Problem with condensing boiler. When hot water is running CH fires up (and no hot water).

30 Nov 2010
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United Kingdom
Hi there,

I have a condensing boiler (Worcester Bosch Greenstar) and am having a few problems with it.

Pressure was low a couple of weeks back and filled up via filling loop. Possibly slowly losing pressure again as black needle has dropped a little since then.

A few days ago I had the situation where the heating was on irrespective of the thermostat settings (thermostat timer was set to off for both CH and hot water but the boiler was firing CH). I had to turn down the CH setting on the actual boiler unit itself to stop the unit running

The main problem at present is with the hot water. When I run the hot I get no hot water in the tank. and the radiators become very hot. This happens even if CH is off, and the system sounds like it's in overdrive.

I’m getting the boiler it serviced this week, but wonder whether you guys have any info in the meantime as to what the problem(s) may be.


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Sounds to me like a faulty motorised valve. Power to operate the boiler in some systems is supplied via the motorised valve(s), (there's one or 2 depending on type of system), if the valve is sticking open or the microswitch within the valve is stuck, then the boiler can keep running irrespective of external controls. I'd suspect you have a Y plan, valve is stuck open to heating, so boiler keeps running and whacking heat around the rads, whilst the cylinder is starved of flow so will never get to temp.
Thanks Mate.

Yeah sounds about right. I took a snap of the system (attached below) and there are 2 motorised Draytone type valves beneath the boiler (I guess these are 2 port valves as they only have two pipes attached). The right hand valve appears to run to the megaflow tank so I guess this could be problematic. I flicked the reset lever which appears to have done the trick. Cheers.
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yes those are the motorised valves you have what is called an S plan system, the little black levers on the top of the valves are for manually opening the valves, turn both the heating and the HW to OFF and try and slide the levers across, they should both move but have resistance, if one slides but has very little to no resistance that is the faulty valve
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The odd part is that you say that you have a fault with both the central heating (being on when it shouldn't be) and the hot water (no hot water) when they have separate motorised valves controlling them. That would mean that both motorised valves developed faults simultaneously, not impossible but....

Has anything happened that you haven't mentioned. For example new controls fitted, or parts changed?

The boiler thermostat will control the temperature of the water leaving the boiler, and going to both the hot water cylinder and also the heating (ie not just to the heating). So if by "no hot water" you just mean that the hot water isn't getting quite as hot as it used to (rather than actually being cold) then that will probably be why, and the fault is just with the central heating valve.

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