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Brian, Plumber
Category: UK Plumbing
Satisfied Customers: 845
Experience:  16 yrs. experience as plumber, including full bathroom and kitchen installations.
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I have an underfloor heating system. when the heating demand

Customer Question

i have an underfloor heating system. when the heating demand comes on from the thermostat the pumps work fine but the boiler doesn't start up. I know the boiler is working as the hot water works fine.
the HW/CH control panel is all set correctly and seems to be ok any thoughts ?
its a Viesmann boiler.
Submitted: 2 years ago.
Category: UK Plumbing
Expert:  Brian replied 2 years ago.
Hi, my name is***** have been in the plumbing & heating trade for approximately 15 years. I will be helping you today. The zone valve that switches the boiler is probably faulty. what you could be hearing is the underfloor heating pump switching on.
Not the actual heating system pump. find where the zone valves are and check that they slide across when you switch the heating on.
Customer: replied 2 years ago.
Hi Brian,well I found the zone valve switch under the boiler in the garage.Its a siemens unit with a small lever under it and when I push it across the boiler fires up and i can feel the hot water going to the underfloor manifold which is still in 'demand'. However when i take my finger off the lever it immediately motors back to off and the boiler shuts down immediately although there is still water flowing through it. Sp obviously Im not an expert but is this flow valve demanding 'off' all the time even though there is a demand?how does it know to open or close? is it demand based on flow or electrical or both.Thanks for the help so far.Brian T
Customer: replied 2 years ago.
sorry - just to be clear :I turn the thermostats up full. I get a demand and the circulation pump comes on but circulates cold water. I go into the garage and the small lever under the flow valve is to one side. when i slide it across the boiler comes on. when I take my finger off it motors back to the other side and it shuts the boiler down. meanwhile the underfloor pump is still pumping in demand mode.hope that makes sense.
Expert:  Brian replied 2 years ago.
Hi yes it looks like the zone valve or the timer is faulty thezone valve wire should come from a wiring centre. Open that up and check the voltage on the brown wire to the Zone valve, if its 240v when you set the timer for your boiler for heating on, then your zone valve is faulty. if its zero then its your timer /controller.
The underfloor heating system is separate to the boiler system. so depending how its wired it will come on even if the boiler is switched off.