Gas Instant Hot Water

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xlinknz

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#322719 15-Sep-2025 20:43

Hi all

Our gas usage has gone up >50% by volume over the last 6 months. For the last month were were extra dilligent on hot water use e.g. showers etc with little change.

Downers confirmed no gas leaks (and meter ok) which only leaves the 20 year old Rinai Infinity 24 to be checked for efficiency etc

Has anyone either found their Gas Instant Hot water heater significantly inefficient or replaced an old unit?

Also if it requires replacing I am tempted to replace it with electricity instant heated hot water if available (but leave gas for oven top)

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#3415262 15-Sep-2025 20:51

If the cold water inlet temp is lower (as in winter) it will use more gas to get to the same outlet temperature. There may well be a fault, but some of the extra may be normal.

Aucklandjafa

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#3415263 15-Sep-2025 20:57

Yeah, house we moved to our gas usage has gone up over 50% - the unit is nearing 20 years old. Electric instant hot water uses a crazy amount of energy, and assuming you’re on natural gas, you may want to look at converting your cook top to LPG (just run it off a 9kg bottle) will save you tonnes in line rental.

timmmay

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#3415264 15-Sep-2025 20:57

Building on what RM said, ChatGPT says if your water is 10C colder in winter it takes 2.3kWh extra energy to heat 200L per day. With gas not being perfectly efficient it could be up to double that. Would that account for the extra costs? If it was power 2.3kWh/day is about $15 per month at 22c/kwh. If your gas heater was 50% efficient it could be $30 per month. Add in slightly longer showers when it's cold, that could be it.

If you replace with electric it might be worth getting rid of all gas appliances. The price keeps going up and you still need to pay the daily charge. I'd probably hold off until summer to see if it improves, but it'd be worth making sure it's safe.

insane

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#3415279 16-Sep-2025 02:54

Well this is very interesting. Our usage pattern doesn't change month on month as we only use gas for hot water (Rinnai Infinity EF24) and cooktops/hobs - but what is interesting is how all over the place our meter readings are. There is no pattern rhyme or reason that I can tell. Some months it's as low as 240kwh, others it's 1100kwh, it's all over the place.

We don't have a digital meter - they showed up three times with incorrect fittings to replace it, after which I gave up chasing them.

Does your increase coinside with a meter replacement?

wellygary

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#3415293 16-Sep-2025 09:58

insane:

We don't have a digital meter - they showed up three times with incorrect fittings to replace it, after which I gave up chasing them.

Before our gas meter was changed to a digital one , ours was only being read bi monthly, some times only every 3 months,

For an accurate comparison you will actually need to compare the 2 months between meter reads, rather than the actual meter read month ( as the previous month was just an "estimate")

Aucklandjafa

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#3415324 16-Sep-2025 11:50

That, or just do your own readings, most energy companies give you the ability to punch them meter number in. Contact would even reissue/adjust the invoice once you gave them an accurate reading if it was an estimate month

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scuwp

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#3415379 16-Sep-2025 14:06

Sorry, but you will have to prise our gas instant hot water system out of my cold dead hands (as the saying goes). Nothing quite like it.

Yes it's got more expensive (what hasn't) and yes future supply is questionable depending who you listen too, but its a long way off being end of life IMHO.

Priced up a replacement heat pump unit at ~$9K, that would take 10+ years before we would break even! And by then it's probably not far off needing an upgrade.

Yes they use a bit more in winter, that's normal. More grunt needed to heat colder water, colder ambient operating temp, and probably longer and hotter showers. It all adds up.

I 've heard there's huge costs involved now to have gas decommissioned from a house? Like several thousand $$.

Lazy is such an ugly word, I prefer to call it selective participation

RunningMan

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#3415390 16-Sep-2025 14:59

Aucklandjafa:

[snip] Electric instant hot water uses a crazy amount of energy,

Huh? The energy to heat a given volume of water over a specific temperature is the same, regardless of the energy source.

wellygary

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#3415393 16-Sep-2025 15:08

RunningMan:

Aucklandjafa:

[snip] Electric instant hot water uses a crazy amount of energy,

Huh? The energy to heat a given volume of water over a specific temperature is the same, regardless of the energy source.

He's referencing it in comparison to normal house power usage, A instant gas hot water unit can consume up to 40-50KW max output,

Even at 1/4 of that, you are looking at 10-12KW output, a stand house pole fuse is ~15KW, (60amp@240

RunningMan

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#3415395 16-Sep-2025 15:22

So power, not energy.

And if numbers above are accurate, the electric one is using 1/4 - 1/3 of the power of the equivalent gas unit under high load, so not really crazy but quite a lot lower.

cddt

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#3415396 16-Sep-2025 15:26

scuwp:

I 've heard there's huge costs involved now to have gas decommissioned from a house? Like several thousand $$.

Interested to hear any details about this.

My referral links: BigPipe | Mercury

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Jase2985

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#3415397 16-Sep-2025 15:26

RunningMan:

Aucklandjafa:

[snip] Electric instant hot water uses a crazy amount of energy,

Huh? The energy to heat a given volume of water over a specific temperature is the same, regardless of the energy source.

You are correct, but you missed the instant bit, it has to deliver that same amount of energy over a very short time period so draws a huge amount of power. Hard to find something suitable for a single phase house, as they just don't/can't give enough temperature rise for the flow rate.

Standard single phase hose connection is 14kW, so 12kW doesnt give much room for anything else.

Jase2985

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#3415398 16-Sep-2025 15:28

cddt:

scuwp:

I 've heard there's huge costs involved now to have gas decommissioned from a house? Like several thousand $$.

Interested to hear any details about this.

https://www.geekzone.co.nz/forums.asp?forumid=141&topicid=296123&page_no=7#3414065

They brought a new heat pump and decommissioned the Gas and Gas hot water.

fastbike

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#3415418 16-Sep-2025 16:14

wellygary:

He's referencing it in comparison to normal house power usage, A instant gas hot water unit can consume up to 40-50KW max output,

Even at 1/4 of that, you are looking at 10-12KW output, a stand house pole fuse is ~15KW, (60amp@240

I'm struggling to find many over 7kW, my inlaws have one that is around half of that.

We installed solar HW when we built 20 years ago with a decent sized tank. I think the payback was about 7 years so hot water is now free in summer and cheap in winter.And that's at latitude 43.5S

If I had to replace it I'd probably just bung a few solar PVs on the roof and get a low voltage element fitted.

Otautahi Christchurch

fearandloathing

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#3415420 16-Sep-2025 16:16

cddt:

scuwp:

I 've heard there's huge costs involved now to have gas decommissioned from a house? Like several thousand $$.

Interested to hear any details about this.

in the media recently

https://www.rnz.co.nz/news/business/572067/customer-told-it-would-cost-7500-to-disconnect-gas

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