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The cold water tank in my loft has two overflows side by side, both of which run parallel and straight out the back of the house. One has a 90-degree bend inside the tank meaning the end of the pipe is slightly submerged, while the other is just a regular opening. I understand that the 90-degree bend might be to either stop wind or bugs getting in, or to cause a siphoning effect to make it more obvious when there's a problem with the fill valve. However, those points are largely invalidated by the one right next to it that's just open.

They're approximately the same height, so I can't see that one is intended to be an early warning or anything. A couple of other houses in my road that I can see the back of also have two pipes coming out, so I'm guessing they're set up the same.

Why would there be two and not just a single one? Could it be to do with the potential volume of water? Also why would one have a 90-degree bend and not the other? I've been searching for ages and can't find any information on this so any insight would be much appreciated! Thanks.

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    My guess is the straight pipe is not meant for overflow but to draw fresh air for the pressure required to allow smooth water circulation between the elbowed drain pipe and the body of water. Also, it services to eliminate the stagnant smell. The straight pipe should have a mesh at its exit.
    – r13
    Oct 24, 2021 at 19:21
  • Interesting, hadn't thought of it being that. I'm pretty sure there's a breather on the top of the tank though, and I can't see any mesh on the exit of either, but it would provide an outside vent as you say rather than just venting into the loft.
    – Martyn Small
    Oct 25, 2021 at 20:38

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When it overflows, both pipes will start flowing. If it overflows to any appreciable extent, the one with the 90 degree bend will continue flowing until the water drops below its intake (it will make a siphon) rather than "constantly trickling out" right at the top level. So, if you have a stuck fill valve, the water level will oscillate between below the 90 intake and up to the top level - whether this is "to make it more noticeable" or some other reason, I don't know.

Perhaps it increases the odds of the fill valve float self-resetting, since the float would move? "Automaticaly jiggling the float" could well be the intent here.

Additional factor - redundancy in case one pipe is blocked, since the typical location for these tanks in the house (AIUI) is high, and overflows not taken out of the house would be bad

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  • Thinking about this more, it probably makes it quite hard for the siphon to get going - it would have to be filling faster than it drains out of both pipes together and it's a pretty long tank so I'm not sure that's even possible. Good points on the "jiggling" and redundancy. I've just never known a tank have two before and none of the pictures or installation instructions I've seen ever show this!
    – Martyn Small
    Oct 25, 2021 at 20:46
  • The tank is designed for hot or cold.
    – Polypipe Wrangler
    Oct 26, 2021 at 7:58

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