Radiators Running Lukewarm when Heating Off

[nb99]

Hi

I'm fairly handy but don't know boilers so well so please excuse me for any schoolboy errors here. We have a De Dietrich (model F67110**correction** - apparently this just means a French-made model. There are 2 "type" numbers on it: GT/GTM/GTU 1104V and MT 130) oil-fired boiler which provides hot water and central heating. It's recently started behaving a little "strangely". When we turn off the central heating using the thermostat (which I've recently replaced in case the old one was causing the problem) the radiators continue to remain lukewarm thereafter, for days on end. I don't know exactly how the central heating/hot water are linked/separated inside the boiler, but it feels to me like the hot water is heating the central heating circuit which is then being pumped around the radiators even though the heating is off. This would be great except presumably the boiler is having to fire up to re-heat the hot water so it is obviously costing extra money. I thought it might be a valve that's stuck somewhere but following the wires, I can't find any electric valves in the system. Does anyone have any ideas of what it might be or anything I can test/try?

I know all of you offer your opinions for free and on your own time so I really appreciate any thoughts/ideas you may have.

Thanks

NB

 

[ericmark]

The C Plan central heating was to start with rather basic with no motorised valve or domestic water thermostat, it did domestic water using thermo syphon and central heating with a pump, the programmer has only 10 options latter ones had 16, and often if the heating has been on it will thermo syphon even after turning the pump off.

 

[nb99]

Hi Eric - thanks for the reply. The problem is we've used this boiler for 10 years and it only started this a couple of weeks ago, so something has changed. My question really is what could have changed/be malfunctioning to cause this effect? For what it's worth there's also a pump on the hot water circuit on our boiler.