Heater plumbing

Does water flow through the heater all the time or is there an on/off valve?
I ask because I don't have a heater, the outlet from the back of the manifold is connected to the metal return pipe with a U shaped hose so I think the heater return is feeding water at engine temperature back into the front of the block via the pump all the time so am wondering if this is why the electric fans are on more than I would expect?
(I'm not overheating but the 2 speed electric fans do seem to be running nearly all the time on slow speed)

There isn't a tap, coolant flows through the matrix all the time and temperature control is done by the flaps.

By bypassing the heater you're reducing the capacity of the cooling system, and losing the extra cooling you would get from the heater matrix.

Many people think that the standard p6 heaters not that great.
Many are now 40+ years old with furred up matrixies and internal flaps missing it's foam sealing.
I have just overhauled my mates 2000 heater unit with a recon matrix and resealed flaps and correct flap adjustments.
Now works correctly with screen and floor settings working as it should.
One other item to check is the interior bulkhead areas behind glove boxes.
There is some foam stuck to a wide seam when new and when this falls off causes a draft and looses power / efficiency to windscreen.
Only one issue now, it's too hot and burns your feet
Clive.

Cheers guys, food for thought there...