At some point, MMC must have removed the on / off water valve. My heater matrix is permanently fed with hot water and there is simply a flap to stop the hot air coming into the car.
I'm going to add a valve so I can turn it off in hot weather as a bit of the hot air always finds it's way past the flap even in the off position.
I found that even with the heter valve off, hot water still found its way into the heater matrix as bizarrely the valve is on the outflow side of the heater. So I put in an extra valve on the inflow side as well to completely isolate the heater matrix in hot weather.
In really hot weather I do as you indicate in your later post, I put the heater matrix back in circuit to get some extra cooling.
