Radiator/hot water temperature

[Meggalar]

Hi,
Our radiators don't appear to be getting as hot as they used to be and neither does the hot water.
We have an Ideal Gas Boiler with a hot water cylinder upstairs which is a unvented indirect cylinder I think.
The boiler fires up ok and hot water leaves the boiler, the boiler stays on for around 2 minutes before turning itself off for a while.
Last weekend the radiators did really heat up properly and kicked out plenty of heat but then went back to how it is now, just warmish radiators which doesn't always take the edge off the cold.
Is it possible that the boiler is switching itself off too early based on the temperature of the water that is coming back into the boiler. From previous experience when we had a similar problem a year or so ago, an engineer replaced the PCB and the thermistor saying that the boiler was switching off too early. It may be the same fault but i'm not sure how likely this is.
Also they had recommended a powerflush but i'm not sure if this is just a sales ploy.

Any thoughts?

Thanks

 

[twgas]

Several potential causes, pump running slow due to sludge is quite high on list of suspects.
What colour is the water when you bleed rads?
If I'm right a powerflush is not a bad bit of advice.

 

[Meggalar]

The colour is quite clear with a tinge of brown to it. It has been drained down a couple of times with centralheating cleaner in it. I've had different views on the powerflush option, one plumber thought the system was clean and didnt recommend it, the chap from eon always recommends it, however its not cheap! And i want to know it definitely needs it before spending the money.
I thought it was odd that it would work really well one day then then the next go back to just warmish.
Thanks for your advice.

 

[twgas]

If the system has been drained a couple of times i would expect the water to be clear(ish).
What, exactly,do you mean by " drained down a couple of times with centralheating cleaner in it"

 

[Meggalar]

I put central heating cleaner a year ago, left it for a month and drained down and let it run through. It also got drained down for a repair earlier this year. You say it could be the pump sludged up, this was replaced a year ago, is there anything I can do to improve this do you think? Many thanks

 

[twgas]

A dirty system can get through pumps pretty quickly,
Try removing the head to see what condition its in - dont forget to isolate the water and electric first.

 

[Meggalar]

Do you have any suggestions as to why it would suddenly work really well and then go back to being just warm? Could this be a partial blockage that cleared then partially blocked again?
Thanks

 

[Meggalar]

Hi,
This evening the boiler stayed on for a lot longer and therefore the radiators got really hot banging out lots of heat and the hot water will have been nicely heated also.

I hadn't changed anything, so not sure why it should suddenly do this. Does anybody have any ideas?
I don't know how long this will last for unfortunately.
I wondered if it might be the pump starting to work better, or the boiler thermistor not shutting the boiler down at lower temperatures?

Grateful for any advice.
Thanks