Air Handler Inquiry

The ROCIS Air Hander – High MERV Filter Inquiry

The ROCIS Air Handler Intervention

ROCIS 24/7 Air Handler Checklist

In order to operate the fan in continuous mode, the following conditions should be met in a typical residential system


  1. The fan watt-draw <150 watts, ideally <100 watts, in the continuous mode.
  2. The air flow is adjusted to 300 - 400 cfm in the continuous mode, while air flow is also in the optimum flow rate for both heating and cooling.
  3. The TESP (total external static pressure) is lower than the maximum on the manufacture’s name plate.
  4. A MERV 13 fat (usually 4”) high quality filter within a filter slot that provides a good seal to minimize bypass.  The fat, high quality filter will not be as subject to clogging and will not offer as much resistance to air flow.
  5. Duct system is primarily within the conditioned space of the building, with little evidence of duct leakage to the outside.

The intervention varies in response to the performance of the individual HVAC system. 

The checklist (above) is followed to identify deficiencies in the system as is, and improvements which need to be made.

The primary actions we take to achieve the conditions in the checklist

1) ECM change-out to achieve lower watt-draw

2) Replace the return drop with a larger return that include a horizontal filter slot for a larger filter, as well as a second filter.

3) Adjust the fan setting for optimum air flow for the continuous, heating, and cooling mode. In most cases, the fan is wired to default to continuous mode. This can also be controlled at the thermostat.

Intervention Items under consideration

  1. Controlling the continuous mode in response to air quality through a smart thermostat and feedback re air quality
  2. Monitoring the system’s performance in terms of resistance over the filter and possibly watt-draw to determine when the filter should be replaced
  3. Providing for charcoal filtration in an additional filter slot in order to reduce other air contaminants in addition to particles
  4. Other control options in response to occupancy

[ PDF here ]

For additional info on the ROCIS air handler intervention, and some results to date:

Healthy Buildings Summit 2018
IEQ, Remediation, & Restoration: Research to Practice
October 25, 2018, Seven Springs, PA 
"Beyond Dilution - Reducing Exposure to Particle Pollution"
[ PDF here ]

Healthy Building Summit: Research Into Practice
November 2-3, 2017, Seven Springs, PA
"ROCIS's Low Cost Monitoring & Interventions: Insights & Implications" 
PDF here ]   Slides 28-39 are of particular interest to the Air Handler Inquiry

Linda WIgington interview at the National Home Performance Conference [Link to YouTube video here]

The ROCIS Air Handler Diagnostic Visit

The purpose of this visit to clarify whether or not the furnace and/or central air conditioner's air handler filter can be a viable tool to reduce indoor particles in ROCIS Low Cost Monitoring Project participant homes. The cost of this visit is covered by ROCIS; we recommend having it done regardless of whether or not LCMP participants are using a high performance filter. 

We have conducted 60 diagnostic checks. Over 50% of the time we have made improvements at the time of the visit to optimize the blower speed - resulting in better comfort, performance, and energy use.

However, there are only a few cases where we can recommend that the air handler be run continuously to provide better filtration. With the right modifications, it appears to be a very effective intervention, but very few air handlers are set up to provide that option without adverse consequences (very high electricity use ($60+/month) or shortening the life of HVAC equipment).  We have not seen a clear reduction in particles when LCMP participants have a high performance filter, and operate their system in the auto mode, which only runs the fan when there is a call for heating or cooling.

The visit usually takes 2 to 2 1/2 hours.  The purpose of the visit is to determine the feasibility of using your furnace/AC filter to substantially reduce particle counts.  If an intervention is recommended, ROCIS may share that cost.  At the time of the visit, participants find out the electrical cost of operating the air handler, afterwards, they receive a written report with a summary of the findings and recommendations.

Depending on the system, there is a possibility that there are modifications that can make at the time of the visit.  In the past, this has primarily been changing the blower speed.

The diagnostic visit will determine the following. The tests will check performance under different operating conditions (heat, air conditioning, continuous fan).

1.      The energy use at different blower speed settings
2.      The total external static pressure (TESP) of the entire system as well as by HVAC system component (return duct, filter, supply duct, and over the coil)
3.      The predicted or measured air flow compared to the system’s recommendation for optimal performance
4.      The potential to change the blower speed to save energy and or reduce (if appropriate) the total external static pressure
5.      The potential for the thermostat to modify the operation of the air handler
6.      Whether or not
             a) a lower or higher MERV filter, and /or deeper filter is an appropriate system upgrade
             b) an ECM replacement is a viable option
             c) duct modifications are a viable option

In addition, we measure the air tightness of the home or workplace with a blower door, and perform a visual inspection of the duct system.

Air Handler Inquiry Findings

What did we find as we have explored the feasibility to modify existing residential HVAC systems to provide effective particle filtration? The graph below summarizes the problems/barriers identified. NOTE: Most of these HVAC systems were located in a basement.


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