radiator stays on (even when central heating off)

[Jennie]

Hi all,
I'm in the process of flushing out my parents central heating system (using F3 cleaner for a few days, then flush clean).
A day into the clean, we noticed the bathroom and landing radiators were coming on intermittently.
The system is an old semi-pumped gravity system, with floor-standing boiler, and one zone valve by the cylinder.
This has happened once before (though not during a clean, etc). About a year ago, and my dad 'fiddled' with the zone valve, which seemed to correct the issue. The zone valve appears to be working OK - the lever is moving across/back OK. Though I haven't ruled it out as being behind the issue.
The boiler seems fine. I serviced it (I'm Gas Safe) a few weeks ago. The boiler isn't running continuously (just when there's a demand for heat or hot water). No particular noise in the system (ie, air). The boiler shuts on/off if you turn the thermostat knob up/down, so I presume the boiler stat is fine. The flow pipe from the boiler seems very hot.
Obviously, there's a surplus of heat, which is making it's way to the 'heat sink' radiators. But why?
I'll be draining the system down and flushing through in a couple of days, which may sort it all out, or not.
Any ideas most welcome,
Thanks everyone,
Jennie

 

[ShaunCorbs]

Depending on age of the zone valves I would replace them as the heating side could be passing

Also they used to put one rad before any zone valves as a leak (something for the boiler to work on)

Have the rads job st started doing this or they always done it ?

 

[Jennie]

The rads have just started doing it.

 

[ShaunCorbs]

i would rule out the zone valve passing first esp if hes had problems with it before

 

[JCplumb]

Some zone valves have a couple of springs that pull the valve closed when not powered. The little plastic clips that the springs are on get brittle with heat and can snap, so at a glance it looks like it's closing when it isn't. If there's a cylinder and the valve isn't closing then the stored hot water will keep local rads hot for hours.
Had this problem in my house and without taking it apart, the zone valve looked to be doing its job.

 

[doitmyself]

The system is an old semi-pumped gravity system, with floor-standing boiler, and one zone valve by the cylinder.

If there is only one zone valve, normally on the return from cylinder to boiler, you have a C Plan system. The zone valve is controlled by a thermostat on the side of the cylinder and allows hot water to travel through the cylinder coil when the DHW is below the thermostat temperature. It is not a normal zone valve as there is a 2-way microswitch inside and there is a 6-core cable. Here are pics showing the piping and wiring:

Check that the microswitch is working correctly - terminals not fused together - and valve is not passing water when it should be off.

 

[SavageW]

If it's gravity hot water and pumped heating I'd look at non return valve in CH return,when you say zone valve by the cylinder is this a 2port purely supplying heat to the coil of the cylinder or is it a 3port valve (flow from boiler,pipe to coil & pipe to central heating) if that's the case as Shaun said I'd look at valve passing.

*edit* how many pipes are coming off the boiler?

 

[Vee]

Sounds like non return valve passing by.

 

[Last Plumber]

In addition to the above and just in case it helps in future. The Boiler stat is designed to operate (open) at 82 degrees C (plus or minus 3 degrees). if you put a thermometer on the flow as close as possible to the Heat ex, and set the Boiler stat on full, you will see if the calibration is out. I would test it with the pump running.