Only get hot water when heating is on.

15 Jun 2009
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United Kingdom

I only get hot water when heating is on.

I have a boiler and a megaflow system with a honeywell 7 day timer.

If the system is set just to hot water. I get no hot water. If I have the central heating on and the hot water set at the same time , I then get hot water.

The timer fires up the boiler when set to continuous. However when I set just the hot water to continous the boiler does not fire up. I have tried replacing the timer thinking that it may be a problem with the timer not telling the boiler to fire up. But I get the same problem

Help ! any ideas would be very appreciated !


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Does your system have 2 2-port motorised valves (usually silver cased)?? If not, and only had 1 3 port motorised valve (illegal with an megaflow to have just 1 of them) then the stat could have failed. If you do have 2 2 port valves, have you manually push the leer across on the hot water one, thus allowing h/w when the heating is on?
It seems that either the hot water motor valve or thermostat has failed.

Its difficult to be precise with seeing it as there are several different ways these can be wired up.

Any fault finding would need electrical measurements.

Any work on safety devices needs a qualified person.

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If the HW zone valve opens on demand, but the boiler does not fire it will be the microswitch in the valve head broken. New head.

If the valve does not open, then although it could be the cyl stat, it is most likely to be the motor in the valve head. These can be replaced on their own, but I always fit a complete valve head.

If the manual lever won't move, then the valve body has jammed and you will need a complete vew valve.

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