Cold downstairs radiators

Combi boiler (Ravenheat) - upstairs radiators hot, most of downstairs cool or cold. Boiler located downstairs and feeds upstairs radiators and then T's off to come back down stairs. Downstairs will get hot if upstairs radiators shut off. New pump fitted but symptoms remain.

On initial firing up of boiler, it doesn't take long before it shuts down and then cycles regularily (on 45 secs off 90 secs). Return pipe still cold/cool

Hot water works ok and return into boiler then red hot (from diverter valve). Boiler not cycling whilst running hot water - modulating to keep temp something like constant. Turn hot water off and return drops cool again so it seems diverter valve is ok.

No air in system. Drained some water off at lowest point and it is clear - appears to be no sludge etc

Should I try a flush with fernox cleaner, refill and then add inhibitor.

I had the same problem with my heating . changing the zone control valve should do the trick .It should be located somewhere near the boiler. You may be able to open the valve manually with a little switch on the valve ,but only for a temporary measure. Good luck.

There is no zone valve. There is a three way diverter valve in the boiler which normally allows the pump to pump water around the heat exchanger and then onto the radiators via the flow and return pipes. If you then call for hot water the diverter valve temporarily creates a loop from the heat exchanger to the pump and back to the heat exchanger and cuts out the central heating heating system. Cold water then flows through another part of the heat exchanger in the opposite direction to the water being pumped around and is heated very quickly and then sent to the hot tap.

The diverter valve seems ok as when you call for hot water this works ok and the pipe leading to the flow side of the central heating remains cool until the hot water is switched off. As soon as this happens the flow to the CH becomes red hot. Whilst calling for hot water the return pipe from the loop is hot and when hot water is no longer called for this pipe then becomes cool as it is no longer used and the return from the CH comes back into play.

Tonight a couple of the downstairs radiators where hot and one of the up stairs radiators remained cold. It seems the pump is struggling to pump the water around even though it is new. Boiler still cycling even though the return pipe to the boiler is nothing like hot. When the boiler fires up it lasts for only a short time and seems as if the water isn't being pumped around fast enough so the boiler then switches off as the temp has risen to the point where it shuts off.

I guess the 3 way diverter valve could be limiting the flow and when used for just hot water it manages to force the water around ok as it is only a very small loop (much much smaller than the CH radiators) ??