Modifying a 22mm vent pipe

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In my loft there is a 25-gallon circular cold-water tank, at least 40 years old. I've just fitted it with a new lid - it never had one before, just the insulation jacket stretched over it. There is a hole in the lid for the 'vent pipe' (correct term???) from the HW cylinder in the room below, but the position and angle of this pipe is causing trouble now, as it's not allowing the lid to sit properly.

The vent pipe rises vertically to about 2' above the top of the tank, then bends 90 degrees and runs horizontal about a foot, then bends downwards at about 45 degrees and slopes down into the tank. I want to alter the end of the pipe so that it goes down vertically through the lid hole when the lid is properly fitted.

Not wanting to try to bend the existing pipework and risk distortion and/or breaking a joint somewhere, I propose simply to cut it off near the top of the vertical riser and make up a new section to run horizontal out to above the tank, then go down vertical and thru' the hole in the lid. Does it matter if the pipe goes down towards the tank vertically, or does it need to be on a slope for any reason? Anything else that I should take care to do/avoid please?
 
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Does it matter if the pipe goes down towards the tank vertically, or does it need to be on a slope for any reason?

No, it isn't critical how it enters the cistern.
 
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Thanks - I was hoping not.

How far below the lid towards the water level should it extend - is it better to err on the side of keeping the end as far away from the water as possible? In the few days that the lid has been in place, I'm finding worrying signs of water on the outside of the lid around the hole - splash-back perhaps?

Once the new pipework has been fitted it won't be possible, as far as I can tell, to get the lid fully off the tank - it will be 'trapped' around the pipe - so I plan to fit a compression coupler somewhere in the downward pipe so that the end section can be 'uncoupled' if necessary. Or are there better ways please?
 
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You should be able to get the lid off with some manipulation. Never found one yet that I couldn't...

Keep it well above the water level. As long as it's properly through the lid you shouldn't have any problems with splashing as it should only be discharging water in a fault situation anyway.
 
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Having looked again at the existing pipework, I have decided that the best/easiest solution will be to cut the existing pipework in the middle of the horizontal section at the top. I can then fabricate a new extension that will start with a short horizontal piece, then bend downwards at an angle using an obtuse elbow, then have another obtuse elbow at the bottom with a short tail that goes vertically down into the tank through the hole in the lid.

The new extension will be connected to the existing pipe by a straight compression coupler (I don't want go waving a blow-lamp about near the rafters and also using a compression joint will enable me to make some fine adjustments before tightening). Given that normally the pipework does not contain any water, how can I test that the compression joints are actually water-tight, or will it actually matter in that sort of location anyway?
 
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Given that normally the pipework does not contain any water, how can I test that the compression joints are actually water-tight, or will it actually matter in that sort of location anyway?

Join the mains cold water to the hot water pipework, usually by covering the kitchen mixer tap with a firm cupped hand. Open the hot tap, then crack the cold tap open slightly. Cold mains water will attempt to run into the hot cylinder, but will be diverted up the open vent and into the cold tank via your new pipework. Turn off the taps, go back to the loft. If the new vent is both dry and warm then your pipework is good.

MM
 
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Given that normally the pipework does not contain any water, how can I test that the compression joints are actually water-tight, or will it actually matter in that sort of location anyway?

Join the mains cold water to the hot water pipework, usually by covering the kitchen mixer tap with a firm cupped hand. Open the hot tap, then crack the cold tap open slightly. Cold mains water will attempt to run into the hot cylinder, but will be diverted up the open vent and into the cold tank via your new pipework. Turn off the taps, go back to the loft. If the new vent is both dry and warm then your pipework is good.

MM
 
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