What is Dew Point?

What is dew point temperature?

In simple terms, dew point is the temperature to which air must be cooled for water to turn from water vapour into liquid water, otherwise known as dew. However, it’s not as simple as saying that dew point is X ℃ - it can vary depending on other conditions, such as the outdoor temperature, humidity and air pressure.

To reach dew point, the air must first be cooled for it to become saturated with water vapour. It is then cooled further to condense and turn from a gas into a liquid, or dew. The higher the dew point, the more moisture is present in the air.

Dew point is frequently associated with relative humidity (RH). RH is a percentage that shows how much water vapour is in the air at any given temperature, compared to how much water it could hold. For instance, an RH of 50 per cent shows that the air is holding half of the water vapour that it’s capable of holding at that temperature.

However, RH can be misleading. For example, if it’s a chilly -1.1 ℃ outside and the dew point is also -1.1 ℃, the RH would be 100 per cent. On a warmer day of 27 ℃ with a dew point of 15.5 ℃, RH would be just over 50 per cent. You would expect the day with the higher humidity to feel more muggy, however it would actually feel more humid in the latter example because the dew point is much higher. At higher temperatures, the air can hold more moisture. This is why dew point has become the preferred way of measuring humidity over RH.

Most people would feel comfortable at a dew point of 16 ℃ or lower. Any higher than this and you may begin to feel ‘sticky’ and uncomfortable. This is because the volume of water vapour in the air slows the rate at which sweat can evaporate, keeping the body from cooling down.  

What does dew point tell us?

So, what can dew point actually tell us? It can be used to determine a range of things, from the height of the base of certain cloud types in meteorology to when you should and shouldn’t  paint the radiators, walls or doors of your home.

As a rule of thumb, you shouldn’t paint your home when the dew point is closer than three degrees to the air temperature. This means that if it’s 16 ℃ outside and the dew point is 14 ℃, you should leave the painting for another day. A high dew point can cause condensation to form on the wet paint, potentially causing problems such as streaks, fading and yellowing (with white paint or gloss).

When you use your shower, the air becomes saturated with water vapour very quickly. It can reach 100 per cent RH at a fast rate, so the air cannot absorb any more water vapour. When this happens, condensation will begin to form on a surface that is cooler than the air temperature, such as tiles, unless you  increase the room temperature. As previously stated, the higher the temperature, the higher the dew point and therefore the surrounding air is able to hold more moisture.

This is why good quality insulation is so important. A house with poor quality or little insulation will be much more prone to condensation. This is because the walls and ceilings may be cooler than in a well-insulated house, meaning the air inside will reach RH more quickly. A house with sufficient insulation will be warmer, meaning the air can hold more vapour

Why is dew point important in boilers?

With regards to boilers, dew point is important as it allows the heat exchanger to work at its most efficient; the condensing of the water vapour releases additional energy into the heating system. The dew point of the byproducts created when burning natural gas is around 55 ℃. Therefore, in order for the boiler to work as efficiently as possible in condensing mode, the surface of the heat exchanger should not exceed this temperature. This is because this is the temperature at which water droplets could form.

To prevent the temperature of the heat exchanger rising above 55 ℃, the  temperature of the water  needs to be lower than this so that your boiler can work at up to 98 per cent efficiency.