above stove microwave venting

I installed a microwave oven above my stove top and vented it to the roof using the internal fan in the microwave. Even on the highest setting it barely sucks smoke up from the stove top. There is 1 90 degree bend in the 6" diameter pipe. The junction of the vent and microwave is a 2 x 8 vent (if I remember correctly).

Just wondering if there is something better I can do? I even thought about pulling the microwave motor out and putting an inline fan in the pipe. Not sure if that would work or even how to large a fan to use should I go that route.

Thanks,
Rut

Did you read the instructions correctly? Most units tell you to invert the motor if you are venting exhaust up instead of the "thru" filters.

Microwave vents have an industry standard of 300CFM and are not recommended to be installed higher than 29" above the cooking appliance.

The rectangular transition, from the MW to vent should be 3.5 X 10", to the vent material.

The 90 will reduce the efficiency, so, if they would fit, two 45's won't have as dramatic an effect.

Unfortunately, because of the internal restriction in the vent path, internally in the MW, a remote in-line exhaust above 400CFM will have little effect.

Whet is the recommended CFM for your cooking appliance, by the appliance MFG?

Have you checked to see if the louvers (or flapper) on the outside of the house are operating?
And hell, i sure hope you mean a microwave range hood and not just any old microwave

Thanks for your replies.

I didn't change the motor around that I remember.
I did have to move a cover from the rear vent opening and put it on top where there was an opening. I'll check the installation manual again (I put this in a year ago). I'm sure the transition is the correct size. It matched exactly the opening on the back of the microwave. I was guessing at the measurements but was close.

The pipe goes straight up through the roof. There is no flap, just the vent cover (solid on top but openings on the sides).

I'm not sure what the stop top cfm recommendation is. Didn't know they came with those specifications.

I'm surprised an inline fan wouldn't be more efficient. I wonder if I removed the internal fan in the microwave, if it would reduce the internal restrictions?

The plate that you moved from the rear to the top is standard for the appliance, however had you instead vented it out the top you would probably have eliminated the 90, and while it's an understandable response to maintain as much usable cabinet space as possible, there are some sacrifices you elect, for efficiency.

Removing the motor still does little to change the narrow and restricted path of air travel
inherent in those appliances. It's a function of the design.

All food preparation appliances will have, from the mfg., a recommended CFM venting, especially those that are fuel gas fired, and while a 30" cooktop may have a recommendation of about 300CFM, a 60,000BYU HOBB may have an 1000CFM.

You can obtain this from the cooking appliance mfg.

The manner in which you prepare food, as well as the height above the cooking appliance also affects the efficiency.

The thought just occurred to me that you would use different filters for the vent, depending on whether it is recirculating air or evacuating it. Could it be something like that?

Refer to page #4 on the attached PDF file on the motor rotation. I know this is probably not your unit, but worth investigating.

https://www.whirlpool.com/digitalassets/WMH75520AS/Installation Instruction_EN.pdf

IMPORTANT: If blower motor is not positioned with flat sides
facing the back of the microwave oven (as shown), performance
will be poor.

I decided to tear into this today and took the microwave down. I wasn't prepared for what I found.

The rear vent at the wall is indeed a 3.5 x 10. However, the vent on the back side of the microwave is much smaller. About 2 x 8. Also, I noticed the flapper on back didn't open all the way when the motor was blowing on its highest setting. I found a couple of metal tabs sticking down from the top causing this issue and bent them out of the way. The flap moves freely now.

Inside the microwave was disgusting. There was a pool of thick grease under the motor. The motor housing and enclosure was covered as well. I started taking things apart and the further I went the more grease I found. It was everywhere.

I took the bottom and side panels off only to find more thick grease. I guess it was sucking some after all. Can't imagine what a fire hazard this could be if a pot caught on fire. Unlikely in my case as I us an induction cook top that doesn't get hot like regular electric burners.

The motor was in correctly and blowing out the back. I didn't go straight up because I have a second floor. Out the back was the only option. With it sitting on the counter and plugged in, the motor runs fast (on highest speed) and blows a lot of air out the back. It just doesn't seem like the draft from the bottom is very concentrated around the intakes.

This isn't surprising after taking it apart. There isn't really any designated air channels in the unit that lead down to the intake filters on bottom. Just opening down the back and sides that lead to the bottom. Basically you are pulling air from the whole unit in a sense. I guess they expect the top, sides, and bottoms to seal off the unit enough that the air will suck up through those holes on the back side.

I also noticed that the vent on top of the microwave, where air would come out if you didn't vent externally, was also covered in a thin film of grease. This leads me to think that it was sucking a good amount of air in these vents as well. When putting it back together, I sealed the top vents off so air would not enter from the top.

Putting it all back together, I can't say I feel any greater suction from the bottom but I guess with all that grease inside it must have been sucking some in the 2 years it has been up.

Bottom line is even though they may be designed to allow for external venting, they do a poor job. And those crappy metal screen filter really don't filter well. This is apparent from all the grease inside the microwave. I wonder if there are better filters available somewhere?

Sorry for this long dissertation but thought I'd share what I found. Not sure there is a good solution to get better venting as long as the microwave stays there. Don't really have anywhere else to put one.

Guess I'll do a better job of keeping those metal filters clean and maybe need to think about taking this thing down once a year or so and clean it out.

We appreciate your post more than you know. I too need to change my vent soon, because the idiot previous owner who installed the unit so far has messed up every installation inside this house.

Dropping the filter into hot water with blue Dawn, on a monthly occurrence, will go a long way to improve the operation.

You can use an incinerated piece of paper to check the venting.

There should be a charcoal filter above the screen.

nealtw said:

There should be a charcoal filter above the screen.

Click to expand...

Unless it is incredibly thin I don't see how it would fit in the filter slot with the metal grease filter.

A charcoal filter is generally used in those models that offer the option of being vented out or recirculating, or are specifically recirculating.

You can run the filters through your dishwasher too.

Mine vents out the back, does a 90 degree turn up to the garage ceiling and then another 90 to take it 10 feet out the back wall of the house. I get plenty of suction at the intake and it opens the flap outside when you turn it on. My unit transitions to 8" round duct immediately after the MW unit.