Water Wont heat up - Water Water Heaters

it could be either but no one that can not put eyes and hands on can really tell

I'll need to know what make--model--- age of this unit is and if it's electronically controlled to help you----your question is too vague do to the enormous variety types of heating systems----I'm going under the info provided that this is a boiler with domestic hot water coil----you say central heating so this throws me off---do you heat with hot water--hydronic-----or forced hot air----if hydronic---it sounds like your circulator in not functioning----a bunch of reasons circulator has failed---aquastat's "circulator control" has failed---thermostat has failed----broken wires----plugged lines----it goes on and on----this is the best I can do with this info sorry Happy Rabbit

check to see if the suction passage on the machine is plugged.

If not tank filling ur heater wont start mostly ur soleniod valve for incomimg water spoiled. Also ur heater might be open or heater'$ thermal fuses open, if have.

to what

most likely the diverter valve.. when hot tap turned on the movement of the water through the valve operates a diaphragm which pushes a pin/button//switch to tell the boiler to fire up

Have you tried turning one of the thermostats off.? They are both talking to the same unit and they may be arguing with each other.

has the unit every been cleand you can check the it in the service door