A little while ago our totally fine bath/shower tap unit broke in our bath. Our landlord's plumber installed a new one and since then we've had big problems. Steps to reproduce:
We get in the shower, and turn the hot tap to the 'on' position. After a minute, the water is scalding (as expected). We turn the cold tap as little as a quarter turn. The boiler switches off. Shower is now freezing. We turn the cold tap back 1/8th of a turn. The boiler is now on and the water is boiling. Turning the cold tap even slightly makes the boiler turn off again. Doing nothing causes the the boiler to turn off again after a minute. The water is now freezing. Doing nothing causes the boiler to turn on again after a minute. The water is now too hot to be used safely.
This has happened only since the new tap was installed. It is new, but its also probably the cheapest in the shop. The plumber commented that our pipes are very old and not of a standard size so I know there is a converter to convert from the old size of our houses' plumbing to the new size used by the modern tap. Is it possible that turning on the cold tap causes a pressure change which the boiler just somehow can't deal with? What could be causing this problem? And what steps could we take to make our landlord fix it?