Re: Furnace filter
I thought I should make sure there isn't any misinformation or myths being spread. Many of the newer furnaces use Variable speed "ECM" motors. These motors match the airflow requirements more precisely than the older less efficient PSC motors. They can adjust to changing static pressure (airflow restrictions) and therefore maintain the roper airflow. Old furnaces might have often had their blower speed set higher than nessesary to make sure there was always adequate airflow.
Originally Posted by Fencepost
That being said... technically, a high efficiency furnace will need MORE airflow. Why? Because a 95% furnace puts out more heat than a 80% furnace with the same fire rate. The maximum temp rise is typically the same, so to move the extra heat, you need more air. Example... a 60k BTU 80% furnace puts out 48kBTU BTU, and at a 50F temp rise needs 810CFM. A 95% furnace the same size puts out 57kBTU and therefore needs 1060CFM at a 50F temp rise. ON an older furnace, that an entire fan speed faster on a 4 speed PSC motor. OR put this way, if fed the same amount of air, will be at a 65F temp rise, and could begin to damage the heat exchanger.
A newer furnace might seem like it needs less airflow because those with VS fans are almost always 2 stage. SO on low stage the heat output is lower. Since most furnaces are oversized, they rarely go to high stage when maintaining temp. Actually low stage is 70% capacity, so even on a proerly sized system it won;t be in high stage very often either. why it might seem that it needs less
1925 Two-Story Stucco Beaux Arts Neoclassical