venting in crawl space

I was in a crawl space, the duct has about a 12 inch diameter opening on top as seen on the right side of the photo. There is air flowing out of it.

This is a vented crawl space, the open is about 75’ direct opposite of the furnace itself. I recommended the home owner contact an HVAC guy for evaluation.

He stated it’s a common building practice in my area. I called my HVAC guy and stated it’s not correct.

My thoughts are this large opening is providing moisture to a vented crawlspace potentially creating an environment for mold. The open is reducing air volume to supply registers. The homeowner is spending extra energy/money needlessly heating/cooling the already vented crawlspace.

I was reading through articles in the NACHI library but could find a direct article. Any have a good resources for education? Please provide a hyper link. I’d like to learn more.


Stephen Rager

seventy five feet? If that is not a typo that is ridiculously long for an HVAC supply duct.
Your HVAC guy is right.
The rectangular duct is not insulated, another defect.
Belay that last, it looks like it may be internally insulated I can see pin spot welds

I spent 11 years installing, designing, and seller HVAC systems. I agree with your thoughts and recommendation.

Was the guy that called it common building practice from the same company that installed it?

If the duct length is 75’. I would venture a guess that you do not have great air flow to begin with. You also have to figure in the equivalent length for the fittings. They add up quickly. Most residential furnaces are only designed for 100’ of total duct length.

Thank you all for the comments and other considerations. I read them all and incorporate suggestions into my inspection habits.


Stephen Rager

Defiantly wrong. A leak in your supply will also cause the house to be under negative pressure since the system is trying to recoup the air that was just pushed outside the conditioned space. A leak in the return creates a positive pressure