What exactly is a dew point, and how does it affect my boiler?

You hear it all the time from your local meteorologist: "today's dew point is..." But what exactly is a dew point, and what does it mean? Chances are, you’ve observed a dew point in action a million times, but never thought twice about it. If you’ve seen condensation on a cold glass on a summer day, or droplets settled across your lawn on an early morning outing, you already know the effects of a dew point. Basically, a dew point is the temperature that air must be cooled to in order for water in the air to turn from vapor to a liquid. That liquid appears as the dew on the grass or the puddle on your coaster (if you’re following your mom’s rules). However, despite this seemingly simple explanation, the dew point temperature is determined by a lot of factors, including the outdoor temperature, humidity, and air pressure. 

Dew point is closely associated with the “relative humidity,” or RH, which we’re all familiar with seeing listed in our weather apps, or experiencing on a muggy summer day. The RH is a percentage of the water vapor in the air at a given time and temperature. For example, a RH of 50% shows that the air is holding half of its total capacity for water vapor at the current temperature. What makes RH a bit tricky is that as the temperature falls, air is capable of holding less water, meaning that a RH of 90% at 5ºF and 80ºF would feel vastly different in terms of that sticky, humid feeling you’re used to thinking about. Because of this fluctuation, dew point is a more reliable way of measuring humidity. 

So…. what does it mean in real life?

Despite feeling a bit scientific and easy to disregard in daily life, the dew point can actually tell us a lot of things, like if it's a good day to repair and paint that door you’ve been meaning to get to. As a rule of thumb, you should not paint when the dew point is within a few degrees of the air temperature on warm days, it will be too damp and won’t dry properly! The excess moisture in the air can also cause wood to expand, causing doors to swell and stick.

Another way a dew point is important is in terms of your boiler. Many modern boilers offer the ability to condense (learn about condensing boilers here). To allow your condensing boiler to work at its highest efficiency and save you the money it promised, it must be able to condense that dew from the air within the heating system, meaning - you guessed it, the dew point comes into play!

When a heating system is being designed, or a new boiler is being installed, the system designer or installing contractor should pay close attention to the temperature of the water coming back into the boiler - if it’s too high, the dew point will not be achieved, and the water will remain vaporized and escape up the chimney. However, when the water that flows to your radiators can be lowered enough for the dew point to be met as it returns to the boiler, magic happens. The water stored as vapor is able to condense into water droplets on the boiler’s heat exchanger (see a picture of that to the right), releasing all that additional energy, stored in the form of heat, back into the heating system. This keeps the fuel usage low, and the efficiency high! 

The byproducts created when burning natural gas to fire your boiler have a dew point of around 130ºF. This means that in order to get that condensation to appear on the heat exchanger, the water cycling through your radiators must be 130ºF or lower. Find a trusted contractor, and ask them to ensure that your boiler is set up to condense properly, or to learn more about condensing boilers, check out this blog post, here.