Problem
The problem being discussed here is the “correct” position of the vent and the cold feed. The other information, about this system and its problems, is background reading. Still, I'd also welcome any suggestions about dealing with those problems.
If CH, but not HW, is on, the boiler heats up fast, cuts out fast and starts up again many minutes later. The cutting out tends to be accompanied by air swooshing through the pump. If HW is on, the cycling is more leisurely and not accompanied by air.
Air seems to be entering the system, at least when the pump turns off. This is so for any pump setting but possibly less so for lower pump settings.
System
Pumped open vent system. However:
- The (22 mm) vent enters the boiler at the top left hand side.
- The (22 mm) flow leaves the boiler at the top right hand side.
- The pump is on the flow.
- The (22 mm) return enters the boiler at the bottom left hand side.
- The (15 mm) cold feed joins the return.
About six heating engineers have looked at the system. Their opinions on the position of the vent, on the position of the cold feed, and on combining the cold feed with the vent near the F&E tank range from “That’s probably causing problems” to “That’s the best place for it”. In this forum, too, there seem to be differences of opinion about at least combining the cold feed with the vent.
I think that the cold feed used to join the flow but was then replaced (to remove scale from it) and joined to the return.
Suggested diagnosis and cure
There are restrictions in the pipework: even after power flushing magnets stay attached to short stretches of the flow (between the pump and the CH motorised valve) and the return (before the join with the cold feed, but not at or in the cold feed). These restrictions (and possibly faults with the calling by the motorised valves) are likely to be partly responsible for the boiler cutting out.
Air could be entering the system if water see-saws into and out of the F&E tank: the water level in the vent would fall when the pump is running and water would be pulled in from the F&E tank when the pump turns off. However, sometimes the water in the F&E tank is warm, so there may also be venting.
There is evidence that the CH motorised valve does not always send live to the boiler when it is open. This might account for the time taken by the boiler to turn on after it turns off.
I don’t live in the house and for various other reasons I can’t conveniently carry out many tests (such as checking that air is sucked in at the top of the vent, or seeing exactly when the water in the F&E tank is warm). However, I intend at the weekend to have the restricted sections of the pipework replaced for the benefit of the tenants. I also intend at least to have the cold feed moved to the flow, to reduce see-sawing.
Questions
Should the vent be moved so that it is between the boiler and the pump?
Should the cold feed be moved so that it is between the vent and the pump?
Should the vent and cold feed be combined?
Why is there supposed to be at most 150mm between the vent and the cold feed?
Why is there supposed to be at least 450mm (perhaps half an atmosphere) between the top of the vent and the water level in the F&E tank?
The problem being discussed here is the “correct” position of the vent and the cold feed. The other information, about this system and its problems, is background reading. Still, I'd also welcome any suggestions about dealing with those problems.
If CH, but not HW, is on, the boiler heats up fast, cuts out fast and starts up again many minutes later. The cutting out tends to be accompanied by air swooshing through the pump. If HW is on, the cycling is more leisurely and not accompanied by air.
Air seems to be entering the system, at least when the pump turns off. This is so for any pump setting but possibly less so for lower pump settings.
System
Pumped open vent system. However:
- The (22 mm) vent enters the boiler at the top left hand side.
- The (22 mm) flow leaves the boiler at the top right hand side.
- The pump is on the flow.
- The (22 mm) return enters the boiler at the bottom left hand side.
- The (15 mm) cold feed joins the return.
About six heating engineers have looked at the system. Their opinions on the position of the vent, on the position of the cold feed, and on combining the cold feed with the vent near the F&E tank range from “That’s probably causing problems” to “That’s the best place for it”. In this forum, too, there seem to be differences of opinion about at least combining the cold feed with the vent.
I think that the cold feed used to join the flow but was then replaced (to remove scale from it) and joined to the return.
Suggested diagnosis and cure
There are restrictions in the pipework: even after power flushing magnets stay attached to short stretches of the flow (between the pump and the CH motorised valve) and the return (before the join with the cold feed, but not at or in the cold feed). These restrictions (and possibly faults with the calling by the motorised valves) are likely to be partly responsible for the boiler cutting out.
Air could be entering the system if water see-saws into and out of the F&E tank: the water level in the vent would fall when the pump is running and water would be pulled in from the F&E tank when the pump turns off. However, sometimes the water in the F&E tank is warm, so there may also be venting.
There is evidence that the CH motorised valve does not always send live to the boiler when it is open. This might account for the time taken by the boiler to turn on after it turns off.
I don’t live in the house and for various other reasons I can’t conveniently carry out many tests (such as checking that air is sucked in at the top of the vent, or seeing exactly when the water in the F&E tank is warm). However, I intend at the weekend to have the restricted sections of the pipework replaced for the benefit of the tenants. I also intend at least to have the cold feed moved to the flow, to reduce see-sawing.
Questions
Should the vent be moved so that it is between the boiler and the pump?
Should the cold feed be moved so that it is between the vent and the pump?
Should the vent and cold feed be combined?
Why is there supposed to be at most 150mm between the vent and the cold feed?
Why is there supposed to be at least 450mm (perhaps half an atmosphere) between the top of the vent and the water level in the F&E tank?