Last week I got hit with a colossal water bill. After contacting the supplier I was told to conduct a stop tap test on my water meter. I was confident this would reveal an external leak as we'd seen no sign water is leaking anywhere indoors. However, the test indicate the leak is indeed internal.

The supplier said that we now need to get a plumber in - at our expense - to locate and fix the leak. But I find it hard to understand how on earth anyone can track down a leak when there's no evidence of any water escaping other than the meter showing.

The supplier suggested the likely culprits were toilets or the boiler. I don't really understand how a boiler can leak water without us knowing, so I checked the toilets and couldn't see any water escaping down the overflows. We don't hear water running anywhere although we do have a water softener which I suppose could be involved somehow.

What I'm really dreading is a plumber coming out and racking up a huge bill spending hours failing to find a leak. Is there anything I can do to try and locate this leak myself, and if not, what can a plumber do that I cannot?

  • 5
    A water softener is the other usual suspect. I've had a bad one with sticky valves that would run water like crazy when it stuck. And it goes down the drain, so you don't see a leak. Easy to test - throw the bypass valve and see if the leak stops.
    – Ecnerwal
    Commented Nov 16, 2022 at 15:19
  • 1
    Are you 100% certain there isn't a leak in a wall somewhere where the water just hasn't yet made it through to somewhere visible/noticeable?
    – FreeMan
    Commented Nov 16, 2022 at 15:46
  • 15
    The plumber has a hammer. "A company calls an engineer to fix a device. Engineer looks at device, taps it with a hammer, and device works. Engineer presents bill for $1000. Company asks for itemized bill. Engineer presents bill: hammer swing: $1, using 20 years of experience to know where to tap device: $999."
    – longneck
    Commented Nov 16, 2022 at 16:20
  • 1
    If you can lift an inspection cover on the drainage system, you can see whether the internal leak is finding its way into the waste, or (presumably) going into the ground directly. You can find an upper limit on the flow rate from the estimated excess usage (either based on previous year's consumption, or on an average family). "Colossal" should not be hard to find: it should have an audible hiss on a quiet night, or you may pick it up with a damp meter. Commented Nov 17, 2022 at 0:28
  • 12
    When we had a mysterious leak like this, the insurers sent a leak detection specialist. He flushed the water out of the system, and injected a special gas. He then walked around with a handheld detector until it picked up traces of the gas, hence identifying the location of the leak (in this case, it was under the floor). Then the plumber came and fixed it! Commented Nov 17, 2022 at 9:49

5 Answers 5


General process would be to shut things off internally and repeat your stop valve test to see which thing or things affect the leak. How well you can narrow it down depends on how your plumbing is set up in terms of valves to shut off various supplies.

So, if you have an "inside the house main shut off," shut that. If the leak stops, it's past that point. If the leak continues, it's between the meter and that valve.

If the leak stops at the main shutoff, reopen the main shutoff. and bypass the water softener, and check the leak again.

Then, if you still haven't found it, shut off each toilet supply and recheck, etc.

If a boiler has a drain (for condensate, typically) that can take away water from a leak, depending where the leak is.

For toilets, some food coloring in the tank can show you a leak you may not hear - if the bowl turns color without you flushing, the toilet is leaking. Toilet leaks are more commonly not "down the overflow" but "around a poorly sealed flapper valve" - down the overflow is only when the fill valve is not shutting off as it should.

As commented, water softeners are a prime suspect, again, because there is a drain connection that takes away the "leak" so you don't get a puddle. Since those are normally installed with a bypass valve, or 3 valves you can use to bypass them in backwards states like Mass that don't allow a single, unscrew-up-able bypass valve, they are normally easy to check, via bypassing. After cleaning the valves in the one I had multiple times, and the problem recurring multiple times, it was an easy decision to replace the thing.

  • 5
    / if the bowl turns color without you flushing/ and also without you using the toilet.
    – Willk
    Commented Nov 16, 2022 at 17:49
  • 27
    @Willk if that part isn't blindingly obvious, hire a plumber.
    – Ecnerwal
    Commented Nov 16, 2022 at 19:05
  • Also note that some toilets have internal overflows because they're tidier - so even the overflow pipe flows into the bowl. The downside compared to an external overflow is that you can't spot it so easily. And if such a toilet was fitted in place of one with an external overflow, the pipe may still be there, just not connected.
    – Chris H
    Commented Nov 17, 2022 at 9:55
  • Also the newer-style (in UK) push-button flush toilets are rather prone to valve leakage. The water trickles down into the pan , and only close inspection reveals it. (That, or shut the service valve if the cistern has one, and see whether the level in the cistern goes down over the next few hours).
    – nigel222
    Commented Nov 17, 2022 at 12:50
  • 5
    Blindingly obvious, yet humorous. We need to allow for that on occasion. ;)
    – FreeMan
    Commented Nov 17, 2022 at 13:20

You don't say what country, but in the UK the pipe from the water company's stopcock to your premises is your responsibility. In older premises this pipe is quite probably iron, and can rust through.

That stopcock is usually in the public walkway outside your house, integrated with or adjacent to your water meter. Trouble is that you need a special key to operate it, that's not generally available to the public (lest anti-social types were to go around turning other people's water off!) Plumbers do have that key.

If this pipe is leaking it will be quite a major job to replace it with modern blue plastic water pipe between your internal stop-cock and the meter location. (Builders, a mini-digger, a complete mess of your lawn or flower-beds ...)

  • Older? Older can still be lead!
    – Chris H
    Commented Nov 17, 2022 at 13:38
  • @Chris_H Outdoors, buried? Would have thought Lead was too soft for that. But it doesn't rust :-)
    – nigel222
    Commented Nov 17, 2022 at 13:44
  • 1
    Lead water pipes are not uncommon. Look up anything on the Flint Michigan (USA) water crisis.
    – SteveSh
    Commented Nov 17, 2022 at 14:06
  • My last house was 30s-built and the incoming pipe was lead under the front path, joined to copper in the hall
    – Chris H
    Commented Nov 17, 2022 at 14:06
  • 2
    Large parts of the UK have hard water. If the water is hard, undisturbed Lead piping is safe. A layer of limescale will have formed on its inside sealing the Lead away from the water, and Lead doesn't dissolve into alkaline water anyway. OTOH if your water supply is soft and acidic, its a very good idea to get a water sample analyzed to make sure there's no hidden Lead piping involved.
    – nigel222
    Commented Nov 17, 2022 at 14:30

To answer your question generically, there are only so many places that water could be transferring from the supply lines to the drain lines. We can reasonably assume that this is what's happening since you have no visible evidence of leaks outside the drain system.

So make a list and investigate by listening and doing additional tap tests. You could also open a primary drain cleanout to help observe flow in various circumstances.

Some general suggestions:

  • Sinks
  • Toilets
  • Tubs and showers
  • Water softener
  • Exterior water heater
  • Radiative heating systems*
  • In-floor heating systems*

* Should normally be closed-loops, but worth a check.

  • 1
    while radiative and in-floor heating systems are closed loop, they should have a pressure-temperature relief valve and probably a source of make-up water. And your list omits the water heater, which also will have a relief valve which can leak. More rare, but if the house has a floor drain, for example in a laundry room, there may be an automatic valve that keeps the drain trap from drying out.
    – Llaves
    Commented Nov 17, 2022 at 4:22
  • @Llaves indeed, the float valve filling the feed-and-expansion tank in a vented system is one of the first places to look. Mine (and the one for the adjacent cold water tank) have overflow pipes leading out under the eaves. I've never worked on one without but I'm sure they exist somewhere.
    – Chris H
    Commented Nov 17, 2022 at 9:57
  • 1
    You'd normally notice water emerging from the overflow pipe and dripping down. But maybe not, if the pipe is positioned over some shrubbery or grass and you don't ever go there. Best then if the overflow is extended to drip somewhere more noticeable, but not obnoxiously so. For example, the flower-bed immediately adjacent to the front door, rather than the path.
    – nigel222
    Commented Nov 17, 2022 at 12:56
  • We don't have exterior water heaters in my part of the world, so that didn't come to mind. A leak there of any size would be painfully obvious indoors.
    – isherwood
    Commented Nov 17, 2022 at 13:53
  • @isherwood interior water heating systems, but overflows routing to the outside
    – Chris H
    Commented Nov 17, 2022 at 14:15

Hire a plumber with a leak detector

If you can’t find the leak and there is no obvious damage it’s likely the leak is underground. There are tools that “listen” for leaks and can pinpoint the leak within half a meter or so.


You're looking for a combination of a) a connection to utility water, and b) a place where that water could go without being observed. So right off the bat we can cross off spigots with sinks under them. You will see water moving from spigot to sink.

So next is toilets. Pop the lids off the tanks and see what's going on. There is never any moogie (human waste) in the tank, this is a fresh water supply. It's gross because it hasn't been cleaned in 20 years. Toilets have two places that can leak in stealth: the refill valve (attached to the float) and the flapper valve that dumps the big shot of water (attached to the handle). Start by observing the water level as compared to pipes in the bowl after the toilet has been quiescent (unused) for a half hour.

  • There should be an open-top pipe risng at or above the top of the water. That is the tank overflow. Note the water level in its undisturbed state (long time since a flush). Go ahead and flush it and watch it refill. When the valve shuts off, you'll see the intended resting place of the water, and it should be about an inch below that overflow pipe. If over time it's climbing to the top of that pipe, that's refill valve leakage.
  • If the refill valve is good, with the tank full block or tie the float in the "up" position so that it cannot fall down. Note tank water level and leave it that way for 10 minutes. Has tank level fallen with the float up? That is a flapper valve leak.

Next, look at your water heater. It should have an over-pressure valve to a pipe. Often these dump on the floor or into a tray under the water heater. If they are fully plumbed to dump outdoors, follow the pipe to its outlet and see if water is dribbling or flowing there.

Next, a leaky water softener valve is a possible culprit. However, water softeners have a special drain for purge water. It isn't dumping water into the supply piping! Follow that plumbing and look at flow there.

Next, if you have a hydronic furnace, it may have the capacity to refill itself from supply. It will also have an overpressure valve- same deal as the water heater. Hunt it down and follow the piping to the outlet.

We can cross off an internal pipe leak within your home. Tens of thousands of gallons leaked that way would cause heinous mold and mildew problems.

Also check outdoor taps for being broken, especially if the valve is kept on 24x7 with a hose attached.

Lastly look for wetter-than-normal parts of the yard, or an extra-green or extra-tall patch. If you lived in the southwest this would be immediately apparent by the grass not being brown. If you have green grass because you have irrigation, shut that off! Both to test if the leak stops when you do, and also to distress your lawn - if some part of the yard does not start turning brown, that reveals the leak.

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