Starting the process of buying a house is a daunting task. Beyond all the forms and stress, are all the inspections. I’m learning there are many household issues that relate to heat and airflow. As a CFD engineer, and a soon to be cash strapped home owner facing a laundry list of repairs, I wanted to prioritize and investigate the various issues using the various CFD tools at my disposal. So please follow along with me while I use some company time analyzing the various ills of my property in my series of blogs that I affectionately call “this old CFD house”.
For my first investigation, I’ll be looking into a problem discovered during the general home inspection. In California, most houses don’t have basements, I think due to earthquake concerns. The houses are 2-3 feet above the ground on piers/posts and have a perimeter foundation. This under floor space is known as the crawlspace, and seems to be a common area for household issues, likely because it’s out of sight.
Our issue is related to the heating ducts. In northern California it does get a little chilly in the winter months, so heating is still a concern. Our furnace pushes air downwards into ducts in the crawlspace that then distributes the air throughout the house via floor registers. According to our home inspector, the ducts should be 4 inches above the ground. Ours?? Directly on the ground in many spots. If you’re like me, you’re instantly thinking about how much worse heat loss from conduction is to convection. I learned in an army first aid class that one blanket under someone is worth 5 on top, but I’d never use this rule of thumb to infer that raising the heating ducts will decrease my heat losses by 5 times. Convection is highly dependent on airspeed so impossible to use general rules of thumb to estimate it in a complex system. Therefore I was interested in quantifying this heat loss using CFD. As I didn’t know the exact layout of the heating ducts, or the conductivity of the crawlspace soil, or the exact CFM or BTU’s of my furnace yet, I had to do some estimating. I created the entire house as a “system level” model, show here.
From that, I created a zoomed in model with a simple duct from the furnace room to the family room as an estimated duct line, shown below. I researched and found a typical CFM for a furnace is about 1000 cubic feet per minute, and estimated there would be about 1/5 of the air going towards the family room/breakfast nook/kitchen area, so the airflow through this duct was 200 CFM. I defined a set inlet air temperature of 74 degF at the start of the duct, as it would make for easy comparison by simply looking at the final temperature of the different cases. Then I ran 4 cases in FloVENT, our HVAC CFD software: a bare duct conduction, 2 inch fiberglass insulation conduction, bare duct 4 inch airspace, and finally 2 inch insulation 4 inch air space.
As shown in the table above, the world of thermodynamics is a strange place. Whether the duct is on the ground or above it, as long as it’s insulated, it doesn’t matter much. Because the conductivity of dry soil is pretty low at 0.36 W/m2K, it doesn’t conduct much. Its conductivity isn’t that much different than the conductivity of air at 0.026 W/m2K. Of course, this depends on a lot of the estimates I made initially. If the soil conductivity increases substantially, say due to moisture, then conductive heat transfer will increase. And, this is just a heat transfer point of view as we are ignoring key factors like if the insulation gets damp or degrades. But we also estimated the ductwork was in constant contact with the ground, a worst case scenario. Still, I know the ducts are wrapped with insulation, and the winter is almost over, so I’d like to procrastinate on this chore till another day, and now I have the results to prove it when my lovely lady asks me to get in that dark, muddy, spider filled crawlspace to fix it.
Join me next time when I investigate my leaky chimney flue. Home energy efficiency nightmare? Or a non-issue, hyped by the chimney industry’s marketing machine?