Thanks for this important

Thanks for this important caveat, David. It’s worth its own article so I’ll add that to the list of articles in this series.

Great question, Steve. That

Great question, Steve. That extension is there for very good reason. Yes, you could put an elbow there for the last takeoff, but it’s generally better for static pressure in the trunkline not to do that. And you don’t want to have the last takeoff right there at the end so you extend the trunk 6″ or so beyond the last takeoff. When air hits the end of the duct, it creates turbulence and you want to take the air off into the last branch before it hits the turbulence.

Good questions, Steve. I’d

Good questions, Steve. I’d have to go back and see exactly which design that was from to know if it’s been installed or not. Sorry. I don’t recall. Also, the photo, duct schematic, and effective length table all are from different homes, and the photo isn’t from one that we designed. In the last article, Mark Johnson made the reasonable request for a case study so once I get through the basic principles here, I’ll do that.

Absolutely true. However, most designers miss an important step when considering equivalent lengths of fittings (junctions, elbows, boots, etc). The values given in the Manual D Appendix 3 tables assume a velocity of 900 FPM and a friction rate of 0.08 IWC (700 / 0.08 for return fittings). However, this ignores the fact that fitting EL’s vary dramatically with velocity, and even if ducts are designed around those values (unnecessary and often a bad idea), velocities and friction rates will necessarily vary throughout the system, so you’re guaranteed to have the wrong EL’s in some or most cases.

Prior to 3rd Edition of Manual D, no one seemed concerned about this simplification. As long as you don’t exceed 900 FPM, the table EL’s are conservative.

In 2008, ACCA issued Technical Bulletin 2008-04, which goes into depth explaining what happens in a duct system when velocity is low, and why Manual D doesn’t stipulate minimum velocities (see my comment in part 2 of this series). This bulletin became Appendix 15 in the latest (3rd) edition of Manual D. Also found in 3rd edition is Appendix 4 “Fitting Equivalent Length Adjustments” (for when velocity and/or friction rate is lower than assumed for the Appendix 3 tables).

HVAC systems designed for homes built to current energy codes are likely to operate at significantly lower than 900 FPM, and as ECM blowers have become much more common, it’s even more important to adjust fitting EL’s for actual velocities as per Appendix 4. Unfortunately, popular duct design software** doesn’t implement these adjustments (although it would be rather trivial to do so). Sigh.

** I haven’t checked this is a while. I trust someone will correct me if any duct design software has implemented these adjustments.

I notice the main horizontal run in the layout ends not in 90* elbows, that with turning vanes would seem to be a better option, but at a termination that provides a short duct extension preceeded by a Tee.

What’s up with that?

You state the layout drawing above is “one we have done recently” and I must ask, are you referring to the drawing or has this system been installed and commissioned? I’m asking because it would be informative to know the outcome of your efforts, seeing no balancing dampers in the drawing. Optimistic, are we? 🙂

Overheard recently on NPR: A pessimist says “Things can’t possibly get worse.” An optimist says, “Yes they can!”

Thanks, Carl! We’ve done a

Thanks, Carl! We’ve done a few WaterFurnace designs.