Glow worm boiler problems

[flinty]

Hi,
the problem is this. The boiler is a Glow worm space saver mk 2.
A while ago I had the problem that if you put it on for hot water only the pump does not seem run and after a very short time the hot water starts venting into the overflow tank and the boiler you get loud banging noises.
while if you put the system on for hot water and central heating then the system pump will run and there is no unusual noises from the system.
A guy came and said it was a reset switch and he reset it. It worked fine for about 3 weeks.
On Wednesday & Thursday this week however I haven't been able to get any hot water at all.. but this morning it all seems to be working again and everything is fine.

Is this just a case of a knackered boiler?

 

[gas4you]

Is this a fully pumped system or gravity hot water, pumped heating?

 

[flinty]

not sure.. how do I tell the difference?
The tank that feeds the hot water cylinder sits above the hot water cylinder.
The pump is located under the floor outside of the airing cupboard.

 

[gas4you]

How many pipes come off the boiler? If 2 = fully pumped, if 4 then usually gravity primary.

 

[flinty]

3 pipes in total on the top of the boiler.
2 large 22mm and one small 15 mm

 

[gas4you]

Sounds like fully pumped then. Is the 15mm the gas pipe?

 

[flinty]

as far as I can tell it is. The pipes are up above the boiler and boxed in, so I removed the front cover.

 

[flinty]

could it possibly be the motorized valve becoming stuck and intermittently freeing itself?

 

[flinty]

or has anyone else got any ideas?

 

[ChrisR]

Is there an air bleed point near the cylinder - atop pipes which go in the side?
If so, let air out.
I called it a Vent when I drew this a year ago!

 

[flinty]

No.. there is no bleed valve.
There is also no diverter valves and no thermostat on the tank.
The system has always been run wiuth both heating and water on.
On water only it goes over into the tank above and makes noises.

We have been in the property for a year.

Sounds like it has been put together wrong to me as there is no way of circulating water through the coil in the tank when the central heating is off.