Boiler short cycling for no reason- temp doesn't get any higher then 50deg

[Hapman73]

Hi
I have a Worcester Greenstar 32 with a Megaflow unvented cyclinder and its been running great for he last 4 years.
This week we noticed the radiators are not as warm as usual, for some reason the boiler heats up but doesn't get much above 50 deg before cycling on and off every few minutes. It's struggling to get the house temp up to the room thermostat setting (granted its gone a lot colder this week). But makes no difference if I turn the boiler up to max, it still cuts out around 50 degrees.

I did have a large radiator fitted a few months ago but have plenty of pressure and have bled all radiators. Also have an automatic bypass valve fitted.

The flow and return pipes do get hot so am wondering if I have an airlock somewhere or circulation issues.

Any ideas?

Thanks

 

[ericmark]

Try easy thing first, lock shield valve setting, if one valve wide open it will return hot water to boiler and boiler will shut down.

So likely it is a radiator that is hot which is problem, so try turning off new radiator completely then once pipes cold turn back on ¼ turn at a time until just a little warmth felt in feed pipe. And see if rest start working.

 

[SpecialK]

Do the pipes both sides of the bypass get hot quickly when the heating is on and radiators are open (TRV's and lockshields). If so the bypass is probably opening too early and causing the loop temp to be satisfied and the boiler goes into anti cycle.

 

[MJN]

Yes, do check the bypass. I found mine to be passing all the time regardless what it was set to, with the same symptoms to yours.

Obviously the new radiator should be viewed with some suspicion too. Are you able to measure the flow and return temperatures to determine if it is allowing too much flow through? Alternatively, does the situation improve if that radiator is turned off?

 

[Hapman73]

Many thanks all for the swift replies and suggestions.
I have TRV's on most major rads and also 22mm pipe for most of the long runs (with 15mm for the final stretches to the rads). So I have checked all these and any without I have done as suggested and opened the lockshields slightly. Is it correct that with a TRV fitted the opposing lockshield should be fully open?

I have also closed the bypass valve fully down now to check. It feels cooler than the flow/return pipes so I am guessing its probably working ok.

The new rad was just replacing an existing one that was there with a larger cast iron type so no major change to the system as such, it was drained though.

I can get to a position where there is a noticeable difference between the flow and return and watch the temp go up but then around 50degs on the boiler the flame light goes out and temp drops swiftly. Previously the rads would be lovely and hot to the touch all over, now even the hotter ones are cooler at the bottom.

I can't understand why the boiler temp doesn't go any higher, I could understand if it was cutting out at high temperature but when set to max I would expect >75 deg on the boiler display at least. Is there a thermistor on both the HW & CH and are these prone to becoming faulty? I will be getting my gas man in to have a look but wanted to rule anything obvious out or at least save some time.

I will check the temp of the pipes to see if it correlates to the display temp and again any further suggestions are most welcome.
Cheers.