O.T. Microwave hood Vent

I am installing an over the range microwave/hood vent and I need some advise on the vent ducting. I haven't purchased the microwave yet, but I am pretty confident that I want the vented vs. the charcoal "filtered" vent. This kitchen is very small and heat does become a factor when cooking, thus I want the vented stuff to go outside. At any rate, as I understand it the microwaves come standard with a 10" by 3" opening/stack for the vent, and I have to run that up 3 feet and over 6.5 feet to an exterior wall. Here's where it gets tricky, and I need some advise. This is a brick house with concrete block behind the brick. I am concerned about drilling a 6 inch hole through the wall, that is to say, I am concerned that it would comprimise structural integrity. This house has NO interior studs for the walls, just brick, then block, then plaster. I asked the guys at Lowes if I could reduce the vent pipe from 6 inches down to four, and still be within code, and not hurt anything, and put a smaller hole through my wall, and he said yes. However, the local appliance dealer told me that the vent fan couldn't push enough CFM through a smaller pipe to be effective. So I don't know what to believe, and I am confused. I hope you folks can help me so I can finish this remodel project and get back to more important things like my 8N!!!!
Thanks in advance for any suggestions, and sorry for the long post.

Brad

When I engineered high-end hood systems, I put a small vent (about the size of a dryer vent) somewhere in the kitchen, maybe behind the refrigerator, stove, etc. . This vent opens a little to allow "make-up" air to be drawn into the kitchen, or else the actual workings of a ducted hood become less effective the longer the fan blows.

A tenant can understand about washing the filters. They sometimes even operate the hood without filters in place and then the ducts can get to a point of being a real mess.

So, I encourage ductless hoods in rental properities. Actually in my own house, I prefer a simple old fashioned fan in the wall type.

If you put in a $150-175 microwave range that is fine but think you should just go with a range hood. It will use the same venting and I cannot imagine having to use a 6" duct. I went thru cinder block in the past and just ended up using my air compresser and small air hammer (we are talking about one used for a car) to knock a hole in the block and sealed it up with "GREAT STUFF" expanding foam.

If you go with a carbon filter all that will happen is they will just plug the thing up and not spring for the $20 for a new filter. Then you will just have a house with a bunch of grease and smoke in it.

If it were me, duct the pipe outside and go with an inexpensive broan over the stove duct.

So, there is a relationship between the ventilation fan in the hood and the btu's being generated by the appliance.

It has been awhile since I did some kitchen engineering, but if you contact the stove manufacturer, they can help you with determining the CFM of the hood blower, and, the dia. of the vent pipe.

Funny thing about hood ventilation. You can put a negative pressure in the house and snuff out pilot lights.

I would make the tenant provide their own micro-wave.

As for the block wall.. keep in mind that not all the cells are filled ( with concrete )... many are hollow.. or at best, are filled with insulation.. etc.

People add pet doors all the time... no problems there either.

Soundguy