Is this logic correct? Off peak hot water heating.

Forums › Home Workshop DIY › Is this logic correct? Off peak hot water heating.

GSManiac

488 posts

Ultimate Geek

#250603 18-May-2019 09:17

My dad is on a electricity plan that provides a cheaper night rate of power between 9pm and 7am. So he now turns off the hot water cylinder out side of those hours so it’s only heating during the cheaper night rate.

My argument is that Surely it’s more energy efficient and therefore cheaper to keep it at a constant temperature 24/7 rather than having it cool right down and have to work over time to heat back up at night.

Any thoughts?

TIA.

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556 posts

Ultimate Geek

#2240356 18-May-2019 09:44

By him switching his water off and on will be costing him quite a lot of money as it uses significant energy to reheat. Quite sad that people are forced to do this. Won't he be receiving the extra heating allowance from the Government?

rogercruse

644 posts

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#2240364 18-May-2019 10:06

GSManiac: ... cool right down ...

Surely, the hot water cylinder is insulated?

GSManiac

488 posts

Ultimate Geek

#2240366 18-May-2019 10:12

Installed in 1993 so, no.

Mattnzl

281 posts

Ultimate Geek

#2240368 18-May-2019 10:32

Why does it cool down when switched off? Because it is hotter than the surrounding air and looses heat to the air.

This still happens when it is turned on though... the thermostat will turn the element on periodically to "replace" the lost heat.

Sooooo, whether it looses it's heat all in one go (turned off) or in small increments (tuned on) it will still loose the same amount of heat (energy) over a period of time and require the same amount of energy to "heat up".

In your dad's case, it IS cheaper for him to replace the lost heat off peak because he gets cheaper power then.

mugs2000

66 posts

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#2240406 18-May-2019 10:43

Mattnzl is quite correct. Several factors come to play here;

1) Newton's law of cooling - heat loss is greater when the temperature difference is greater, so there is no topup of heat during the day at the higher temperature.

2) Re-heating only replaces the heat lost during the day. It doesn't require any extra heating.

3) The price at night-time is cheaper.

I have done the exercise of comparing the difference with a 5 Gallon heater, and saved $1 per day without any price differential.

Geese

1028 posts

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#2240416 18-May-2019 11:15

What about the risk of Legionnaires from doing this?

https://i.stuff.co.nz/national/health/70285141

rugrat

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#2240418 18-May-2019 11:37

I’m on night and day. My hot water only runs at night.

Cheap power 9pm to 7am, and hot water is switched on 8 hours of that period, which the lines company picks, they don’t want to turn everyone’s hot water on at same time so different two hours of no heating for different people.

I know in the past controlled water heating gave a cheaper rate as well.

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sidefx

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#2240419 18-May-2019 11:46

Geese: What about the risk of Legionnaires from doing this?

https://i.stuff.co.nz/national/health/70285141

From that article the issue doesn't appear to be turning off heating at particular times, but rather turning it down so it doesn't heat up to 60degC or above.

"I was born not knowing and have had only a little time to change that here and there." | Octopus Energy | Sharesies
- Richard Feynman

rugrat

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#2240421 18-May-2019 11:53

Geese: What about the risk of Legionnaires from doing this?

https://i.stuff.co.nz/national/health/70285141.

"These recommendations state hot water must be stored at 60°C inside the water heater by ensuring, at least once a day, the temperature reaches at least 60°C in the entire tank."

So only have to reach 60 degrees once a day to kill.

Source.

sqishy

470 posts

Ultimate Geek

#2240423 18-May-2019 11:59

Very hard to answer unless you know what wattage it draws to reheat and cost of electricity per kW during day vs night.

You could check the meter reading each day and write them down during peak/off peak charging then try turning off/on at different times until you get the right efficiency.

One thing is to run the hot water at lowest temperature is a good start.

lxsw20

3552 posts

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#2240424 18-May-2019 12:01

GSManiac: Installed in 1993 so, no.

A tank from 1993 will be insulated.

GSManiac

488 posts

Ultimate Geek

#2240466 18-May-2019 12:11

lxsw20:
GSManiac: Installed in 1993 so, no.

A tank from 1993 will be insulated.

Sorry yes it’s insulated on the inside but nothing on the outside.

Thanks for the replies.

wally22

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#2240499 18-May-2019 13:16

If the cylinder was heated to 60C all the time, i.e. power on all of the time, then heat loss would be only marginally greater as the larger the temperature difference, the greater the heat loss.

I am currently in a unique? situation, house sitting for an old friend in hospital. I changed his account over to Electric Kiwi and I only turn the cylinder on during the 1 hour of free off peak power. That suffices for my showering. The only hot water I use. The dishwasher and washing machine heat their own water, again, only used in the hour of free power, so on a good day I have achieved 43% of my power being free.

That being said, both the DW and WM take longer than 1 hour to complete their respective cycles. We just have to wear that. However, one night I was up late, set the free hour to 11pm to midnight, started everything at 11pm. Then at 11:30 started a luxuriously long shower and waiting until after midnight I set the next day's free hour to run from Midnight to 1am.

You can change the hour of a particular day anytime up until midnight on that day on the phone app or internet.

sidefx

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#2240574 18-May-2019 15:52

wally22:

I am currently in a unique? situation, house sitting for an old friend in hospital. I changed his account over to Electric Kiwi and I only turn the cylinder on during the 1 hour of free off peak power. That suffices for my showering. The only hot water I use. The dishwasher and washing machine heat their own water, again, only used in the hour of free power, so on a good day I have achieved 43% of my power being free.

Is that definitely long enough to actually get up to 60degC or above though? I'd want to make sure.... legionnaires disease doesn't sound fun.

"I was born not knowing and have had only a little time to change that here and there." | Octopus Energy | Sharesies
- Richard Feynman

wally22

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#2240585 18-May-2019 16:18

I haven't tested it. But the shower is far too hot unless I put lots of cold into the mix. Perhaps I should test it. Later. Just arrived at Good George for meetup.

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