Experimenting first would be wise as the sail switch inside the furnace requires a certain amount of airflow to function without hiccups. Restricting the airflow with a filter material right over the vent holes in furnace body may reduce volume of air enough to trigger the sail-switch while building a framework around the furnace and using multiple rigid furnace filters to encase the furnace in a larger cavity of filter material may not.
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Sandra, Bruce
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