microwave exhaust vent

what’s with the microwave exhaust fans that suck the air from the cooktop and blow it back into the room from the top of the microwave? Am i missing something with this setup?

Same as a recirculating vent hood, just another way to get usage out of available space.

reason i asked is cause i did an inspection for an indian family and they asked me about it cause of the spices they use. They didn’t seem to please that it didn’t vent to the outside.

I venture to say the neighbors probably are pleased it doesn’t vent out (on Midway Island all the base galley cooks were Sri Lankans so I have experienced the aromas and cuisine). Most hood vents and microwaves that do not vent outside or through the roof do little more than circulate the same air back into the room after running it through a sad little charcole filter that probably hasn’t been changed since Col. Sanders was a corporal. Basically a waste of time, energy and space. BTW, I HATE curry after having been on Midway.

A range hood of any type is not required. Here’s what I say about the recirculating type: “The kitchen contains a re-circulating range hood. This type of hood does not actually “exhaust”, but moves approximately 40 CFM of air through a filter and returns it to the kitchen.”

My comment on a re-circulating range hood of any type:

“They’re only good as a stove top light!”

Most users have never changed the interior charcoal filter assembly. it’s normally located behind the upper discharge grill. Changing that filter and cleaning the exhaust vent airway will help a great deal in removing any foul odors.

The most efficient way to remove foul odors is divorce, no more in laws, no more odors! Simple really.


The amount of charcoal in one of those flimsy filters is pitiful and will be used up in about two weeks of normal use…so it’s out to buy another at what $10-15!!! The only way to go is “Get it outdoors!!” I find residential ventilation is one of the poorest understood items by the field from homeowners to contractors and inspectors.

Get this: did an 11 month warrranty inspection on a 1.5 million home last April. found none of the 5 bath exhaust fans met code and one was already dead.

Are any of the recirculating hoods ‘convertable’ to external exhaust?

Just about everyone I’ve seen are convertible, but you have to get the original or another OEM blocking piece for the front exhaust also.

The most common error I see with the vented hoods is the size of the recommended ducting (5,6,7" diameter or 3.25" X 10/12") is reduced to 4"…all most people can relate to is 4" dryer duct, I guess! The metal knock-out on the top or rear of the rangehoods/microwave exhauster is usually 3.25" X 10 or 12"- a standard pre-manufactured store shelf piece (know as “wall stack” by many for taking air upstairs in 2X4" walls). What I see is a “boot” (transition piece) of 3.25"X10" at the unit reducing to 4"!!! It starts with 32.5 sq in and ends up with 12.6 sq in…choked off…try and work well now you little…and all this is with a poor, bottom-of-the-barrel $40, no strength, propeller type range hood in the majority of cases. Start with crap, add more crap and wonder why it won’t work well plus being noisy!!!

In Canada, we (since 1995) have had codes that require quiet fans (who’ll turn and leave on a noisy fan?). All bathroom fans must be 2 sones or less and kitchen exhaust fans must be 3.5 sones or less. I go to the big box stores with their “wall of fans” you can try and find 90% or so don’t meet the codes in effect…let the homeowner buy the cheap fan (that’s all he’s willing to pay for it anyways and a fan is a fan…we don’t do that with our cars–a Yugo is not a Lexus but they are all cars) and put it in poorly…we made our money…it’s all we really care about! I just saw a $750 (cost of unit only) brushed stainless steel Italian range hood that you could only really use on low speed due to noise if you were working right beside it

Yeah, but it was Italian. The Gucci of Vent hoods, style is what matters to someone who spends that kind of money on a vent hood, no?


I found this thread on exhaust venting of microwaves while searching for information on a proper installation. I’m looking because my house does not vent outside and I’m looking for information on the proper way to vent it outside. I’ve found a lot on going through a roof and through an exterior wall. I have a two-story colonial, so going straight up would be difficult. The range is not on an outside wall, so going horizontal to an outside wall isn’t practical either. On the opposite side of the range is a laundry room that is cut into a corner of a double car garage that is only one-story. The range is on a non-load bearing wall. I’m just looking for proper options to vent this thing outside, because my wife hates it. If anyone has any links to websites or helpful information I would appreciate the feedback.



Have you considered building a soffit ,Stephen ?
Obviously the only other way is to use charcoal vents($$$) or go up.
There are down draft stoves and ovens or totally redesigning the kitchen if you wished to go that far.
Otherwise enjoy the grease.
Last not as good of an option would be a wall vent.(often found in older homes).


Would it make sense to angle out of the top of the microwave hood and go through the wall in the laundry room and then straight up through the roof of the one story laundry room/garage area? That would be a couple of quick elbows and then straight up.

I can hide pretty easily in that laundry room by hanging a couple of cabinets in there. Exactly how straight up does it have to be in the vertical section of pipe? I’ve never done any ventilation work except to repair some open seams in an HVAC duct.



if we’re talking combo micro/range hood vent get the make and model off your unit and follow mfr recommendations and code

M1503.1 General. Range hoods shall discharge to the outdoors
through a single-wall duct. The duct serving the hood
shall have a smooth interior surface, shall be air tight and shall
be equipped with a backdraft damper. Ducts serving range
hoods shall not terminate in an attic or crawl space or areas
inside the building.
Exception: Where installed in accordance with the manufacturer’s
installation instructions, and where mechanical or
natural ventilation is otherwise provided, listed and labeled
ductless range hoods shall not be required to discharge to
the outdoors.
M1503.2 Duct material. Single-wall ducts serving range
hoods shall be constructed of galvanized steel, stainless steel or
Exception: Ducts for domestic kitchen cooking appliances
equipped with down-draft exhaust systems shall be permitted
to be constructed of schedule 40 PVC pipe provided that
the installation complies with all of the following:

  1. The duct shall be installed under a concrete slab
    poured on grade; and
  2. The underfloor trench in which the duct is installed
    shall be completely backfilled with sand or gravel; and
  3. The PVC duct shall extend not more than 1 inch (25
    mm) above the indoor concrete floor surface; and
  4. The PVC duct shall extend not more than 1 inch (25
    mm) above grade outside of the building; and
  5. The PVC ducts shall be solvent cemented.
    M1503.3 Kitchen exhaust rates. Where domestic kitchen
    cooking appliances are equipped with ducted range hoods or
    down-draft exhaust systems, the fans shall be sized in accordance
    with Section MI507.3.

I dug up the owner’s installation manual on line. It only says you can roof vent it or wall vent it, but doesn’t provide any details of the venting after the connection to the unit.

Rather than us trying to guess it all Stephen this link has exact requirements for installation on these hood models.
Check it out as it will help you.

Back in my install days we would just run flex to a side wall.Often we would ask the client to install cabinets to hide the vent run.
It is really pretty straight forward as there are no worry s about clearance like a furnace vent.


I think you’re dead on about going to the side wall. In the long run, a soffit might be the easiest way to get it done. I’ll check out the shortcut you posted as well.



Easy to do.
Just drill a series of holes with a masonry bit and then chisel it out with a sledge hammer.
Use urethane instead of silicone in my opinion as the silicone does not stay sealed well to the brick often.
Remember to chisel from the outside to avoid spall gaps or pop outs.

If you get any my trick was to use brick pieces and dust rubbed into silicone to blend it.
Extra cabinets are always nice.:slight_smile: