Is my boiler behaving normally?

[gemius]

I have a Vaillant ECO max 800 (approx. 17 years old) running 8 rads in a medium sized semi. Recently the last two rads on the circuit haven't been heating up fully. They don't need bleeding and I don't think the system can be furred up, because one of the rads is only a year old and there was no evidence of sludge when it was changed.
I turned the Hall thermostat up to full ( about 30 degrees) and shut off all the rads except the two culprits and the Hall rad which has no TRV fitted. They did heat up, although the new rad was only warm at the bottom.
I watched the boiler and it was firing for about 30 seconds until the temperature went up to around 90 degrees then going into anti cycling mode for about 4 mins before firing up again when the temperature had dropped to 60 degrees. I had a thermometer next to the Hall Thermostat and it never reached above 20 degrees so in theory it was calling for heat.
My question is. Do you think that this is normal behaviour from the boiler or should it be firing for longer?

 

[Madrab]

Possible circulation issue ..... pump may be getting tired.

 

[Doggit]

Although there wasn't much evidence of sludge a year ago, the rad being cold at the bottom suggests you may have it now. The easiest thing to do first, would be to remove the rad, and flush it out. Did you top up the inhibitor when the rads were changed.

 

[John D v2.0]

Agreed with madrab, water in the boiler is getting up to 90c but it's not really leaving the boiler in a hurry. Weak pump or too much resistance in the pipe work to those rads for some reason.
If you have another rad open elsewhere, does it heat quickly when the boiler comes on?
If so it's a blockage or air loch, if not it's more likely a pump issue.

 

[durhamplumber]

Possible circulation issue ..... pump may be getting tired.

Rofl

 

[Dan Robinson]

Rofl

And....?

 

[durhamplumber]

And....?

what?

 

[Dan Robinson]

You blithely dismissed Madrab, but offer no alternative idea.

 

[durhamplumber]

You blithely dismissed Madrab, but offer no alternative idea.

you the wit and content moderator then?

 

[Dan Robinson]

Yes.

 

[durhamplumber]

Yes.

rofl........Again...

 

[Dan Robinson]

So you have no suggestions for the problem? Madrab's solution seems reasonable considering the information given.


Or do you think it is normal behaviour for the boiler?

 

[gemius]

Thank you all for your input.
For a bit more clarification if all the rads are open the ones nearer to the boiler heat up quickly to their full temperature. It's just the last two which never get more than luke warm.

 

[John D v2.0]

Must be a restriction in the pipework to those or the rads themselves. If the valves are open and it came on gradually over time, the issue, I'd either take the rads off the wall and see if anything needed flushing through, or drain the system and try to flush things through those rads at mains pressure which has a lot more power than your pump.

 

[D_Hailsham]

For a bit more clarification if all the rads are open the ones nearer to the boiler heat up quickly to their full temperature. It's just the last two which never get more than luke warm.

Have you tried balancing the system?