Hot water cylinder getting too hot

[Doc955]

Hello,

We're having a new problem with our DHW getting too hot.
It's a pumped system with 2 zone valves controlling CH and HW seperately. Potterton Promax 24 boiler.
With the stat on the cylinder set to 55 degrees, it's possible to turn it to about 75 degrees where you can hear it click and the zone valve opens. Turning it back down causes it to click again and the valve closes. This suggests to me that the stat is working but water is still circulating through the cylinder and 'overheating' the DHW. Does this sound like a leaky zone valve? If not, what else could it be?
CH is working as per normal as far as I can tell.

Thanks.

 

[dal5band]

Just check that the system is wired correctly : when both CH and HW thermostats are satisfied, the pump and boiler should go off - i.e the pump should not be pumping when both valves are closed - (there may may be a bypass and the pump may run on for a short while - "overun")

The most likely cause then is that the HW valve is not closing properly - either due to debris or corrosion etc. In this case I would sugget you replace the whole valve (not just the actuator head) - this will mena a drain down etc. As a temporary measure, turn the boiler thermostat down a bit.

 

[leakydave]

Important you get it checked for 2 reasons

1) you will be wasting fuel (and £££)

2) Potentailly dangerous, especially to the young or elderly... hot water can kill...

 

[Doc955]

Thanks guys,

The system does shut off when both stats are 'satisfied' so I'm fairly happy that that part of the control system is working OK.

The system was fitted by Grittish Bas nearly 5 years ago and as such is still under their service contract so they'll be getting a call (again - this will be the 6th time they've had to come out and fix it since we moved in (December), all different faults!)[/i]

 

[GasMan1965]

If the cylinder is quite old then the build up of scale can false read the stat ,, as the surface temp is lower than internal temp ,, another possibility !!

If you take the stat off the tank then you can check the surface temp with a thermometer ..