Intermittent Overflow of Feed and Expansion Tank

[Kenspark]

Hello my parents have a house with an open vented system. It has an oil boiler feeding 4 circuits of radiators and also an indirect hot water cylinder. It also has a solid fuel fireplace back boiler (with 4 pipes) to it. The fire can heat both the radiator and the hot water. The fire is never lit. The feed and expansion tank is regularly overflowing every so often, mostly when the hot water heating circuit stops (I think). The feed and expansion tank gets sporadic water fill from the vent pipe and I can also feel a small pulse, or in flow, of water from the feed and expansion tank outlet. Here is a diagram. The system has been like this for years, but only started giving this problem a few months ago. Be gentle.

 

[Kenspark]

Here is the diagram

 

[steadyon]

1. No diagram visible.
2. Could be blocked / partially blocked feed from F&E into system.
3. If you can identify the feed pipe, try running a magnet along the outside. Assuming it's not a steel pipe, the magnet should not be attracted, unless there is some magnetite (basically rust off the inside of radiators) inside the pipe. The presence of magnetite points (not infallibly) to a blockage.

 

[Kenspark]

hopefully i can attached it now. sorry i tried to attached a pdf earlier.

 

[Kenspark]

1. No diagram visible.
2. Could be blocked / partially blocked feed from F&E into system.
3. If you can identify the feed pipe, try running a magnet along the outside. Assuming it's not a steel pipe, the magnet should not be attracted, unless there is some magnetite (basically rust off the inside of radiators) inside the pipe. The presence of magnetite points (not infallibly) to a blockage.

thanks - i will give that a try!

 

[John.g]

Maybe I didn't look hard enough but it looks as if the OF boiler system is sealed? as the vent is blanked and no cold feed. If sealed, can you indicate where the expansion vessel is teed in.

 

[Kenspark]

Maybe I didn't look hard enough but it looks as if the OF boiler system is sealed? as the vent is blanked and no cold feed. If sealed, can you indicate where the expansion vessel is teed in

The system doesn’t have an expansion vessel fitted anywhere, but the water in the CH/HW system is expanding and getting into the feed and expansion tank somehow. The only way I can see for expanding water to escape the HW/CH system is via the blue arrowed line in the attached picture. i.e. the water has to go via the back boiler (assuming all 4 pipes lead into one common chamber in the back boiler). Does it make any sense? I'm guessing it is not the way it should be...

 

[John.g]

No, that's certainly not correct, here is a idea of the correct way.
The gas boiler is showing gravity flow DHW heating like the solid fuel but doesn't have to be this way, it can be fully pumped.

 

[Kenspark]

No, that's certainly not correct, here is a idea of the correct way.
The gas boiler is showing gravity flow DHW heating like the solid fuel but doesn't have to be this way, it can be fully pumped.

View attachment 47045

Thanks for your post, the comments and diagram are very useful, and I appreciate the time you took. It is food for thought. The current oil boiler has only got two pipes running to and from it, so it would require a bit of digging in the floor. But if it is the only way to fix it then needs must...