It should be easy and cheap enough. It really is just cutting a hole and replacing that section. I'd guess $200 but it would really depend how long it is 6 inches.

What you have to wonder is why someone did that? Was there an obstacle? Honestly I think the transition would cost more than doing it 10 inches to begin with.

Could the electrician mean a 4" x 10" rectangular duct? Not everyone understands the square inch area of ducts. Do you mean a range hood requiring a 10" diameter round duct- for a residence? I suspect that some confusion surrounds that belief.

are you changing to a stove vent hood that
requires 10" ducting?

usually high cfm stove vents have larger
venting duct.

is this the case in your situation?

best of luck.

Most Modern-Aire ones require either 8" or 10".

I am attaching the link but you need to click on specs to see them (IDK how to attach a PDF file, sorry).

http://www.modernaire.com/PSL_Liners.html

I know that Futuro and Faber make a lot of 6" ones but the ones I have seen all had mesh filters.

But many other brands who make hoods with baffle filters require at least 8" ducting (to the best of my knowledge), so the labor should be same, regardless of the size, right?

you need to realize that these fans remove as little
as 400 cfm (cubic feet of air per minute) to as
much as 1400 cfm.

removing 1400 cfm of air..like in a commercial application
with multiple burners & oven on for 8+ hours..is
much different from residnetial where burner & oven
usage is much less on most ocassions.

these oversized stove vents are causing problems
in residnetial homes by putting the house under
a negative pressure when the fan is in use.

solutions are..buy smallest cfm fan of this type
not larger..unless you cook on all burners all day
and into the night.
or provide a fresh air intake dedicated to the stove
vent hood.

when the house goes into negative pressure then
make up air takes the path of least resistance.
recessed cans will pull in attic temps & insulation
particles if not air tight, any leakage to outside
or attic will become active. even the cut for the venting
of the fan will pull air into the house.

labor to install should be about the same, but
the fresh air for the fan would be addtional charge.
don't know if it is code in your area or not, but
it is something you should check into.

also, take a look at the sones levels of the fans.
3.8 to 5.9 is pretty loud. don't expect to have
normal conversations near the stove when fan is
on high speed.

personally, I'd look for lower cfm & sones levels.

best of luck.

Yes, I know that. I was going to get one with ~900 cfm and then run it at lower speeds while cooking, reserving the high speed for "extreme" situations like when I burned something and need to vent fast (never mind having a conversation, LOL).

I think that with a 900 cfm hood, I'll be fine without providing a MUA return. There are two windows nearby in case I need MUA and we live in a warmer climate, so opening them wouldn't be a problem even in winter.

Now, another reason for wanting higher CFM was a longer duct run in the attic as we are only allowed to vent to the back of the house. Longer runs need more powerful hoods as I understand.

All I want to know if I can have the duct enlarged w/o spending $$$$. The guys who came to the house to tell me what can or cannot be done, told me they could do any of it but they wouldn't give me the price. It's been weeks and I am very frustrated as I need to order the hood.

Yes, for $1M I can demo the house and build anything I want but that is not the case here as I am on a very tight budget due to other unexpected house repairs. They seem to be missing the point. Urghh...

I hear you. it is frustrating. but just hang in there.

glad you aren't going with high cfm hood.
unless your house is exceptionally tight you shouldn't
need make up air.

things don't always go as planned..for instance..
I had everything in place for my new stove vent
hood to be installed..told SO to close end of installed
venting until the new vent hood came in.

you'd think he would have closed the end
(just a tempory thing) at the end inside
the house...but instead..he closed it at
the attic end. and promptly forgot it was
blocked off.

new hood came in..we installed it..then
for a week he complained that the vent hood
wasn't sucking..so we took it back down
to return it. something made me shine a flashlight
up into the vent..and saw the paper he had
put over the attic end..
LOL!

teased him for days about it. me, I'd have blocked
if off where I could see it (cause I'm forgetful)
...but he got into
the attic and finished venting it out of the attic
space sooner than he had planned.

so it worked out.

on my vent hood the opening was round, & the venting
was round, so we made a transition piece out of
ductboard. we are both diy people & had it on hand
from building supply plenum for hvac system.

hope you get yours installed with minimum of
hassle.

best of luck.