A Remedy For Falling Into The Trap Of Bathroom Pressure Fluctuations

min read time
2021-07-11 08:52:54

As house design becomes more complex, and as more and more buildings are converted into flats, there is greater potential for pressure fluctuations to occur in plumbing systems. This might not sound too serious at first glance; however, it can lead to the standard water seal traps fitted under sinks, basins and showers failing. This can in turn lead to foul air entering the building from the drains, breaching Part H of the Building Regulations not to mention causing a health issue and being unpleasant for users.



Examples of some of the design flaws in bathrooms that can lead to pressure fluctuations include the following (which might be found for example in an adjacent bathroom and ensuite):


  • Excessive length of branch pipe from waste
  • Combined branch pipes
  • Alternative branch runs incorporating vertical drops below WC
  • Vertical drops to bath waste
  • Shallow seal traps


Wavin OSMA water seal traps

Water seal traps have been habitually installed by the industry for many years, however, they are vulnerable if 

such pressure fluctuations occur. This will cause the tell-tale gurgling which indicates that siphonage is occurring in the system and that the water seal has been broken, and foul air allowed in. As Hep vO is waterless, it prevents foul air from entering a building when siphonage might have otherwise been a factor.

Part H details the maximum length of branch pipework, which is permitted for different appliances in an unvented situation. However, installing a Hep vO instead of a water trap effectively vents the branch pipe. So, for instance, a 32mm waste run can extend to 3m without increasing the pipe diameter to 40 mm, once a Hep vO is introduced.


HepVo valve under bath tub


The 10 ways water seals can fail:
  1. Self-siphonage – atmospheric pressure causing water seal to be sucked out of the trap
  2. Induced siphonage – atmospheric pressure plus appliance causing water seal to be sucked out
  3. Compression – water discharging from above combines with a bend in the system to create positive pressure
  4. Evaporation – reduces seal depth by 2.5 mm per week
  5. Wind effect – positive or negative pressure depending on wind direction
  6. Foaming – backing up of foaming detergent causes depletion of water seal
  7. Momentum – water poured at high speed above outlet carries seal away
  8. Capillary action – caused by material hanging over the edge of the trap drawing water away
  9. Leakage – caused by damage to ‘U-bend’
  10. Movement – in mobile applications eg caravans, boats, trains


Wavin HepVO waterless trap


There are many situations where a water trap might fail, but these are all avoided with a waterless trap such as Wavin’s self-sealing waste valve HepVO. Its unique membrane design allows fresh air in until equilibrium is achieved and negative pressure removed and then closes to create an air-tight barrier between the living space and the drainage system. No venting is required and siphonage cannot occur. The product also allows straight or vertical pipe runs to be created, allowing for a more straightforward system design, in addition to longer runs from the waste, providing increased design freedom.


Book your Part H CPD today

This CPD covers an insight into the requirements of Part H, key design considerations and information on installation and testing. Book now