Controlling a Radiator Flow Booster is achieved by turning the power supply on and off. This can be done by either manually or automatically (see below):-
At ShowerPowerBooster we are expert at making pumps turn off and on in response to flow and It would be possible to make a radiator flow booster which detects flow in order to turn on. The reason your radiator is cold is because there is no flow so the radiator booster would not turn on. The pump causes flow so once it did turn on it would never turn off.
There are however several options which allows the radiator to get hot when you want it to get hot.
Methods which require control of the 240 volt AC supply to the transformer:-
1.0 Wire the transformer in to the power supply used by your primary circulation pump so that both pumps on at the same time.
2.0 Use a radio remote to turn it on an off (we can sell you a radio remote for £10)
3.0 Use a manual timer (we can sell you a 24 hour timer for £5)
4.0 Use a wi fi heating control to automate a plug socket.
Methods which require hard wiring into the 12 volt dc output from the transformer:-
5.0 Use a room thermostat (available from a local plumbers merchant (£10 to £15))
6.0 Use an emersion heater thermostat (available from a local plumbers merchant (£5 to £10))
7.0 Use a hot water tank thermostat (available from a local plumbers merchant (£10 to £20))
My Personal Recommendation
Use a manual timer set to the same on off times as your central heating (we can sell you a 24 hour timer for £5) plus a room thermostat fitted directly behind the radiator (available from a local plumbers merchant (£10 to £15). This will ensure that the radiator gets hot when only if the radiator is cold and only when the central heating is running. Adjusting the thermostat to higher temperature than the room is needed as it will be hotter behind a radiator than in a room and you can fine tune this to suite. Just turning the transformer on and off is simpler and of benefit if the room is only used occasionally.