Water Heater Problem

I have a Main Medway multipoint water heater. When I open a hot tap fully nothing happens but if I decrease the water flow through the tap the heater will work. Any ideas as to what is wrong.
DAB
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I am not familar with that brand
Looks like a gas fired small mass high velocity on demand boiler/water heater.
It probably uses a flow sensor to know when to turn on; this might be bad. Is it a new problem? Can you turn it up to full flow once it is going, or does that make it stop?

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Yes, it is 24kW output (80K BTU). It should give around 11 litres per minute at shower temperatures (i.e. 40C) and around 7 litres per minute at washing up temperatures (55C), assuming incoming mains of 10C. Ask for more than that and it will start getting colder.
Christian.
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At full pelt, is it cold or tepid? If the latter, your heater probably isn't capable of heating that much water. If so, find the isolating valve for the tap and turn it down until the water runs nice and hot to prevent you asking for more water than can be provided. If is runs totally cold, then you probably have a dodgy flow switch.
Christian.
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This is what I was thinking. I have installed a flow regulator on hot outlet to prevent heater from "feeling the pain" of an overload. The user would just get luke warm water. Your heater might cut out when it senses a load it can't possibly keep up with. I would check for this and the possibility of a bad flow sensor.
Just a side note, on gas stuff I always keep in mind how sensitive they are to having a good ground.

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