Is it more economical to heat water whilst heating is on?

[Zippyzoppy]

We have a boiler like most, which heats our hot water tank as well as serving the radiators.

When it is set so that both the heating and hot water are on together, it seems that the flame is the same as whether we are only heating the water or heating the house.

So, my question really is, do you think we will get better fuel economy by only heating water when we want the heating to be on (which would be fine), or does it not work like this?

The flame will modulate up and down so it won't make much difference.

Although the amount of useful energy is the same, the amount of wasted energy is less during the heating season.

If the boiler is just heating HW, it may go and off frequently, for quite short periods. Each time it turns on, it has to warm the metalwork and water inside the boiler and pipes. Each time it goes off they lose heat to the air. When a boiler is running continuously until it has finished, it only needs one "on" and one "off" so there is less wastage.

This is why, if you have a cylinder, it is better to have a large, well insulated one and to heat it once or twice a day. If it runs for five minutes every time you turn on a tap, to top up the cylinder, this is less efficient. This is also why Combis cost more in gas than cylinders in summer. They are heating up and going cold every time they are used.