Heatilator TD46B1 question

You'll have to determine what your actual opening will be when you remove
some or all of the guts from the Heatilator. Once you know what will fit,
then you can decide which ones the woman of the house likes.
Make sure that the surround is correctly sized to cover whatever opening
is left after the insert is in. This may also require a firebox riser to make
up for any excess in the opening height.
If the damper unit in the vent system is large enough, you may be able to
leave it in place, & slip the co-linear liners by, but it's best to remove the
damper plate (Sawzall) so you can jam some unfaced fiberglass insulation in there
around & between the liners. Once you remove the refractory material from
the Heatilator, you may find that there are gas "knock-outs" on either sidewall
for gas line access.
So, basically, you run the liners down the vent, secure the cap at the top &
insulate the bottom. It's a good idea to ID the exhaust side cuz if you mix
them up at final vent hookup the wife's not gonna like it.
Run the gas line into & thru the box so you can make your connections to the
insert before you install it. I always pull enough gas line thru so that it reaches
the front of the hearth.
You'll also want to run an electrical feed in at this time for the blower unit or the IPI.
Tapcon or Zipscrew a metal box to the floor of the Heatilator somewhere clear of the insert.
Install an outlet & cover plate.
It's also a good idea to use a piece of the cardboard packaging to set under the unit
to prevent damage to the hearth as you jockey it around.
Set the box in the opening & bleed the gas line while there's nothing in the way
& you can get your nose in there to smell the fuel.
Plug in the lead from the insert.
Light the pilot.
Set up the refractories/embers/logs.
Install the glass, then the surround, then the front.
Make your adjustments on the pilot (if necessary)& install the T-Stat/remote.
Once the newness of the unit burns off & the paint cures, warmth & happiness!
HTH