Why Some Rooms Are Always Cold: Ductwork in Older Houses
If one room has never been comfortable, the equipment is probably not the problem. What goes wrong in older duct systems and what is worth fixing.
By Chris Wheeler, Co-owner ·
Every house has one. The bedroom over the garage, the room at the end of the hall, the addition that never quite matched the rest of the place.
People replace furnaces trying to fix this. It rarely works, because a bigger furnace pushing air through the same inadequate duct still cannot get air into that room. The equipment was not the constraint.
What we find in older New Hampshire houses
Ductwork added to a house that never had it. A lot of the housing stock here was built with hot water or steam heat and got forced air added later, often in a renovation, often routed wherever there was room rather than wherever made sense. Long runs, sharp turns, undersized trunks.
Undersized returns. This is the most common and least understood problem. Everybody thinks about getting air to a room. Air also has to get back. If a bedroom has a supply register and no return path, closing the door pressurizes the room and the supply air has nowhere to go, so it stops coming in.
Easy test: does the room feel better with the door open? That is a return air problem, and the fix might be as simple as undercutting the door or adding a transfer grille.
Leaky joints. Older duct was assembled with a lot of confidence and not much sealant. Ten or fifteen percent of the air leaving the furnace never reaching a room is not unusual, and if the ducts run through an unconditioned basement or attic, you are heating that space instead.
Cloth duct tape, despite the name, is not a duct sealant. It dries out and falls off. Mastic or foil backed tape is what actually holds.
Uninsulated duct in unconditioned space. Running warm air through an attic in January means it arrives cool. Same for cooling through a hot attic in July.
Flex duct problems. Flexible duct is fine when installed properly, pulled tight and supported. It is very often not. Sagging runs, kinks at the takeoff, sharp bends. Every one of those costs airflow, and flex has much higher resistance than metal to begin with.
The addition on the end of the run. Somebody tapped into the existing trunk to feed a new space without checking whether the system could carry it. Now the addition is cold and so is everything downstream of that tap.
What is worth doing about it
Sealing and insulating is the best value in the category. Sealing accessible joints with mastic and insulating duct in unconditioned space is not glamorous, costs a fraction of equipment replacement, and often produces a bigger comfort improvement than a new furnace would.
Fixing return paths. Adding a return, cutting in a transfer grille, or undercutting doors. Cheap and effective.
Balancing. Adjusting dampers so air distributes according to what each room needs. This is real work that takes measurement, not guessing, but it costs far less than equipment and it is often what the house actually needed.
Replacing bad runs. Sometimes one long crushed flex run is doing all the damage and swapping it for properly sized rigid duct fixes the room.
Point solutions. Sometimes the honest answer is that the duct will never serve that room properly without opening walls, and a single mini split head in that space is cheaper and better than the alternative. We recommend this more often than you might expect, particularly for additions and bonus rooms over garages.
What we do before quoting equipment
If somebody calls about one cold room, we do not start by talking about furnaces. We look at where the ducts run, measure what is coming out of the registers, check the returns, and find out whether the system can even deliver what the room needs.
Sometimes the answer is new equipment. Frequently the answer is that the existing equipment is fine and the distribution is the problem. A contractor who quotes you a furnace without ever going in the basement to look at the duct has not diagnosed anything.
If you have a room that has never been right, call us and we will come look at the distribution first. Free estimate, and the fix is often smaller than you are bracing for.