Got a new above the range Samshlong microwave with exhaust fan last June (Model #ME19R7041FS) and it's supposed to have 400CFM fan but it seems to barely pull. It'll hold a paper towel against the screen but steam from a frying pan that's slightly in front of the microwave (over the big burner) just goes straight up and bypasses the microwave exhaust fan altogether.
The grease screens are clean.
The exhaust fan is connected to a round duct through the roof - total lift approx 15' straight up. Duct joints are taped and it should be pretty air tight. I have not yet gone up on the roof but will do that soon.
Otherwise.. WWGRMD?
They should be able to vent straight up, out the back, or out the front. You have to take the fan assembly out of the unit to change the vent direction. It could be turned the wrong way and not getting airflow. You would have to take the microwave down to check.
11GTCS
SuperDork
3/5/24 9:22 p.m.
In reply to OHSCrifle :
So as with many things, it depends. What size duct is connected to the exhaust fan? I see that you have about a 15 foot duct run, are there a bunch of offsets / elbows or is it a pretty straight shot? The fan being rated at 400 CFM will depend on what external static pressure that rating was achieved at; it could be 400 CFM at 0.0" water column (free air) or it could be 400 CFM at 0.5" and there's a pretty significant difference between the motor and fan size required between those two ratings.
Most of the over the stove microwaves I've seen use a small centrifugal fan that are not designed for the higher static of long duct runs so a straight out the wall behind the fan would be the desired installation / best performance. For your situation with a 15 foot run I'd like to see at least an 8" round exhaust duct for 400 CFM especially if there's an elbow or two.
If it helps, we had an old GE over the stove microwave vented out the back wall that worked great. Changed that with a new Kitchenaide that was better in every way except the exhaust part, very similar lack of performance to what you're describing. No change in the duct, just the fan in the microwave. When we did a full gut reno of the kitchen 5 or so years ago we put in a dedicated exhaust hood over the stove and the microwave is a separate appliance under the counter now, so much nicer. Let me know on your duct size, etc and I may have some suggestions.
In reply to 11GTCS :
Believe the microwave's top duct outlet is 3x10 which transitions to 6" round. Possibly 8 I need to check. Definitely not smaller than 6".
The duct runs literally straight up, no elbows. The fan block off is correctly configured for "up". The wall behind the stove is the family room so I can't go directly out the back.
Is there such a thing as an inline booster fan for a kitchen hood?
A real hood not a microwave may be the best answer here.
11GTCS
SuperDork
3/5/24 10:25 p.m.
In reply to OHSCrifle :
Well it sounds like you've got a pretty decent set up for a residential installation. The 3 x 10 is the weak link; if that's just the transition at the microwave and it goes right to 6" round that's way better in terms of airflow / friction loss, if 5-6 feet up in a wall stud cavity not as awesome and maybe the problem. I believe you're describing a 3 x 10 outlet at the fan that goes right to 6" round and that's pretty decent. Nominally a 6" round exhaust would be good for 100-300 CFM depending on the fan and the length of run you have.
Check the dumb things first. Make sure the duct is open at the point it leaves the roof, no stuck back draft damper in the roof vent or bird / insect nests for example. Sometimes there's a backdraft damper right in the microwave so make sure it's not stuck closed. If those check out I was thinking an inline booster as well but it would take a relay to sequence the booster fan with the microwave.
The dedicated hood is awesome, it's basically a mini version of a commercial restaurant hood. I did not ask for the breakout price for the hood from the cabinet contractor....for reasons. LOL.
In reply to 11GTCS :
Thanks. And yes the transition is right on the top of the microwave and that's the outer size on the microwave. I will check the cap this weekend. Hopefully something simple.
Any way to temporarily remove filters and ductwork to check the suck? 400 cfm is a fair amount of air, but as noted above, the small forward curved squirrel cage blowers typical in this application can't stand much resistance to airflow, so transitions, length of run, dampers and filters will all act to reduce that number.
Capture is another aspect altogether, and if your microwave fan is like mine, there is no hood to help contain vapors whatsoever. It would take a crazy amount of air to literally suck out every bit of steam coming off the cooktop. That's why commercial kitchens all have hoods that overhang the cooking surfaces, and the hood would typically be 6" longer and wider than any part of the cooking surface. All mandated by code.
In a previous lifetime, I sold commercial hoods and ventilators (fans) to installers and worked with engineers to specify this equipment.
In reply to 1988RedT2 :
That's a great point about coverage. A traditional hood may be the best solution for containment since the big induction "burner" sits up front.
11GTCS
SuperDork
3/6/24 1:06 p.m.
In reply to OHSCrifle :
As 1988RedT2 points out exhaust is one thing and capture is another. In your picture the front edge of the microwave looks like it's only about halfway out on the width of the stove top below so there might not be much improvement just with increasing the airflow. If you think you might consider the booster fan path something like this may be suitable. Fantech FG6XL - FG Series Round Inline Exhaust Fan, 6" Duct (483 CFM) - SupplyHouse.com
The trick would be integrating a relay into the microwave motor power wiring to sequence the booster fan that would likely need to be installed in the attic.
More than you ever wanted to know about commercial kitchen exhaust hoods:
https://kitchenventilation.com/2019/11/07/what-you-get-for-free-when-specifying-short-circuit-exhaust-hoods/
The "short circuit" concept was very popular when I was in the business. Because exhausting large quantities of air at the hood put a significant load on the heating and cooling systems, manufacturers built hoods that would introduce up to 80% of the total exhaust directly into the hood (make-up air) that eliminated the need to condition that air. The thinking was that the unheated/uncooled air being introduced at the front of the hood would move directly to the back and be exhausted along with cooking vapors. It apparently didn't work terribly well in actual practice.
In reply to 1988RedT2 :
Yep very familiar from my years in a hotel/restaurant/banquet commercial kitchen
thanks again
Ironically I think the question I asked was because it's "too close to home".
11GTCS
SuperDork
3/10/24 8:38 p.m.
In reply to OHSCrifle :
I just checked that link out and agree, that's a nice hood that should do the job. 6" duct recommended per the installation manual, I was surprised they didn't show that on the spec sheet. Built in grease separator and collection cup, 39 db fan sound level so pretty quiet too.
