When it comes to improving indoor air quality in the home, you have three options in dealing with pollutants. You can remove the pollutants, encapsulate, or dilute them.
- A spot ERV would be an example of solving a problem by dilution. A spot ERV may not provide enough air flow to adequately reduce the problem.
- Encapsulation would require you to identify the dust particles in the air and prevent them from entering the conditioned space by sealing potential access points.
- Elimination would involve identifying the pollutant (insulation, dust, pollens, etc.) and eliminating it.
So you have options.Before I comment on locating a spot ERV, I would like to discuss some of the other issues you raised in your question.
- The MERV 13 filter should be helpful in reducing the amount of airborne particulate, but it only works when the air handler is operating. Three-month replacement intervals are fairly typical based on manufacturers' recommendations. The frequency of replacement does not necessarily indicate a significant indoor air quality issue.
- Vacuum cleaning daily is only effective if you are using a HEPA-rated vacuum. Standard vacuums tend to recirculate dust particles too small to be captured in the vacuum filter system. Recycled particulate tends to remain in the air for hours (or even days) due to its small size (under 1 micron).
Consider a HEPA bypass filter
Consideration should be given to installing a HEPA bypass filter on your HVAC system.
HEPA filters can operate independent of the HVAC system, cleaning the air even when the air handler is not operating.
Sizing and installing a spot ERV
A spot ERV can be very helpful in improving the indoor air quality of your home.
- When dealing with pollutants in the house, a spot ERV improves indoor air quality by diluting the pollutants in the air.
- Spot ERV units have a limited capacity: 20 to 40 CFM.
Based on limited capacity, your best bet would be to install the unit in one of two areas.
- First, you might consider installing it in the room where you spend most of your time. This would most likely be a bedroom. This would give you an area of the house that would be a safe place to spend the bulk of your time with reduced airborne pollutants.
- If you have more than one bedroom, it should be installed in the room with the person who is most vulnerable (elderly, compromised health system, very young, etc.).
- Alternatively, you could install the unit in the area nearest the pollution source. If the source of pollution is the converted garage, you should select the largest habitable space available.
Will a Spot ERV be adequate for your home?
To determine if a spot ERV is adequate, you should determine the quantity of makeup air required to meet industry requirements.
- A simple rule of thumb requires 7.5 CFM per bedroom plus 7.5 CFM plus 0.01 CFM per square foot of conditioned floor space.
- A 2,000-sq-ft, 3-bedroom home would require 50 CFM 24/7 to ensure adequate makeup air.
- That is: 7.5 + 7.5 + 7.5 + 7.5 + (0.01 x 2000) = 50 CFM
You can plug in information concerning your home to determine if a spot ERV is adequately sized for your home.
Address the source of the pollution instead?
Since your question seems to suggest that the IAQ problems are related to the remodel of your garage, it might be prudent to address the source of pollution rather than filtration (HEPA) and/or dilution (ERV).
Since you've had a variety of problems with the installation of the HVAC system, you might want to have the system professionally cleaned, sanitized, and sealed.
Duct sealing would reduce potential communication between your attic and framing cavities. Ducts hidden in finished structures can be sealed using a process developed by AeroSeal. Ducts are sealed using an aerosolized polymer (think radiator Bar's Leak for ducts). The cost varies by the size and condition of the duct system.
For more information:
Read "How can you add fresh-air ventilation to an old home with a forced-air system and only supply ductwork throughout?" a Q&A answered by Danny Kelly.
Also, check out Mick Dalrymple's Q&A "What is the most effective air exchange system I should employ in a new three-level townhouse in Texas?"