See how a simple mistake can cost you 10-12% on utility bills


I went to a home in Munster IN to look at the hvac system. The homeowner was concerned his utility bills were higher than than needed. When I looked at the furnace in the attic I noticed the filter access cover was missing. The homeowner said he didn't recall ever seeing a cover.  He audibly scoffed when I told him replacing the cover would save him more than any of the other measures he was considering. A missing cover in an attic allows the furnace blower to draw cold attic air into the duct work. After mixing with the house air brought through the returns the overall temp entering the heat exchanger is considerably lower making the furnace run longer than it should. I guessed he kind of believed me but didn't really want me to do any testing. Without testing I can only give some estimates but it will still make the point. 




The upstairs furnace blower should have been set at 1000 cfm. The open filter cabinet could allow 350 cfm through depending on the static pressure of the duct work, to be safe I'll use only 200 cfm.
The house is 70 degrees and the attic is 35 degrees.


So what happens? The thermostat calls for heating, starting the furnace. An 80% furnace should have a 55 degree temp rise. That just means for 70 degree return air the supply should be 125 degrees. But we have only 800 cfm of 70 degree air and 200 cfm of 35 degree air, yielding 1000 cfm of 63 degree air and supply air of 118 degrees.


What does that mean? First that seven degree loss equates to 12% of capacity and about 11% loss in efficiency, and thats at 35 degrees the colder it gets the worse the effect. Losing that much capacity and efficiency will be clearly felt in comfort and seen on high utility bills. I didnt even touch on the effects during air conditioning season 
I made some assumptions in the calculations:

1. The furnace was tuned correctly.
2. The insulation around the attic duct was correctly installed.
3. The furnace was truly 80% efficient.

Overlooking seemingly small items can add up quickly in cost and comfort.