Sunday, August 25, 2013

Better In-Attic Furnace Ducting

An in-attic furnace duct system need not be the on-scene creation of an HVAC mechanic, done unhappily in darkness and danger, then defying inspection and service, perhaps never competently reviewed. The found system examined in this post, helter-skelter thoughtless, contributed to inaccessibility of the attic, and to the darkness and danger.

A fire wall defying access and good work in a "main attic" had no redeeming value, where it was necessarily violated. Someone finally threw a heavy wood door off its hinges. Two passes through the wall by a 12" flexible duct would readily have propagated fire.

The better way to block a garage fire is to complete a ceiling to required fire rating. Here an unblocked access hole and several slots through ceiling drywall were patched with superior Densarmor drywall. Safer, better access to the attic is via a one-hour fire rated Calvert ladder. New Calvert models in 2013 include fire rating at $70 average added cost.

Four fat ducts and an exhaust B-vent crossed over the main attic, chest high or lower, like the last one in this view. Tying everything to roof joists is stupid.

Half of the attic had been accessible only in a remodel, before new drywall ceilings were installed. All new drywall was left uninsulated. Who could ever know?

It is radical to think of destroying the fire wall, and taking down all ducts, as the work of a weatherization contractor. Almost none would have the courage. There is need to convince a home owner that all will be put back together, better. 

Think of the promised outcome as a circuit diagram. A map is detailed in the end by measurements vs. the regular system of decking. The beginning was a sketch upon satellite photos. Symbols are invented for the duct elements. Red for warm air ducts. Blue for return ducts. Wyes and reducers quite evident as diameter products. Take-offs I imagined but did not invent, as groups needing some consensus, here only in 12" flex ducts, as diameters leaving one or two cone saddles. Everything connects to ceiling registers, identified by diameter of serving flex ducts. The furnace is evident with its plena.

Please read on in a captioned web photo album.

The pleasing outcome is noted in these photos:

This portion of the attic, before inaccessible, was completed first. Access is via a Fakro LWS-P 25/47 attic ladder. As with the garage fire-rated ladder, the limit arm pivots were custom-set to have ladder deployment at a 60° angle. At that angle, one may carefully ascend the ladder with both arms encumbered. The many trips for proper work easily justify the modest cost of the ladder. Such a ladder, beautiful, strong, well insulated and gasketed, adds $1000 to the value of a home. One use, avoiding a fall, is worth a lot more. One moves more easily and a lot safer upon the raised floor, than upon trampled insulation hiding floor joists. Composite beam construction of decking supports efficiently aligns and levels the plywood flooring.

Please note the wonderfully-formed warm and return headers here, lined, not needing exterior insulation and not contributing to cycling thermal mass. These are not "D-boxes" crudely hacked-in by an installer on-the-job. They were fabricated by a very competent sheet metal business. For me, in Portland, Oregon, that fabricator is Vinje & Son. If you don't have such convenience in your town, Vinje & Son will serve you anywhere. Please see a subsequent post where I describe a similar installation in a crawl space . We must no longer permit thoughtless D-boxes that may be impossible of airtight sealing, robbing air flow and adding to operating cost, through high flow resistance.

I will do better with the warm header the next time a 180° return is needed. The header (next photo) was formed as two pieces, where a one-piece unit with turning vanes would not have cost more and would have served the customer much better. I was still somewhat accepting of rectangular shapes. I did at least insert a round far wall of the second 90° turn. Even without the turning vanes this is a big improvement upon an octopus D-box.

The label Skylight Insulation is added, as innovation with a skylight is reported in another album for this job, Dosch Attic. Innovation is in finding a use for old flex ducts now scrap. Scrap included about twelve feet of 12" duct ruined by crawl-upon. As a determined recycler, I separated the twisted liner wire for my bin of steel scrap. Remnant very long R8 fiberglass insulation 24" wide is useful in filling irregular spaces on skylight drywall, between on-flat 2x4s. The tough R8 fiberglass is superior to flimsy R11 commonly found, in filling 1 1/2" pockets between on-flat 2x4s.


John Terry said...
This comment has been removed by the author.
John Terry said...

Found your blog. It is full of really good information. Thank you for sharing. If you ever need service on your furnace, air conditioner or water heater please visit us at We would love it if you would have a look at some of our blogs and let us know your thoughts.

John Terry said...

Found your blog. It is full of really good information. Thank you for sharing. If you ever need service on your furnace, air conditioner or water heater please visit us at We would love it if you would have a look at some of our blogs and let us know your thoughts.

jose palmer said...

Great idea,and that would be useful in summer. But how about winter season.
Heating and Cooling Hamilton

Phil Norman said...

Efficient ducts, buried as much as possible under insulation and securely assembled, is better in all seasons.