A few ideas here..while you wait or the plumbers.
In my last house, I had to run a garden hose up to the loft & poke it in the hot out pipe in the cold water tank, always worked..what a palava.
Don't think it is an airlock. Sounds as though you are getting the higher pressure cold through the hot tap until you turn the cold off and the hot can flow. Don't know how you've managed that but, assuming it was ok until you changed the washer, you should check the tap was put together properly once you were done.
Can you turn off the hot supply and leave the cold on. If it still runs cold through the hot tap then you've definitely done something while putting it back together. Was it a simple tap washer you changed, or some kind of cartridge?
It was just an ordinary tap washer, but while I was changing it cold water started to flow through. I then turned the mains off and re-fitted the tap. When all the water was turned on the cold water came out of the hot tap. When the main was turned off, hot water flowed out. When mains was turned back on, cold came out.
Before changing the washer, the tap did not work. When trying to fix, I noticed that the washer was dislodged from the tap valve, so blocked off the entry hole completly.
Would it make any difference to the hot water tank being on the same level as the bath and the mains pressure has changed since the tap stoped working (to a greater amount)?
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