pumping over without pump running

[cooldude1066]

I have a conventional indirect system with a gas boiler and 12 rads. When the boiler is getting up to temperature the pump cuts out (controlled by the boiler stat) and shortly after there is a massive rush of water up the expansion pipe into the F&E tank. This is accompnaied by loud bangs and crashes from the whole system. We have changed the heat exchanger in the boiler as we thought it was causing kettling and producing gas. I have added an air release valve at the top of the system to replace the old screw cap type.The rads have been removed periodically and when you drain the system the water is clear. Dont think there are any blockages and once it sorts itself out the heating is fine. The expansion pipe is 22mm and fitted on the negative side of the pump along with the supply pipe which is correct I think. The 3 port valve goes A to heating and B to cylinder which is correct. Could it be a blocked manifold? Its the only part of the system I have never looked at.

 

[stevetheplumber]

I have a conventional indirect system with a gas boiler and 12 rads. When the boiler is getting up to temperature the pump cuts out (controlled by the boiler stat) and shortly after there is a massive rush of water up the expansion pipe into the F&E tank. This is accompnaied by loud bangs and crashes from the whole system. We have changed the heat exchanger in the boiler as we thought it was causing kettling and producing gas. I have added an air release valve at the top of the system to replace the old screw cap type.The rads have been removed periodically and when you drain the system the water is clear. Dont think there are any blockages and once it sorts itself out the heating is fine. The expansion pipe is 22mm and fitted on the negative side of the pump along with the supply pipe which is correct I think. The 3 port valve goes A to heating and B to cylinder which is correct. Could it be a blocked manifold? Its the only part of the system I have never looked at.

you dont say which boiler you have but most boilers have pump overun which allows the heat in the boiler to be pumped away after the boiler cuts of it sounds as though yours isnt doing this and this causes the boiling water to escape up the vent its also a possibility your stat isnt working and the boiler is shutting down on the limit stat get it checked properly

 

[wnjswozo]

is it a back boiler,check boiler stat,pump really should be controlled by room stat,water must be expanding rapidly up f/v pipe ,is pump located near to boiler.need more info

 

[cooldude1066]

The boiler is a Myson Apollo model about 20 years old. Your theory about the pump over run might be the problem - I have since checked and the pump just cuts out for no reason whilst the boiler is heating which then causes this massive series of bangs and the water spewing up the F&E pipe. Do you know if you can replace the pump overun controllers on these? I dont think its the boiler Hi stat that is faulty as it does this on the Lo one too and it seems unlikely they have both failed.

the pump is not near the boiler and its not a back boiler. I dont think its the limit stat as it does it whther its on Lo or Hi.

I have just checked the manual and it looks like the pump over run thermostat might be the faulty bit although when the boiler turns off at night on the timer the pump does keep going for a while like it should. But the pump definately turns off when the boiler is running and causes this massive bang.

 

[cooldude1066]

We have decided to bypass the controls in the boiler and wire the pump up direct to the mains with a (Temporary) manual switch. This will allow the pump to run continuously and then we can see if this prevents boiler over heating. Presumably the hi and lo thermostats control just the boiler not the pump. Any ideas on this?

 

[doitmyself]

The boiler is a Myson Apollo model about 20 years old.

They are the symptoms of a faulty over-run thermostat.

When the boiler is cold the pump gets its supply from the switched supply to the boiler from the room stat etc. When it has warmed up a bit the over run thermostat changes over so the supply comes from the main unswitched connection on the boiler. When the boiler is turned off, the pump continues to run because it is powered from the unswitched supply. When the temperature has dropped the thermostat changes to the switched supply, which is now dead, so the pump stops. Your stat is not switching over properly.

Which model Apollo do you have - it's on the back of the outer cover?

 

[RoryD]

good call, changed one recently. The vent pipe is doing what it is designed to do. It is the excessive temperature and pressure release pipe after all

 

[paul H]

your problem sounds like lack of water flowing around your boiler when too hot or still being heated, the heat exchanger wont like it if it gets very hot and will push out the water from the vent
probably the boiler will sound like its trying to jump off the wall as well
check pump and blockages in the heat exchanger, check to see the gas is shut down when the thermostat is satisfied

 

[cooldude1066]

Sorted - we by passed the thermostat and wired the pump up direct to a socket with a timer and its fine now. No more noise and a nice warm house. So have sent away for an over run thermostat. Thanks to all - appreciated.

 

[panikmkanik]

nice to know it was sorted. can you tell us where this thermostat is located in these boilers. cheers!

 

[cooldude1066]

its screwed to the rear of the boiler frame at the right - take off the outer and inner covers and its there.

 

[panikmkanik]

thankyou!

 

[cooldude1066]

Just to conclude this - new over run thermostat turned up today (£24) and once fitted solved the problem. Boiler switches off once room stat calls and pump carried on running until the temperature of the boiler frame drops (about 12 minutes) and then switches pump off. Boiler switches on when room stat calls and pump starts straight away. Cheers!