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Our combi boiler in the loft (less than 3 years old Vaillant EcoTec) is heating the upstairs radiators nicely, but the downstairs ones are lukewarm to cold.

I have opened all the radiator valves fully downstairs, and reduced the upstairs ones to try to increase pressure to downstairs. This worked for a few weeks, but again downstairs have become cool. Opening the upstairs ones fully makes the downstairs ones dead cold.

The pressure on the boiler reads 1.7 bar. Increasing it to 1.9 doesn't help and I don't want to go any higher. The thermostat is now set to 23°C and downstairs radiators still don't get beyond lukewarm. All the radiators have been bled, there is no air trapped.

One thing to note, and I don't know how related it is, but when we turn on any hot water tap, all the radiators get hot, even if the heating is off, even the downstairs ones!

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  • Has it always been like this or has it just started recently?
    – Niall C. ♦
    Commented Feb 22, 2013 at 23:29
  • Hi, it has been intermittent for over a year. If I play around with radiator valves or decrease and increase the boiler pressure it starts working again for another 2-3 weeks, and then starts playing up again, with the downstairs getting cold. One radiator downstairs, however, always stays hot (may be a different pipe?). Also I notice, if I watch the boiler start heating up, it goes from 55C to 70C very quickly (about 10 seconds) and cuts out, so it doesn't even give time for the radiators to get hot (except upstairs).
    – nLinked
    Commented Feb 23, 2013 at 13:15

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Had an engineer look at everything. The magnet filter was covered in thick black gunk. Took ages to clean. The verdict is all pipes and radiators are filled with this gunk and blocking flow very badly. The system will be power flushed and the pump replaced.

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