Recently we purchase a combination Samsung microwave / range hood. The venting system is 4-speed using 2 backward impeller fans. I measured the exhaust flow at the highest speed on the output to be ~200 CFM while the unit was still sitting on the ground. We hooked up the unit in its final destination using a 3-1/4-in x 10-in to 5-inch diameter adapter, about 7 feet of aluminized flex duct, then to a roof vent. Even though the instructions said to use 6-in diameter duct, my calcs showed this was not necessary and measurement eventually showed the exhaust at the end of the 5-in pipe before hook up to the roof vent was about 220CFM...i.e. in the range what I had measured initially.

 

Getting to my point...when I measured the face velocity at the extraction point of the metal-mesh filters (which are very porous, not much pressure drop), I am lucy to get 75CFM... hardly enough to bring a stream of vapour into the flow of the exhaust. There seem to be many bypasses for the original 200 CFM, I am not sure why, and Samsung has absolutley no tech support that I can find to comment on the low range hood extraction.

 

Can anyone comment on whether 75 CFM is sufficient for over the range extraction. I have always understood that 100 CFM was the absolute minimum.

 

Many thanks, Roman

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Very few if any microwave range hoods are HVI tested, so you will likely not find an HVI-rated airflow.  

Which device/method did you use to measure the airflow at the inlet and at the vent cap outlet?  There is a lot of variation in the accuracy of different airflow measurement devices; in fact there is significant variation in the accuracy of the same device when used to measure inlet vs. measuring outlet flows.  This may play some role in explaining the discrepancy between the inlet and outlet flow measurements.  

But it sounds like there are multiple inlets, which I understand to be common, especially on microwave range hoods.  This makes measuring accurate airflow at the inlet(s) very difficult. As others have suggested, it's not unusual for actual installed flows to be a fraction of advertised flow, because advertised flow claims are not third-party (HVI) tested.  (And for that matter, most HVI-rated flows are higher than actual installed flows, likely because the duct pressure airflow test conditions (0.10 IWC) are not representative of typical installed conditions (which are probably closer to 0.25 IWC) -- but that's another issue.) 

In general microwave range hoods are problematic.  We need to fix this.  But this doesn't help you in the short term.

   

The rated CFM by Samsung is 400.

We measured the airflow at the exhaust using a shroud and a Reed vane anemometer to be ~200 CFM on the floor before the unit was installed. We repeated the measurement after installation at the end of the 5-in exhaust pipe just before connecting to the roof vent cap. Again ~200 CFM...i.e. the 7 ft pipe and transition pieces had minimal negative effect on CFM.

 

The measured CFM was repeated at the intake to the range hood (see photos); each branch gave 50 - 60 CFM for a total of around 110 CFM. The strength of air flow is so weak that it can hardly raise a Kleenex tissue. There is no perceptible difference in air flow at the intakes whether or not the transition boot and 5-in diameter exhaust pipe is connected to the roof vent cap.I dont believe the duct pressure is very high in our case; the limiting factor appear to be the microwave adapter plate which has an open area of only 16 sq-in.

 

There is also an intake of 25 CFM on the top of the unit to cool the electronics. as well as vents inside the unit which appear to circulate or convect air during cooking. I presume this must make up the balance of the 200 CFM.

 

But in no way can we reach what Samsung quotes as 400 CFM.

 

I agree, it is unlikely there has been any 3rd party testing of these combination range hood / Microwaves; we went to Home Depot and Lowes and tested 12 units. Most are rated between 300-400 CFM, but few can raise and hold a Kleenex tissue.

 

The best of the ones we have seen appear to be Panasonic, but I would be open to suggestions and expereince of others.

 

Roman

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Hi Roman -- this may sound too simple but this test answered some questions for me about my microwave/exhaust combo.  Open your kitchen door or window after you turn your fan on and see if it doesn't improve the draft.  My 1990's built ranch was kinda leaky and then I sealed a bunch of bypasses, wiring & plumbing penetrations, etc.  My kitchen didn't have 200 CFM of air to give the microwave with all the doors & windows in the house closed.  Opening the kitchen door made it instantly better at removing the steam/smoke.  (The basic rule is 1 CFM out = 1 CFM in)

Hello Eric,

Great question! You have run into the dilemma of installing a microwave / range hood! None of them pull enough air(as you said, 100 cfm), as tested by the only trusted authority on appliances and home energy, Consumer Reports.

If there's an all electric range under it, no problem. If it's natural gas, not that much of a problem. If it's a propane based range, real problem! For whatever reason, I have never seen a propane based unit that doesnt put out NOx and SO2, even with the right jet and air shutter tuning. Hope you are able to work this all out.

There are several factors affecting the effectiveness of range hoods, height, length and width, and CFM. A fairly accurate way to measure the CFM flow is tape a corrugated cardboard box across the range hood to completely seal it off, then cut a hole in it, say 10"x10". Then tape a smaller piece of thin material (stiff plastic sheet) over the hole with a 5"x5" hole.  You want a hole with a sharp edge for accuracy.  Stick a manometer probe through the cardboard near a corner to measure the pressure across the hole.  Ideally, you want to be in the 4-8 Pascal range.  Too much pressure suggests the hole is too small and will reduce the CFM you're trying to measure. Keep the hole opening away from the fan intake for more accuracy.

Use the equation CFM = √ΔP x A x 1.07  (square root of the pressure measured times area of the hole times 1.07) to calculate the exhaust flow.  

Dale

We have measured the airflow with a cardboard tunnel and anemometer.

Can you provide a sketch of your method and we will try it.  There is some confusion where you are suggesting in placing the manometer probe in relation to the 5 x 5 orifice and the type of tip required to do this measurement.

Home Energy Magazin has a picture of how to use a box to measure flow at a range hood from an August 2012 article. You can use the Energy Conservatory Flow Meter, or make a similar box and cut a hole.  Simply poke a hole for a manometer probe or hose, no special tip needed, just keep it away from the hole you cut.  You're measuring pressure, the hole size, and calculating the CFM.  Residential Energy Dynamics has an online calculator you can use instead of the equation.

Dale

A duct tester can be used to test a range hood pretty effectively if you can create a box to go under the whole assembly.  The duct tester works as a powered flow hood so the air going through the fan in the hood should equal the air going through the duct tester.

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I second Paul's suggestion to use a duct tester and a cardboard box to measure the range hood flow.  

It sounds like there are multiple inlets on this model, though.  So you have to be sure that the box captures all of them.  

Use a manometer probe to measure the difference in pressure inside and outside the box, and adjust the duct tester fan flow to make that pressure difference go to zero (i.e. match the duct tester fan flow to the range hood flow).  It's cumbersome and makeshift looking -- but this method yields the most accurate measurements.  

Hi Paul and Chris

 

I appreciate your helpfull suggestions regarding different airflow measuring techniques.

 

I chose the anemometer approach due to its simplicity as well as being able to easily average 9 readings over the scanned open face of the short cardboard duct interfaced to the range hood opening as i show in earlier photos. Perfect sealing is not so important as there is not other pressure drop transition to worry about causing significant bypass. In this sense, I dont see any significant physical reason why the results with the anemometer should be appreciably different than if I had measured pressure drop across an orifice or used a duct blaster; can you?

 

The bottom line I fee will still be the same, combination microwave /range hoods have poor airflow, barely able to lift tissue paper at opening to the air intake point.  

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