There are two elements to your heating system that need to work in tandem to deliver warm air in an efficient manner. The first is the furnace, and you’re aware of the need to change the filters and perform any other routine maintenance that the manufacturer recommends. The other half of the system — unseen and largely ignored — is the duct system, though which the heated air is delivered into your home.
The duct system has a number of components, and to work efficiently it needs to be well sealed and well insulated. Whether you do it yourself or have a heating contractor do it, taking the time to inspect the system and, if necessary, repair it can do a lot toward keeping your home comfortable and your heating — and air conditioning — bills to a minimum. Here are a few things to keep in mind during the inspection:
1. Physical damage
You might be amazed to find that people who have done work under your home or in the attic over the years may have damaged the duct work — sometimes even knocking entire duct runs loose! One of the first things to look for is physical damage to any of the ducts, including dents in sheet metal pipes, tears in flexible ducts, or ducts that have been loosened from their connection points or from the supports that hold them up to the bottom of the floor. Also part of this examination is to look at the insulation on the ducts to make sure it covers the entire system uniformly — duct insulation can sag down away from the pipes, and is also subject to damage from people and any animals that may have found their way into your crawl space.
2. The plenum
The plenum is a large sheet metal box that’s attached to the top or bottom of the furnace. It’s the delivery and transition point for all of the ductwork, and is the point where all of the main or individual duct runs connect. Plenums are typically custom made at the sheet metal shop at the time the furnace is installed, and then insulated on-site. 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.
At the end of each of the ducts is a sheet metal fitting called a boot. The boot is actually the opening into the floor or ceiling of the room, and also serves as the attachment point for the register. Here, you would want to check to see that the boot itself is fully wrapped with insulation — it’s easy for the insulation to slip down in this area, and to no longer fully enclose the fitting. You also want to look at the connection between the duct and the boot to make certain the duct is firmly attached, and that the joint between the two is sealed to prevent air leakage.
Around the boot, 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.
Ducts in garages are sometimes overlooked under the mistaken impression that the garage is part of the house and the ducts there don’t need to be wrapped. Because the garage is unheated space, ducts that are not insulated will loose a tremendous amount of heat into this area — the result is a warmer garage and soaring utility bills. Make sure any garage ducts are fully insulated.
5. Duct wrapping
If you have a heat pump or a heating system that also has central air conditioning, the outside of the duct insulation needs to be wrapped to prevent condensation. This is typically done with a white vinyl material that is wrapped in a spiral fashion around the ducts. If you have flexible ducts in all or part of the system, the vinyl wrap will be part of the duct, and may be black or gray in color. Examine all of the ducts, including the boots and plenum, and check that the wrapping is intact and covers all of the insulation. Repairs can easily be made using tape obtained from your local heater repairman — resist the temptation to use standard duct tape — it won’t hold up for very long.