Is a flow switch really necessary?

[siricosm]

If that was the case then only a combi boiler would be acceptable for hot water Any system which used stored hot water would fail the test. Recharging the cylinder is a legitimate reason for having the boiler on. The regulations also specify how well the cylinder needs to be insulated, so reducing the heat loss from the cylinder.

Exactly, so a cylinder stat is a legitimate boiler interlock as well.

In the system I am talking about, recharging the cylinder is the only reason to have the boiler on.

 

[doitmyself]

Exactly, so a cylinder stat is a legitimate boiler interlock as well.

In the system I am talking about, recharging the cylinder is the only reason to have the boiler on.

In your system the boiler is turned off when the cylinder is up to temperature. But the regs say the boiler has to turn off when there is no demand for space heating or hot water, which is not the same thing. The boiler could continue heating the cylinder for some time after the ch/hw demand had ended.

 

[siricosm]

The boiler could continue heating the cylinder for some time after the ch/hw demand had ended.

This happens in all normal unvented cylinder installations.