hot water problem

16 Aug 2006
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United Kingdom
i have noticed a problem with my system since using it just for hot water rather than for both hot water and central heating.

there is a gloworm 18hxi condenser boiler in a Y-plan style setup. when the boiler is set for both central heating and hot water, everything is fine.

However, as soon as there is demand only for hot water (either by turning the room thermostat right down low or by flicking the switch on the boiler control to hot water only) the boiler burns very hard for a few seconds, the temperature gauge on the boiler shoots up and then the boiler turns off. the pump continues to run and the boiler temp slowly drifts down to about 30-40 degrees, then another brief hard burn of the boiler. this is repeated continually. because the pipe going into the cylinder to heat the hot water is therefore intermittently going very hot, then lukewarm (as the boiler burns and then cuts out), it's taking an absolute age to generate any hot water at all.

there doesn't appear to be a problem with the mid-position valve (which is a honeywell v4073a) as i can see it moving correctly when i turn the room thermostat either right up or right down.

the pump also seems to work fine (grundfos, not sure of model).

we recently had a radiator moved but everything seemed fine after that until about 2 weeks ago.

as i say, this problem only occurs when there is no demand for central heating but there is demand for hot water. if both are required, everything is fine.

many thanks in advance for any help!
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The flow of hot water from boiler to cylinder coil (and return) is restricted by something. Could it be an air lock (is there an air vent on the pipe near the cylinder?), or a partially shut valve, or lack of water in the system (feed tank)?
thanks for your help.

i can't see an air vent near the cylinder. how can i tell if there is an air lock? and if there is, how can i clear it? presumably it is not in the central heating system as this works fine (i have bled the radiators in any case).

i'm pretty sure the valve is OK from eyeballing it.
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Agree with chrisH - look for something to undo, on the upper pipe which goes into the side of the cylinder from the boiler.
there's nothing on the pipe going into the side of the cylinder other than an isolation valve which is open.

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