC with more insulation, and save money that way. Simple and effective. Just do it. Mitre 10 and Bunnings sell kits.
Get a heatpump HWC
This would definitely be my pick for a replacement hot water system.
All the faff in the rest of this blog post is for trying to optimise the massive power drain a regular cylinder consumes. Heat pump cylinders use the magic of heatpumps to do their job on 20-30% of the power of a regular cylinder. So you just don't need to worry about the kWh and the $ anymore.
Oct 2023: My work colleague Richard has just bought this one - Sanden - Rheem . It has two benefits: the heat pump heating system can be retrofitted to an existing cylinder, and it uses CO2 as its refrigerant. That second one means it's hydrocarbon free, and it runs a lot more efficiently and at a lower outside temperature.Other examples include:
- https://www.stiebel-eltron.co.nz/wwk
- https://www.tradedepot.co.nz/midea-heat-pump-water-heater-280l
- https://www.reclaimenergy.co.nz/home-hot-water-heat-pump/
Consider a stainless steel HWC
If a heatpump HWC is too expensive for you, think about spending just a little extra on a stainless steel HWC with a smart mixer valve. (For example, https://greenglo.co.nz/greenglo-range/ or the standard brands like Rheem). These will work with other heat sources like wetbacks, because they aren't fussy how hot you get the water. They also give you the option with solar to over-heat the water when you can, so you can use them like a battery.
Gas infinity systems are great - but be wary
A gas 'infinite hot water' system, with high pressure gas bottles or reticulated gas, works really well and avoids that whole electricity bill. But I'd personally steer away from these since in NZ the govt has a plan to phase these out in the next 10 years. The gas industry is pretending that hydrogen will replace natural gas but this is basically vapourware (see what I did there?) - it won't work, and if it does work you'll need a new appliance anyway. (Rosie, and Paul Martin, explain why at length in this video.)
If you must, I'd say bottles will be more long lasting than reticulated gas, and you want an appliance that can run on different sorts of gas with just a jet change.
Consider an electric infinity system!
Edit Jan 2023 My workmate is looking at ripping out his gas cylinder and replacing it with an instantaneous electric system. Something like what Stiebel-Eltron offer. He's thinking about two of these units, one in each end of the house. Watch this space!
Watch this video instead
Technology Connections has gone into how hot water cylinders work and how to maximise their efficiency in lots of detail. It's a great watch if you like nerds explaining simple things in extreme detail, just like I do.
So why do I care so much about hot water?
With solar on our roof now, I was expecting to use the hot water cylinder as our battery. Use a timer and heat the water really hot when the sun is shining, use it up when it's dark. Easy.
No, not that easy. Three main reasons:
Our HWC is ceramic-coated inside, not stainless steel or copper, which means it can't hold really hot water. And our mixer valve is simple and dumb, it cannot adjust how much cold water it mixes in to keep the water temperature constant at the tap. The whole system is designed to just maintain a steady 60°C in the cylinder, and deliver 50°C to the tap. So it's not an option to get it to say 90°C when the sun is shining, and store a good lot of hot water for when it's dark.
Our HWC is either off (0W) or on (3000W). 3000W is far more than our panels will generate except in the middle of a very sunny day. No option to throttle the cylinder down to "low" or something, other than replace its elements completely - a very permanent solution! This means that if the HWC runs during the day, it's mainly using grid power in spite of the solar panels on the roof.
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Why can't our hot water cylinder do what our $70 heater does? A 24h timer and a Low/Med/High power setting as well as a thermostat. |
One element is halfway up the cylinder and the second one is at the bottom. Only one element will be on at any time. - they are "non-simultaneous". The idea is that when you run low on hot water, the top half heats first (giving you hotter water faster since the hotter water will "float" on top of the cooler water). Then when the top half is hot, the cylinder switches to heating the bottom half and the rest of the water. Normally the two elements work like this, handing over from one to the other, but you can rewire the bottom element to only run off-peak on your timer. Basically you keep just half the water hot during peak times and get the whole cylinder hot only at off-peak rates. (This seems to be a bit of a waste of time to me).
Legionnaires!
Let's get the Legionnaires disease elephant in the room out of the way. If you run your cylinder luke warm, under 60°C, then you run the risk of Legionnaires disease getting into the system and putting you in hospital or worse. But, as long as you get it up to temperature at least once a week you are fine (source: EECA via Radio New Zealand).
So it's safe to use a timer to get the cylinder to 60°C each day on cheap power, then let it slowly cool while power is expensive. Turning the thermostat down to keep the cylinder permanently cooler to use less power, is NOT SAFE.
Hot Water Timers - options and links
Updated March 2023
OK this is the long story about hot water timers. Unfortunately, fitting a HWC timer fell off the manifest for our solar install so I had to investigate options myself. I found it pretty hard to find out this information. It's sparsely documented, the sparkies aren't really up with the play and their suppliers don't really care. You've got to find it out for yourself and figure out how to source things too.
If a hot water cylinder plugged into a regular 3 pin socket, you could go to Mitre 10 and buy a $10 timer plug and you'd be done. But because it uses more like 16A instead of 10A, it's not that simple.
I started out with the sensible path and rang a few electrical wholesalers/suppliers. They must be able to help, surely? No. They were either a bit baffled about why I would want such a thing, or confidently asserted that they were nearly impossible to get. I had to go elsewhere.
Here is the list of timer units I found, and the pros and cons. It will go out of date fast, but the principles will stay relevant.
PDL Iconic kit from Eurotech $200
This was my initial second choice (and Angela's first choice!), but after I "saved" money with the $26 Sinotimer below, and it failed after just a few months - I went with this one.
Eurotech - https://www.kiwisparks.co.nz - have taken some Schneider/PDL off the shelf components and bundled them to give you everything you need to put a Bluetooth or Z-Wave driven timer into the wall 'socket' that your HWC is wired into. Less invasive than a switchboard install (assuming you even have the right switchboard!) and you can change the settings from your phone.
You can find all these components on the PDL website - this is the bluetooth/ZWave device and here are all their timer options - but it's very confusing and hard to follow there. Their website seemed to be aimed at electrical suppliers who know exactly what they want to order already. Eurotech have made this and a lot of other home automation jobs much easier by putting together the things you need and making them easy to buy.
The installation was a little tricky because the timer is about 55mm deep and an external mounting box is only 34mm deep. After trying various ways to make it fit I settled on stacking two mounting boxes on top of each other. It actually worked tidily and well, with long screws thoroughly securing both boxes to the wall.
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| Anticlimactic looking for the effort that went into it |
Sinotimer digital timer $26
I bought this in April, because I thought it was cheap and it did the job. But, it lasted six months before it failed. I don't recommend! The cost of getting the Sparky to wire it in is far more than the money I saved by going cheap.
It's a Chinese unit that they also white-label for anyone who wants to put their "home brand" on it. That's why you'll find any number of timers like this that all look suspiciously similar. It goes into your switchboard, so it's definitely something a sparky must fit, and you need to have a modern switchboard with breakers, not one of those old black panels with fuses mounted on it.
http://www.sinotimer.com/products/din-rail-weekly-programmable-timer-ID191.html
I sourced it from TradeMe, from this crowd who seem to bring in lots of electrical goods.
https://www.trademe.co.nz/a/marketplace/stores/SafenSecure
https://www.safensecure.co.nz/
Gewiss 16 Amp 7 Day Electronic Timer $160
There are more expensive (and hopefully more durable!) versions of that electronic switchboard mounted timer you could consider instead, such as this Italian brand.
https://www.kiwisparks.co.nz/products/timer-gewiss-16-amp-7-day-electronic
Theben SYN160a Daily Analogue Time Switch $126
This is a mechanical version of those electronic switchboard timers. I like this one. It's simple and mechanical so it'll arguably outlast anything electronic or Bluetooth/wifi, and it's 'Made In Germany' so you'd expect it to be good quality. It's probably the one to pick if you think paying a sparky to fit a $26 Chinese electronic timer is a false economy.
My sparky pointed out, though, that any mechanical timer will be noisy so if your switchboard is close to a bedroom it might not be a great choice.
It also goes into your modern switchboard.
Lightforce Solar sell this, it's what they'll give you if you buy a solar panel install from them. It's also available on TradeMe from sellers like MyTools with listings like this one.
Honeywell ST9100C timer $585
This is what I would have got from Harrisons Solar if it had made the quote. It's beautiful, fully featured and has more options than you can shake a stick at. But I think it's just too expensive. It also doesn't connect with home automation systems.
https://www.harrisonssolar.co.nz/Downloads/Assets/3335/1/
Tuya wifi switches
These look ideal if you want a smart timer that talks to your home automation system. I just didn't know enough about them to take the gamble that they would work, or even be legal, in a NZ house.
Sparky Direct
https://sparkydirect.com.au had a much better range of electrical goods, in a much friendlier form, than most websites I was browsing. Australia seems a safe bet too since they share our electrical standards. I did consider this timer unit which does look quite familiar (remember what I said about SinoTimer white-labelling their stuff?).
A regular timer + a relay
In hindsight perhaps I should have investigated this.
Get a more regular timer, one designed for lights or something, and hook it up with a relay that has the oomf to control the HWC. This avoids having to mess with the switchboard and gives you more options for your timer.
Other options for efficient hot water heating
There are a few other options, that weren't useful for my circumstances. They might be worth looking at if you have something like a spa pool or a stainless HWC, or you are off-grid and every joule counts.
Though I wish I'd known about that Fronius Ohmpilot back when I was sorting out my timer.
Hot water Power Diverters
This is the 'holy grail' that I naively assumed would be
straightforward to set up for a solar installation. Talk to the smart
meter/inverter which has a standards based API to tell you how much
excess power is coming off the roof, and funnel the right amount of
amps to the hot water cylinder to maximise its use. Add in some
periods where grid power is cheap and the unit can just suck down all
the amps the grid will give it, too. Done! Optimum use of your solar
power and your grid sources.
The truth is, these units are quite expensive ($1000 or so), and
really need to be paired with a stainless steel cylinder so they can
over-heat the water when the sun is shining. The ROI just wasn't
there. That's serious money you could just put into a heatpump hot
water cylinder instead and not use the power in the first place.
Here's a great article that explains the differences and the pros and
cons.
A couple of examples of Power Diverters are
http://www.paladin.nz/ and
https://catchpower.com.au
(available from Harrisons Solar)
Mar 2024: my sis-in-law Iona has a Fronius Ohmpilot that was put in as part of her solar install. It looks perfect! A power diverter that talks directly to her Fronius Inverter and just works. I want one.
This is the 'holy grail' that I naively assumed would be straightforward to set up for a solar installation. Talk to the smart meter/inverter which has a standards based API to tell you how much excess power is coming off the roof, and funnel the right amount of amps to the hot water cylinder to maximise its use. Add in some periods where grid power is cheap and the unit can just suck down all the amps the grid will give it, too. Done! Optimum use of your solar power and your grid sources.
The truth is, these units are quite expensive ($1000 or so), and really need to be paired with a stainless steel cylinder so they can over-heat the water when the sun is shining. The ROI just wasn't there. That's serious money you could just put into a heatpump hot water cylinder instead and not use the power in the first place.
Here's a great article that explains the differences and the pros and cons.
A couple of examples of Power Diverters are http://www.paladin.nz/ and https://catchpower.com.au (available from Harrisons Solar)
Mar 2024: my sis-in-law Iona has a Fronius Ohmpilot that was put in as part of her solar install. It looks perfect! A power diverter that talks directly to her Fronius Inverter and just works. I want one.
Solar hot water
Why take sunlight, turn it into DC electricity, put it through an
inverter to make it AC, run it in wires to a hot water element and
heat the element to make hot water - when you could just heat the
water directly by shining sunlight on it? Quite a few years ago
my sister followed that logic and got a solar hot water system
installed.
Unfortunately the system didn't really work. For efficiency it uses a
glycol heat exchanger. It seems to be very problematic - the glycol
leaks away, the pump fails, it seems to need regular expensive
maintenance. She also needed to replace her cylinder with a stainless
steel one to let it over-heat the water.
I have a friend with this system too, she had a better experience but
it still requires a bit of maintenance. The pipe joints have a habit
of leaking because the water temperature inside them varies so much
and they're constantly expanding and contracting.
One advantage is that you have two energy sources - so if your HWC
elements fail then you still have lukewarm hot water to get you
through the fix. Though a power cut still means no hot water since the
pump to the roof doesn't work anymore, and a HWC leak is still
catastrophic too.
These systems have fallen out of favour, since for your money you'd be
better off installing a heatpump hot water cylinder for similar
efficiency gains.
Why take sunlight, turn it into DC electricity, put it through an inverter to make it AC, run it in wires to a hot water element and heat the element to make hot water - when you could just heat the water directly by shining sunlight on it? Quite a few years ago my sister followed that logic and got a solar hot water system installed.
Unfortunately the system didn't really work. For efficiency it uses a glycol heat exchanger. It seems to be very problematic - the glycol leaks away, the pump fails, it seems to need regular expensive maintenance. She also needed to replace her cylinder with a stainless steel one to let it over-heat the water.
I have a friend with this system too, she had a better experience but it still requires a bit of maintenance. The pipe joints have a habit of leaking because the water temperature inside them varies so much and they're constantly expanding and contracting.
One advantage is that you have two energy sources - so if your HWC elements fail then you still have lukewarm hot water to get you through the fix. Though a power cut still means no hot water since the pump to the roof doesn't work anymore, and a HWC leak is still catastrophic too.
These systems have fallen out of favour, since for your money you'd be better off installing a heatpump hot water cylinder for similar efficiency gains.
Ripple Control
For completeness, I better mention "Ripple control". This is where the
lines company (not your power company) can switch off your cylinder
remotely in peak usage times to smooth out peaks and troughs. It was
common in the 1970s and 80s, it's still a thing today but I don't know
anyone who still has it. EECA have written a report that talks about Ripple control in terms of the greater good but
don't offer any reasons why as a consumer I would want to have it (or
for that matter why the lines companies would even bother either). The
conclusion seems to be, it's a Good Thing but it will be gone in 5-10
years anyway as smarter management systems come online.
For completeness, I better mention "Ripple control". This is where the lines company (not your power company) can switch off your cylinder remotely in peak usage times to smooth out peaks and troughs. It was common in the 1970s and 80s, it's still a thing today but I don't know anyone who still has it. EECA have written a report that talks about Ripple control in terms of the greater good but don't offer any reasons why as a consumer I would want to have it (or for that matter why the lines companies would even bother either). The conclusion seems to be, it's a Good Thing but it will be gone in 5-10 years anyway as smarter management systems come online.
Do you have any advice for me?
If you've got this far you must be quite interested in electrical efficiency! What did I get wrong? What did I miss? Do you have any advice for me? Do you know anyone or any company that gives good straightforward advice and has a range of products that can help?
Please tell me in the comments.
My experiences are also based entirely around living in my particular suburban house in New Zealand. Different countries have different energy options with different costs and carbon footprints. Hot water systems are also quite conservative and one country's common system will be unheard of and difficult to get in another. If you are rural and/or off-grid, you'll have completely different needs too. Even for me, if electricity prices or solar buyback prices change, the equation will change. If I ever add a battery or buy an EV with vehicle-to-grid capability, everything will change again. It's an exciting time!
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