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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?

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    I assume it's an instant-type boiler (not sure how they are called in english). It should switch off due to lack of water flow or overheating (due to not enough water flow). Can you set the boiler water temperature to a lower value? – Dan Feb 16 '15 at 19:34
  • Agreed @Dan, On-Demand water heaters will shut down with lack of sufficient flow; opening the cold side may lower the total system pressure and the HW will drop out. OP has an actual boiler though. – Mazura Mar 26 '15 at 21:17
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We spoke to a plumber. Speaking off-record, he thought the problem might not be related to the new tap but may be incidental.

We now think its because of the pressure in our boiler. About a week after I posted this question, the pressure reached zero. We didn't realise that we had low pressure until it briefly stopped working! After re-pressurizing it, the shower temperature seems to behave reasonably. We still have an over-sensitive tap, but at least the shower temperature no longer changes by itself!

If you have this problem as well, check that the pressure in your boiler is within the expected range. For us, that's between 1 and 1.5 millibars.

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