I want to install a Stainless steel chimney liner in my exterior
masonry chimney for my new high efficiency oil boiler. Boiler vent
pipe is 6" dia. Chimney interior is 7"x10".

The boiler installer said I need to install a tee section _inside_ the
chimney where the vent pipe (from the boiler) meets the chimney so
that gases can go up and any condensate can drip down into the
existing chimney cleanout below. He said to break out a section of
the chimney around the existing vent pipe opening in order make enough
space to insert the tee. Does this sound right?

If so, is there any special type of mortar I should use to repair
opening?

Thanks for the advice.

-Theodore