Diagnosing drainage odour problems

On this page we describe the most common reasons why unwanted drainage smells occur. There are some technical terms so diagrams are provided for clarification.

Click on images to enlarge

When an appliance is discharged

When nothing is being discharged

Trap Seal

Smells inside the property.

Cause 1: The water in one or more trap seals has dried out through evaporation. This allows sewer gas to escape via the plug hole in a shower, hand basin, bath, etc. (see diagrams 2 and 3) Properties not permanently occupied are especially vulnerable to this problem. There are some expensive solutions but the cheapest and easiest remedy is simply to run some water into infrequently used appliances on a regular basis and/or when you first arrive back at your holiday home.

Don’t forget that hot water heaters and central heating boilers have an overflow that may also be connected to the drainage system via a trap seal. These traps are particularly prone to evaporation due to their close proximity to a heat source. Fortunately they also usually have a valve that allows you refresh the trap quickly and easily.

Cause 2: The water in a trap seal has been depleted by siphoning. Usually happens in properties where the Soil Stack Vent is too small or has not been fitted at all (yes it does happen). This means that the partial vacuum that is formed when an appliance is discharged cannot be equalised via the vent, with the result that water may literally be sucked out of adjacent traps (see diagram1).

Cause 3: The soil stack is terminated in the loft but there is no air admittance valve. This happens quite frequently. There will be drafts in the loft space that disperse the gas but on still days the heavier-than-air components, such as Hydrogen Sulphide, may find their way behind wall boards into other areas of the property. If you find an open soil stack vent in your loft space fit an Air Admittance Valve to solve the problem.

Cause 4: A window is too close to the Soil Stack Vent. This means that in certain wind conditions odours will be carried from the vent and find their way back into the house via an adjacent window.

Cause 5: No Fosse-Septique vent. If your septic tank is not properly vented on the output side, it will be pressurised every time an appliance is discharged. This pressure may, in turn, force sewer gas out of weak pipe joints, trap seals and inspection covers.

The new regulations require that such a ventilation pipe is fitted so if you haven’t got one you should get one fitted. Anyone with a concrete fosse-septique should also note that Hydrogen Sulphide is highly corrosive to reinforced concrete and that, without proper ventilation, its continued presence will cause the concrete to crumble in a remarkably short time.

Cause 6: One or both of the vent pipes are open and not covered. At certain speeds and directions, prevailing winds will cause an open vent pipe to become positively pressurised. This additional pressure will exacerbate any or all of the causes previously mentioned. All ventilation pipes should have a suitable cover that deflects wind currents and prevents the pressure from forming. The Maxi-Filtra will eliminate all such affects due to wind direction and velocity.

Diagnosing smells outside the property.

Assuming that there are no broken pipes or badly fitted joints or covers, the most likely source of external odours is the Septic Tank vent pipe. It is performing an essential job and has to be there so don’t be tempted to block it off.

To get rid of the pong fit the Studor Maxi-Filtra. This will allow two-way ventilation but will block the sulphides that cause the smell.