Hot Water cylinder timer

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SteveM2002

5 posts

Wannabe Geek

#289593 14-Sep-2021 23:02

Hi, looking for some advice. Like most people, I’m trying to bring my power bills down and my electric hot water cylinder is one of the biggest contributors. Does anyone have any experience with the Bobbie - Smart hot water cylinder controller? Or a similar device? Lots of websites sell the device but I can only find one review, on Amazon Germany. I know it’s not a DIY project and I’d need a sparky to hard wire anything. With Contact Energy offering free power between 9pm-7am my plan would be to heat the water to max temp during those hours then no heating during the day.
Any advice or experience anyone has with hot water controllers would be much appreciated.

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3968 posts

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Lifetime subscriber

#2778522 14-Sep-2021 23:38

Few points.

The contact energy free power plan is from 9PM to midnight, so only a three hour window, rather than the 10 hours you have stated (unless you are on something special).
kWh rate on the above plan is a bit more than typical plans, so only worthwile if you can move a lot of energy use to the 9PM to midnight window. (Which actually shouldn't be too hard to heat water, run dishwasher, run tumble dryer, charge EV if you have one etc. in that window). Would want to run the numbers to see if this plan actually delivers you value.
I don't see the point of anything smart to optimize for a 9pm to midnight window. A dumb timer should be fine, any sparky can procure and fit one.
A family member has a timer on their hot water system, installed to optimist self consumption of solar. Kicks in mid morning, and shuts of late afternoon. Has some kind of over-ride button so they can disable it with a single press if the house is full and they want to ensure they don't run out of hot water. Timers for this purpose are quite common, so residential sparkies will be familiar with them.
It would be good to run an experiment for a couple of weeks where you manually switch the cylinder on at 9pm, and off when you wake up in the morning. Bulk of heating will be done in the 9pm - midnight window (perhaps with some overflow into the small hours of the morning). The rest of the time will just be the odd pulse to replace heat loss from the cylinder itself. This should let you know if your cylinder is big enough for the operational pattern you propose.
Cranking up the thermostat from the recommended 60C to maximum (90C ???) will slightly increase thermal losses into the inviroment, but will increase your storage capacity. Double check your tempering valve is working (max temp at bathroom tap should be 55deg, regardless of cylinder temp)
My hot water cylinder is 180L and about 3kW. Makes for a recovery time of 3hr, 9min based on 15c cold water and 60C set point. If your cylinder is bigger, or you are running hotter, recovery time will be longer with the same element size, but you won't typically be doing a full recovery every day.
Would want to set the timer to have a decent run beyond midnight so you do get a full recovery each day.

Note that some of the people doing more aggressive management of hot water cylinders have large, dual element versions. Essentially a top element that will kick in 24/7 if you get down to your last 50L of water, and a bottom element that will heat the rest when power is cheaper / solar is available...

SteveM2002

5 posts

Wannabe Geek

#2778523 14-Sep-2021 23:49

Thanks for the info. I got my rates muddled. Yes Contact Energy is free 9-12 and Mercury who I’m currently with do 20% extra disc on power used 9pm-7am as I’ve got an EV. I’ll be doing some tests to see if it’s worthwhile switching to Contact or keeping Mercury.
Good to know about the dumb timer thanks. I would rather that as I have enough apps on my phone already!

Scott3

3968 posts

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Lifetime subscriber

#2778524 14-Sep-2021 23:58

SteveM2002: Thanks for the info. I got my rates muddled. Yes Contact Energy is free 9-12 and Mercury who I’m currently with do 20% extra disc on power used 9pm-7am as I’ve got an EV. I’ll be doing some tests to see if it’s worthwhile switching to Contact or keeping Mercury.
Good to know about the dumb timer thanks. I would rather that as I have enough apps on my phone already!

First thing I would do is run a trial of manually switching your cylinder on at 9pm (use the switch next to it, not the breaker on the circuit board), and off when you wake up. See if that causes you to run out of hot water etc. If no issues then you can get a timer added to automate that process.

sparkz25

750 posts

Ultimate Geek
Inactive user

#2778529 15-Sep-2021 00:11

what part of the country are you in? if you are in the WEL network area it might be worth taking a look at ourpower.co.nz 23.5C flat rate isn't too bad.

SteveM2002

5 posts

Wannabe Geek

#2778530 15-Sep-2021 00:13

I’m outskirts of Dunedin which as it’s semi-rural the kWh is 34.5c. Similar for all companies.

Scott3

3968 posts

Uber Geek

Lifetime subscriber

#2778531 15-Sep-2021 00:59

SteveM2002: I’m outskirts of Dunedin which as it’s semi-rural the kWh is 34.5c. Similar for all companies.

Sounds like a low user rate.

Given you have an EV and electric hot water, might be best to confirm if the low use rate is getting you best value.

SteveM2002

5 posts

Wannabe Geek

#2778533 15-Sep-2021 01:13

Yes it is a low user. To go standard the daily rate is over $3.20 a day and only brings the kWh down by about 12c which results in a higher annual cost to low user charges if I use approx 10,000 kWh.

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timmmay

20579 posts

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#2778539 15-Sep-2021 06:36

I had an electrician install a digital timer on my hot water circuit, cost from memory $250 - $300 for the timer (which they supplied) and their time. It's worked well for at least a few years. I had it heating overnight while I was on Flick, now I use the hour of power with EK.

In three hours you can probably do a large chunk of your water heating for the day. An hour isn't near enough but it reduces our bill by about 65c / day $230 per year, so it's paid for itself easily. With more hours it would pay for itself even more quickly.

traderstu

332 posts

Ultimate Geek

#2778578 15-Sep-2021 07:47

I think you are on the right track by tackling your hot water cylinder. This is where you make the most savings.

We have a Senztek controller on our cylinder (we don't use the solar functions). This is a bit smarter than a dumb timer. Supposedly, it works out your usage and only heats accordingly, but gives the water temp a boost every so often to kill the legionnaires. I reckon we get all of our hot water heating done in our hour of free power with EK, but there is only 2 of us in the house. We also run 2 heat pumps, the washing machine and the dryer during this time and end up with a saving of $46 per month.

tripper1000

1617 posts

Uber Geek

#2778618 15-Sep-2021 09:37

+1 what Scott3 said.

I am also a low user with an EV. I have a dumb timer with an override switch on a 3kw cylinder. Had the timer for 20 years, and it has paid for it self many times over.

It is pretty normal that my off-peak usage is greater than my on-peak now days.

IMHO everyone should have timers on their hot water cylinders, because many lines companies don't manage hot water for the greater economic or environmental good anymore. The cluster we had last month when we ran out of power highlighted this. Power prices were going through the roof but many lines companies didn't shut off hot water until the grid was in imminent collapse. The whole point of a controlled system was to knock the tops off the peaks - hot water should be turned off at peak demand every day, not just in dire situations. Transpower shouldn't have needed to order the lines companies to do their job's. Also the lines companies obstinance narrows the choices of power plans that power retailers are able to offer, harming customers.

SteveM2002

5 posts

Wannabe Geek

#2778728 15-Sep-2021 12:44

Thanks for all your responses. Much appreciated.

Chuckman

38 posts

Geek

#2797549 19-Oct-2021 11:37

I've just installed one of these. V happy with it so far. Not v expensive, wifi enabled and reports on energy usage. Biggest cost is Sparky's time, but I needed some work doing anyway so get it added in at the same time.

https://smarthomeshop.co.nz/products/shelly-1pm

Am on Electric Kiwi so have timer set to come on just before my free hour of power.

Ge0rge

2052 posts

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#2797561 19-Oct-2021 11:56

Chuckman:

I've just installed one of these. V happy with it so far. Not v expensive, wifi enabled and reports on energy usage. Biggest cost is Sparky's time, but I needed some work doing anyway so get it added in at the same time.

https://smarthomeshop.co.nz/products/shelly-1pm

Am on Electric Kiwi so have timer set to come on just before my free hour of power.

Keep an eye on the device temperature. I know that it says it's good for 16A, but I was not happy with the size of the contacts when connecting it to a HWC - and the temp rise of the device when it was connected and running was alarming enough that I removed it.

Chuckman

38 posts

Geek

#2797580 19-Oct-2021 12:50

Ge0rge:

Keep an eye on the device temperature. I know that it says it's good for 16A, but I was not happy with the size of the contacts when connecting it to a HWC - and the temp rise of the device when it was connected and running was alarming enough that I removed it.

Thanks - could you see the temp in the app? Meant to be an internal temp sensor which will trigger a shutdown if necessary but I can't see where to find the value. My HWC is drawing about 14A.

Ge0rge

2052 posts

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#2797590 19-Oct-2021 13:15

You might be able to see it in the app, I'm not sure - I use mqtt to talk to mine so get it from there.

I wasn't happy with something measuring 85c inside a wall cavity so it got removed. From memory shut-down is 90-95c.

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