First, turn the heater valve to its closed position.
If you then disconnect the two heater hoses at the ends furthest from the
heater, and connect the one that doesn't go to the heater valve to a garden
hose, trying to get the fit fairly watertight. When you turn the hose on,
you will find that all sorts of discoloured water and crud comes out the
other hose. When the exit water is running clear, get someone else to turn
the hose off (but leave it connected to the tap) while you put your thumb
gently over the other end - you are not trying to pressurise the heater,
just stopping air entering the pipe when the flow stops.
Connect this free end to the heater valve as quickly as you can,
(remembering to put the jubilee clip on) so that the minimum amount of water
is lost. You are only trying to join the hose to the heater valve at this
stage, so don't waste time doing the clip up. Then disconnect the hose,
putting your thumb over the other pipe to minimise water loss and put this
pipe back where it came from. Then do up the hose clips on both pipes.
Start the engine and let it warm up until the thermostat opens and allows
any air in the cooling system to go into the radiator. Then with the engine
still running, open the heater valve. Once the cold water in the heater has
been replaced by hot, you should get hot air.
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