Continuous Electric Hot Water Heaters

Forums › Home Workshop DIY › Continuous Electric Hot Water Heaters

1 | 2 | 3 | 4

1723 posts

Uber Geek

Trusted

Chorus

#3295752 10-Oct-2024 22:38

We have a Rinnai 26 gas califont at our bach, supplied by 2 x 45kg gas bottles and honestly it's the best setup for us.

We arrive, we turn the gas on, and we have instant and virtually endless adjustable temperature hot water - whether it be for one or 12 people. And we pay nothing when nobody is there.

Our gas bottles are supplied by Frank Energy - $164 per bottle delivered, and because we have our power with them, we pay no bottle rental fee (and their electricity plan in our area is one of the lowest too).

We chose to not have a gas regulator that automatically switches from one bottle to the other when the first is empty - that way we always know when the first bottle has run out so we can order more via the Frank app (it takes like 3 taps in the app). If we have paying guests or friends staying there, we always ask them to tell us if a bottle runs out while they are there so we can order another - and they always have.

Our bach is quite rural (up the Waitaki Valley on the border of South Canterbury/North Otago an hour inland from Oamaru), but Frank delivers to the area once a week via the Genesis distribution network.

I can't accurately estimate how many guest nights our bach has in any given year, but at the height of our AirBnB operation we maybe went through 4 or 5 gas bottles a year. Since we stopped AirBnB, we only use around 1 bottle per year, but we visit regularly (at least monthly) and can sometimes have up to 12 people staying for 3-4 days at a time (but usually it's between 2-4 people staying except at public holiday weekends when it's a full house).

To be honest, I'd never consider another option. Having to turn something on and wait for it to heat up upon arrival is a pain, but on the flipside we wouldn't want to pay for hot water storage when nobody is there - that's just a waste of money.

dimsim

848 posts

Ultimate Geek

Trusted

Lifetime subscriber

#3295753 10-Oct-2024 22:43

SomeoneSomewhere:

It looks like almost all cylinders here use a 1 1/4" BSP fitting for the element. Assuming the cylinder is of sufficient diameter, there's no reason you couldn't add a circuit and replace the thermostat to fit a 6kW element like this: https://smithsnz.co.nz/collections/tank-elements/products/6kw-low-watt-density-element-450mm-long

Pretty much all tanks have a TPR valve good for 10kW.

More hot water capacity is really ambulance at the bottom of the cliff, though. Fix whatever is letting people empty a 300L tank, unless this is being used as something like a ski bunkhouse with two dozen people turning up for a hot shower and to crash for the night.

It really is sometimes like that. its a large house, sleeps approx 12, sometimes can have 12 adults staying. Typically we don't have many issues but when we have it's been "overseas" visitors who treat it like a hotel with endless hotwater. We've never run out of hot water and have other families staying with us sometimes along with teenagers and their friends staying often. Regardless it is still something id like to try and avoid, while at the same time

replace the aging cylinder
optimise energy efficiency

@SomeoneSomewhere - I'm thinking a definite increase in heater element kw is on the cards. As the cylinder is old, its likely that a new cylinder with either a 2x3kw or 2x5kw system would be sensible or like this maybe just a single 6kw. From what I understand if the supply is depleted, the the dual coil/element model will create usable hotwater quicker, but I may have that wrong.

@WWHB - a friend is in the middle of a build and has just purchased a heat pump hot water system with indoor cylinder - has a trade discount and that system is approx $10k. I do however like the idea of being able to remotely control/monitor the system. Also the location is Taupo, so it does get cold, but we do have heatpumps inside for heating and where the outside units are located is sheltered and we haven't had any issue with these.

@nickb800 - understand the payback argument and agree - our powerbill isn't exorbitant in yearly terms

By the look of our latest bill below - we do have ripple control and this looks like it's saving us $10/month - so possibly not worth the bother.

SomeoneSomewhere

1802 posts

Uber Geek

Lifetime subscriber

#3295756 10-Oct-2024 23:08

Yup, you have ripple control. It's pretty common for peak shower time to be peak energy usage time when ripple is going to operate. Consider getting the lines company to disconnect the ripple (this will probably be charged for because they probably have to roll a truck).

I haven't been able to find any details on how Unison (the lines company) decides to use ripple control, but they list it as being able to be switched off for up to seven hours per 24 hour period, though they may go past that in times of extreme shortfall (AKA they would otherwise have to shed customers altogether).

You also now know exactly how much hot water is costing you, separated out from the rest of the electricity bill: the hot water is the controlled line, and no other usage is controlled. You might even be able to get a separate half-hourly breakdown for controlled vs 24 hour.

A tank with an element midway up it will recover somewhat faster from completely empty than a tank with all the element capacity at the bottom. If not emptied completely, there should be no difference. Complete recovery will take longer because the top element will cut out early.

Next time you visit, I'd definitely recommend doing a bucket test on the shower and maybe trying out a couple of modern showerheads. I'm not certain whether this is work that legally requires a plumber.

I don't think I'd recommend replacing the cylinder with another electric cylinder unless it actually fails. New cylinders merely have better insulation and that's totally DIY-able and retrofittable. Elements are 100% efficient regardless of how old they are.

dimsim

848 posts

Ultimate Geek

Trusted

Lifetime subscriber

#3295759 11-Oct-2024 01:33

SomeoneSomewhere:

Yup, you have ripple control. It's pretty common for peak shower time to be peak energy usage time when ripple is going to operate. Consider getting the lines company to disconnect the ripple (this will probably be charged for because they probably have to roll a truck).

I haven't been able to find any details on how Unison (the lines company) decides to use ripple control, but they list it as being able to be switched off for up to seven hours per 24 hour period, though they may go past that in times of extreme shortfall (AKA they would otherwise have to shed customers altogether).

You also now know exactly how much hot water is costing you, separated out from the rest of the electricity bill: the hot water is the controlled line, and no other usage is controlled. You might even be able to get a separate half-hourly breakdown for controlled vs 24 hour.

A tank with an element midway up it will recover somewhat faster from completely empty than a tank with all the element capacity at the bottom. If not emptied completely, there should be no difference. Complete recovery will take longer because the top element will cut out early.

Next time you visit, I'd definitely recommend doing a bucket test on the shower and maybe trying out a couple of modern showerheads. I'm not certain whether this is work that legally requires a plumber.

I don't think I'd recommend replacing the cylinder with another electric cylinder unless it actually fails. New cylinders merely have better insulation and that's totally DIY-able and retrofittable. Elements are 100% efficient regardless of how old they are.

thanks for your insights, all very helpful.

will def do a bucket test and do some research on showerheads.

I'm seriously considering replacing the cylinder however. i think 20 years from what I've been told is a good run and as it's remote to us, the last thing I want is for it to burst/leak while we're not there.

Spyware

3761 posts

Uber Geek

Lifetime subscriber

#3295790 11-Oct-2024 09:03

A 300 litre cylinder being cycled on 24 hour rate is going to be using at least 18-20 kWh/day all by itself.

Spark Max Fibre using Mikrotik CCR1009-8G-1S-1S+, CRS125-24G-1S, Unifi UAP, U6-Pro, UAP-AC-M-Pro, Apple TV 4K (2022), Apple TV 4K (2017), iPad Air 1st gen, iPad Air 4th gen, iPhone 13, SkyNZ3151 (the white box). If it doesn't move then it's data cabled.

SomeoneSomewhere

1802 posts

Uber Geek

Lifetime subscriber

#3295964 11-Oct-2024 12:51

Spyware:

A 300 litre cylinder being cycled on 24 hour rate is going to be using at least 18-20 kWh/day all by itself.

I think you're off by nearly an order of magnitude there.

I'm struggling to find a definitive source because this is all under Standards lock-and-key and the energy ratings label scheme doesn't seem to cover it, but various sources quote 1.7kWh per day as the current legal requirement (since 2005) for a 180L tank, and a 300L tank is probably in the 2.5kWh/day range.

The daily chart posted upthread here shows roughly 4.5x 0.75kWh periods of use per day, or 3.4kWh/day with the house empty. Some of that will probably be loss through uninsulated pipes. I wouldn't be surprised if a good cylinder lag halved it.

OP's bill shows they're using 5.8kWh/day with unknown hot water usage - given 500+kWh of normal usage, someone was probably there for part of the month.

Spyware

3761 posts

Uber Geek

Lifetime subscriber

#3296109 11-Oct-2024 15:04

SomeoneSomewhere:

I think you're off by nearly an order of magnitude there.

By being cycled I mean people using the hot water in these many long showers, It takes 18 kWh to heat 300 litres from 20 to 70C. To true they would have been having cold showers if only heated during the night. To expect that a 300 litre tank can supply daily hot water to 12 people when only heated during an 8 hour interval is ridiculous. Even 5 minute showers would push things.

Equinox IT

Cloud spending continues to surge globally, but most organisations haven’t made the changes necessary to maximise the value and cost-efficiency benefits of their cloud investments. Download the whitepaper From Overspend to Advantage now.

mattwnz

20141 posts

Uber Geek

#3296113 11-Oct-2024 15:24

Wheelbarrow01:

We have a Rinnai 26 gas califont at our bach, supplied by 2 x 45kg gas bottles and honestly it's the best setup for us.

We arrive, we turn the gas on, and we have instant and virtually endless adjustable temperature hot water - whether it be for one or 12 people. And we pay nothing when nobody is there.

Our gas bottles are supplied by Frank Energy - $164 per bottle delivered, and because we have our power with them, we pay no bottle rental fee (and their electricity plan in our area is one of the lowest too).

I have the same sort of setup and also with Frank so get free bottle rental and it works out a lot cheaper and convenient for me..

TBH I think instant gas water heaters are still a great option for many and can work out cheaper in some situations, and you are not losing energy from the heat loss in heating a tank. Plus you never run out of hot water. Gas seems to have got a bad reputation in recent years, but it is consider a transition fuel and NZs electricity network is already struggling to cope with NZ current electricity usage. So everyone switching to electricity for their home heating and water heating is going to put extra load on the network. Our next door neighbor also had their house flooded by a newish hot water cylinder that failed and leaked water through the house, despite being on a drainage tray so that was off putting too.

SomeoneSomewhere

1802 posts

Uber Geek

Lifetime subscriber

#3296169 11-Oct-2024 16:05

Spyware:

SomeoneSomewhere:

I think you're off by nearly an order of magnitude there.

By being cycled I mean people using the hot water in these many long showers, It takes 18 kWh to heat 300 litres from 20 to 70C. To true they would have been having cold showers if only heated during the night. To expect that a 300 litre tank can supply daily hot water to 12 people when only heated during an 8 hour interval is ridiculous. Even 5 minute showers would push things.

Oops, sorry, thought you were continuing the conversation on standing losses and just talking about it being powered 24/7.

HWC ripple channels normally guarantee around 16-19 (Unison is 17) hours per day powered, only being switched off during peak times. That implies that with a 3kW element you can technically use about 50kWh/day, but because the storage capacity is limited that only works if you evenly space things and someone has a shower at midnight...

Regardless, if we consider draining the tank in e.g. a 3-hour period (a reasonable period for ripple control to be off), an unheated tank will deliver 300L of hot water while one with an operating 3kW element (51L/h) will deliver about 450L, and a 5/6kW element pack (85/103L/h) would deliver around 550-600L.

If we considered a 7L/min showerhead drawing about 5L/min from the tank (cold water mixing), we'd get about 60, 90, and 110/120 minutes of showering respectively.

Scott3

3963 posts

Uber Geek

Lifetime subscriber

#3297133 14-Oct-2024 11:17

I have specified a continuous electric water heater once.

Was for a generally unstaffed industrial facility, which due to it's nature had several hundred kW's of electricity demand, so the 20 - 30kW extra to cover the shower was small fry (and I told the electrical designer if it bumped us over the a transformer sizing threshold I could swap it out for a cylinder). And I put it on the non-critical list, so it is unavailable when the site is running on generator.

Generally pretty happy with the selection. Compact, zero standing losses (important given infrequent use), and unlimited running if ever needed.

I specified this brand.

https://www.stiebel-eltron.co.nz/water-heaters

And the below model (can't remember if I specified the big one or the small one).

https://www.stiebel-eltron.co.nz/download/268

While continuous electric water heaters are extremely common in Asia (Typically 3.6 - 4.5kW units in the style that is mounted high on the bathroom wall, and where the outlet doubles as the vent (no taps on the discharge side), they are very rare in NZ.

In short with NZ cold water temperatures, to get a decent flow rate you need a heap of electrical capacity.

As a ballpark indication, WELS 3 star shower is 9L/min, 6L/min would be considered very low flow. High flow is likely 12+ L/min. If we assumed 15 Deg C inlet temp, and a 25 degree raise for a 40C shower, then the larger three phase unit below could do two weak showers at once (~ 7L/min each), and the single phase units are only copiable of giving a really low flow shower.

As to all your questions:

Venting is required on some units, but not the one I specified.
Single phase is fine, as long as you are OK with a really low flow shower (and only one at a time), also note if the house has a lot of other electrical stuff, you could exceed the 63A pole fuse.
Generally fairly expensive to have three phase installed.

3 phase system example:

Single Phase example:

In NZ the most common power configuration is single phase 63A, with the following being available but much more rare: single phase 100A, three phase 63A, three phase 100A. From the tables above, the grunter water heaters are consuming a decent chunk of this power.

While I recommend people get high power connection with new builds (mostly to accommodate an all electric home with multiple EV's in the future), generally the cost of upgrading is prohibitive, but feel free to get a price.

Yes, you are right that electric continuous would be ideal for a batch, but for most it is not a cost effective option.

Suggest you will likely end up with another large cylinder, perhaps with a rapid recovery (higher than normal power elements), perhaps with a remote control so you can remotely turn it on before you arrive.

As per other comments, draining a 300L cylinder is unusual. Most of us do fine with much smaller cylinders. Suggest trying new lower flow shower heads.

LightbulbNeil

57 posts

Master Geek

#3300778 24-Oct-2024 06:50

We currently have a gas califont for hot water from 45kg bottles. With the rental and the 4 bottles per year, currently it is about $550 a year for 2-3 people. The home has 3phase power. The house design has no hot water cylinder. The gas hot water heater is in the house roof, near the middle of the house.

My question is, how do I find out the kWh equivalent energy in a 45kg bottle ? I would like to work out the costs differences between the gas califont versus a 3phase power califont . Our current califont is rated for I think 27 litre per min? We can have both showers running and not be low in hot water .

Just looking at planning for when the has hot water unit will require to be replaced. Do we stay with the gas or switch to a 3phase electric unit.

Thanks for any help.

Neil Lickfold

itxtme

2102 posts

Uber Geek

#3300825 24-Oct-2024 09:51

LightbulbNeil:

My question is, how do I find out the kWh equivalent energy in a 45kg bottle ? I would like to work out the costs differences between the gas califont versus a 3phase power califont . Our current califont is rated for I think 27 litre per min? We can have both showers running and not be low in hot water .

LPG is about 13.8 kWh/kg. So its approx 621kWh.

LightbulbNeil

57 posts

Master Geek

#3300889 24-Oct-2024 10:49

Thank you for that information. The gas is definitely cheaper than electricity.

tweake

2391 posts

Uber Geek

#3301154 24-Oct-2024 17:18

LightbulbNeil:

We currently have a gas califont for hot water from 45kg bottles. With the rental and the 4 bottles per year, currently it is about $550 a year for 2-3 people. The home has 3phase power. ............ Our current califont is rated for I think 27 litre per min?

the other issue is an electrical version would probably use about half, maybe a bit more, of your available power. (assuming 60amp 3 phase)

i know someone overseas who has them, but they have a 400 amp connection.

LightbulbNeil

57 posts

Master Geek

#3301174 24-Oct-2024 17:54

We could upgrade the pole fuses to 100amp as we went with the larger wire supply to the house. But chose the 63 at the time. May have been a mistake who kbows .
Thanks for the information thats been shared. Very interesting reading.

1 | 2 | 3 | 4