Thread: HVAC Filter location

I've never seen it or heard about it but after today I can never say "The filter is always before the fan" because there it was at the outlet side of the horizontal furnace tucked in a filter housing within the duct box. Who's seen that?

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The filter is there to protect the system, not to protect the occupants. That needs to be corrected.

I should have removed the fan cover just to see the filth.

One of the issues this arrangement presents is excess resistance to the blower fan. The system will be less efficient and there will be more strain on the blower motor than was intended.

Not only is this system not properly filtered it is paternally creating a situation where the fan motor can be overloaded. The load on a motor trying to "suck" a vacuum drops off as the air supply is restricted, as when a filter is filling with debris. When the fan motor is required to maintain a head pressure (trying to push air through a filter) the motor load increases as the filter becomes plugged and the motor is trying to maintain increasing head pressures.

Ditto every comment..........but call for a complete system evaluation because if an installer was this stupid, who knows what else is wrong.

Originally Posted by Lon Henderson

Ditto every comment..........but call for a complete system evaluation because if an installer was this stupid, who knows what else is wrong.

Ya mean like the gas line?

Seriously, this could be an easy miss if your attention was diverted or you were rushing.

Originally Posted by Rich Goeken

Ya mean like the gas line?

and the vent

and the NM cable

and ...

I understand why the location is incorrect as far as the filter not catching dirt before it gets to the furnace but I do not understand why it should affect the load on the fan. Is there much difference between a fan sucking air through a dirty filter as opposed to pushing it through a dirty filter?

Originally Posted by Bert de Haan

I understand why the location is incorrect as far as the filter not catching dirt before it gets to the furnace but I do not understand why it should affect the load on the fan. Is there much difference between a fan sucking air through a dirty filter as opposed to pushing it through a dirty filter?

Bert,

With the filter before the fan, the interior of the AHU stays clean ... er ... 'cleanER' , including the blower blades.

With the filter after the fan, the blower blades will be impacted with all that dirt and dust before it gets to the filter, and this will build-up rather quickly on the blower blades when there is no filter in front of it. That dramatically reduces the efficiency of the blower and adds a heavy load to the blower.

That dirty blower reduces its ability to move air and thus reduces the cfm the unit is capable of moving.

Ah. Thanks.

Originally Posted by Bert de Haan

I understand why the location is incorrect as far as the filter not catching dirt before it gets to the furnace but I do not understand why it should affect the load on the fan. Is there much difference between a fan sucking air through a dirty filter as opposed to pushing it through a dirty filter?

Big difference when talking about a squirrel cage blower as opposed to a propeller type fan. Reducing the amount of air on the suction side will reduce the load on the fan much like on a centrifugal water pump. play with a blower and an amp meter when you get the chance, it can be an eye opener. It is kind of counter intuitive unless you under stand the physics behind air flow and centrifugal blowers.