Boiler short cycling on CH only?

[mish78]

Hi,
I have a Worchester Bosch Greenstar 35cdi boiler with Underfloor heating & rads upstairs. Im looking for any advise or things to check before calling out a service engineer to service it and also repair it. The problem is that it keeps cycling when on Central heating mode only.

UFH only - Boiler will heat up to whatever temp I set on boiler control then cuts out, temp ramps back down, then boiler starts up again and the same thing happens
Rads only - Boiler will heat up to whatever temp I set on boiler control then cuts out, temp ramps back down, then boiler starts up again and the same thing happens
DHW only - fine, boiler remains a set constant temp.
Its not the Room thermostat switching as its just been replaced ( issue occurred before this ) and its usually calling for heat.
The flow looks ok as the rads get warm and the flow rate in the UFH shows as 2 ( litres i think )
Pressure in the boiler is ok - around 1.2 bar
no error codes on the display.
The heating UFH & RADS do work and get up to temperature but the cycling cant be doing the boiler any good and UFH takes ages to heat as is..

I was thinking it may be the thermistor in the boiler but am unsure as DHW is fine.

Any ideas anyone? my minds fried?

 

[Agile]

I would say your boiler is working fine!

But changing some settings would probably reduce the cycling.

Unfortunately, Worcester do not make changing settings very easy. But I am told that they are there if you can work out how to access them!

Tony

 

[mish78]

Thanks Tony,

I thought the boiler should keep the heat going for much longer.
this cycle takes around 5 mins at most and the temp display on the boiler is never constant, always increasing or decreasing?

If theres a settings change to reduce the cycle though then id love to have a bash if there's any Worcester gurus out there that can advise

The service manual shows loads of stuff but im not sure what to change and dont want to make things worse

Mish

 

[Agile]

You probably only need to reduce the CH output power in your case.

You have not said how long it stays off for! Should not be reduced below about 3 min if its currently longer ( and adjustable ).

It may also be just the consequence of having a boiler massively overrated for supplying heating and inevitable!

Tony

 

[mish78]

Ive tried reducing the temp level knob on the boiler to 2 but then the boiler just fires upto 45 -55 degrees then cuts out. It takes a few mins for the temp on the boiler guage to drop down then as soon as it gets to 29 it will fire on again...
when set to 4 it will fire up to 60-70 degrees then cust out, It then takes 3 to 4 mins for the temp on the boiler guage to drop down then as soon as it gets to 29 it will fire on again...

It may be oversized but I`m sure it worked better ( remained fired up for longer ) before which would rule that out.

Its not been serviced in a while so I might look into having this done and see what comes up.

 

[ianmcd]

if you go for a service be honest with your engineer when you book him and tell him the fault you are having do not expect him to stumble accross it while servicing the boiler

 

[mish78]

Will do Ian, was looking to get it fixed ( if it is broken ) and while they are there get them to service it.
That way if its not broke at least they wont have came out for nothing.
Hamish