Q: Jason, My fireplace was constructed with a 42″ metal heatelator box. The walls above the damper taper to a 16″x12″ opening about 28″ above to face. This seems to be the nearest location that has any ledge to grip. However, just above this metal, the ledge has an 18″x16″  dimension that tapers up to a 12″ square flue liner several feet above. Of the sizes you list, only the 15″x36″ would seem to be big enough at this location, but it’s disproportionately long for the 18″ width of that opening. Would maybe consider using a 15″x15″ and an extender, placing it higher up in the chimney? Suggestions, please. .DW

A: Dear DW, You may have a wood burning fireplace insert instead of a built-in Heatelator, but either way this would be my advice… Order a custom 18X18 Chimney Balloon and also get a HEK extender for the valve. This will allow you the reach to get to this higher height in the flue. That area above the metal ledge is where you want to install it as long as that metal edge does not have any dangerously sharp edges. A 18X18 Chimney Balloon with an extender will cost $64.99 including USPS shipping. Custom Chimney Balloons do take 14 days from your order to ship, due to building time. You can call the customer service line if you have any further questions about this particular application. – Jason