Ideal Classic FF250 only running for 60-90 seconds following new HW cylinder installation

5 Oct 2014
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United Kingdom
Hi all,

Here is my setup....

Ideal Classic FF250
Brand new Gledhill 250L unvented cylinder and associated pump, zone valves etc (installed 2 weeks ago)
Nest Gen 3 Thermostat (installed 2 weeks ago)
All radiators have been bled

As mentioned above I had a new HW cylinder and Nest programmer installed a couple of weeks ago. Everything was working absolutely fine for around a week until I started to notice that the hot water wasn't as hot as it should be and the rads were only getting warm rather than hot. Initially I thought it may have been how I had configured the schedules on the Nest, but after some testing I dont think its anything to do with the Nest.

I started to notice that when the boiler fires up, it runs for around 60-90 seconds before stopping again. After about 5 minutes, the boiler fires up again, runs for 60-90 seconds and then stops again. This process then repeats. This results in warm (not hot) radiators, and the HW in the cylinder only getting to about 40c.

I have the installers coming back out to take a look this evening, but I want to be prepared with any thoughts/suggestions that I can talk to them about. I fear that they are just going to tell me I need a new boiler due to the age, but it seems a bit of a coincidence that it this is just happened after the cylinder installation (although it did function correctly for a week or so).

I have done a bit of Googling, and most suggestions seem to be that this occurs when there is an issue with the circularity system. This then causes the boiler to overheat and shut down. Once it cools down it fires up again and the process repeats. Does this sound right?

I have checked the pump and it is on the max (3) setting and all radiators have been bled. The installer has said that the pump is self-venting so there is nothing I can bleed in the airing cupboard.

What do people think might be happening here? Is the boiler dying? Is it possibly an airlock somewhere? Is it possible that the draining of the system has dislodged some muck which after a week has moved around the system and caused a blockage?

Thank you
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You have motorised zone valves to seperate the CH and HW circuits, these have manual levers on them and they are there to be manually opened when draining and filling systems, I will bet this wasnt done when you drained the system so causes air locks

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