Air Handler Inquiry

The ROCIS Air Handler – High MERV Filter Inquiry

The ROCIS Air Handler Intervention

Operating the fan continuously in a typical residential air handler requires certain conditions. This checklist enables the identification of deficiencies in the current system and improvements which need to be made. The appropriate intervention varies depending on the performance of the individual HVAC system. 

ROCIS 24/7 Air Handler Checklist: Optimizing Residential Air Handlers for 24/7 Filtration

In order to operate the fan continuously in a typical residential air handler, ideally, the following conditions should be met.

  1. The fan watt-draw is <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 specified on the manufacturer’s name plate.
  4. A MERV 13 fat (usually 4”) high quality air filter is 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. The duct system is primarily within the conditioned space of the building, with little evidence of duct leakage to the outside.

* Follow this checklist to identify deficiencies in the current system and improvements which need to be made. The appropriate intervention varies in response to the performance of the individual HVAC system. 

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 cross sectional area return that includes 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 air handler run-time 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

[ Checklist PDF here ]

[ AHU return drop modification diagram here ]

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

2020 Home Performance Regional Education Series: New England 
Nov. 13, 2020  
"Particle Filtration With Central Air Handlers: Folly or Opportunity" 
[Linda Wigington's presentation PDF here]
Rhett Major's presentation PDF here]
[Video here]
[Audio recording here]

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

The ROCIS Air Handler Diagnostic Visit

The purpose of this visit to clarify whether 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. 

The visit usually takes 2 to 2 1/2 hours.  If an intervention is recommended, ROCIS may be able to absorb some of that cost.  At the time of the diagnostic visit, participants find out the electrical cost of operating their air handler. Afterwards, they receive a written report with a summary of the findings and recommendations.

Depending on the system, it is possible that modifications could be made at the time of the diagnostic visit.  For example, during more than 60 diagnostic checks conducted, over 50% of the time we have made improvements to optimize the blower speed - resulting in improved comfort, performance, and energy use.

However, there are only a few cases in which we have recommended that the air handler (as is) be run continuously to provide better filtration. With the right modifications, it appears to be a very effective intervention. And yet very few air handlers are set up to provide that option without adverse consequences such as 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 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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