Mystery Inspector HVAC: A Big Bend, But Not In Texas

An innocent HVAC system is choked and suffocated in a mid-south closet—its air supply slashed to nothing!

The Mystery Inspector has resurfaced on after uprooting from the Washington DC area and moving to southern New England. But that won’t limit the geographic scope of Mystery Inspections.No, far from it.

The Case of the Asphyxiated Air Handler — Justin Wilson was all over the mid south investigating a string of HVAC complaints among homeowners of 1 year old houses. This is one of the perps.

Mystery Inspector has just gained three of the nations foremost building science experts who travel all over the US and Canada consulting for builders and product manufacturers on problems, solutions, and better procedures. These guys see a lot of problems. In the course of seeing these problems, these three building science experts do the dirty work — crawl through attics, crawlspaces, and knee walls. And they send pictures.

HVAC problems in the mid-south

A furnace that was designed to push air UP, is being used to push air down into the floor system in what is typically called and down-flow configuration. This is 180 degrees different from the design. Also, the bend in the flexduct is a hard 180 degrees — all in one turn and (worse) Directly off the top of the air handler and AC coil. (I typically takes about 3 feet of straight ducts to smooth our the turbulent airflow downstream of the AC coil.

This is very, very wrong.

It seems likely that the HVAC technician who installed this air handler had little understanding of how air actually moves, or the equipment required to create a down flow system.

Why does this matter?

For every 90 degree turn in an smooth metal air duct, it typically has an equivalent length of 20 feet for smooth metal duct and 2x that for a turn in flex duct. This job has the equivalent of two 90 degree turns all together in one bend directly off the top of the air handler. It is literally off the charts for airflow resistance — nearly four times the resistance (this is a lot). This 180 degree turn with flex duct off the air handler is something that is hard to estimate or measure, but is staggering what it does to the airflow. We estimate this turn is equivalent to at least 205 feet of flex duct run straight, fully extended and uncompressed.

Right out of the gate is a turbulent place

And there’s the next problem with this: the whole ‘right out of the gate’ thing — bends shouldn’t occur closer than 3 feet from the fan. Because the friction and turbulence from bends need room to back up without pushing against the fan. When big bends are this close to the box, the static resistance of this home-made fitting create nearly 75% of the acceptable airflow resistance of the fan. Forget about the resistance of the AC coil, additional ductwork (including the return), filters, registers and grilles.

How should/could it have been done?

Clearly, this is a furnace that was designed to sit in a basement or crawlspace and push air up into the house. It is on the second floor in a mechanical closet. Here are six ways to improve on this system:

  1. Use an air handler that is designed as a down flow/counterflow condition. They are essentially upside-down from a traditional basement furnace and the return air is on the top and supply discharge air on the bottom. Now the air handler will push air directly into the duct system located in the floor (minus 206 feet of equivalent duct work)
  2. Install a coil with a counterflow condensate pan intended specifically for this purpose.
  3. Check and re-balance the system to meet the intended airflow targets from the manual J load calculation.
  4. Re-check the total system airflow of the air handler and perform a refrigerant charge test again on the system.
  5. Educate the technician who either didn’t understand, or didn’t care about, the implications of his/her actions.
  6. Gord Cooke has another post-construction engineering option: Re-duct so thet there are two 90 degree bends, the first one occurring not closer than the air handler unit, the second occurring another three feet or so down stream. This will not solve the problem, it will just reduce the effect by about half.

Links to other resources:

National Renewable Energy Lab:
Advanced Strategy Guideline: Air Distribution Basics and Duct Design
Residential Duct Systems – Selection and Design of Ducted HVAC Systems


Mystery Inspector In Other Places:


2016-10-18T21:52:56+00:00 Technical Articles|