Hot water keeps venting into header tank

[buckparka]

I have an Ideal Icos HE15 open vent boiler which is 8 years old.

Recently, the boiler keeps cutting out only a few minutes after firing, meaning the radiators only get warm, nowhere near hot. Just before it cuts out, hot water "spurts" a few times into the header tank. Two or three minutes later, the boiler fires again for another minute or two before cutting out, and this just keeps repeating.

Raising the pump speed above the lowest speed lets the boiler run longer before cutting out, but then a continuous stream of hot water is sent into the plastic header tank via the open vent. Previously, the slowest pump speed worked fine.

The actuator/diverter and the pump were replaced last year, and the system was powerflushed.

The top of the open vent pipe is only 35cm above the header tank water line, however as the header tank is mounted on a plinth in the loft, and the open vent pipe is almost touching the roof lining, the pipe can't be easily raised. I think the header tank in the loft is on a plinth since the boiler is mounted fairly high on a wall in a bungalow. This arrangement seems to have worked fine, until now.

Is it possible the heat exchanger is somehow clogged up and restricting flow through the boiler resulting in overheating and cutting out at lower pump speeds? Or maybe some other blockage is restricting flow and increasing pressure in the system?

Thanks for any advice.

 

[Reg Man]

Sounds like a water circulation problem. Have you checked that the cold feed is not blocked?

 

[Rickster123]

Was the system fully power flushed when it was done? Or was it a quick flush through with the hose? I'd agree with reg man sounds like a restriction somewhere and cold feed is most likely.

 

[buckparka]

Was the system fully power flushed when it was done? Or was it a quick flush through with the hose? I'd agree with reg man sounds like a restriction somewhere and cold feed is most likely.

Thanks for both your replies.

Yes, the system was fully power flushed last year with a Kamco device. A Spirotrap MB3 was also fitted afterwards to prevent further sludge build up (which I empty every 6 months).

The header tank is filled with plenty of water, as is the cold water storage tank, and mains pressure is fine. How would/could I check for a cold feed blockage, and would the power flush have cured/picked this up?

 

[moogwai]

Drain the system down and see if the water level drops in the tank. A powerflush wont get rid of a blockage. Chances are you have a blockage around the pump, cold feed and open vent pipe. I personally would just renew the whole section.
The powerflush wouldve squeezed the water through what space was available in the pipe and the pipe would continue to block up after the powerflush was complete.
You could get a magnet and run it around said pipework to see if it pulls.... If it pulls you have magnatite in the pipework.

 

[Whyme]

Definately sounds like cold feed full blockage or partial like above, like moogwai says get a magnet and see cos coppers not magnetic and black sludge and blockages are..

 

[buckparka]

The header tank does empty when the system is drained. I think the cold feed being partially blocked sounds like the most likely.

Just so I can rule out the boiler, is it unlikely a damaged/blocked boiler heat exchanger can cause similar symptoms?

 

[Whyme]

The header tank does empty when the system is drained. I think the cold feed being partially blocked sounds like the most likely.

Just so I can rule out the boiler, is it unlikely a damaged/blocked boiler heat exchanger can cause similar symptoms?

Usually if its boiler related ie blockages, the boiler would overheat and turn off prematurely do to not being able to shift the heat.