Unvented cylinder and boiler cycling

[Paul 1978]

I recently had an unvented cylinder installed and it is causing the boiler to cycle every 3 mins due to the return from the coil leaving the cylinder being so hot. This in turn means I get no heating as the boiler doesn't stay on long enough. I am sure the cylinder stat is function correctly. Any advice on a remedy to this would be good as the guy that installed it is coming back so having a little knowledge would be a good thing.

 

[SavageW]

The guy that installed it is coming back which is good! I would say at guess making sure there’s a balancing valve on HW return.How do you know the return from the coil is too hot out of interest? Other things could make your boiler short cycle.Im a big believer in if someone’s provided you in a service an your not happy call them back might not be something they have done

 

[Paul 1978]

Evening. It would be much better for yourself to get the installer back if you can? Is it under warranty still?

Yes, I have got him coming back just wanted a rough idea to cause/cure

 

[SavageW]

Very hard to say mate,what boiler,system set up do you have?

 

[dan_the_plumber]

I'd say that maybe the motorised valve is not closing once the coil temp is satisfied.
If the motorised valve is opening to heating then you should have heating.
This leads me to suspect a wiring fault, especially if it's a new install

 

[Paul 1978]

It's an Alpha sy9 24 boiler, sealed system. I know the valve is closing properly because when I turn the stat right down it closes and the boiler fires up and stays on providing me with heating.

 

[Paul 1978]

There is not a balancing valve on the return from the coil. It joins up with the heating return then goes into a magnaclean from there into the boiler. Should a balancing valve been fitted when cylinder installed?

 

[dan_the_plumber]

Maybe you are just heating too much hot water and not actually using enough.
The coil is only going to loose its heat if the water in the cylinder is cooler than the coil is.

 

[fezster]

A balancing valve on the HW would allow you to control how much heat goes to your cylinder and how much to the radiators. However, many installers prefer to have the cylinder take the full boiler capacity so that it heats up with priority. Most modern unvented cylinders have 20+ KW coils and recovery times of 20 mins or so, so your heating is not off for long.

In your case, if the return from the cylinder is almost as hot as the flow, then no heat is being transferred and the boiler is cycling. One cause of this could be the cylinder stat is set too high OR the boiler flow is set too low. The cylinder stat should be roughly 60 degrees (to kill off legionella) and your boiler flow should be set 10 degrees higher. What is your boiler flow set to at the moment? Try increasing it temporarily and see if the cylinder stat is satisfied.