Dew Point and Acid Dew Point

We are all aware of condensation when we wake up on an autumn morning and the lawn outside, the bedroom windows or van windscreen is covered in water droplets. We see the same thing when boiling a kettle near the kitchen window, where water vapour in the form of steam condenses back to its liquid form and runs down the window. Digging deep into memories of science lessons at school, we might recall something called the water cycle. The sun shines, causing warm air to rise carrying water vapour with it. This warm moisture loaded air cools as it rises, the tiny droplets get bigger and bigger until they eventually fall as rain. The point at which the vapour reverts back to a liquid is called the ‘Dew Point’. 

This phenomenon is important when applied to chimneys, fuels, and fires. Burning any hydrocarbon fuel (gas, oil, wood, coal etc.) produces lots of water vapour in the flue gasses. E.g. burning a single molecule of methane (natural gas) will result in two molecules of water vapour. You only need to look at the white plumes of steam coming from the balanced flue of a gas boiler on a cold morning to see just how much water is created. Any additional water in solid fuel (e.g. wood at 20%) will also pass up the chimney. Sweeps can better understand certain chimney problems with a basic understanding of dew point. If the water vapour in flue gasses cools too much, then we get liquid water droplets forming in the chimney at the ‘Dew Point’. This is bad news. By introducing moisture in the form of water droplets into a flue system all sorts of weird and wonderful things begin to happen. 

Generally, flue gases from the combustion of coal, gas, biomass or oil is made up of Carbon Dioxide (CO2) and Water Vapour (H20). Also present is gaseous Nitrogen (N2) and excess to combustion Oxygen (O2). There are small amounts of particulate matter, Carbon Monoxide (CO), Nitrogen Oxides (NOx) and Sulphur Oxides and Trioxides (SO2 & SO3). The sulphur compounds can combine with water vapour to form gas phase Sulphuric Acid (H2SO4).  

Low flue gas temperatures will allow any acid gasses to reach their Dew Point and condense inside the chimney. These acids are capable of damaging stainless steel, ferrous metal, masonry, and even glass.   

For sweeps, the important thing is to understand is that the lower the flue gas temperature and the higher the moisture in the gasses, the bigger the potential problem. Understanding the factors that contribute to high moisture and low temperature combinations can help you give practical advice to your customers and get a good result.

If you found this information useful, this topic is just a small section of the many technical areas covered in our training courses. Our Refresher course is open to anyone and packed with up to date information and industry best practice. You get to spend an interactive day (online option) with a group of very professional sweeps. Refresher Course 2021 – The Guild of Master Chimney Sweeps