Hot water cylinder recovery

[CliveM]

What would be a reasonable time for a HW cylinder to recover (from cold water to 65deg)?

[Neil Jones]

depends on its size and what method is used to heat it.

[CliveM]

Assumeing say a gas boiler and pumped system.

Should not the boiler be rated in relation to the cylinder - i.e the bigger the cylinder the higher output boiler so not an issue.

[IMM]

To 60C with a part L cylinder with the correct sized boiler and pump flow,

30 minutes from cold. get a quick recovery cylinder, they drop down to 15 mins, well worth it.

[stefek.zaba]

for goodness' sake, it's sub-O-level/GCSE physics. It takes 4.2kJ to raise

1 kg which is 1 litre of water through 1 degree Celsius - this quantity is called the "specific heat" of a substance, and water's remarkable for having a quite high specific heat compared to metals, rocks, and so on. J (Joules) is a measure of energy; Watts is energy-per-unit-of-time, and 1W is 1J-per-second; so a 1kW source is punping out 1kJ per second. To relate to "relevant" figures for a CH boiler, a 20kW-output boiler is pushing out 20kJ/sec, which will raise 5l of water through 1 degree (in the ideal, unrealistic, but close-for-first-approx case where all the energy is indeed transferred to the water cylinder). So, if you've a 150l tank, it'll take 30s for all 150l to be raised through 1 degree - that's half a minute. To go from a cold-inlet temp of, say, 15 degrees (ass-U-ming summer) to the target 65 degrees will take 50 times as long, what with there being 50 degrees between 15 and 65; so that's 25 of your earth minutes.

Adjust as you like for your actual boiler output, tank size, and transfer efficiency; and note that you won't have to wait all that time for usefully hot water to appear after a full discharge of hot water, since hot water rises and the draw-off for your hot taps is cunningly arranged to be towards the top of the tank; so you can draw off a basin or kitchen-sinkful after say 10 minutes, as rather than being at a uniform temp of 35 degs which the above calcs suggest, you'll find water on the top of the tank at maybe

45 degs ("hand-hot") and stuff at around 25 at the bottom. (Details depend on where the coil is, "direct" versus "indirect" heating of the stored water, shape of tank, location of draw-off point, etc., all of which influence the convection currents by which heat is transferred from your boiler's "primary" circuit to the mass of stored water in the hot water tank).

HTH - Stefek

[IMM]

It is much easier to say 30 minutes with a Part L and approx 15 minutes with a quick recovery cylinder, as long as the boiler is man enough.

[IMM]

Boy is that complicated. The simple formula is;

No. of kW =

Quantity of water in litres X Temp rise degrees C X Specific heat of water (4.2) ________________________________________________________

Number of seconds in an hour

So for 150 litres with a 45C temp rise to get to 65C (which is too hot anyway)

150 X 45 X 4.2 _____________ 3600

= 7.88 kW/hr

So, it takes 7.88 kW one hour to raise 150 litres from 20C to 65C. Double that result to get 1/2 hour, which is, 15.75kW (53,738 BTU/hr) to raise 150 litres from 20C to 65C.

Very easy.

[Peter]

Hmm, these days that will get you an 'A' at advanced level!

[Neil Jones]

For future reference, 4.2kJ/g/K is an approximation to 2 significant figures. You cannot gain more accuracy as you try to here, going to 3 (7.88 kW), then 4 (15.75 kW) and finally 5 (53,738 BTU/hr) significant figures.

[Neil Jones]

Of course not - it's just that the syllabus is constantly being reduced...

[Ric]

Surely you are not implying that GCSEs/A-Levels are getting easier???

[Andy Hall]

It used to be first form physics. Nowadays this is practically degree material.......

.andy

To email, substitute .nospam with .gl

[Andy Hall]

It is, but it wouldn't be true, because it depends on the temperature of the cold water, among other things.

.andy

To email, substitute .nospam with .gl

[IMM]

No it isn't. Not at all.

[IMM]

You don't need finite accuracy to find the time a cylinder heats up. There are a number aspects that influence the heat up time, such as the ideal pump speed, the efficiency of the heat exchanger, etc. No one is fussed if it is

29.5 or 30 minutes.

[Ric]

I covered it in A-level physics about eight years ago.

[Ric]

Why are you making such a big deal of it then?

[IMM]

I am not, others are.

[Ed Sirett]

What would be reasonable depends on your expectations.

Typical size, modern kit, properly installed would be around 30 minutes. That would be from cold. In practice a bath full would take extract about half of the HW therefore the subsequent reheat would be around 15 minutes.

Fancier kit can do better but is not needed IMHO. Unless you spent most of you time away from home (=HW off) , and liked to have a bath asap you get home.

[mike ring]

Peter wrote in news: [e-mail]:

Gotta feeling it's pushing BSc

mike