Heating water question

9 years 4 months ago #38618 by Hondo
Heating water question was created by Hondo
We are off the grid, and planning to heat our hot water by wetback coal range (burning wood). We plan to attach an electric hot water cylinder, but not use the electrics. Our question: if we buy a second hand large hot water cylinder, does it have to be full to get hot water out of it? If we buy a 180 litre cylinder, can we put less water in it? Or will the hot water not be able to get out because the hot water outlet pipe is at the top? We don't want to heat 180 litres of water, but small cylinders are few and far between on Trade Me. Are we going about this the right way? We are on solar power, but don't have enough solar to heat water. We have already obtained a wetback coal range, and are trying to buy a hot water cylinder now. Any advise would be appreciated.

Two humans, one dog, two cats, one milk cow and her calf, four lambs, numerous chickens and roosters.

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9 years 4 months ago #497275 by spark
Replied by spark on topic Heating water question
Hi Hondo,

Most hot water cylinders work by drawing hot water from the top of the cylinder and replacing it by introducing an equivalent volume of cold water into the bottom of the cylinder. There is a very good reason for this - hot water is less dense than cold water, so hot water "floats" on top of cold water. If the cylinder is well insulated and the water is not stirred or otherwise disturbed, then the conduction of heat from the hot water in the top of the cylinder to the cold water in the bottom of the cylinder is very very slow (still water is actually a surprisingly good thermal insulator).

Provided that your cylinder is well insulated, being a bit larger than it needs to be simply means that it will take a bit longer to heat from cold the first time you heat it but that it will "hold the heat" better than a smaller cylinder (more mass of water per unit surface area). Provided that each day you either have good sun, or you light the fire, you should have plenty of hot water.

The building code says that hot water systems heated by uncontrolled heat sources (like wetbacks) must be "open vented" to atmosphere to reduce the risk of boiler explosion (this is what the vent pipe sticking out the roof of the house is for)


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9 years 4 months ago #497312 by kernels
Replied by kernels on topic Heating water question
All spark's information is on the money, also, I believe you need to use a 'low pressure' copper cylinder for wetbacks because the high temperature can crack the enamel on steel cylinders.

Some of our neighbours have just put a wetback + solar hot water system into their brand new house, they are using a mains pressure cylinder, but bought a stainless steel one, so probably cost a bomb. They have been living there for a month and have not yet turned on the power to the hot water cylinder.

I got a little engineering-gasm when they showed me their hot water setup.

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9 years 3 months ago #497581 by Mainlander
Replied by Mainlander on topic Heating water question

In our offgrid system, the hot water cylinder is 300L - heated by 3 means! The solar hot water system (40 tubes), the wetback, and the electric element (this is permanently turned off at the wall, but wired in)....

The tank also has sensors on it - right now (11pm) the top of the tank is at 66 degrees, and the bottom at 27 degrees (and the water on the roof still at 21 degrees).

The tank is mains (well full pump) pressure, and has a relief valve setup (with outside venting, to ground level rather than through the roof).

I would assume that a second hand tank with a wetback coil in it would be suitable for the purpose! And have all the right inlets, outlets and pressure relief setups.


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9 years 3 months ago #497654 by lars arsbjorn
ok we are having heating water by usage wood for burning when requiring hot waters

ha det bra, adjø

( i am from the Scandinavian my translation poorest in english)

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