Does a Furnace Decrease Humidity?

Does heating the air cause the humidity in the air to decrease? Yes and no

Heating air causes the RELATIVE humidity percentage to decrease but it does not change the overall moisture content in grains of moisture per lb of air.

Many old timers will swear a blue blaze that oversizing a furnace will directly result in lower humidity, cracking furniture etc…

The problem with that theory is that no matter how much you heat the the air you don't change the overall moisture content and when you blow that air into the space it quickly acclimates with the room.

But there still may be some truth in this oversizing dries stuff out theory

In the Winter during cold climates the moisture content is very low outside regardless of the relative humidity. When you use a larger furnace than you need you also tend to move more air than you need to.

When you move more air there is often greater negative or positive pressurization of the conditioned space due to zonal imbalance and duct leakage. This pressure imbalance will tend to drive more dry air into the space or more of the inside air out resulting in lower humidity.

Neil Comparetto also pointed out that when the appliance takes its combustion air from the space this can cause significant negative pressures which also draws dry air in from outside. The larger the BTU output the greater volume of air that must come in for combustion.

The other factor is the supply air temperature itself. If the hotter supply air is blowing directly on an object it will tend to dry it out more quickly due to the increased temperature of the object itself.

In conclusion –

Furnaces don't reduce air moisture quantity directly no matter how big or small

There are other reasons why oversizing can cause issues so don't do it.

— Bryan

Related Tech Tips

Don't Use Barometric Pressure For Calibration
Take a look at the screenshots above. The one on the left is for Death Valley at 282′ below sea level and the one on the right is Denver, CO at 5,280′ above sea level. Notice the barometric pressure, they are almost the same This means that barometric pressure is corrected or “normalized” to sea […]
Read more
A job well done….. Almost
This tip is written by HVAC Applications and Technical Specialist Dakota Brown. Thanks Dakota! As a technician it was always the same old story Are you done yet? There was always pressure from the office; either from the dispatcher, service manager or project manager to get the job done and get it done quickly. When […]
Read more
Trades and the Skills Gap - A Manifesto
  This article is a year old and I'm recycling it because it's on my mind today. I had a fun conversation with Richard Trethewey on the podcast that has me thinking along these lines today. The link is HERE if the player isn't showing up. I have a confession to make. I'm a bit […]
Read more

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

en English
X