On Tue, 19 Jun 2007 07:27:17 +0100, "Nigel"
wrote:

I've just installed a 65lt hot water tank, it has 2 heating coils and an
immersion. I haven't got around to connecting the 2 heating coils so for now
it runs on just a 1 kw immersion heater... I connected up my 1.2kw
generator ( just a cheapy from B&Q, not a marine genset) and it took 2 hours
to get warm and about 3 hours to hot.... For no very good reason I had
thought it would heat in about an hour. I have tried looking up calorific
values of water etc, but just got myself confused... anybody here shed
some light for me..

If we assume that the immersion heater is fitted to the top of the
tank, we "might" be heating 65/2 - 32 litres of water. Assuming
initial water temperature is 15degC and final temperature will be
70degC we are increasing by 55degC.
The heat capacity of water is 1.16 Watt hours per deg C.
Therefore for the data above the total Wh is 1.16 x 32 x 55 = 2041.6

In the case as above 2 (assuming constant running) the thermostat will
tend to cycle towards the top of the temperature range, therefore
increasing the time. Heat losses from the hot area to the unheated
cold area of the tank will also increase the time.

Generally it sounds about right.

--
Richard

Nb "Pound Eater" Parkend G+S
"Governments are like Nappies, they should be changed often."
(For the same reason)