boost your furnace output with this simple trick

if you have a room that’s always a little colder in the winter or warmer in the summer, try this simple trick to boost your furnace’s blower speed.

it’s a common occurrence in the heat of the summer or the cold of the winter — one room doesn’t get as warm or as cool as the others. it can happen for lots of reasons, from improper furnace sizing and dirty filters to air duct insulation failures to the room being farthest from the furnace.

fortunately, there’s a simple fix if you look at your furnace’s integrated furnace control (ifc) board. this trick only works on gas furnace systems with a multi-speed blower.

to understand how this works, let’s look at the wiring on your furnace’s blower. there are often five wire colors coming from your blower to your ifc. each is associated with a different blower speed. typical setups include these wire colors with associated blower speeds.

  • black: high;
  • yellow: medium-high ;
  • blue or brown: medium-low;
  • red: low;
  • white or purple: common.

furnace wiring with arrows to indicate cool heat and parkfamily handyman

to begin, turn off the power to your furnace. mine has a wall switch. if there isn’t one in your home, turn it off at the breaker box. most furnaces have a relay switch on the door cover. when you open the panel, it kills the power to the furnace.

on my furnace ifc, the black (high speed) wire is connected to the air conditioning (cool) terminal, and yellow (medium-high speed) is connected to the (heat) terminal. there are also red and brown furnace speed wires. these connected to (park) terminals, which are not used and simply hold those two additional wire speeds.

it’s not advisable to slow your furnace blower speed. if you do, you may not have enough air flow over the heat exchanger, resulting in overheating and eventually a shorter exchanger life span.

when the summer air conditioning season transitions to the fall heating season, increase the blower speed by switching the wires on the cool and heat terminals. only swap these two wires. if you remove the common and place it on another terminal, it can burn out your motor — not how you want the new furnace season to begin.

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shay tilander
shay tilander is a senior editor at family handyman. when he's not enjoying family time with his wife and three boys, he loves tinkering with projects and geeking out on electronics.