Water not flowing through pipes from boiler

[LittleSole]

Hi,
Hot water/Heating/Boiler ... problem with water flow; any advice please:

Boiler is 15 years old ... pump & mid-position valve are new.

Our boiler fires up but turns off after a couple of minutes ... when the water in its pipes gets hot. If it's left to cool it will fire up again but turn off again after a couple of minutes.

The pump is working ... it is going round and is triggered when the Hot Water or Heating is turned 'on'.

The pipes in/out of the pump do not get hot when the boiler is operating for a couple of minutes.

The mid-position valve is working ... the lever moves to indicate which position it is in.

The control units all work (wiring checked and the heating/hotwater/thermostat controls all work to trigger the boiler & pump.

There does not seem to be any obvious/visible blockage in the boiler flue.

Water drains satisfactorily ought of the system ... drain valve below boiler ... and the header tank empties.

There IS (not surprisingly; having drained the system) some air in the system.

SO ... why is the heated water not leaving the boiler and being pumped through the system ????

 

[Lagerkhan]

Perhaps the system is empty ?
Check header tank in roof or wherever and make sure it has water in.
Boiler will cut out quickly with no water

 

[LittleSole]

Perhaps the system is empty ?
Check header tank in roof or wherever and make sure it has water in.
Boiler will cut out quickly with no water

The system is definitely full of water (plus a small amount of gurgling air!)

 

[dal5band]

Sounds like an air lock - some trapped air in a horizontal section. Try pump on full speed, make sure pump valves are open. Otherwise a sludge blockage due to sludge being disturbed and then lodging against bend or a valve. Is there a dribble of water when you take off the pump rotor bleed cover ?

Extreme measures involve connecting a mains hose at the drain and turning it on for a 2-3 seconds (otherwise the header tank will fill up & overflow).

 

[LittleSole]

Sounds like an air lock - some trapped air in a horizontal section. Try pump on full speed, make sure pump valves are open. Otherwise a sludge blockage due to sludge being disturbed and then lodging against bend or a valve. Is there a dribble of water when you take off the pump rotor bleed cover ?

I've got the pump on full speed and I'm sure pump valves are open;
I've also tried 'locking' the mid-position valve in mid-position.
Yes there is a dribble of water when I take off the pump rotor bleed cover.

Extreme measures involve connecting a mains hose at the drain and turning it on for a 2-3 seconds (otherwise the header tank will fill up & overflow).

This idea had crossed my mind ... now you've also suggested it I shall try it!

I've even considered taking the mid-position valve off and seeing if water comes out of the top of the pump ... BUT that will be messy water-wise!

 

[ChrisR]

Have you bled the pipes by the HW cylinder?

Remove pump motor, make sure it's spinning, has good torque and is clean.
Open pump valves and water should flood out. If it's much less on the exit side, put the pump in backwards for a minute's operation, with gas to boiler off, which will clear airlock.

 

[LittleSole]

Many thanks 'dal5band' ... we tried the 'extreme' measure of putting mains water UP the system which did push a lot of air up into the header tank ... BUT the Water was still not flowing through the pump.

MANY MANY THANKS to ChrisR ... what a brilliant idea to temporarily reverse the pump ... the water is now flowing through the pump & heating both hot water & central heating. We have just one radiator which will not heat up (there's water in it & no air) so I suspect whatever was blocking the pipes could now be at the inlet to that one radiator.
We will need to drain the system again soon (to fit a new radiator) ... have you any thoughts on the pros/cons of using a 'flushing agent' (to remove debris etc from the system?