Posted by & filed under chimney sweeping tips and tricks.

Hello all and welcome

In one of our previous blogs smoking back symptoms causes and cures you learned about different types of smoking back and their causes. In today’s blog we are going to look into the specifics of one such situation which is the competition for available air and back syphonage.

Firstly when looking to understand the phenomenon of the competition for available air we must first look at the chimney and how it interacts holistically with the rest of the house. Ok, so we know that when we burn our natural draught appliance it consumes air for combustion, it also warms air which then  rises through buoyancy affect and this also leaves via the chimney. This same air in general comes from within the fabric of the property after it enters through air vents so in some ways you can think of the entire house as an extended  mouth of the  fire place or stove feeding it the sir that is required.

Natural draught

Following on from above, as air exits the fabric of the building via the chimney it is likely that the pressure within will be lowered Unless it is fully replaced by X route or method. luckily a natural draught chimney only works on a very low negative pressure typically <25 Pascals and as such does not demand so much air from the building. Now let’s add a second appliance into the the same air space, it is now plausible that there may not be enough available air for both appliances simultaneously, if one of the appliances has a stronger draught due to extra heat or flue length then the weaker of the two chimneys may become the source of combustion air in a back syphonage scenario. Sucking both air and toxic smoke back into the building.

Mechanical extract ventilation

So, we learned above that a natural draught chimney has a vacuum rating of <25pa that’s a pretty small number when you consider that one atmosphere of the air we breath has a pressure rating of 101325 PA and we all know we still cannot sense or feel that, makes you wonder why the old timers say that they can feel the chimney draught with their hands?. Anyway I’m getting off track so what if we now have a scenario where we have a natural draught chimney and a mechanical extract fan in the same place. An electric extract fan is in effect a pump for air and can move huge volumes quickly, here’s a quick example a typical bathroom or kitchen extract fan with suck 47L per second  or 168m3 per hour.  So if the room has a volume of 55m3/h this little fan will empty it of all of its available air three times per hour. Now compare that to an average 5kw stove that requires no air vent within the uk, the stove will typically utilise 40 to 80m3/h.Now if we add our 168m3/h to or 40 m3/h we have a whopping 208m3/h which is hard to achieve without a sizeable vent or even an open window. In some countries in Europe it is now required to fit a magnetic switch on the windows of any room that has extract ventilation and a natural draught appliance so that the fan cannot run unless the window is open.

Appliance to be aware of that can contain mechanical fans include; tumble dryers, other boilers specifically gas and oil, kitchen over hoods, ventaxias, some mechanical air to air heat exchangers and air conditioners.


Open upstairs windows, loft hatches and leaky uninsulated roofs.

Thinking about the house a a whole again it is possible and likely that hot air within hallways and connecting spaces can and will rise up as it is heated by the appliance or central heating working like a chimney in itself. Air can  exit through the badly insulated roof, an open upstairs window specifically ones in the roof or even an open loft hatch. As this air exits the building it leaves a depressurised zone behind if the pressure is low enough then it can reverse the flow of working chimneys causing smoking back and possible poisoning by CO.

Conclusion what must you do?

  • Close all  external windows, loft hatches and doors prior to the chimney testing
  • Run all extract ventilation equipment at 100% for up to 15 minutes and measure the negative pressure in the room utilising the 4 Pascal test developed by Wohler. This can be checked with smoke but it can be messy and inaccurate.
  • Check you local laws and bylaws to see if extract ventilation is allowed in the room or the shared air space of a natural draught appliance alternatley if a window switch is required.
  • In the UK no two natural draughty appliances should share the same room, check to see if the adjoining wall gas been removed.
  • Check all air vents serving the air space and ensure that they too are clean and compliant.
  • A chimney sweep must educated themselves continuously

  • I hope that you have found today’s blog useful, if you have any questions or comments please feel free to ask. If you would be so kind as to like and share too. If you require further advice from a professional chimney sweep, clean sweep chimney service, London please feel free to ask.

Kindest regards

Daniel Hodgson master Chimney sweep





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