You set your thermostat, you hear your furnace come on, and you feel warm air coming out of the registers. All must be OK with the system, right?
Maybe not.
The furnace and the thermostat are only two of the elements in the system that heats and controls the warm air in your home. The third is the duct system, and even though it’s delivering warm air to the registers, there’s a pretty good chance that it’s not doing it at peak efficiency. And every bit of warm air you’re losing is money that’s coming straight out of your pocket!
Start with an inspection
Whether you do this yourself or pay a home inspector or, better yet, a licensed HVAC contractor to do it for you, tuning up your duct system begins with a thorough inspection. You may be surprised to find what age and even damage from others who’ve worked under your house have done to your duct system over the years. It’s going to mean some crawling, but it’s worth it.
Turn the furnace on, even if it’s just to the "fan" setting. That way there’s air moving through the ducts, which makes it easier to both hear and feel any leaks. Take a strong light — preferably a cordless one so you’re not dragging a long extension cord around behind you — and follow each duct run from the furnace all the way to the end.
If you’re not planning on doing the repairs immediately, plan on also taking some bright pink or yellow flagging tape with you. It’s available at any home center, and you can use it to mark any problem areas so you can easily find them later.
Look and feel for areas where joints between pipes and fittings may have come loose, or where there may be small gaps. Also look for areas where support straps are missing, sagging, or otherwise not providing adequate support for the ducts. This is especially important with flexible ducts, where large sags or kinks in ducts can impede air flow.
Another thing to look for is areas where insulation is missing or nonexistent. You’ll also want to make note of any areas where the ducts are resting directly on the ground, as contact with the cold, damp soil can wick heat out of the ducts, and can also cause rust and other moisture-related problems.
Inspect each of the boots, where the duct attaches to it. Again, look for loose joints, missing insulation and sagging ducts. Around the boots, look to see that large gaps are not present where the floor or ceiling was cut out to accommodate the fitting. Occasionally you will find a floor that has been cut too large for the boot and then not patched, which can leave an opening for drafts to come into the house from unheated areas.
Also inspect the plenum, which is the large sheet metal box attached to the top or bottom of the furnace from which all the ducts originate. You’ll want to check that the plenum is fully insulated all the way around, and that all of the ducts are well sealed at the connection points.
Be sure that you inspect the garage and unheated basement as well. These areas are sometimes assumed to be part of the house, and people have the mistaken impression that ducts there don’t need to wrapped. But remember — if it’s unheated space, ducts that are not insulated will lose a tremendous amount of heat into the surrounding air, so you’ll have a nice warm garage or unfinished basement, and higher utility bills to go with it.
Make the repairs
Put together a simple repair kit of some basic tools, including a hammer, tin snips, utility knife, cordless drill, and anything else you may have identified during your inspection.
You’ll also need some short sheet-metal screws, a roll of metallic foil duct repair tape (get the good stuff, not your basic, run-of-the-mill gray cloth duct tape), and some duct strapping. Assemble everything into a 5-gallon bucket or other tote that’s easy to work with under the house.
In hard ducting — solid sheet metal as opposed to flexible ducts — repair loose joints using sheet-metal screws. After the joints have been secured, seal them up using the foil tape. Be sure there’s air moving in the ducts, and check to be sure you can no long hear or feel any air leaks.
Flex ducts typically use a clamp system to secure the flex duct to a hard duct. If a flex duct joint has come loose, check to see if you can reuse the original clamp. If you can’t, you can typically use a large worm-drive clamp or a flexible plastic clamp to secure the joint. After re-securing, wrap the duct’s inner insulation blanket and outer shell back into place to cover and seal the joint.
If ducts need to be re-supported, use duct support strapping that’s made for this purpose. Attach the strapping to a joist, girder or other solid support, using nails or screws. Don’t use wire or string, as it doesn’t provide adequate support and can also pinch and kink the ducts. Make sure all of the ducts are up off the ground.
Insulate the ducts
Remember that you’re pushing heated air through ducts that live in a very cold environment under your house or up in your attic. That heated air is constantly trying to move out of those ducts and into the unheated air that surrounds it, and when in goes, it happily takes your hard-earned dollars with it.
The best way to prevent that from happening is to insulate the ductwork. All of your ducts, both hard ducts and flex ducts, should be insulated to at least R-8, and I prefer R-11 if you can get it. If you live in a warm climate and you’re not worried about losing heated air, remember that the reverse is true as well, and you’re also wasting money whenever you lose air-conditioned air.