Energy Conservation...

[stulew]

What is the cheapest way to run a Hot Water Cylinder ?
I can't remember the answer to this question !!!
If a household has a domestic hot water cylinder that complies with BS1566 and a cylinder stat (Guess who has just done the Energy Efficiency Course) Less said about that waste of time the better..
Is it cheaper to have the hotwater on all day controlled by the stat and then timed to go off during the night, or is it cheaper to have it switching on and off during the day at various times, and does this theory also carry through for an unvented high efficiency cylinder as well.

 

[sine]

My opinion (not scientific at all) is the less the system starts up but the longer it runs the more efficient it will be. A boiler works more efficiently if it has a greater load on it.
In the winter there may be no difference as its probably on for heating anyway but in the summer I expect there will, although the difference may be so small as to not worry about.
Each time the boiler comes on it will have to heat up the water in the heat exchanger and the flow and return pipes before it heats the HW. Therefore once its running it makes sense if its got a large quantity of water to heat rather than a tiny bit at the bottom of the tank.

 

[croydoncorgi]

A boiler works more efficiently if it has a greater load on it

Not necessarily true and definitely NOT true if the boiler is a condenser.

If it is a condensing boiler used for heating water at (say) 55 degrees, the Flow temperature from the boiler would need to be close to 70 to get a reasonable cylinder recovery time. But then the Return temperature would tend to be above 56 (unless the cylinder was almost completely full of cold water) so no condensing. In this case, I (sort of) agree with you that it would theoretically use less energy to heat up a whole cylinderful of hot water in one go, rather than a series of shorter firings to heat the same total volume. This would be because the Return temperature would be held down below 56 until the average temperature of the cylinder got to (say) 40 degrees.

But given that the cylinder is well-insulated and the thermostat is nearer the bottom than the top, I doubt it will make that much difference.

The worst-case scenario is when the user finds the water runs cold (because the boiler was still waiting to start) and turns on the immersion heater as well!