March, 2015
Inspection News and Views from the American Society of Home Inspectors

Filters and Electronic Air Cleaners



Filters can either be conventional mechanical filters or electronic air cleaners. There are other options as well, although these are the two most common.

Filters help clean the house air, making the environment more pleasant. Filters also clean the air before it passes through the blower and heat exchanger. This helps to keep these furnace components working efficiently.

Electronic Air Cleaners Beyond Standards
Inspection of electronic air cleaners is beyond the Standard, since the Standard says we do not have to inspect these. However, we will touch on them here since they are common and most inspectors do check them. Let’s look at conventional filters first.

Mechanical filters

Materials and Types
Conventional air filters are typically made of fiberglass in a cardboard or plastic frame. There are other types of systems, including pleated fabric-type filters. Some filters are disposable and are intended to be thrown out. Other types are washable and can be reused. Let’s look at a few of the common conditions. Common problems include –

  1. Missing
  2. Dirty
  3. Installed backwards
  4. Wrong size
  5. Loose or collapsed

We’ll go over a few of the common problems.

Filters Installed Backwards
Conventional filters have an arrow on the frame that indicates the direction of airflow. This is an installation issue. The implication is that the filter is more likely to be pulled into the fan and become tangled if it is installed backwards.


The strategy for inspecting this is to pull out the filter and make sure that the arrow indicating the airflow is in the right direction. Again, as a courtesy, you can explain to your client how the filter should be installed. Do not correct a filter installed backwards. All the trapped dirt will be released into the furnace if you do. Recommend replacing or cleaning the filter and then installing it properly.

Wrong Sizing
Another common issue is wrong sizing. This is also an installation issue and the implication is that some of the air will not be filtered. The strategy is to check to see that the filter completely covers the air path. If two filters are needed, make sure both are in place.

Electronic air cleaners

More Efficient
Electronic air cleaners are more efficient than a conventional mechanical filter. They are able to capture small particulate, including pollen and cigarette smoke particles. They are not 100 percent efficient, but are better than mechanical filters if they are well maintained.

Location and Description
The electronic air cleaner is located in roughly the same place as a mechanical filter would be. It includes a metal cabinet and an electrical connection (120-volt). There is a power switch and sometimes a test button on the outside. There is an access door to get at the internal components.

How They Work
These cleaners operate with an electrostatic charge. The small wires carry a high-voltage electrical charge and the dust particles passing those wires receive the same charge. The collector plates downstream of the wires have an opposite electrical charge. The dust particles are attracted to the plate because of the opposite charge.

Clean Frequently
The dust collects on the plates until the units are cleaned. Because these filters can be quite efficient, they need to be cleaned frequently, perhaps more often than conventional filters, since electronic filters collect more dirt.

Charcoal Filter for Odors
Sometimes there is a small, charcoal filter downstream of the cells to pick up ozone that is produced by the static charge. This is typically replaced from time to time, but is not an integral part of the system. Problems we frequently encounter on electronic air cleaners include the following –

  1. Dirty
  2. Missing components
  3. Damaged cells
  4. Inoperative
  5. Improper orientation
  6. Miswired
  7. Restricted Airflow

Again, we’ll go over a few of the more common problems with electronic air filters and discuss them in terms of causes, implications and strategies for inspection

Missing Components
If the cells or prefilters are missing, this is usually a homeowner mistake. Sometimes this is because the units have been damaged. The air cleaner will not work properly with missing cells. In many air cleaners, there are two cells, installed one behind the other. Make sure when you push the cell in that it goes all the way to the back of the cabinet. You don’t want air to get around the cell. In addition, make sure there is a prefilter located upstream of the cells.

Damaged Cells
This is usually caused by careless cleaning. The damage is usually to the wires on the cells. The plates can also be damaged. The efficiency of the unit will be reduced where wires or plates are damaged. When you take the cells out, look for damaged wires or plates. If several wires are broken, repairs may be warranted. Many feel it’s not worth the cost of repairing just one broken wire.

Installed Backwards or in Wrong Location
The filter should be installed in the return air duct just before (upstream of) the furnace. When you take the cover off, the first thing the air stream should hit is the prefilter. The air should go through this filter first and then go into the electronic cells. The arrows on the cells should point in the direction of the airflow.

Just like the mechanical filters, the cause for improper location of the filter itself is an installation problem. Improper orientation or location of the filters is usually a homeowner mistake. Implications are that the unit will either not work at all or the efficiency will be greatly reduced. Strategy for inspection is to make sure the filter is in the right location and the cell orientation is proper, with the prefilter before the electronic portion. Electronic air cleaners can usually be installed in a vertical or horizontal orientation.

We have introduced filters and electronic air cleaners. How well these components work will have a significant effect on the performance and condition of the heating and cooling system, as well as the comfort of the occupants. More information regarding the other conditions can be found in the ASHI@HOME training program.