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## elpenguino

669 posts

Ultimate Geek

Huchiz: The model is a Rheem 135L low pressure one, manufacture date was 3/2001.

So the 24 hours temp loss is about 8 deg, for 135L of water it is about 1.26KWH. (4.2*8*135*1000*1/3600*1/1000) Considering during normal use the temp will be higher in average, so let's say the normal daily heat loss is 1.5KWH max.

It's not that simple. The heat loss is related to the differential between the hot object and the ambient environment. At the start of your test it was 60-18(right?) but at the end of the test it was 52-18.

What you have to include is an 'e' to reflect that the transfer of heat is a first order logarithmic system.
Perhaps someone who paid more attention in maths class will come along now :-)

Another way as pointed out is to measure the input electrical power over the time period. You could get away with measuring the current and assume the voltage is constant which is almost true.

## Huchiz

214 posts

Master Geek

elpenguino:
Huchiz: The model is a Rheem 135L low pressure one, manufacture date was 3/2001.

So the 24 hours temp loss is about 8 deg, for 135L of water it is about 1.26KWH. (4.2*8*135*1000*1/3600*1/1000) Considering during normal use the temp will be higher in average, so let's say the normal daily heat loss is 1.5KWH max.

It's not that simple. The heat loss is related to the differential between the hot object and the ambient environment. At the start of your test it was 60-18(right?) but at the end of the test it was 52-18.

What you have to include is an 'e' to reflect that the transfer of heat is a first order logarithmic system.
Perhaps someone who paid more attention in maths class will come along now :-)

Another way as pointed out is to measure the input electrical power over the time period. You could get away with measuring the current and assume the voltage is constant which is almost true.

We are talking about the already measured heat loss, not intending to predict that. You don't need calculus to know the joule loss for a given mass and two certain temperature.

Yes the temperature can't be 100% certain for sure, but your 'e' method won't improve that. What we need is a physically comprehensive controlled measure environment, it has to be 'perfect' and thus ridiculous for home use.

The KWH measurement method is really a good way, but probably need more than 24 hours depends on the heating interval which might be quite long. We don't need to worry about the voltage fluctuation, even cheap kill-a-watt takes care of that.

## mclean

350 posts

Ultimate Geek

Subscriber

The latest MEPS for hot water cylinders (and the building code) requires a 135 litre cylinder to have a standing loss of 1.4 kWh/day or less. Your test, albeit a bit crude, indicates a standing loss of 1.3 kWh/day.

You'd be doing well if you saved a third of that by doubling the insulation thickness.  Depending on your hot water tariff that might save \$25-\$35 per year.

On the other hand, if the HWC heats your airing cupboard then your wife won't thank you for it.  Also if you live where you house has to be heated then standing losses from appliances aren't necessarily losses at all.

McLean

## Huchiz

214 posts

Master Geek

mclean: The latest MEPS for hot water cylinders (and the building code) requires a 135 litre cylinder to have a standing loss of 1.4 kWh/day or less. Your test, albeit a bit crude, indicates a standing loss of 1.3 kWh/day.

You'd be doing well if you saved a third of that by doubling the insulation thickness.  Depending on your hot water tariff that might save \$25-\$35 per year.

On the other hand, if the HWC heats your airing cupboard then your wife won't thank you for it.  Also if you live where you house has to be heated then standing losses from appliances aren't necessarily losses at all.

Thanks. By searching MEPS I found this file http://www.otago.ac.nz/csafe/research/otago055640.pdf
A
s my result was based on a temperature difference quite lower than 55 deg, I think it is well worth to give it a wrap.

Yes it might not be a waste in winter. :D

## Hammerer

1674 posts

Uber Geek

I read a US research paper years ago that had economic insulation levels at least as high as 15 inches/38cm. I looked for it again but couldn't find it.

Interesting that "The  New Zealand MEPS levels apply equally to unvented and vented water heaters" (Australian/New Zealand Standard ™ Electric water heaters Part 2: Minimum Energy Performance Standard (MEPS) requirements and energy labelling) but that doesn't include the vented losses which will also add heat to a hotware cupboard.

## timmmay

14138 posts

Uber Geek

Trusted
Subscriber

Hammerer: I read a US research paper years ago that had economic insulation levels at least as high as 15 inches/38cm. I looked for it again but couldn't find it.

I went from four inches to eight inches of insulation in my ceiling (twelve in some places as I had leftover) and it did make a noticeable difference.

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## Aredwood

3098 posts

Uber Geek

Subscriber

There is a big variance in gas cylinder efficiency. Those links have only considered the inefficient kind. You can get outdoor condensing gas hot water cylinders. (Rheem Stellar, and the Vulcan Freeloader. Although they no longer make the Vulcan) When the burner is on, they produce clouds of "steam" from the flue terminal. Due to them capturing so much heat that the water vapour created by burning the gas is condensing. Their standing heat loss is also far lower.

And even an inefficient gas cylinder running on Natural gas will probably still be cheaper to run than an electric cylinder, Due to how cheap Natural gas is. (Approx 6c per kW/hr instead of approx 20c per kW/hr for electricity)

Myself, I cant recall having ever installed an indoor gas Hot water cylinder as a new installation. (Im a Plumber Gasfitter) Have installed a few as replacements. And installed lots of continuous flow gas heaters. Both new installs and alot to replace gas cylinders as often it is cheaper to install a continuous flow heater than another gas cylinder. Assuming that the gas pipes can be upgraded in size to support the required flow.

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