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Hot water cylinder question! Answered

Ok, so you have a hot water cylinder full of hot water... no matter in which way it is heated, how does it stay hot all the way until all the hot water is used up when you having a shower or whatever?... which seems like a full tank, piping hot until the end.

If you are replacing the hot water with cold water as it is being used would this not just regulate the water temp and make it colder straight away?

If you have a hot cup of tea, just add a little cold water... why does this not happen with a hot water cylinder?



it a process called "thermal stratification" google will help you.


What type of hot water heater do you have?

There are tanks like Steve and the others describe and then there is an energy efficient tankless model that heats the cold water as you use it.

Without storing hot water and keeping it hot until used the energy savings is enormous as well as you never run out of hot water.

Im on instant gas hot water at the moment with no cylinder... it actually costs quite a bit to run... about $15-20 a week.
We are looking at building a system with a cylinder that has connections for wet back from the solid fuel range and solar collector on the roof.... looking at cylinders to suit my needs confused me to how they work.
Great to have this cleared up... i still am quite astonished that water stratifies!

If I lived where you do I would have made sure I got a place over a hot spring.

How much is gas per cubic meter where you live?

We heat the house the water the cloths dryer for that much in mid winter and we get as low as - 40 C and more.

Gas is measured in KG here... its around $4 a KG
We get ripped off for services in NZ... power is bloody expensive so try to do what ever we can to lower the cost.

There is only one major area in NZ where there is a lot of thermal activity... and i would not like to live there!
Dont judge my country by the "100% pure" slogans... its all bullshit... most of our rivers are polluted.

Wow I wonder what that is in Canadian?

We do natural gas by the cubic meter and propane by the kg and propane here is $1 Canadian a kg.

Geothermal heating is big in places around here, but at 300% efficient it is still not as cheep natural gas. Most people use it because the only piped in service they have is electricity.

You can say the same thing about Canada don't believe the slogans.

If you are looking into solar I would suggest you look into your weather history stats. Where I live a couple months a year we get as little as 75 hours of bright sunlight, that is to little sunlight to live on solar power or even as a supplement. We use solar as an emergency power source and most of the time it isn't that useful, some people pump it back into the grid when they don't need it for emergencies.

Well got to go.

Where are all good here for sun all year round.

"earning the nickname 'Sunny Nelson' with an annual average total of over 2400 hours of sunshine"


I have seen those and watched install problem resolution until the happy final result.

They are a tweaker but there isn't the long wait for hot water to go from one end of the house to the other nor the heat loss when they work right.

My friend had dirt in his propane line and a faulty controller.


4 years ago

The water tends to stratify, cold water enters from the bottom and the hot water is drawn from the top. The water in the tank is heated at the bottom and rises to be drawn off as cold water is added.

If the hot water usage is balanced with the heating of the colder water you will have a nearly constant supply of hot water

Hot water rises to the top of the tank. If you add cold water right at the bottom, and you can add it fairly gently, mixing doesn't occur.

Its largely due to the internal design of the cylinder, cold water is added at the bottom in a manner that prevents mixing with the hot layer which is extracted at the top of the cylinder.


Hot water cylinders are tall enough that "hot water rises" is a significant effect, and narrow enough to make it hard for a convection current to start up.