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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ASHRAE 62.2 specifies a 100 cfm minimum but some recent experimental work at LBNL suggests that 150-200 cfm might be nearer the mark for most range hoods. The question is - where is the other 125 cfm coming from?  Microwave range hoods often have extra vents in the top and other locations - so you should check these.  Or could the ducts be disconnected?  FInally, are you sure about the 75 cfm into the metal-mesh filters?  How did you measure this air flow?

Thank you for your reply, all connections were disconnected and rechecked, and we are exhausting over 200 cfm.

 

We measured 75 cfm by averaging 9 face-velocity readings for each of the two 4x5-in mesh filters. The readings of the two were comparable. We found another 25 cfm on top-front of the MW cooling the electronics.

 

I am starting to wonder if some of the fan capacity is being used to circulate air inside the microwave since there are perforations on the sides of the inner chamber.

Even if the fan was circulating within the microwave, that doesn't explain the 220CFM output from the roof. How sure are you the ductwork or roof cap isn't causing any restriction? Was the roof cap on when you did the intake test?

We measured the face velocity at the range hood before and after disconnecting the 5-in diameter duct just below the roof vent connector. The face velocity hardly changed by disconecting the 5-in duct, yet we were measuring about 220 CFM from the open duct.

How about pulling the microwave and do the test free standing w/o ductwork? The extra air has to be coming from somewhere or your test method/equipment is faulty.

Assuming I am reading this right...

Bathroom vents typically only extract 75 - 80% of rated due to duct losses & that is when they are done right

You said you went with a smaller vent pipe - that causes issues right there, then you topped it off with using flex which doesn't even meet codes from my recollection - you should be using 6" or larger straight pipe with the least amount of bends & connectors as possible.

Thank you for yor reply; I would normally agree, however:

 

1. when calculating the losses from flex 5-in diamter vs 6-in diameter, it is small compared to the transition losses from rectangular to round transition at the microwave exhaust, as well as the roof vent.

2. The open area immediately at the microwave exhaust consists of only two 2x4-in openings, which is smaller than the effective open area of the 7 ft duct (approx 20 sq-in).

3. The measured airflow at the exhaust of the 7 ft duct is almost identical to the airflow immedialtely at the microwave exhaust (within error of temperature correction).

 

Roman

I done a lot of testing with small fans-  30% of rated CFM is norm.  If two damper in line  it can cut the air flow in half.  My bath fan rated for 50 CFM  now 4"  steel pipe sealed very well,  35 feet of pipe and 2 elbows  one damper, just 11 CFM.   I took up the size to 6" steel pipe and now 38 CFM - WOW.   then very cold out side and water vapor  froze in side pipe dropped to 28 CFM.   For fun map the SP in the pipe each 10 feet.  

We disconnected the exhaust end of the pipe as mentioned above; the air flow is almost the same as at the exhaust flange of the microwave itself ~200 CFM. Dispite the 5-in flex pipe, it is short run - only 7 ft.

is it HVI.org rated?

No, i dont see it rated by HVI.org.

 

I have iploaded the product specs.

 

As you can see, Samsung rates this unit at 400CFM, yet before it was installed over the range, the best we could measure at the exhaust while still on the ground was 200 CFM at full speed and damper open.

 

The service tech cam last night - checked out the entire installation and operation then sent his report stating the installation was excellent.

 

The service tech did not have any solution to offer appart from contacting Samsung. 

Attachments:

I do not see a fan chart or fan curves.   Hard to a test and balance  with out a start point.   

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