air in system, continuous make up from header tank

[50h9j]

I have an old Grant Euroflame (Gemini) oil boiler (1980s?) running DHW and CH. When the system is off the header tank is quiet. When the DHW or CH is running the header tank inlet is running (not dripping: a slow flow) and there is air getting into the system. When the system goes off the header tank soon stops. The pipework that is common to both systems is visible in the boiler room and there is no leakage. The only place I can see the water going is up the flue. The inside of the boiler is fairly clean and there is no sign of a leak; although there is some corrosion it is not too bad and cleans up. It seems to me that there must be a hole in the heat exchanger but could there be another explanation?
Is there a test I can do to confirm it? I don't have helium leak test kit.

 

[SimonG]

Does the open vent from the boiler terminate over the flowing tank? If you had leak in boiler it would run constant and not just when running. I ask the question because it maybe that the open vent from the heating is vented over the hot water feed and expansion tank and may be pumping over into one tank and being re fed from the other. Difficult to tell without seeing.

 

[50h9j]

There are two vent lines that terminate over the header tank which is about 7m above the boiler. One is off a line that used to be connected to a back boiler on a stove - it is in air at both ends as the stove is gone and the pipework is drained. The other is from the pumped circuit (I assume). The pump is sucking through the boiler but the feed line is just upstream of the boiler, so the boiler should be under pressure. There is no water coming from the vent line. I have even tried corking it to make sure it is not sucking in air. I'm not sure where the vent line is connected to the circuit.

I agree it is odd that the header tank only fills when the boiler is running, but it could be that the leak opens up at temperature. There may well be a more logical explanation, as it is a hell of a lot of water to go missing.

The primary circuit feed line is always cold (in a hot boiler room) so I am wasting a lot of energy somewhere and my system is corroding.

 

[50h9j]

I will have to check the hot water tank when the CH tank is filling. It is a good idea. Thanks.

 

[50h9j]

Ok, I feel a bit stupid now, because the vent line I had tried plugging was actually the DHW vent off the tank. For some reason this is routed into the CH header tank. There is no vent off the primary circuit. The one I disconnected must have been the only vent - I think at the time I assumed there was another since there are two going into the tank.

Any more suggestions as to where my water is going? I thought it could be leaking under the floor somewhere. That is why I did the test running just the DHW and CH separately. I think because it leaked in both cases it means it is in the common part of the system.

 

[ecowarm]

You dont have an overflow pipe running outside do you?

 

[50h9j]

I'm fairly certain the answer to that is no. There are no other visible connections inside and I've never noticed a pipe outside. All the pipes in the boiler/laundry room are surface run so, although it is designed like a plate of spaghetti, it is possible to trace everything.

 

[ecowarm]

Ah the old plate of spaggetti school of plumbing