How hard is HVAC ductwork for the uninitiated?

October 15, 2015 11:12 AM

The heat registers in one room of my house do not output very much air. I estimate less than 10% of the flow that comes from any other register in the house. I have easy access to all of the ductwork in my unfinished basement. I would like to open up the duct from the furnace as the first step of troubleshooting this problem. I've never done any ductwork before. Is ductwork straightforward, at least in a take-it-apart and put-it-back together sense?

I'm handy around the house (I routinely handle minor plumbing and electrical repair, bicycle maintenance, etc). The house was built about 100 years ago, with forced air HVAC installed, I'm guessing, in the 1970s. I've been there over 5 years, the furnace itself, I think is about 10 years old. It all works well except for two heat registers that are in one room. These are the only registers on one of the 'branches' from the manifold on the furnace. If I open up that duct and look around inside to start trouble shooting, is it as easy as it looks to put it back together?

Brass tacks: Winter is coming, I start a (short) business trip on Sunday, so if I open this up on Saturday, is there any chance that I'll be leaving my family with is limping HVAC system while I'm on the road?

I understand that the 'short answer' to the 'overarching question' is to 'call a pro and get your HVAC system rebalanced.'

posted by Doc_Sock to Home & Garden (8 answers total)

Have you looked to see if there is a flue on that branch? Look for a little lever on the side of the duct. This is used to open/close/adjust the flue on the inside of the duct. The lever may be the same material as the duct and thus hard to see.

Yes I know this isn't answering the question and I hate it when people do that but I just thought I'd toss it out there before you tear apart your duct work
posted by bondcliff at 11:21 AM on October 15, 2015 [1 favorite]

We hire HVAC contractors regularly, even if it's only to move a couple of supply/return air vents. They not only have the knowledge but they have all of the proper certifications, equipment, and relevant suppliers at their fingertips. They know a wide range of systems, old and new, inside out and upside down.

Also, our municipality requires a letter from a certified HVAC contractor any time we pull a permit with that sort of work involved, and a letter reading "Customer did it himself" probably wouldn't suffice. Pay a professional.
posted by futureisunwritten at 11:26 AM on October 15, 2015

I personally find ductwork easy to deal with. But I deal with odd airflow setups for my job on an irregular basis. You could probably tackle it on your own; if you can do basic electrical and pluming, you've got enough mechanical aptitude and patience to do it. This is totally realistic. Chances are what you're looking to do won't need a permit pulled to do the work…but, it easily could.

But. Given the dates of the house construction, and when the HVAC installation…you're probably looking at asbestos in some form or another. In fact, I'd put a beer down on the fact that you've got a bunch of that asbestos-duct-tape junk holding together half the pipes. This is problematic. And not something to be fucked with. Again. This is not to be fucked with.

Depending on your jurisdiction, you working on this, and finding asbestos can do any of the following: void your homeowners insurance until you've professionally abated, be liable for cleanup and abatement down the line, not be able to sell your house until you proceed with professional abatement.

In certain jurisdictions, even knowing about the asbestos triggers your legal requirements to abate, ASAP.
posted by furnace.heart at 11:33 AM on October 15, 2015 [2 favorites]

You are probably not going to find that anything is blocking the duct. You lose pressure the more turns the duct has and the further away it is from the blower so if your low output ducts are the furthest ones from the furnace and the ductwork has a lot of angles then that would explain the low output.

If output is low everywhere it could be a mechanical issue with the furnace/blower.
posted by eatcake at 11:59 AM on October 15, 2015

I have done my own duct work, in the attic of my home. This is with the soft insulated tubes, rather than the sheet metal ductwork, however. I purchased the ducts in boxes at the local big-box hardware store, and duct sealing compound. I was also able to get some of the sheet metal connections I needed there.

My task was to replace ducts that had deteriorated over time, and were leaking. I also replaced a duct that had been misinstalled decades before. I had to build a plenum for one connection, which was relatively simple, with sheet metal flashing and sheet metal screws. I wrapped the plenum with leftover duct work for insulation. One tricky aspect is connecting the duct to the ceiling outlet. In addition to duct sealing compound you can use large zip ties to attach ducts to outlets.

It was messy work, and I had to redo some of it. Go over your work with your hand, feeling for leaks when the system is running. Tie your work out of the way so that it does not get damaged.
posted by Midnight Skulker at 12:19 PM on October 15, 2015

I work on an energy efficiency program that provides rebates for properly-performed HVAC work. Because my work is focused on quality, safety, energy efficiency and room comfort as an the outcome (as opposed to low-cost, incremental comfort improvement, or DIY), my program would recommend that you have a professional contractor do this work.

(I can't say whether a DIY project would provide sufficient incremental improvement. I can say a DIY project would not provide high-quality improvement.)

With HVAC, installation quality is critical to getting safe and reliable results. In my area, any duct work will require pulling a permit for safety and compliance purposes (this does vary by jurisdiction). Additionally, for homes of your house's era, we almost always identify asbestos concerns with duct work. This is bad because asbestos causes problems when you breathe it in, so you definitely don't want it near your air ducts and recirculating through your home. Professional abestos abatement is cost effective and worth it (and required) when paired with an HVAC upgrade job.
posted by samthemander at 12:48 PM on October 15, 2015 [1 favorite]

One thing you might check, forced air systems need a return air path. If someone covered up these registers, you'd get some heat, but the airflow would be diminished.
posted by Marky at 2:41 PM on October 15, 2015 [1 favorite]

I think I'd check really close for leaky ductwork before I took it apart... this house is a young 85 years old, HVAC installed probably about the same time as yours. Round sheet metal ducts. I was underneath doing some re-wiring, a couple times I lightly bumped a duct, it fell apart, condensation had rusted the metal duct inside the insulation until there wasn't much left. Air was leaking out from the insulation wrapping - which isn't designed to be duct. Also found similar conditions a couple other places I checked. Ended up replacing about 20 feet of duct between two different runs, and replacing a "boot" where it turned up to a floor register. Man, hot air in the bathroom in the winter! What a treat!
Consider that if you take it apart and it's a rusty mess inside, you might not be putting the same pieces back together.
posted by rudd135 at 5:37 PM on October 15, 2015 [1 favorite]