Help please. Intermittent airlock problem

[airlocked]

I wonder if somebody can help me.
I have a Potterton gas boiler which heats water for heating and hot
water. I have been using this for over 10 years without issue,
but recently an intermittent problem has developed.
When I switch on, everything works fine, but after a
while (could be a few hours), the water stops
circulating. The problem seems to start if the heating is
switched off for a short period of time and then on again, maybe by the
thermostat. The hot water is affected in the same way.
The boiler is still heating water, the pump is still getting power
(I can hear it humming when I switch on) but water does not circulate. the
only way I can get it to work again is by switching
everything off for about half an hour. then it works.
I imagine this is some sort of air lock problem, or air in the pump but I don't
understand why the problems should go away after 30 mins.
I beleve there are 2 pumps in the loft or maybe just one.
I had a new hot water tank fitted a few years ago. I used inhibitors when refilling the system and the problem didn't start until 2 or 3 years afterwards.
I hope somebody can help.
Many thanks

 

[Plumbstar Tom]

a full check up and diagnosis would be required for this sort of inteermittent problem

i would there recommend you get a local plumber round

you could try the screw on the front of the pump and check all rads are clear of air by bleeding them

 

[mkflow]

def full check up. could be scaled up system/air etc

 

[airlocked]

thanks for comments.
I doubt there is a scale problem since the system is very effective when it is working. All rads get hot quickly and hot water is quick as well.
I have bled all rads and found no air there.

I suppose it must be air in the pump, but I am confused as to why the problem should go away after 30 minutes with the system off, especially since the pump is at a high point in the circuit.
I will try to find the screw on the pump. i have not found it yet because it is in cramped corner in the loft covered in insulation.

 

[darrenangel]

I could be barking up the wrong tree here, but maybe your pump is overheating if its in a cramped corner, covered in insulation. Just a thought.

 

[mkflow]

good shout.

 

[airlocked]

Overheating pump. Interesting thought. thanks darrenangel.
things is, when the heating is working continuously, eg. when it's cold outside, the house is cold and the thermostat is up high. then the water is heated and circulated for hours on end, until the thermostat is satisfied and shuts things down. That would suggest to me that it is not an overheating problem.

The cut out only happens after the boiler is shut down, either manually or by the thermostat, for a relatively short period (maybe 10 or 20 minutes.)

As an example, I might come home late in the evening after having left the thermostat low to save fuel. When I get home, the house is a bit cold, the heating is on, but no heat because the thermostat is satisfied. So, I turn up the thermo a bit. It clicks, and the power is applied, but all I hear is a faint hum which says, 'pump is powered but the water's going nowhere'. if I leave it, this situation will continue indefinitely, unless I switch off.
now if i turn it off for 30 minutes, then power up again, I hear gurgling and the whole system functions normally.
OK. So air in the pump impeller would prevent the water from being circulated. but since the pump is at a high point, why would the problem go away after 30 minutes of downtime? why would the air that so mysteriously appeared in the pump, suddenly decide to move off elsewhere?
Puzzled.

 

[GrahamM]

What is the exact model of boiler?

 

[dhgas]

When switching back on and no circulation, does the boiler actually fire at all?

Edit to add as before model please, as some potty's have flow switches.

 

[airlocked]

thanks guys
the model is Potterton Flamingo 40.
Yes the boiler does fire up when the problem happens, but only for a short while since there is no circulation.

 

[airlocked]

some more on this. I've been up and checked the pump. It is Myson unit three.
there is no screw for bleeding, but a big nut in the centre. Does that do the same thing?

when the heating was powered up, but not circulating, the pump was silent and there was no vibration at all. It's looking more like a sensor problem in the boiler, or an electric valve somewhere.
???

 

[dhgas]

Yes the nut does do the same thing, but also goes round with the pump impellor, if it is turning so be careful.

Is the pump controlled from the time clock or wired into the boiler?? If the boiler I would start to check pump over run stats etc

You need to find out if the problem is air or pump related. When you have the problem is the pump running? If yes then probably pipework or air related.

If the pump is not running, is it because it has jammed or because there is no power to it. If it has power to it then probably the pump is at fault. If no power then I would be looking at pump overrun arrangements as stated before

 

[airlocked]

Thanks to all for comments and suggestions. It all completely died on me yesterday so I've had somebody round to fix it. it was the pump.
3 hours from getting on the phone to job done with an hours labour. 5* service I'd say.
thanks again to all.