Old duct work is leaking
I live in a 1961 year old peer and beam one story house in Dallas. about 3500 sq foot. I platoon living here for 15 more years then it will bulldoze it for a new house when I sell.
Lots of dust penetration, Have pella windows and doors. dust is coming from all of the air system. The returns under the floor space that have been newly mastic and sealed. Two main units are in the hallway- 2011 Traine system. Office new in 2014 up in attic.
The majority of the dust is coming in from the old metal duct work(attic) with wrapped 1961 fiberglass insulation that is not intact in some places- gaps. Have done thermoimaging and it shows where the duct work is leaking- and sucking in old original gray attic insulation into the house. And in some places you can visually see it.
I need honest advice.
3 bids AC people and 1 from Insulation co.
One advised to do an attic rehab that included removing the old metal duct fiberglass insulation, patching/mastic and then spraying with closed cell to insulate and re-seal.R- 13, among sealing all seams/open areas of pipe electrical insulation, covers for the can lights. Sealing pocket door’s, top plate sealing etc. First step was to remove the old nasty(it is really bad and filtering into the house) attic insulation with rodent and pest droppings then reinstalling with cellulose R38. Along with spray foam on the vaulted ceilings “knee walls” that butt up to living space. (includes attic tents for attic access). Will the closed cell foam rust my metal air duct’s? Estimate $18,500 to 20K
Second is to remove all the old hard pipe duct work, add new R-8, two new plenums with dampers, (third unit is office only and new in 2004), sealing boots and changing vent registers if needed. Add in 3 Air ranger air cleaners. Remove and reinstall 18 inches of cellulose insulation and 6 in batting insulation on the vaulted walls. Estimated price $16,701. Does not include attic door tents (will need to add) No can light covers
Third bid- aeroseal attic duct work $7800, If mastic seal on exterior is needed of the duct work remove and replace old insulation at 5 foot junctions additional $4704 add in Micro Power Guard air purifier each unit x 3 for a total of $2625. $15,129. Does not include any insulation removal or re-install.
Waiting on the blow in insulation bid. Might use fiberglass vs. cellulose?
Thank you for your help.
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Replies
Odds are pretty good that without all the parasitic load of leaky ducts located in the attic the design cooling load is about 3 tons, maybe a little less, for just the conditioned space. You probably have what, 5-6 ton's of air conditioner, to cover the parasitic duct load/losses with some margin to spare?
In my neighborhood you can get 3 tons of fancy cold climate heating/cooling ductless heat pump for substantially less than most of those quotes. Your best bet might be to mothball or scrap the existing systems and install high efficiency ductless. The better class cold climate ductless runs $3000-4000 per ton (all in, installed price) in my area, simpler versions or cooling-only systems usually come in under $3000/ton. No ducts, no problem!
An aggressive Manual-J load calculation performed by a competent third party (and NOT an HVAC contractor) would be the first and most cost-effective step. That costs $500-1000 in my area, depending on complexity. There are competent people who can do it for that online as well, if given sufficient information about everything from shingle colors & R values to shading factors and window-orientation & type, etc.
A couple of years ago I was involved with a project on a ~3000' house in a gold-plated community in MA (Martha's Vineyard) where the eventual solution was 6 ductless heads, two compressor units, 5 tons of compressor total (needed for heating capacity, not cooling), that came in just under $15K. I'm pretty sure you can get a ductless solution for your loads for less than that. (Their Manual-J cost $800, so call it $16K total cost.)
If adding attic insulation, cellulose is usually quite a bit more air-retardent than fiberglass blowing wools, and cellulose is opaque to infra-red radiation off a hot roof deck, whereas fiberglass is somewhat translucent, absorbing that heat in the upper inches. The partial translucency results in a higher temperature an inch or so into the fiberglass than the attic's air temp, so you're insulating against a higher temperature with an inch or so less insulation.
Sanity checking on the AC sizing. This graphic was compiled by Allison Bailes (working in the Atlanta GA area), of the results of Manual-J's performed by his firm on a variety of houses, most of which are in the gulf coast states:
https://www.greenbuildingadvisor.com/sites/default/files/images/Bailes%20graph%20for%20Manual%20J%20blog.preview.png
The middle of the cluster in the 3000-4000' house range is about a ton per 1300', which puts a 3500' house in the 3 ton range, give or take.
Bailes is a frequent contributor to this site, but also has his own blog:
https://www.energyvanguard.com/blog/air-conditioner-sizing-rules-of-thumb-must-die
My Old Home,
I'm providing a third vote in favor of abandoning the bad ductwork and installing a couple of ductless minisplits.
Your description suggests the house does not have much value. Is that accurate? Do you live in the entire 3,500 square feet, or is there a lot of surplus space?
With your plans to move and bulldoze, I would be reluctant to sink too much money into air sealing and insulation. Perhaps you can do some or all the duct sealing work yourself. That would certainly help with the dust. But it might make more sense to abandon the forced air system and simply install some window units or a couple of split minis.
Thank you to all for the good feedback.
I appreciate it so much.
Sincerely, My old home.