hot header tank, any ideas?

[KnappGate]

My CH system is the unpressurised sort with a header tank in the roof. It all seems to work, but I've noticed that when the boiler is running there is an intermittent dribble of hot water going into the header tank from the vent pipe, and the header tank gradually gets hotter and hotter. I've followed the pipes through and I can't see anything obviously wrong with the installation:

The hot feed from the gas boiler goes into the pump, the outlet from the pump goes to a two-way valve that diverts the flow to heat the radiators or hot water tank, the returns join and go back to the boiler.

In the hot pipe between the boiler and the pump there is a little swirl pot the size of your fist, with a vent pipe out the top that goes into the loft and ends over the top of the header tank. Immediately downstream of this there is a pipe teed in that goes up into the bottom of the header tank. These two connections are at the highest point in the circuit and seem well placed to remove air from the circuit and replace it with fresh water.

When the boiler comes on the main pipes all get hot very quickly indicating that there's a good flow of hot water. The vent pipe slowly heats up and eventually gets to the same temperature as the rest of the system. The 'fill' pipe remains cold. In the loft I can see an intermittent dribble of water from the vent pipe into the header tank, perhaps a cupfull of water in five minutes, the header tank very slowly warms up.

Final point, the vent pipe goes up about 18" above the level of the header tank before coming down and ending level with the top of the tank.

The fill and vent pipes are on the 'inlet' side of the pump which I think is correct. Since they're connected to more or less the same point on the circuit I'd expect the bottom of the fill and vent pipes to be at the same pressure, and given that the vent pipe extends above the water level in the header tank I'd expect any water pushed out of the system to go back up the fill pipe in preference to the vent pipe. But clearly it *is* going out the vent pipe. The only theory I can come up with is some sort of expansion causing a tidal flow between the circuit and the header tank, and the fill pipe having something preventing flow back up it into the tank i.e. forcing the flow to go via the vent pipe. I can't imagine why there would be a one-way valve in there though. It all seems very strange to me.

Two questions:

Is it normal for the header tank to get hot?

Any idea why it's happening?

Pete

 

[gas4you]

The header tank does usually get warm, but I would not say hot. Is the vent pumping over everytime the pump starts? Does it do it on hot water, ch, or both? In theory the air eliminator you have should eliminate this problem.

 

[KnappGate]

The header tank does usually get warm, but I would not say hot. Is the vent pumping over everytime the pump starts? Does it do it on hot water, ch, or both? In theory the air eliminator you have should eliminate this problem.

It's hard to say exactly when the water flow via the vent starts and stops compared to the pump going on and off. From what I've been able to see so far, water has dribbled out of the vent in circumstances where I'd expect the pump to have been running continuously for at least a few minutes. The flow from the vent isn't particular steady but it often seems to go on for several minutes and then stop.

My best guess so far is that's it caused by thermal expansion in the heating circuit pushing excess water back into the header tank, but I'm baffled by the fact it goes via the vent and not the fill pipe.

 

[KnappGate]

An update on the problem: I think the fill pipe is blocked, or very nearly blocked. I've tried blowing down it hoping to get bubbles back up the vent pipe, and absolutely nothing happens. I guess I will need to cut out and replace the fill pipe and hope that whatever is blocking it hasn't got into the rest of the system.