Installing OTR question

[I RFD]

I am installing an OTR in my new home. I have removed the builders hood fan.
Question pertains to the opening of the exhaust vent. OTR comes with template which instructs to cut an opening in the bottom of the overhead cabinet. The suggested dimensions of said opening are 11 3/4 X 5. Builder has cut an opening of 10 1/4 X 5. The OTR's exhaust vent (on the top of the OTR) is 10 X 2 3/4.

Question is, why do I need to have such a big opening in the overhead cabinet as suggested by OTRs install instructions if the OTRs vent is already smaller then the opening? Do I need to widen the opening or can I use the existing setup? OTR is GE profile. The vent is vertical.

Also holes need to be put through the tiled backsplash to secure the OTR mounting bracket, is using a "regular" drill bit alright for doing this? as I was told to use "masonry" drill bits, which I presume are those "diamond" drill bits which give you a cleaner cut. However if thats all they do , then the "clean cut" is not so important as the holes are concealed behind the OTR. I want to make sure I dont damage the tiles with the regular drill bits.

[CSK'sMom]

The vent hole needs to be bigger because depending on the installation bracket the microwave will need to be tipped and angled to be put on the bracket. The slightly larger vent hole allows for this. As for drilling the holes through tile... use the proper ceramic tile bit if you don't want to break them. You will probably need to drill at least a couple of those holes large enough to get a toggle through them where they don't hit a stud. And make sure the micro is withim the specs for final height from the stovetop. ;)

[tallAJ]

good explanation CSK...it might be hard to find the studs throught the tile. or you can line up the screws from the overhead cabinet and follow that. code states that your outlet for your OTR must be a dedicated line. your exhaust should be a 6' line also.

[Pete_Coach]

They want the hole in the cabinetry bigger than the OTR vent so that the cabinetry does not become an obstruction in the airflow and the grease being drawn up does not permeate the wood. It would quickly become a fire hazard.

[I RFD]

Thanks for the replies, I see what you guys mean now. So it appears I need to make my existing vent hole in the bottom of the cabinet bigger. How would I go about doing that , use a reciprocating saw? Thanks for answering my noob questions.

[CSK'sMom]

Yes you could use a recip. saw or a jigsaw.