Hi Everyone, I’m hoping to get some thoughts on an ongoing leak we’ve been trying to track down for a while now. We’ve had long-term leaking from our shower into the hallway (shared wall). The leak occurs when hot water is used from the shower and also when hot water is used from the bath. The bath is downstream from the shower on the same copper pipe. A few key points: * The leak only occurs when hot water is used (from the shower and (we think) the bath). * Using cold water only does not cause any leaking. * Once the area is dry, it takes about 5–6 days of hot water use for the leak to become visible again in the hallway. * Multiple plumbers have been consulted over time. They’ve provided advice and carried out various works, but none have resolved the issue. * The shower tiles have been professionally re-grouted and sealed. * The shower breech was replaced, but the leak eventually returned. * We’ve carried out multiple waterproofing and plumbing tests: * The shower basin was filled with water for 7 days with no leak. * Buckets were filled with hot water from the shower and poured down the toilet (so no water entered the shower basin). After about 5 days, water appeared in the hallway. Currently, we’re only using cold water and there is no apparent leak. This morning, we ran hot water from the bath to fill a bucket and took a thermal image inside the shower wall, at the junction in the pipe that runs to the bath and up to the breech. My question is: does this thermal image look typical when taken immediately after running hot water, or could the hot spot indicate a potential leak at that joint? This area also aligns with where moisture comes through the concrete into the hallway. For scale the wall tiles in the image are approx. 100x200mm. For context, this is a 1970s double-brick apartment with rendered walls and original tiling. Thanks in advance for any insights or experiences you’re willing to share.

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How to interpret thermal image for leak detection?

Hi Everyone,

I’m hoping to get some thoughts on an ongoing leak we’ve been trying to track down for a while now.

We’ve had long-term leaking from our shower into the hallway (shared wall). The leak occurs when hot water is used from the shower and also when hot water is used from the bath. The bath is downstream from the shower on the same copper pipe.

A few key points:

The leak only occurs when hot water is used (from the shower and (we think) the bath).
Using cold water only does not cause any leaking.
Once the area is dry, it takes about 5–6 days of hot water use for the leak to become visible again in the hallway.
Multiple plumbers have been consulted over time. They’ve provided advice and carried out various works, but none have resolved the issue.
The shower tiles have been professionally re-grouted and sealed.
The shower breech was replaced, but the leak eventually returned.
We’ve carried out multiple waterproofing and plumbing tests:

The shower basin was filled with water for 7 days with no leak.
Buckets were filled with hot water from the shower and poured down the toilet (so no water entered the shower basin). After about 5 days, water appeared in the hallway.

Currently, we’re only using cold water and there is no apparent leak. This morning, we ran hot water from the bath to fill a bucket and took a thermal image inside the shower wall, at the junction in the pipe that runs to the bath and up to the breech.

My question is: does this thermal image look typical when taken immediately after running hot water, or could the hot spot indicate a potential leak at that joint? This area also aligns with where moisture comes through the concrete into the hallway. For scale the wall tiles in the image are approx. 100x200mm.

For context, this is a 1970s double-brick apartment with rendered walls and original tiling.

Thanks in advance for any insights or experiences you’re willing to share.

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MitchellMc

Welcome to the Bunnings Workshop community @kbb. It's wonderful to have you join us, and many thanks for your question about leak detection.

You’ve done some really solid detective work already, and the testing you’ve described rules out a lot of the usual suspects, so you’re not chasing ghosts here.

In a thermal image taken immediately after running hot water, it is completely normal to see a bright hot trace where a copper pipe runs inside the wall. Copper conducts heat extremely well, so even a short burst of hot water will show up very clearly, often brighter than people expect. A straight, well-defined line usually just indicates pipe location rather than a failure.

What makes your image more interesting is the shape and intensity of the hot area. Instead of a clean linear trace, you’ve got a concentrated hot spot that flares out at what appears to be a junction. That pattern can still be normal, because fittings and joints hold more metal mass and therefore retain heat longer than straight pipe runs. However, when a joint is leaking hot water, even very slowly, the surrounding masonry or render will also warm up and hold that heat for longer, which creates a broader, less defined glow rather than a sharp line.

The timeline you’ve described strongly points toward a hot water pipe or fitting issue rather than waterproofing. The fact that cold water never triggers the leak, that it takes several days of hot water use to reappear, and that pouring hot water directly into the plumbing system without wetting the shower still causes moisture in the hallway, all suggest thermal expansion or a very slow seep at a joint. As copper heats and cools, a marginal joint or pinhole can open just enough under hot conditions to allow water to escape, then effectively seal itself again once cooled, which is why it’s so hard to catch and why plumbers often miss it during short tests.

The location lining up with where moisture presents in the hallway slab further supports this. Water can migrate surprisingly far through concrete and masonry before becoming visible, so the source is often slightly offset from where the damage shows.

On its own, the thermal image isn’t definitive proof of a leak, but combined with your testing and the behaviour over time, it’s a strong indicator that this junction is worth further investigation. A useful next step would be to take comparative thermal images, one after running cold water only and one several hours after hot water use, to see whether that hot spot persists longer than the surrounding pipework. Persistent heat in the wall long after use is one of the clearest thermal signs of an active leak.

Given the age of the building and the fact it’s original copper in a 1970s apartment, I’d be leaning towards a concealed joint or elbow that’s failing under heat rather than anything to do with the shower itself. You’re absolutely right to keep pushing on this now, because left long enough, slow hot water leaks can cause significant structural and mould issues even if they seem minor on the surface.

Please let me know if you have any questions.

Mitchell

kbb

Thanks for your detailed answer @MitchellMc

As an added note, It takes significantly longer use of the hot water for the pipes either side of the hot spot to show up in the thermal image. I wonder of that is of significance? (and not on our list above, is the fact that a recent dye test of the shower basin did not reveal any dye in the hallway)

I will do as you suggest and do timed thermal images to get an idea of how the heat dissipates over time.

thanks again.

@kbb

MitchellMc

My money is on an internal leak in the wall @kbb. You have almost entirely ruled out a leak from within the shower.

Yes, I do believe it is very significant that it takes longer for the pipes to show up in the thermal image. It's not unusual that once the pipes have shown up for there then to be a buildup at that joint section, which gets brighter than the pipes. But, for that area to be radiating so much prior to the pipes showing up, to me as a layperson, that would indicate water escaping at that point.

You'd really need to have a leak detection specialist run their camera over the area, as it's likely to be more sensitive than the consumer-grade phone Flir cameras.

Mitchell

kbb

thanks for your help on this @MitchellMc. The site was down for a bit, so apologies for the late reply. All these details are very helpful. Fingers crossed we find the culprit soon

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