Why Does the Air Coming Out of My Furnace Start Out Cold?

We had a call today from a client who wanted to know how to turn up the temperature on her furnace. She didn’t want to adjust her thermostat. She wanted to adjust the temperature of the air coming out of her furnace. How do you do that? She was surprised to learn that the air leaving her furnace is at a consistent, hot temperature. Generally speaking, there is no way to adjust it up or down.

As with many questions though, the answer is not always that simple. Occasionally, we can make some adjustments to the airflow which can make the air coming out of the vents seem warmer, but this can have some unintended negative impact so it is not always an answer.  During a comprehensive tune-up, the technician can determine if this option is open to you.

Then why do I sometimes feel cooler air coming out of my registers when the air first starts to circulate?

When your furnace is running, the thermostat determines when the home is warmed to the temperature you desire and turns the furnace off. The air that remains in your ductwork after your blower stops begins to cool down. When the thermostat determines that the house has cooled again it turns the furnace back on. The cooler air in the ductwork is then pushed out of the registers by the hot air coming out of the furnace. So initially, the air will always feel cool until the air in the ducts is pushed out.

In addition, the ductwork itself cools down when the furnace isn’t running, especially if it goes through a crawlspace or other unconditioned space. This problem can sometimes be helped by insulating the ductwork to keep it from getting too cold too fast when the furnace is not running.  During longer off cycles the air in even well-insulated ducts will eventually cool, too.

Sometimes we find that the duct system has some defects either with restrictions or leaks and these can have a negative impact. Gaps in the ducts can allow cold air to be drawn in and cool the air in the ducts.  Regrettably, many duct systems have been shown to be really leaky—as much as 30-40% is not uncommon. These problems can be identified during a thorough home performance assessment of its HVAC system.

If you are concerned about cooler air coming out of your registers, contact your heating and cooling contractor. The professionals at P.K. Wadsworth Heating & Cooling would be happy to help you get the most out of your heating and air conditioning system.