This might have been asked in controls but, here goes...Have older roof exhaust fans that seem to be causing negative pressure in several kitchens. Might be asking the age old question...speed control of psc motors. Basic 115 vac psc motors (with 3L belts) and have heard that introducing a "speed controller" will cause over heating and thermal ov/ld to open. Have looked into the new vari-green motor from Greenheck and this seems pretty cool. Any ideas besides the expensive JCI S66 or nimbus? Anybody installed the vari-green yet?