Discuss Intermittent bad smell in the UK Plumbers Forums area at Plumbers Forums


I have an intermittent bad smell that's really hard to pin down, I was wondering if I could get your thoughts on it.

I can go months without smelling that sulphur smell and then one day it kicks in. Sometimes its really faint, others its strong and lingers. I never know when it will stop, sometimes days, others weeks at a time. It seems to only be linked to hot water usage upstairs (1 bathroom with a shower and sink tap too). And its strongest in the bedroom which is directly above the kitchen and boiler. So I'll wash my hands or shower and then walk past that bedroom and smell it. The pipework that runs from the bedroom to the bathroom goes through some eaves, meaning that you can smell it coming through there but it seems to be emanating from the bedroom first.

A few things that I've done, checked or considered:
  • Hot water from the tap or shower doesn't seem to smell or taste bad (filled a cup, taken it outside and tested)
  • The property used to have connecting pipework in that bedroom to a water tank in the loft. That has been cut (before I moved in) and has been drained/dried out, no smell. The working pipes, coming up from the boiler in the kitchen have then been connected with plastic pipes to serve the floor's network. As far as I can tell, there are no dead legs under the bedroom's floorboards.
  • Since I moved in there's always been some kind of pressure issue that I've never looked into. The bathroom shower is generally fine (sometimes I'll notice a slight dip but nothing much) but the sink tap when turned to a low flow, stutters or glugs. If I turn the flow up it stops.
  • The other day I disconnected the radiator in that bedroom (to skim). The water drained from the radiator was clean. The last few glugs as I tipped it up, kicked out some black residue but nothing sludgy. And it didn't smell.
  • The boiler is a Worcester 24i Junior GC 47 311 86 Natural Gas. I've read about anode rods etc. I can't work out if this boiler has them but surely if there was a problem with the boiler it wouldn't be intermittent and its likely that I would have the issue downstairs too


Not really my area but since no one else has answered yet...

From the information you've given I don't think you can 'rule out' much. My guess is that you probably have a leak somewhere in the pipework, e.g. an incorrectly assembled joint, and inadequate venting. So, running some water increases the pressure in the pipework and forces sewer gas out through a leak.

I'd start with a careful visual inspection. If no joy, either a pressure/smoke test.

We can probably help more if you provide some photos / diagrams showing how the system is configured. It maybe that it's simply not installed correctly, e.g. wrong falls, incorrect ventilation but it's difficult to diagnose such problems from a text description.

To be honest, you don't want sewer gas in you home. Don't mess around for too long before getting a professional who knows what they are doing in to help.
Boiler been serviced ?
Hi all,

@ShaunCorbs As far as I can tell, not since 2017 (full flush was carried out)

@Chuck Here's some more details. I've numbered the pipes to help.
  • PDF detailing second-floor network.
  • A few photos. The only thing that looks odd to me, are 7 and 8 which disappear downstairs towards the boiler but they go behind the wall, whereas 1, 2, 3 & 4 do not
Let me know if you think anything is odd




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First off let's identify the smell so that we all 'think' we know the smell you're describing.

I know you said sulphur but are we talking:

Hydrogen sulphide rotten eggs, stick bombs etc? The sort of smell that you might get from wetting the products of combustion or other sulphur containing materials, old batteries etc.

Or is it more a smell like rotting or decomposing organic matter in a drain, or stagnant water.

I know it's difficult to describe a smell but might be worth a go.
When I originally read your first post, I couldn't understand why you were concentrating on the smell coming from your central heating system. My first thought was that it would be from the drains. Maybe coloured by having drain smell problems in our property.
The obvious thing to check is to ensure all traps are full of water (unless you've got Hepvo traps), sinks, basins, showers, any separate vertical wastepipes for washing machine/ dishwasher etc. Particularly looking at any that are infrequently used.

There should be a vent open to atmosphere (outside the building!) somewhere in the drainage system, possibly even in an adjoining property. The reason is to relieve any occasional positive pressure in the system, and without this, smells can find their way by blowing past traps, or through a faulty Air Admittance Valve, or through any leaking joint, into the house. It's become the norm to install AAV's in new bathroom or toilet conversions, but they are only to allow air into the drainage system, to stop a vacuum forming from say a toilet flush, which could suck water out of a trap. So here again, a 'stuck' AAV could result in a trap emptying enough to allow smells into the room.

I bought one of these to track down the source of sewer gas. It can be a bit sensitive to set up/ adjust, but it did work!
Amazon product
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That would do it need extending externally
Would an air admittance valve work here?
Yes. It would certainly improve matters regarding smell.
I think many would fit an AAV in your situation.
In theory there should be a vent to atmosphere somewhere in the vicinity, so if you, or even your neighbours, have another open soil stack on the outside of the house, I believe an AAV would be in order here.
Is there any evidence that the pipe in your photo used to go through the roof? Another issue to consider is if there are any opening windows above where the pipe would come out.
If you have another vent to outside then an aav would be fine

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