The Quiet fans have a blower wheel that is almost twice the size as the Fasco Model B30 it replaced, even though the quiet fan turns half the speed and therefore it was supposed to have more CFM than the B30. We have since learned that the quiet fans while rated by the manufacturer at 55CFM (hence the inlet damper cover we installed) actually produce less airflow than the B30 fans, which are rated at 45 CFM (we measured both at zero static pressure). Static pressure in the plenum under the grate will vary as coal size allows more or less air through, but basically the inlet cover on the quiet fans should be removed tp provide proper combustion air. In some extreme cases the quiet fans can cause the Coal-Trol settings to be reduced (both min and max) as some coal burns slower with the slightly lower CFM air flow.
Please do not hesitate to contact us if any reduction in burn performance is noted while using the quiet combustion fan. The good news is that now we have been able to get the folks at Fasco to improve the noise level of the model B30, reducing the need for a quieter fan, they were really pretty noisy before!
As always, thanks to all of you for the feedback and patience as we work to continually improve our products.